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    INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY


    ENVIRONMENT HEALTH CRITERIA 23





    LASERS AND OPTICAL RADIATION





    This report contains the collective views of an international group of
    experts and does not necessarily represent the decisions or the stated
    policy of the United Nations Environment Programme, the International
    Labour Organisation, or the World Health Organization.

    Published under the joint sponsorship of
    the United Nations Environment Programme,
    the International Labour Organisation,
    and the International Radiation Protection Association

    World Health Orgnization
    Geneva, 1982


        ISBN 92 4 154083 4

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    to reproduce or translate its publications, in part or in full.
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    (c) World Health Organization 1982

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    names of proprietary products are distinguished by initial capital
    letters.






CONTENTS

ENVIRONMENTAL HEALTH CRITERIA FOR LASERS AND OPTICAL RADIATION 

1. SUMMARY AND RECOMMENDATIONS FOR FURTHER STUDIES
     1.1. Summary
           1.1.1. Scope
           1.1.2. Optical radiation exposure
           1.1.3. Present health and safety standards
     1.2. Recommendations for further studies

2. DEFINITIONS OF OPTICAL RADIATION
     2.1. The electromagnetic spectrum
     2.2. Interaction of electromagnetic radiation with matter
           2.2.1. Interaction at an interface
                   2.2.1.1   Reflection
                   2.2.1.2   Refraction
           2.2.2. Interaction with a medium
                   2.2.2.1   Transmission
                   2.2.2.2   Attenuation
           2.2.3. Interference, diffraction and scattering effects
                   2.2.3.1   Interference and diffraction
                   2.2.3.2   Scattering

3. SOURCES OF RADIATION
     3.1. Molecular and atomic transitions

4. LASERS
     4.1. The laser medium
     4.2. The pumping system
     4.3. The resonant optical cavity
     4.4. Types of lasers
           4.4.1. Active media
           4.4.2. Temporal modes of operation
     4.5. Spatial (TEM) modes
     4.6. Beam characteristics
           4.6.1. Beam diameter
           4.6.2. Beam divergence
           4.6.3. Beam irradiance versus range for a circular beam
           4.6.4. Hot spots
           4.6.5. Coherence

5. RADIOMETRIC CONCEPTS
     5.1. Radiometric and photometric terminology
     5.2. Extended sources versus point sources
     5.3. Inverse square law

6. RADIOMETRIC AND PHOTOMETRIC MEASUREMENT
     6.1. Introduction
     6.2. Measurement instrumentation
           6.2.1. Thermal detectors
           6.2.2. Quantum detectors
           6.2.3. Detectors to resolve short pulses
           6.2.4. Safety meters
           6.2.5. Spectroradiometers
     6.3. Biological weighting of spectroradiometric data

7. BIOLOGICAL EFFECTS
     7.1. Thermal injury
     7.2. Photochemical injury
     7.3. Threshold of injury
           7.3.1. Means of determining thresholds of injury

8. EFFECTS OF OPTICAL RADIATION ON THE EYE
     8.1. Anatomy and physiology of the human eye
           8.1.1. The cornea
           8.1.2. The lens
           8.1.3. The retina and choroid
     8.2. Spectral properties of the eye
     8.3. Injury to the anterior portion of the eye
           8.3.1. Effects on the cornea
           8.3.2. UVR lenticular effects
           8.3.3. Infrared cataract
     8.4. Retinal injury
           8.4.1. Determining the retinal exposure
                   8.4.1.1   Pupil size
                   8.4.1.2   Spectral transmission of the ocular 
                             media and spectral absorption by the 
                             retina and choroid
                   8.4.1.3   Optical image quality
                   8.4.1.4   Small images
                   8.4.1.5   Retinal pigment epithelium (RPE) 
                             absorption
           8.4.2. Chorioretinal thermal injury
           8.4.3. Location of retinal burns
     8.5. Photochemical retinal injury
           8.5.1. Very long-term exposure
     8.6. Flash blindness
     8.7. Discomfort glare
     8.8. Flashing lights

9. THE SKIN
     9.1. Anatomy
     9.2. Body heat regulation
     9.3. Optical properties
     9.4. Penetration depth and reflection
           9.4.1. Injury to the skin
           9.4.2. The sensation of warmth and heat flow
           9.4.3. Thermal injury threshold for the skin
           9.4.4. Delayed effects
           9.4.5. Ambient environment and heat stress
           9.4.6. UVR effects on the skin
           9.4.7. Photosensitization
           9.4.8. Photoallergy

10. LASER SAFETY STANDARDS: RATIONALE AND CURRENT STANDARDS
     10.1. Introduction
     10.2. Laser hazard classification

11. EXPOSURE LIMITS
     11.1. Rationale
     11.2. Assessment of the "safety factor"
     11.3. Environmental considerations
     11.4. Limiting apertures
           11.4.1. The 1-mm aperture
           11.4.2. The 11-mm aperture
           11.4.3. The 7-mm aperture
           11.4.4. The 80-mm aperture
     11.5. Spectral dependence of exposure limits
     11.6. Repetitively pulsed laser exposure
     11.7. Restriction for special applications (Class 3a)
     11.8. Present standards of exposure
           11.8.1. Laser standards
                   11.8.1.1  Exposure limits
                   11.8.1.2  Repetitively pulsed lasers
                   11.8.1.3  Extended source laser exposure
                   11.8.1.4  Restrictions on ELs
           11.8.2. Standards for non-laser sources
                   11.8.2.1  Introduction
                   11.8.2.2  UVR criteria
                   11.8.2.3  Retinal health criteria
                   11.8.2.4  Retinal thermal risk evaluation
                   11.8.2.5  Retinal blue-light risk evaluation
                   11.8.2.6  IR-A risk analysis
           11.8.3. Infrared standards

12. RISK EVALUATION
     12.1. Laser hazard classification
     12.2. Environmental considerations including
           reflection and the probability of exposure
           12.2.1. Reflections
           12.2.2. Retroreflection
           12.2.3. Optically aided viewing

13. ACCIDENTAL INJURIES

14. CONTROL MEASURES

15. HAZARDS OF LAMP SOURCES AND PROJECTION SYSTEMS

16. PROJECTION OPTICS

17. SAFETY GUIDELINES FOR HIGH-INTENSITY SOURCES

18. WELDING ARCS

19. EYE AND SKIN PROTECTION
     19.1. Laser safety eyewear
     19.2. Welders' filters
     19.3. Eye protection for furnace radiation
     19.4. Eye protection filters for solar radiation
     19.5. Skin-protecting agents for UVR (Sunscreens)
     19.6. Protective garments

20. MEDICAL SURVEILLANCE (RATIONALE)

21. FORMAL TEACHING FOR LASER WORKERS

REFERENCES

GLOSSARY


WHO/IRPA TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR LASERS

 Members

Dr B. Bosnjakovica, Ministry of Health and Environment,
   Rijswijk, Netherlands

Dr L. Court, Commissariat ŕ l'Energie Atomique, Département
   de Protection, Fontenay-aux-Roses, France

Dr P. Czerskia, Bureau of Radiological Health, Food and
   Drug Administration, Rockville, MD, USA

Dr M. Fabera, The Finsen Laboratory, Finsen
   Institute, Copenhagen, Denmark  (Chairman)

Dr M. Garavaglia, Centre of Optical Investigations CIOp,
   La Plata, Argentina

Dr F. Hillenkamp, Institute of Biophysics, Frankfurt
   University, Frankfurt am Main, Federal Republic of Germany

Dr H. Jammeta, Département de Protection, Commissariat ŕ
   l'Energie Atomique, Fontenay-aux-Roses, France

Mr R. Landry, Bureau of Radiological Health, Food and Drug
   Administration, Rockville, MD, USA

Dr J. van der Leun, State University of Utrecht, Institute
   of Dermatology, Utrecht, Netherlands

Dr J. Marshalla, Department of Visual Science, Institute of
   Ophthalmology, London, United Kingdom  (Co-Rapporteur)

Dr Z. Naprstek, Department of Surgery, Institute for
   Clinical and Experimental Medicine, Research Department of
   Surgery, Prague, Czechoslovakia  (Vice-Chairman)

Dr Qin Jianan, Department of Biophysics, Second Shanghai
   Medical School, Shanghai, China

Dr M. Repacholia, Radiation Protection Bureau, Environmental
   Health Centre, Health and Welfare Canada, Ottawa, Ontario, Canada

Dr Y. Skorapad, Institute of Medical Radiology of the
   Academy of Medical Sciences of the USSR, Moscow - Obninsk, USSR

Mr D. Slineya, Laser Microwave Division, US Army Environmental Hygiene
   Agency, Aberdeen Proving Ground, MD, USA  (Co-Rapporteur)

Dr B. Tengroth, Department of Ophthalmology; Director,
   Karolinska Hospital (FACK), Stockholm, Sweden      

---------------------------------------------------------------------------
a   From IRPA/International NIR-Committee

 Members (contd.)

Professor A. Tsyb, Institute of Medical Radiology of the
   Academy of Medical Sciences of the USSR, Moscow - Obninsk, USSR

 Observers

Dr S. Charschan, Western Electric Engineering Research
   Laboratory, Princeton, NJ, USA

 Secretariat

Dr E. Komarov, Environmental Hazards and Food Protection,
   World Health Organization, Geneva, Switzerland
    (Secretary)

 IRPA Secretariat

Mrs A. Duchęne, Commissariat ŕ l'Energie Atomique,
    Département de Protection, Fontenay-aux Roses, France

NOTE TO READERS OF THE CRITERIA DOCUMENTS                          
                                                                   
    While every effort has been made to present information in the 
criteria documents as accurately as possible without unduly        
delaying their publication, mistakes might have occurred and are   
likely to occur in the future.  In the interest of all users of the 
environmental health criteria documents, readers are kindly        
requested to communicate any errors found to the Division of       
Environmental Health, World Health Organization, Geneva,           
Switzerland, in order that they may be included in corrigenda which 
will appear in subsequent volumes.                                 
                                                                   
    In addition, experts in any particular field dealt with in the 
criteria documents are kindly requested to make available to the   
WHO Secretariat any important published information that may have  
inadvertently been omitted and which may change the evaluation of  
health risks from exposure to the environmental agent under        
examination, so that the information may be considered in the event 
of updating and re-evaluation of the conclusions in the criteria   
documents.                                                         


                            *  *  *


     A detailed data profile and a legal file can be obtained from
the International Register of Potentially Toxic Chemicals, Palais
des Nations, 1211 Geneva 10, Switzerland (Telephone no., 988400 -
985850)

ENVIRONMENTAL HEALTH CRITERIA FOR LASERS AND OPTICAL RADIATION

    Further to the recommendations of the Stockholm United Nations 
Conference on the Human Environment in 1972, and in response to a 
number of World Health Assembly resolutions (WHA23.60, WHA24.47, 
WHA25.58, WHA26.68) and the recommendation of the Governing Council 
of the United Nations Environment Programme, (UNEP/GC/10, 
3 July 1973), a programme on the integrated assessment of the 
health effects of environmental pollution was initiated in 1973.  
The programme, known as the WHO Environmental Health Criteria 
Programme, has been implemented with the support of the Environment 
Fund of the United Nations Environment Programme. 

    A joint WHO/IRPA Task Group on Environmental Health Criteria 
for Lasers and Optical Radiation met in Paris from 1-5 June 1982.  
Dr E.I. Komarov, Division of Environmental Health, WHO, opened the 
meeting on behalf of the Director-General, and Dr H. Jammet, 
Chairman of IRPA/INIRC made some introductory comments.  The Task 
Group reviewed and revised the draft criteria document, made an 
evaluation of the health risks of exposure to lasers and optical 
radiation, and considered rationales for the development of 
exposure limits. 

    In November 1971, the WHO Regional Office for Europe convened a 
Working Group meeting in The Hague which recommended,  inter alia,  
that protection of man from laser radiation hazards should be 
considered a priority activity in the field of non-ionizing 
radiation protection.  To implement these recommendations, the 
Regional Office has prepared a publication on "Nonionizing 
radiation protection", which includes a chapter on laser radiation 
(Suess, ed., 1982).  In October 1974, the Regional Office convened 
a Working Group in Dublin, hosted by the Government of Ireland, to 
discuss laser radiation hazards.  This provided one of the first 
opportunities for the exchange of information on the biological 
effects of laser radiation and threshold data, at an international 
level. 

    The International Radiation Protection Association (IRPA) 
became responsible for NIR activities in 1974 by forming a Working 
Group on Non-Ionizing Radiation which became the International Non-
Ionizing Radiation Committee (IRPA/INIRC) at the IRPA meeting in 
Paris in 1977 (IRPA, 1977).  Dr M. Faber, Dr J. Marshall, 
Mr D. Sliney (members of IRPA/INIRC) and Dr L. Court, all acting as 
WHO temporary advisers, prepared the draft criteria document on 
lasers and optical radiation during 1980-81, and revised it after 
receiving comments from the national focal points for the 
Environmental Health Criteria Programme and individual experts.  Dr 
Marshall and Mr Sliney were responsible for the final scientific 
editing.  The Secretariat gratefully acknowledges the work of these 
experts without whose help the document could not have been 
completed. 

    The document is based primarily on original publications listed 
in the reference section.  Additional information was obtained from 
a number of general reviews, monographs, and proceedings of 
symposia including:  Urbach, ed. (1969), Goldman & Rockwell (1971), 
Wolbarsht (1971, 1974, 1977), Sliney & Freasier (1973), Fitzpatrick 
(1974), Magnus (1976), Rubin (1977), Parrish et al. (1978), Lerman 
(1980a), Pratesi & Sacchi, ed. (1980), Sliney & Wolbarsht (1980), 
Williams & Baker, ed. (1980), Goldman, ed. (1981), and Goldman et 
al. (1982).  Radiometric terms, units, and spectral band 
designations used in this criteria document are in accordance with 
the SI recommendations (Lowe, 1975) and those recommended by the 
Commission Internationale de l'Eclairage (CIE, 1970). 

    Modern advances in science and technology have changed man's 
environment, introducing new factors which, besides their intended 
beneficial uses, may also have untoward side effects.  Both the 
general public and health authorities are aware of the dangers of 
pollution by chemicals, ionizing radiation, and noise, and of the 
need to take appropriate steps for effective control.  The rapid 
growth of electro-optics and laser technology and the increasing 
use of electro-optical devices and lasers, including optical 
scanning equipment, high-intensity lamps, welding arcs, and UV 
photo-curing equipment, alignment lasers, and medical lasers have 
increased the possibility of human exposure to optical radiation 
and, at the same time, concern about health effects. 

    This document provides information on the physical aspects of 
electromagnetic radiation in the optical spectrum, within the 
wavelength range of 100 nm - 1 mm.  Optical radiation includes 
ultraviolet radiation (UVR) from approximately 100 nm to 400 nm, 
light (visible radiation) from approximately 400 nm to 760 nm, and 
infrared radiation from approximately 760 nm to 1 mm.  Each of 
these spectral regions can be arbitrarily divided into subregions.  
Lasers are capable of producing optical radiation in all three 
major divisions of the optical spectrum.  A brief survey of lasers 
and other man-made sources of optical radiation is presented.  It 
is known that optical radiation interacts with biological systems 
and a summary of knowledge on biological effects and health aspects 
has been included in this document.  In a few countries, concern 
about occupational and public health aspects has led to the 
development of radiation protection guides and the establishment of 
exposure limits for laser radiation and UVR.  Several countries are 
considering the introduction of recommendations or legislation 
concerned with protection against untoward effects from non-
ionizing radiation in the optical spectrum.  In others, efforts are 
being made to revise and update existing standards.  It is hoped 
that this criteria document may provide useful information for the 
development, at a national level, of protection measures against 
non-ionizing radiation. 

    Details of the WHO Environmental Criteria Programme, including 
definitions of some of the terms used in the documents, may be 
found in the general introduction to the Environmental Health 
Criteria Programme, published together with the environmental 
health criteria document on mercury ( Environmental Health Criteria 
 I - Mercury, Geneva, World Health Organization, 1976), now available 
as a reprint. 

1.  SUMMARY AND RECOMMENDATIONS FOR FURTHER STUDIES

1.1.  Summary

1.1.1.  Scope

    The potential hazards of optical radiation from wave-lengths 
between 100 nm and 1 mm, i.e., ultraviolet radiation (UVR), visible 
light, and infrared radiation (IR) are considered in this document, 
and known adverse health effects, standards, and control measures 
are reviewed.  Emphasis is placed on the health risks of laser 
radiation, but those of other sources are also covered.  The health 
effects of UVR are discussed only briefly, as UVR has already been 
considered in depth in the WHO Environmental Health Criteria 14 
(1979).  Risks to the general population are considered as well as 
those of occupational exposure. 

    The clinical treatment of different disorders or non-medical 
problems, such as cosmetic surgery, where risk versus benefit must 
be addressed, is outside the scope of the document.  However, it 
should be emphasized that the doses used in such exposures are 
entirely the responsibility of the persons authorized to give such 
treatment. 

    Although a certain amount of light is necessary for human 
health, this document does not attempt to determine a lower 
exposure limit or whether certain wavelengths are more necessary 
than others. 

1.1.2.  Optical radiation exposure

    Despite the great increase in the use of man-made optical 
sources, the sun remains the principal source of optical radiation 
exposure for man.  Though the development of the laser in 1960 
aroused great interest in the potential hazards of optical 
radiation, many other artificial sources pose similar hazards.  It 
is often more difficult to evaluate the risks of non-laser sources 
since, typically, they emit over a broad band of wavelengths.  When 
broad-band sources emit in all parts of the optical spectrum, each 
of the potential hazards must be considered separately, as well as 
collectively. 

    Beneficial effects of sunlight and UVR for man have been 
reported in the literature and are treated in the WHO Environmental 
Health Criteria 14, Ultraviolet Radiation.  The reported beneficial 
effects of medical and environmental exposure are important to 
public health, but a careful benefit versus risk analysis must be 
carried out. 

    Because optical radiations are not very penetrating, the eye 
and the skin are the organs of concern.  The main acute effects are 
photokeratitis and thermal and photochemical retinal injury for the 
eye, and erythema and burns for the skin.  Delayed effects include 
cataractogenesis and possible retinal degeneration for the eye, and 
accelerated aging and cancer for the skin. 

    The biological effects of all optical radiation can be divided 
into three major categories:  thermal (including thermo-
mechanical), photochemical, and direct electric field effects.  At 
threshold levels, the predominant mechanism depends on maximal 
exposure rates, total exposure, and on wavelength regimes.  The 
thermal effects are characteristic of the IR region extending into 
the visible.  The photochemical effects are mainly characteristic 
of the ultraviolet region, but also occur in the visible.  Acoustic 
and other anomalous effects depend on acute thermal impulses of 
nanoseconds (ns) duration, which may induce acoustic or mechanical 
transients, damaging the tissue.  For sub-ns exposures, direct 
electric field (non-linear) interactions with biological molecules 
appear to play a major role in the mechanism of injury. 

    Sources of optical radiation exposure may be categorized as: 
    (a)  sunlight (natural illumination);
    (b)  lamps;
    (c)  lasers;
    (d)  other incandescent (warm-body) sources.

    In industry, in addition to lasers, there are continuous 
optical radiation sources, such as compact arc lamps (as in solar 
simulators), quartz-iodide-tungsten lamps, gas and vapour discharge 
tubes, electric welding units, and pulsed optical sources such as 
flash lamps used in laser research and photolysis, exploding wires, 
and super-radiant light.  Common lasers and their applications are 
listed in Table 1 and some sources of optical radiation exposure 
and potentially exposed populations in Table 2.  In most 
applications, maintenance and evaluation workers may be exposed.  
The general population may also be exposed on occasion, and it is 
the responsibility of the operator to minimize this exposure. 

    Until the advent of the laser, the principal hazard recognized 
in the use of optical sources was the potential for injury of the 
skin and eye from exposure to UVR at wavelengths of less than 320 
nm.  The spectral band of less than 320 nm is often called the 
"actinic ultraviolet" and consists principally of the 2 bands known 
as UV-B and UV-C.  The high attenuation afforded by many optical 
materials, such as glass in the spectral range 100 - 300 nm, 
generally resulted in the empirical safety approach in which 
optical sources were enclosed in glass, plastic, or similar 
materials to absorb this actinic radiation.  If injurious effects 
were noted, the thickness of the material enclosing the source or 
the filter protecting the eye was increased. 

    The widespread use of sources that emit high levels of UV-C/B 
in industry has been the cause of many corneal injuries.  The UVR-
rich industrial sources circumvent the natural defences of the body 
by allowing direct exposure of the cornea at normal angles of 
incidence, unshielded by the brow or eyelids.  In many cases, the 
hazards of these UVR-rich sources are greater as they are 
incorporated into optical systems, the elements of which are 
selected for either high transmission or high reflection in the 
UVR.  Welding is a prime example of potentially hazardous 

industrial exposure. The presence of possible photosensitizers 
makes the use of UVR in the chemical industry for the manufacture 
of photosetting plastics potentially much more dangerous. 

    Until recently, it was felt that chorioretinal injury would not 
result from exposure to visible light in industrial operations.  
Indeed, this is still largely true, since the normal aversion 
response to high brightness light sources (the blink reflex and 
movement of the head and eyes away from the source) provides 
adequate protection against most bright visual sources.  However, 
the recent increased use of high intensity, high radiance optical 
radiation sources with output characteristics that differ 
significantly from those seen in the past may present a serious 
potential for chorioretinal injury.  The recent findings of 
photochemically-induced retinal injury, following long-term 
exposures, reinforce this conclusion. 

    Since organic macromolecules absorbing the radiant energy would 
have broad spectral absorption bands, the monochromatic nature of 
laser radiation would not be expected to create any different 
effects from those of radiation emitted by conventional sources; 
this conclusion is strongly supported by experimental evidence.  
The coherence of laser radiation is also considered not to affect 
the hazard potential for thermal or photochemical chorioretinal 
injury.  Though a speckle pattern resulting from the interference 
effects of laser light at the retina does exist, the very fine 
gradations in retinal irradiance resulting from this effect 
(Considine, 1966; Fried, 1981) would certainly be lost, as soon as 
the pulse duration was greater than a few microseconds (µs).  Both 
thermal conduction and ocular tremor would smooth out the 
distribution of light and localized temperature elevations 
resulting from the 1-10 µm gradations of the speckle pattern and 
these non-uniformities would be blurred.  Chorioretinal injury from 
either a laser or a non-laser source should not differ, therefore, 
if image size (retinal irradiance distribution), exposure duration, 
and wavelength are the same. 

1.1.3.  Present health and safety standards

    Because of widespread concern regarding laser hazards, 
substantial progress has been made towards the development of both 
product performance standards and human (both occupational and 
general population) exposure limits.  Separate environmental 
quality standards are unnecessary.  Several national standards have 
been promulgated and substantial progress has been made towards 
international agreement in these areas, since there appear to be 
only minor differences between the most recent national standards.  
The laser exposure limits are complex functions of wavelength, 
exposure duration, and viewing conditions and cannot be summarized, 
without the use of complex tables.  Based on present knowledge, 
most of these extensive sets of laser standards appear adequate for 
the protection of the health of those potentially exposed.  Several 
areas of concern still exist regarding exposure limits for 
ultrashort pulse, repetitive pulse, long-term, and multiwavelength 
exposures. 


Table 1.  Common laser devices and applications
----------------------------------------------------------------------
Type                       Wavelength(s)    Applications
----------------------------------------------------------------------
argon (Ar)                 458-515 nm       instrumentation;
                           + 350 nm         holography;
                                            retinal photocoagulation;
                                            entertainment
----------------------------------------------------------------------
carbon dioxide (CO2)       10.6 µm          material processing;
                                            optical radar/ranging;
                                            instrumentation;
                                            surgery techniques
----------------------------------------------------------------------
dye(s)                     variable 350 nm  instrumentation
                           1 µm
----------------------------------------------------------------------
excimer lasers             180-250 nm       laser pumping;
                                            spectroscopy
----------------------------------------------------------------------
gallium arsenide (GaAs)    850-950 nm       instrumentation ranging;
                                            intrusion detection;
                                            communications;
                                            toys
----------------------------------------------------------------------
helium cadmium (HeCd)      325, 442 nm      alignment;
                                            surveying
----------------------------------------------------------------------
helium neon (HeNe)         632.8 nm         alignment;
                                            surveying;
                                            holography;
                                            ranging;
                                            intrusion detection;
                                            communications;
                                            entertainment
----------------------------------------------------------------------
neodymium glass            1.06 µm          material processing;
(Nd-glass)                                  instrumentation;
neodymium yttrium-                          optical radar/ranging;
aluminium garnet (Nd-YAG)                   surgery
----------------------------------------------------------------------
ruby                       694.3 nm         material processing;
                                            holography;
                                            photocoagulation;
                                            ranging
----------------------------------------------------------------------

Table 2.  Some examples of optical radiation exposure
---------------------------------------------------------------------------
Sources        Principal wave-    Potential        Potentially exposed
               length bands       effects          populations
               of concern
---------------------------------------------------------------------------
sunlight       ultraviolet (UV),  skin cancer;     outdoor workers
               visible            cataract;        (e.g., farmers,
               near-infrared      sunburn;         construction
                                  accelerated      workers); sun-
                                  skin aging;      bathers; general
                                  solar retinitis  population
---------------------------------------------------------------------------
arc lamps      UV, visible,       photokeratitis;  printing plant
(Xe, Xe-Hg,    near-infrared      erythema;        camera operators;
Hg)                               skin cancer;     optical laboratory
                                  retinal injury   workers;
                                                   entertainers
---------------------------------------------------------------------------
germicidal     actinic, far UV    erythema;        hospital workers;
(low-pressure                     photokeratitis;  workers in sterile
Hg)                               skin cancer      laboratories
---------------------------------------------------------------------------
medium-        UV-A and blue      retinal injury   street lamp
pressure       light                               replacement
Hg-HID lamps                                       personnel; gymnasium
(broken        actinic UVA        photokeratitis;  users; general
envelope)                         erythema         population
---------------------------------------------------------------------------
carbon arcs    UV, blue light     photokeratitis;  certain laboratory
                                  erythema         workers; search light
                                                   operators
---------------------------------------------------------------------------
He-Ne lasers   visible            retinal injury   construction workers;
(0.5-5.0 mW)                                       users of alignment
                                                   lasers; some members
                                                   of general population
---------------------------------------------------------------------------
argon laser    visible            retinal injury,  observers and operators
1-20 W                            localized skin-  of laser light shows;
                                  burns            laboratory workers;
                                                   medical personnel
---------------------------------------------------------------------------
metal halide   near UV, visible   cataract;        printing plant
UV-A lamps                        photosensitive   maintenance workers;
                                  skin reactions;  integrated circuit
                                  retinal injury   manufacturing workers
---------------------------------------------------------------------------
sunlamps       ultraviolet,       photokeratitis;  suntan-parlour
               blue light         erythema,        customers; home users
                                  accelerated
                                  skin aging;
                                  skin cancer
---------------------------------------------------------------------------

Table 2.  (contd.)
---------------------------------------------------------------------------
Sources        Principal wave-    Potential        Potentially exposed
               length bands       effects          populations
               of concern
---------------------------------------------------------------------------
welding arcs   ultraviolet        photokeratitis;  welders' helpers;
               and blue light     erythema;        welders
                                  UV cataract;
                                  retinal injury
---------------------------------------------------------------------------
ruby or        visible            retinal injury   scientific
neodynium      near-infrared                       investigators;
laser                                              military personnel
rangefinders
---------------------------------------------------------------------------
industrial     infrared           radiant heat     steel mill workers;
infrared                          stress;          foundry workers;
sources                           infrared         workers using
                                  cataract         infrared drying
                                                   equipment
---------------------------------------------------------------------------
                                                              
    Health and safety standards for lamps and other non-laser 
sources are almost non-existent.  Some exposure limits have been 
proposed for ultraviolet, visible, and near-infrared radiation, but 
these are quite tentative.  The spectrum of the source must be 
measured and weighted against several action spectra for risk 
analysis - a complex process.  Progress has been made, in several 
countries, towards product performance safety standards for 
specific lamp products such as high intensity discharge (HID) 
lamps, sunlamps, and germicidal lamps. 

1.2.  Recommendations for Further Studies

    The following comments cannot hope to be comprehensive in an 
area of such rapidly expanding technology and whilst many of the 
listed problems may be currently of importance or under 
investigation in various research laboratories, others hitherto 
unsuspected may assume paramount importance.  Current problems are 
discussed in the same order as the list of contents of this 
criteria document, beginning after the background information 
sections 1 - 5; the order does not assert priority ratings. 

    (a) Radiometric and photometric measurement

    Further development of simplified, inexpensive laser or broad-
band survey instruments is desirable for monitoring the health 
risks of optical radiation. 

    (b) The eye

    The transmission characteristics of the ocular media are based 
on averaged data from relatively few eyes.  The variations with 
age, in transmission and absorption in individual ocular 
components, have not been clearly defined. Present understanding of 
the effects of UVR on the cornea and the lens is poor, particularly 
of the role, if any, of UV-A in the exacerbation of cataracts.  
Further studies, especially in the field of epidemiology, are 
needed to establish the possible involvement of short-wavelength 
optical radiation in accelerating senile degenerative conditions in 
the retina.  The special problems of the aphakic (lens-less) eyes 
or eyes with intraocular lens implants require attention in 
relation to the increased retinal exposure to UVA and short-
wavelength visible radiation, particularly in the elderly.  Some 
further work is required concerning the spectral dependence of both 
retinal damage and changes induced in the vitreous between 750 and 
950 nm.  It would also be of benefit to obtain a better 
understanding of the role of the choroid in both the absorption of 
optical radiation and production of damage and its involvement in 
the healing process.  In conclusion, further studies must be 
undertaken on the quantification of the upper limits of flash 
blindness and persistent after-image production and the lower 
limits for oedema and irreversible damage. 

    (c) The skin

    The optical properties of the skin require further study on the 
relationship between penetration depth and absorptivity and 
wavelength, skin pigmentation, and the angle of incident radiation.  
Epidemiological studies should be undertaken to further clarify the 
involvement and wavelength dependence of chronic exposure to 
optical radiation in the induction of skin cancers.  Such studies 
should be encouraged in areas where direct comparison can be made 
between negroid indigenous and Caucasian immigrant populations in 
tropical and sub-tropical countries.  Work is also required on the 
possible additive or synergistic effects of different wavebands, 
for example UVB, and UVB plus UVA.  Finally a better understanding 
is required of the additional protective factors that must be 
applied to counteract the effects of specific photosensitizers 
(WHO, 1979). 

    (d) Exposure limits (ELs)

    To date, the exposure limits and various recommended standards 
have been based mainly on empirical studies of acute effects on 
animals and extrapolation of limited epidemiological information.  
While these figures represent the best current knowledge, it should 
be emphasized that standards should be sufficiently flexible to 
enable the rapid incorporation of new data.  Information is lacking 
in many fields, especially with regard to the long-term health 
risks associated with the adoption of present standards. 

    Further information on chronic effects is required and should 
be obtained from multicentre epidemiological studies. Such studies 
could be either retrospective or prospective but should clearly 
isolate ethnic, environmental, sociological, and age-related 
variations within participating populations. The systemic effects 
of optical radiation have not been adequately studied.  These 
investigations should also take into account the effects of 
progressively increasing exposure to artificial sources in industry 
and the home.  Active liaison is required between architects, 
illumination engineers, and health physicists to establish exposure 
limits and recommended lighting levels in relation to a variety of 
visual tasks. 

    The large number of variable parameters associated with 
repetitively pulsed exposures means that present ELs have been 
established in relation to a limited number of research studies.  
Further studies are needed and should include the problems of 
repeated exposures to a single system and repeated exposures to 
several sources within a relatively short period, i.e., a working 
day. 

    The difficulties involved in measurement of ultra-short, 
(sub-ns) pulses have resulted in few bioeffect studies being 
undertaken and thus a large degree of extrapolation in establishing 
ELs.  More work is required and a better understanding is needed of 
the bioeffects related to non-linear optical effects. 

    (e) Evaluation and control measures

    In many countries, more than one executive office has some 
responsibiities for regulating optical radiation exposure and 
optical sources.  Lack of clearly defined division of 
responsibilities between different agencies has created confusion 
for manufacturers and users of lasers and lamp sources.  National 
agencies should make every effort to work towards uniform and 
compatible standards.  Clear-cut criteria are needed to define 
conditions under which lasers can be used in public places. 

    Current efforts to achieve international harmonization of laser 
classification and control of health risks should be encouraged and 
extended. 

    Radiation product performance and user standards should be 
developed for lamps and lighting systems. 

    (f) Laser accidents

    With the exception of a few published cases, medical and 
biophysical details relating to laser accidents are difficult to 
obtain.  It would be helpful if individual countries established 
national accident-reporting protocols together with a central 
referral agency, in order to provide statistical evaluation of 
problems in safety procedures. 

    (g) Eye protection

    The investigation of new types of eye protection filters should 
be undertaken and further attempts should be made to standardize 
existing filters on an international basis (e.g., welding filters, 
laser safety goggles). 

    (h) Medical surveillance and epidemiological studies

    Occupational medical surveillance of workers may be necessary 
in certain cases; of great importance is the need for 
epidemiological studies on workers exposed for long periods to UVR 
and visible radiation.  A study of central visual function and 
colour vision in comparison with an age-matched control group would 
be very informative.  An epidemiological study of workers exposed 
over long periods to infrared radiation is also needed. 

    (i) Education

    Since control measures for unenclosed lasers rely largely on 
the laser operator, training programmes must be instituted for such 
individuals.  Education of the general population is also required 
both to allay unwarranted fears of accidental laser exposure and to 
give some background information against which elective optical 
radiation exposures (medical and paramedical, e.g., cosmetic) may 
be assessed. 

2.  DEFINITIONS OF OPTICAL RADIATION

2.1.  The Electromagnetic Spectrum

    Electromagnetic radiation consists of oscillating electric and 
magnetic fields.  Radio frequency (including microwave), infrared, 
visible (lighta), ultraviolet, X, and gamma radiation are all 
electromagnetic radiation and are propagated in both free space and 
matter.  Collectively, this electromagnetic radiation forms the 
electromagnetic spectrum, when arranged according to frequency or 
wavelength.  A chart of the spectrum is shown in Fig. 1. 

FIGURE 1

    Equation 1 can be modified for electromagnetic radiation by 
giving the velocity of the radiation the value of the velocity of 
light, usually written as c.  In a vacuum: 

    co = lambda nu                                     Equation (1)

    The velocity rhoo has been set as 299792458 m/s or about 3 x 108 
m/s = 3 x 1010 cm/s.

    The ratio of the velocity of light co in a vacuum to the 
velocity c in a medium is termed the refractive index n of that 
medium (n = co/c).  Equation 1 can also be expressed as 

    lambda = c/nu, or nu = c/lambda                    Equation (2)


-------------------------------------------------------------------
a Light by definition is visible radiation; hence, it is incorrect 
  (but commmon) to speak of "ultraviolet light" or "infrared 
  light".

The inverse relationship between frequency and wavelength is      
clearly evident in Fig. 1.  If n is constant at all points within 
the medium, then the medium is called optically homogeneous;  if n
is independent of direction, the medium is termed isotropic.  If n
is considered independent of the amplitude or intensity of the    
optical field, the interaction with the medium is termed "linear";
if not, it is "non-linear".                                       

    As the frequency increases from microwave radiation through the 
optical radiations to gamma radiation, the wavelength becomes       
shorter and shorter.  The electromagnetic radiations have a         
characteristic energy associated with each photon and the photon    
energy increases with an increase in frequency.  Reference to one   
region or another as the "gamma-radiation region" or the "microwave     
region" is arbitrary and no internationally accepted set of terms   
exist for specifying all of the spectral regions.                   

    The spectral bands represent wavelength intervals within which 
a common state of the art and technology exists in sources, 
detectors, or in modes of interaction with matter.  The upper and 
lower limits of the entire electromagnetic spectrum have not been 
defined at present.  The units used to describe energy, wavelength, 
and frequency often differ between spectral regions, as a matter of 
convention. 

    Ultraviolet, visible (light), and infrared radiation, 
collectively known as optical radiation are described in terms of 
wavelength.  Sometimes, the spectral region of wavelengths shorter 
than approximately 100 nm is termed ionizing radiation, and 
wavelengths longer than 100 nm are placed in the non-ionizing 
radiation spectrum.  These terms are useful for those who wish to 
distinguish  between the biological effects of different types of 
radiation, but divisions between adjacent spectral bands vary 
according to different disciplines.  For the physicist, the optical 
spectrum generally consists of 5 decades of wavelengths between 
10 nm and 1 mm.  On the other hand, photobiologists and health 
specialists, who are not concerned about vacuum ultraviolet 
radiation, begin at approximately 180 - 200 nm (which is the 
approximate long-wave edge of the vacuum ultraviolet) and go to 
far-infrared radiation at 1 mm.  The Commission Internationale de 
l'Eclairage (CIE) Committee on Photobiology has provided spectral 
band designations that are quite convenient in discussing 
biological effects.  Three common schemes of dividing the optical 
spectrum are given in Table 3 (CIE, 1970). 

2.2.  Interaction of Electromagnetic Radiation with Matter

    Electromagnetic radiation interacting with matter is absorbed, 
transmitted, reflected, scattered, and diffracted. In most 
instances, one of these effects dominates, almost to the exclusion 
of others.  However, all effects are always present to some extent.  
For instance, if a beam of light passes through a sheet of 
transparent glass, at least 4% of the incident light is reflected 
from each surface.  On the other hand, only a very small percentage 
(less than 1%) is usually absorbed within the clear glass, even 

when marked refraction or bending of the light takes place.  
Similar effects occur in all spectral regions including the 
radiofrequency and gamma-radiation bands. 

2.2.1.  Interaction at an interface

2.2.1.1.  Reflection

    Reflection takes place at an interface.  There are two basic 
types of reflections that are of interest, i.e., specular (mirror-
like) and diffuse.  Specular reflection is sometimes referred to as 
regular reflection.  With specular reflection from a mirror or 
other very smooth surface, light obeys the law of reflection, which 
states that the angle of reflection equals the angle of incidence. 

Table 3.   Several schemes for dividing the optical spectrum
-----------------------------------------------------------------------
Physical No. 1       Physical No. 2         Photobiological (CIE)a
-----------------------------------------------------------------------
extreme UVR          vacuum or extreme UVR  UV-C
(1-10 nm to 100 nm)  (1-10 nm to 180 nm)    (100 nm to 280 nm)

far UVR              middle UVR             UV-Bb
(200 nm to 300 nm)   (180 nm to 300 nm)     (280 nm to 315 nm)

near UVR             near UVR               UV-Ab
(300 nm to 400 nm)   (300 nm to 400 nm)     (315 nm to 380-400 nm)

light                light                  light
(380 nm to 760 nm)   (400 nm to 700 nm)     (380-400 nm to 760-780 nm)

near IR              near IR                IR-A
(760 nm to 4000 nm)  (700 nm to 1200 nm)    (760-780 nm to 1400 nm)

middle IR            middle IR              IR-B
(4 µm to 14 µm)      (1.2 µm to 7 µm)       (1.4 µm to 3 µm)

far IR               far IR                 IR-C
(14 µm to 100 µm)    (7 µm to 1 mm)         (3 µm to 1 mm)

submillimetre
(100 µm to 1 mm)
-----------------------------------------------------------------------
a   Based on the recommendation of the Committee on photobiology of the
    Commission Internationale de l'Eclairage (CIE, 1970).  The scheme 
    was originally proposed by W.W. Coblentz of the US National Bureau 
    of Standards in the 1930s.

b   The dividing line between UV-B and UV-A is often taken as 320 nm, 
    but may be taken as 315 nm.  For the purposes of this document, 
    315 nm is used unless otherwise stated.

    Specular reflection can occur, when the size of surface 
irregularities or roughness is less than the wavelength of the 
incident radiation.  This description of specular reflection is 
important to keep in mind.  Diffuse reflection occurs, when  the 
surface irregularities are randomly oriented and are much  greater 

than the wavelength of the incident radiation; for example, when 
light is reflected from chalk or a rough granite surface.  Perfect 
diffuse reflection obeys Lambert's Law, i.e., the Cosine Law of 
Reflection.  A useful formula in radiometry is: 

    E = phi rho x cos theta/pi r12                     Equation (3)

where E is the irradiance reflected from the surface at angle theta 
relative to the surface's normal, phi, the optical beam power upon 
the surface, rho, the diffuse reflection coefficient of the surface 
for the wavelength, r1 the distance from the beam spot on the 
diffuse target to the detector, and phi equals 3.14159. 

    It is important to remember that diffuse and specular 
reflections are strongly dependent on wavelength.  A given surface 
may produce a reflection that is specular at one wavelength but may 
or may not be specular at a different wavelength. 

    The fraction of incident radiation specularly reflected from 
the surface of a transparent medium depends on the index of 
refraction, the polarization of the incident beam, and the angle of 
incidence.  This is illustrated for glass in Fig 2. 

FIGURE 2

2.2.1.2.  Refraction

    Refraction also takes place at an interface.  Refraction 
occurs, whenever a beam of light passes from one transmitting 
medium to another having a different refractive index (n).  For 
example, refraction is the bending of light at air-water and air-
glass interfaces.  The law of refraction, which is also known as 
Snell's Law, states that the angle of incidence (theta1) and the 
angle of refraction (theta2) are related by the equation: 

    sin thetal/sin theta2 = n2/n1                      Equation (4)

where n1 and n2 are the indices of refraction of the first medium and 
the second medium respectively. 

    Lenses and prisms are optical components that depend 
principally on the phenomenon of refraction.  The variation in the 
index of refraction with wavelength is termed dispersion.  Thus a 
simple prism bends blue light and red light differently for the 
same angle of incidence, the two angles of refraction differ, and 
blue light can be separated from red light.  In lenses, this effect 
is called chromatic aberration.  It can be reduced by choosing a 
glass with very little dispersion or by combining two lenses that 
have complementary dispersion characteristics. 

2.2.2.  Interaction with a medium

2.2.2.1.  Transmission

    The nature of transmitted light that emerges from a medium 
depends on the phenomena of absorption and scattering and also on 
the reflection of some of the light at the interfaces between 
media.  The transmittance of a medium is usually represented by tau 
and specified at a certain wavelength and for a certain path length 
at normal incidence.  The transmittance of most materials varies 
markedly across the optical spectrum. 

2.2.2.2.  Attenuation

    The absorption and transmission of a beam of optical radiation 
in any homogenous, isotropic medium is expressed in terms of the 
following equation: 

    phi = phioe-(alpha+rho)x=phioe-µx                   Equation (5)

where phi is the radiant power (radiant flux) leaving the medium, 
phio the initial radiant power in the beam entering the absorbing 
medium, x the thickness of the medium (path length of the beam), 
alpha the absorption coefficient, rho the scattering coefficient, 
and µ (Greek "mu") the attenuation coefficient of the absorbing 
medium. 

    In the Exponential Law of Absorption (Beer's Law), the constant 
alpha is the absorption coefficient.  The law shows that the 
radiant power diminishes exponentially with distance during 
transmission through a uniformly absorbing medium.  For a 

scattering medium, the same approach may be applied with the 
absorption coefficient alpha being replaced by a scattering 
coefficient sigma.  The attenuation coefficient varies with 
wavelength as does the scattering coefficient sigma.  Equation 5 is 
only an approximation for weak scattering.  It can be rewritten to 
the base 10 instead of to the base e of natural logarithms and the 
constants µ, rho, and alpha will then be different.  "Attenuation 
depth" and "absorption depth" are useful terms to describe 
attenuation and absorption in tissue.  The most popular convention 
is to define this depth as the distance into the tissue at which 
the incident irradiance is reduced to 1/e (37%) of its initial 
value.  Another convention sets the value at 1/10. 

    Absorption in all substances is strongly dependent on the 
wavelength of the incident radiation.  Atoms or molecules become 
excited when they absorb a quantum of radiant energy.  Following 
absorption, this energy may be released in a variety of ways.  When 
the energy is released as more photons of radiant energy, it is 
known as luminescence. 

    At very high irradiances, non-linear effects can occur as a 
result of the direct interaction of the high electric-field 
intensities with matter.  Saturable absorption and enhanced 
absorption are examples that alter the absorption coefficient. 

2.2.3.  Interference, diffraction, and scattering effects

2.2.3.1.  Interference and diffraction

    When considering interference and diffraction effects, it is 
convenient to use the wave description of light.  The bending or 
spreading of waves after passing an edge or passing through a small 
aperture is a wave phenomenon termed diffraction.  The diffraction 
effects result from the constructive and destructive interference 
of adjacent waves.  When the size of the barrier is comparable or 
smaller in size than the incident wavelength, the wave is bent 
around the barrier considerably.  Thus, particles diffract light 
most dramatically, when they are approximately the size of the 
wavelength of the incident radiation.  In this case, the sum of the 
diffraction effects is known as scattering. 

    In the treatment of plane waves impinging on an aperture such 
as a circular aperture, Huygens principle may be employed.  Each 
point within the area of the aperture is regarded as a source of 
wavelets to explain the interference effects that produce a 
diffraction pattern on a screen some distance away. 

2.2.3.2.  Scattering

    Small particles, the size of which approaches that of a 
wavelength of light, scatter light, as do atoms, and molecules.  If 
the particles are much smaller than the wavelength of light (e.g., 
gas molecules), Rayleigh scattering takes place.  For Rayleigh 
scattering, the fraction of scattered radiation from a beam is 
inversely proportional to the fourth power of the wavelength of the 

radiation.  That is to say, that this type of scattering increases 
dramatically for shorter wavelengths.  Rayleigh scattered, non-
polarized light goes in all directions and becomes polarized to 
some extent.  If light is scattered by particles the size of the 
order of the wavelength of light or greater, this strong wavelength 
preference is not seen in the scattered light.  The type of large-
particle scattering is termed Mie scattering. Unlike Rayleigh 
scattering, Mie scattering is strongly directional.  Normally, the 
forward component of Mie scattered radiation is much greater than 
the backscatter. 

    The scattering of a beam of light passing through a homogeneous 
medium can be expressed in terms of the exponential function 
(Equation 5). 

3.  SOURCES OF RADIATION

    Sources of optical radiation can be grouped according to the 
type of emitting material, the type of apparatus, or the manner in 
which the radiation originates. 

    Incandescent bodies are probably the most common sources of 
optical radiation.  When the temperature of a body is elevated, 
more photons are emitted.  If the temperature of the body is 
approximately that of the human body (37 °C or 310 K), most of the 
emitted photons have wavelengths in the far infrared, in the 
vicinity of 10 µm.  If a material body is heated to incandescence, 
e.g., to 2000 K, the material may be described as "red hot".  The 
higher the temperature, the greater the percentage of high energy 
photons released.  But, in all cases, a wide range of photon 
energies is associated with the emitted incandescent radiation.  A 
theoretically perfect incandescent source has a characteristic 
"black-body" spectrum.  Fig. 3 shows the black-body spectra for 
several different temperatures.  In practice, no material actually 
emits a perfect black-body spectrum, but some materials such as 
solid tungsten or molten metals approach this distribution. 

FIGURE 3

    The ratio of the theoretically possible spectral emittance to 
the actual emittance of a grey body is the emissivity.  For 
instance, the emissivity of tungsten throughout the visible is 
approximately 0.4. 

    A useful relation for black-body sources is the  Stefan-
 Boltzmann Law, which states that the radiant exitance W integrated 
over all wavelengths of a black body is proportional to the fourth 
power of the absolute temperature of the body, i.e.: 

    W = sigmaT4

3.1.  Molecular and Atomic Transitions

    Other sources of light such as carbon arcs, gas-filled arc 
lamps, or gas discharge lamps, depart widely from black-body 
characteristics, i.e., vary greatly with the wavelength in the 
visible region.  In these cases, a stream of electrons flowing 
through a gas induces an emission of photons, characteristic of 
that particular gas.  If gas has a low pressure and the current is 
not great, a line spectrum is emitted.  Line spectra are the result 
of atomic transition.  As the gas pressure and the current density 
increase, the gas temperature increases and a continuous spectrum 
appears.  At high current densities and gas pressures, this type of 
emission (a continuum) predominates. 

    The energy Qq of a single photon, emitted because of an atomic 
transition, is determined by the frequency of the emitted radiation 
as defined by the condition: 

    Qq = xi1 - xi2 = h nu                              Equation (6)

where xi1 and xi2 are energies corresponding to the initial and 
final energy states, h is the Plank constant (6.625 x 10-34J x s), 
and nu is the frequency of radiation (in Hz). 

    Energy transitions in molecular systems can result in a 
radiation emission according to rules similar to those that apply 
to atomic systems.  The energies (0.001-0.1 eV) of molecular 
vibrational or rotational transitions are, typically, less than 
those characteristic of electron transitions in atoms or molecules 
(1-100 eV).  In addition to the electron "orbital" potential and 
kinetic energies, part of the energy of molecular systems is 
associated with rotational and vibrational modes.  Emissions of 
this type occur in the infrared and microwave regions of the 
electromagnetic spectrum.  Heat is the vibrational energy of 
molecular systems. 

4.  LASERS

    All lasers have three basic components: (a) a laser (active) 
medium; (b) an energy source (pumping system); and (c) a resonant 
optical cavity.  Lenses, mirrors, shutters, saturable absorbers, 
and other accessories may be added to the system to obtain greater 
power, shorter pulses, or special beam shapes, but only the three 
basic components (a, b, and c) are necessary for laser action. 

4.1.  The Laser Medium

    Laser action depends on the ability of the laser (active) 
medium to undergo population inversion (i.e., more atoms or 
molecules in the excited state than in the lower state).  Once 
population inversion occurs, an avalanche of photons can be 
generated by stimulated emission.  Initial, spontaneously emitted 
photons stimulate other excited atoms to emit photons of the same 
energy in phase with one another.  This process is Light 
Amplification by Stimulated Emission of Radiation, with the 
acronym, LASER. 

    Fig. 4 shows a simplified 3-level energy diagram for a laser 
material.  This is just one of the many possible systems of energy 
levels.  Though laser action is possible with only 3 energy levels, 
most such actions involve 4 or more levels. 

FIGURE 4

4.2.  The Pumping System

    Pumping systems are necessary to raise electrons to a higher 
energy level in lasers.  These systems pump energy into the laser 
material, increasing the number of atoms or molecules trapped in 
the metastable energy level, until a population inversion exists 

large enough to make laser action possible (Fig. 4).  Several 
different pumping systems are available including optical, electron 
collision, and chemical reaction. 

    In optical pumping, a strong source of light is used, such as a 
xenon flashtube or another laser (e.g., an argon or nitrogen 
laser), generally of a shorter wavelength than that emitted by the 
medium. 

    Electron collision pumping is accomplished by passing an 
electric current through a laser medium, usually a gas (e.g., 
helium-neon laser) or a semiconductor junction (e.g., gallium-
arsenide laser), or by accelerating electrons in an electron gun to 
impact on the laser material, as in some semiconductor or gas 
lasers. 

    Chemical pumping is based on energy released in the making and 
breaking of chemical bonds.  For example, some hydrogen fluoride 
(HF) or deuterium fluoride (DF) lasers are pumped in this manner. 

4.3.  The Resonant Optical Cavity

    A resonant optical cavity is formed by placing a mirror at each 
end of the laser medium so that a beam of UVR, light, or IR 
radiation may be reflected from one mirror to the other.  Lasers 
are constructed in this way so that the beam passes through the 
laser medium one or more times and the number of emitted photons is 
amplified at each passage.  One of the mirrors is only partially 
reflecting and permits part of the beam to be transmitted out of 
the cavity at each reflection (Fig. 5).  The alignment, curvature, 
and separation distance of the mirrors determine the shape (mode 
structure) of the emitted laser beam. 

FIGURE 5

4.4.  Types of Lasers

    Lasers can be categorized in a variety of ways, e.g., according 
to the active medium or temporal mode of operation. 

4.4.1.  Active media

    Lasers are often designated according to the type of laser 
medium, as follows: 

    (a)  Solid-state lasers:  a glass or crystalline medium
         into which active atoms are introduced;
    
    (b)  Gas lasers:  a medium of pure gas or a mixture of
         gases; this category also includes metal vapour
         lasers;

    (c)  Semi-conductor lasers:  a medium of n-type and p-type
         semiconducting element material;

    (d)  Liquid lasers:  a liquid medium containing an active
         material, such as an organic dye, in solution or
         suspension.

    Optical pumping (both coherent and incoherent) is usually used 
in the production of solid-state and liquid lasers while collision 
pumping is usually employed to produce gas lasers.  However, 
chemical-reaction  pumping is also used for some types of liquid 
and gas lasers.  Semiconductor lasers may be optically pumped by an 
electric current, another laser beam, or electron-collision from an 
electron beam.  Table 4 provides an abbreviated list of 
commercially available laser wavelengths. 

4.4.2.   Temporal modes of operation

    Some lasers operate continuously, and are termed continuous 
wave (cw).  In this type of operation, the peak power is equal to 
the average power output; that is, the beam irradiance is constant 
with time.  Many lasers that appear to be cw may actually have a 
temporal structure that can only be resolved with very 
sophisticated systems of measurement. 

    The different temporal modes of operation of a laser are 
distinguished by the rate at which energy is delivered.  In 
general, lasers operating in the normal pulse (or "long pulse") 
temporal mode have pulse durations of a few tens of µs to a few ms 
(Descomps, 1981). 

    Pulsed lasers can be operated to produce repetitive pulses.  
The pulse repetition frequency of a laser is the number of pulses 
that a particular laser produces per unit time duration.  Lasers 
are now available with pulse repetition frequencies as high as 
several million pulses per s (MHz). 

Table 4.  Common lasers
-------------------------------------------------------------------
CIE band  Wavelength (nm)  Medium                Typical operation
-------------------------------------------------------------------
Excimer   UVC + B          XeCl, XeFl, etc.      pulsed
UV-A      325              He-Cd                 cw
UV-A      337              Nitrogen              pulse train
UV-A      350              Argon                 cw
-------------------------------------------------------------------
Light     441.6            He-Cd                 cw
Light     458, 488, 514.5  Argon                 cw
Light     458, 568, 647    Krypton               cw
Light     530 or 532       Nd frequency-doubled  pulsed
Light     632.8            He-Ne                 cw
Light     694.3            Ruby                  pulsed/Q-pulsed
Light     560-640          Rhodamine 6G dye      cw/pulsed
-------------------------------------------------------------------
IR-A      850              GaAlAs                pulse train
IR-A      905              GaAs                  pulse train
IR-A      1060             Nd: glass              pulsed/Q-pulsed
IR-A      1064             Nd: YAG                cw/pulsed/Q-pulsed
-------------------------------------------------------------------
IR-C      5000             CO                    cw/pulsed
IR-C      10 600           CO2                   cw/pulsed
-------------------------------------------------------------------

    The resonant quality of the optical cavity of a laser can be 
altered by rotating one mirror or by placing a shutter between the 
mirrors.  The shutter may be active (e.g., pockels cell) or passive 
(a saturable absorber).  This enables the beam to be turned on and 
off rapidly and normally creates pulses with a duration of a few ns 
to a few µs.  This operation is normally called Q-switching (or 
Q-spoiling or giant pulsing) (Fig. 6).  The "Q" refers to the 
resonant quality of the optical cavity.  A Q-switched laser usually 
emits less energy than the same laser emitting normal pulses, but 
the energy is emitted in a much shorter period of time.  Thus, 
Q-switched lasers are capable of delivering very high peak powers 
of several megawatts or even gigawatts.  Fig. 6 shows a variety of 
oscilloscope traces including a normal pulse and a Q-switched 
pulse. 

    When the phases of a number of oscillating modes in a laser 
resonator are forced to maintain a fixed relation to one another 
through a non-linear absorber placed in the resonator, the laser 
output observed is a train of regularly spaced ultra-short pulses.  
This is termed a mode-locked laser.  In a train of pulses, each 
pulse has a duration of a few picoseconds (ps) to a few ns.  A 
mode-locked laser can deliver higher peak powers than the same 
laser when Q-switched (Dautray & Watteau, 1980).  Fig. 6 also shows 
a mode-locked pulse train from a pulsed Nd-YAG laser. 

FIGURE 6

4.5.  Spatial (TEM) Modes

    A cross-sectional wave pattern is characteristic of all laser 
beam geometries (transverse electromagnetic wave or TEM).  These 
wave patterns across the beam are identified with TEM mode 
notation.  Fig. 7 illustrates how some of the more common modes 
would appear in cross section.  The TEM01 mode is similar to the 
TEM10 mode rotated through 90°. 

FIGURE 7

    Longitudinal (or axial) modes do not influence the emergent 
beam profile, but influence the degree of coherency of the spatial 
and the temporal frequency spectrum and are, therefore, of no great 
significance in the consideration of biological effects (unless 
they are intentionally or accidentally mode-locked to produce ps 
pulses). 

4.6.  Beam Characteristics

4.6.1.  Beam diameter

    The beam diameter of a laser operating in the TEM00 mode has 
been variously defined as the circle where the irradiance or 
radiant exposure is 1/2, 1/e, 1/e2, or 1/10 of the maximum (Fig. 8).  
In almost all discussions in the health and safety literature, the 
edge of the beam is defined as 1/e or 0.37 of the maximum, whereas 
the beam diameter is almost always defined at 1/e2 points in the 
laser industry. 

FIGURE 8

4.6.2.  Beam divergence

    The wave nature of light prevents lasers from producing 
perfectly collimated beams.  However, the divergence or beam 
spreading is much smaller than that of a searchlight or other 
conventional sources of optical radiation. 

4.6.3.  Beam irradiance versus range for a circular beam

    To define potential exposure conditions, it is necessary to 
characterize the beam emitted from a laser.  The beam's 
characteristics may be required near the output of the laser or at 
some considerable distance from the laser, after it has been 

collimated or focused.  The optical radiation emitted by most 
lasers is confined to a rather narrow beam that slowly diverges or 
fans out as the beam propagates.  The beam diameter DL increases 
from an initial diameter a at the laser exit port as a result of 
the divergence phi: 

    DL = a + r phi                                     Equation (7)

where r is the range (distance from the laser).

    With the beam diameter defined as a function of distance from 
the laser, it is a simple matter to derive a formula to estimate 
the beam irradiance or radiant exposure at any distance r.  The 
beam irradiance E (in W/m2 or W/cm2) would be the total power phi 
in the beam (in watts) divided by the area of the cross section of 
the beam (usually expressed in m2 or cm2).  For health risk 
assessment, a defining aperture of irradiance measurement is 
normally specified (e.g., 7-mm circular aperture). 

    The effect of atmospheric attenuation may become a major factor 
in evaluating the irradiance or radiant exposure at distances 
greater than a few kilometres.  This attenuation is the result of 
three effects:  (a) Mie (or large particle) scattering; (b) 
Rayleigh (or molecular) scattering; and (c) absorption by gas 
molecules.  Rayleigh scattering is the most wavelength dependent; 
shorter wavelengths are predominantly scattered.  The atmospheric 
attenuation may best be expressed by an exponential function.  The 
attenuation of optical radiation could be described by a term (eµr) 
where µ is termed the attenuation coefficient of the medium.  It is 
the sum of scattering coefficients and absorption coefficients of 
the medium through which the laser beam propagates.  One equation 
that is a close approximation for calculating the axial beam 
irradiance is: 

    E = 1.27 phi e-µr/(a + r phi)2                     Equation (8)

and a corresponding equation for radiant exposure H from a pulsed 
laser of output energy Q is: 

    H = 1.27 Qe-µr/(a + r phi)2                        Equation (9)

    These equations can be adjusted for other beam profiles using 
simple geometry.  The emergent beam diameter term, a, may be 
dropped at distances where a << rphi.  Equations 7, 8, and 9 are 
close approximations to rigorous formulations of Gaussian optics. 

4.6.4.  Hot spots

    Hot spots are defined as areas of the beam, where the localized 
beam irradiance is much greater than the average across the beam.  
As the irradiance of hot spots may be many times higher than the 
average beam irradiance, they are of considerable concern in 
relation to health.  There are several sources of hot spots:  
inhomogeneities in the laser cavity or areas of the active medium, 
where more energy is emitted than in other areas; imperfections in 

the mirrors and lenses of the laser system; and changes caused by 
atmospheric conditions.  Atmospheric inhomogeneities along the beam 
path produce lenticular effects (scintillation) that are 
responsible for atmospheric hot spots.  Fog, rain, snow, dust, 
smoke, or haze absorb and/or scatter the laser beam but do not 
cause hot spots.  Such scattering reduces the severity of hot 
spots. 

4.6.5.  Coherence

    Two types of coherence are characteristic of laser light: 
spatial and temporal.  The term spatial coherence indicates that 
the optical radiation is spatially in phase, i.e., electromagnetic 
waves at different points in space oscillate in synchronism.  Laser 
speckle is a consequence of spatial coherence.  Temporal coherence 
indicates that the radiation is strictly monochromatic (of one 
wavelength).  No light source is either totally coherent or totally 
incoherent; the differences between individual lasers and, for that 
matter, non-laser sources are merely a matter of degree.  The term 
"coherence length" is used to describe the degree of spatial 
coherence. 

5.  RADIOMETRIC CONCEPTS

5.1.  Radiometric and Photometric Terminology

    Two systems of quantities and units are used to describe 
optical radiation.  One is a physical system called the radiometric 
system.  The other, the photometric system, attempts to describe 
the optical radiation in terms of its ability to elicit the 
sensation of light by the eye.  Table 5 gives the most commonly 
used quantities and the preferred units for each system.  There are 
generally analogous units in each of the 2 systems.  The table is 
arranged to illustrate these similarities.  Though the radiometric 
system of units may be used across the entire spectrum, the 
photometric system is limited to describe light (i.e., 
electromagnetic radiation that is visible) from approximately 
380 - 400 nm to 760 - 780 nm. 

    It is important to remember that some terms refer only to 
extended sources (e.g., radiance) and other terms (e.g., radiant 
intensity) refer only to "point" sources. 

5.2.  Extended Sources Versus Point Sources

    Lasers are often treated as "point" sources, whereas most 
conventional light sources are considered to be extended sources, 
at least at close distances.  An extended source is one that 
appears to have some angular extent as seen by the viewer.  The 
moon is an extended source; a star is a "point" source.  Apart from 
lasers, a light source can be considered a point source only at a 
great distance, or if a pinhole diaphragm is placed in front of the 
light source.  All other light sources are considered to be 
extended sources. 

5.3.  Inverse Square Law

    The inverse square law for calculating the irradiance or 
radiant exposure at a distance from a source applies only to a 
point source.  For example, the ratio of irradiance E1, at one 
distance r1 to E2 at another distance r2 is: 

    El = r22                                          Equation (10)
    E2   r12

For practical purposes, extended sources can be considered as 
"point" sources at a distance many times greater than the source 
dimension.  Both r1 and r2 should be at least as great as 10 source 
diameters for a diffuse lambertian source.  This also applies to 
equation 3.  The irradiance E at a distance r from a point source 
is: 

     E = I/r2                                         Equation (11)

This equation applies to collimated extended sources (e.g., 
searchlights) or lasers, only at considerable distances from the 
source. 


Table 5.  Useful CIE radiometric and photometric quantitities and unitsa,b
-------------------------------------------------------------------------------------
                              RADIOMETRIC
-------------------------------------------------------------------------------------
Term            Symbol   Defining equation              Quantity     SI units &      
                                                        applicablec  abbreviations   
-------------------------------------------------------------------------------------
Radiant energy  Qe                                      S, R         joule (J)       
-------------------------------------------------------------------------------------
Radiant energy                dQe                       F            joule per cubic 
density         We       We = dV                                     metre (J/m3)    
-------------------------------------------------------------------------------------
Radiant power                   dQe                     S, R         watt (W)        
(radiant flux)  phieP    phie = dt                                                   
-------------------------------------------------------------------------------------
Radiant                       d phie                    S            watt per square 
exitance        Me       Me = dA                                     metre (W/m2)    
                                                                                     
                            = /Le x cos x d omega                                    
-------------------------------------------------------------------------------------
Irradiance or                 d phie                    R            watt per square 
flux density    Ee       Ee = dA                                     metre (W/m2)    
(dose rate in                                                                        
photobiology)                                                                        
-------------------------------------------------------------------------------------
Radiant                       d phie                    S            watt per        
intensity       Ie       Ie = d omega                                steradian       
(W/sr1)                                                                              
-------------------------------------------------------------------------------------
Radiance d                    d2phie                    S, F, R      watt per        
                              ------------------------               steradian and   
                Le       1e = d omega x dA x cos theta               per square metre
                                                                     (W/sr/m2)       
--------------------------------------------------------------------------------------
Radiant                       dQe                       R            joule per square
exposure        He       He = dA                                     metre (J/m2)
(dose in
photobiology)
--------------------------------------------------------------------------------------
Radiant                         P                       S            unitless
efficiencye     etae     etae = Pi
(of a source)
--------------------------------------------------------------------------------------
Optical         De       De = -log10taue                R            unitless
--------------------------------------------------------------------------------------

Table 5.  (contd.)
------------------------------------------------------------------------------------ 
                               PHOTOMETRIC                                          
------------------------------------------------------------------------------------ 
Term              Symbol   Defining equation                     SI abbreviations   
                                                                 & units            
------------------------------------------------------------------------------------ 
Quantity of       Qnu      Qnu = /phinudt                        lumen-second       
light                                                            (lm x s)           
------------------------------------------------------------------------------------ 
Luminous                         dQnu                            lumen-second per   
energy density    Wnu      Wnu = dV                              cubic metre        
                                                                 (lm x s/m3)        
------------------------------------------------------------------------------------ 
Luminous flux     phinu                  d phie                  lumen (lm)         
                                        -----------------------                     
                           phinu = 680 / d lamdaV(lamda)d lamda                     
------------------------------------------------------------------------------------ 
Luminous                         d phinu                         lumen per square   
exitance          Mnu      Mnu = dA                              metre (lm/m2)      
                                                                                    
                               = /Inu x cos phi x d omega                           
------------------------------------------------------------------------------------ 
Illuminance                      d phinu                         lumen per square   
(luminous         Enu      Enu = dA                              metre (lm/m2)      
density)                                                                            
------------------------------------------------------------------------------------ 
Luminous                         d phinu                         lumen per          
intensity         Inu      Inu = dr                              steradian (lm x sr)
(candlepower)                                                    or candela (cd)    
------------------------------------------------------------------------------------ 
Luminance d       Lnu            d2phie                          candela per square 
                                 -------------------             metre (cd/m2)      
                           Lnu = dr x dA x cos theta                                
------------------------------------------------------------------------------------ 
Light exposure                   dQnu                            lux-second         
                  Hnu      Hnu = dA                              (lx x s)           
                                                                                    
                               = /Enudt                                             
------------------------------------------------------------------------------------ 
Luminous                       phinu                             lumen per watt     
efficacy (of      K        K = phie                              (lm/W)             
radiation)                                                                          
------------------------------------------------------------------------------------ 
Luminous                            K    K                       unitless           
efficiency        V(*)     V(*) = / Km = 680                                        
(of a broad                                                                         
band radiation)                                                                     
------------------------------------------------------------------------------------ 
Luminous                          phinu                          lumen per watt     
efficacye        etanu    etanu = Pi                             (lm/W)             
------------------------------------------------------------------------------------ 

Table 5.  (contd.)
------------------------------------------------------------------------------------ 
                               PHOTOMETRIC                                          
------------------------------------------------------------------------------------ 
Term              Symbol   Defining equation                     SI abbreviations   
                                                                 & units            
------------------------------------------------------------------------------------ 
Optical                                                          unitless           
densityf         Dnu      Dnu = -log10taunu                                         
                                                                                    
------------------------------------------------------------------------------------ 
Retinal                                                          Troland (td) =     
illuminance       Et       Et = Lnu x Sp                         luminance of 1 cd/ 
(in trolands)                                                    m2 times pupil     
                                                                 area in mm2        
------------------------------------------------------------------------------------ 
a The quantities may be altered to refer to narrow spectral bands, in which case 
  the term is preceded by the word spectral, and the unit is then per unit of 
  wavelength and the symbol has a subscript lambda.  For example, spectral 
  irradiance Hlambda has units of W/(m2 x m) or more often, W/(cm2 x nm).
b While the metre is the preferred unit of length, the centimetre is still the 
  most commonly used unit of length for many of the above terms and the nm or µm 
  are most commonly used to express wavelength.
c Some radiometric quantities refer only to the source, field, or receiver.  This 
  noted in this column.
d At the source,              dM         ; at a receptor,               dE        
                 L = d omega x cos theta                  L =  d omega x cos theta
e Pi is electrical input power in watts.
f tau is the transmission; Dnu is also abbreviated as O.D.

6.  RADIOMETRIC AND PHOTOMETRIC MEASUREMENT

6.1.  Introduction

    Reliable radiometric techniques and instruments are available 
that make it possible to analyse risks for the skin and eye from 
exposure to lasers and other sources of optical radiation.  
However, the cost of accurate equipment remains relatively high 
compared with that of survey equipment now available to evaluate 
many other environmental risks.  Radiometric formulae and 
manufacturers' specifications of lasers will often be an adequate 
substitute for measurement. 

    There are many types of measurements for defining conditions or 
characterizing a source's output.  The types of measurements 
considered in this section fall under the broad term of 
"radiometric".  For characterizing a conventional light source, 
radiance is generally the most useful.  For laser output 
measurements, radiant power and radiant energy are by far the most 
important.  Irradiance and radiant exposure are of greater 
importance in defining hazardous exposure conditions from all 
optical sources. 

    In any discussion of the measurement of laser radiation for 
purposes of evaluating health risks, it is important first to 
clarify the conditions and requirements for such measurements.  
Industrial and environmental health specialists and health 
physicists usually rely heavily on instruments to detect or 
estimate a chemical or physical agent that their own human senses 
cannot detect.  The presence of a laser beam can generally be 
detected by the human eye or through the use of an image converter, 
thus raising the question "Why should the laser beam be measured?".  
It soon becomes evident that, in most cases, even routine 
monitoring of either a work area or an individual by 
instrumentation is a hopeless task.  A more logical approach to 
risk assessment is to develop a means of analysing the potential 
risk of a laser, based on the laser's output parameters. 

    As a general rule, present standards require only measurement 
of the laser-output characteristics for laser risk classification.  
Routine monitoring is seldom considered necessary and all 
measurements are normally performed only once, by (or for) the 
manufacturer of the laser equipment.  Periodic measurements may be 
considered worthwhile for certain lasers that are near the 
borderline between two classes in the hazard classification and 
field measurements of outdoor laser propagation paths have often 
been found useful.  Unlike most noxious agents, a high-powered 
laser beam is almost always hazardous for a considerable distance 
and its hazard generally exceeds exposure limits by orders of 
magnitude.  Diagnostics of the beam profile and measurement of beam 
divergence have been discussed. 

    The evaluation of more conventional broad-band sources is more 
complex, since spectral characteristics and source size must be 
considered.  To evaluate a broad-band optical source, it is 
normally necessary to determine the spectral distribution of 
optical radiation emitted from the source at the point or points of 
human access.  The spectral distribution of the accessible 
emission, which is of interest for a lighting system, may differ 
from that actually being emitted by the lamp alone because of 
apertures or filtration by any optical elements in the light path.  
Secondly, the size, or projected size, of the source must be 
characterized in the retinal hazard spectral region.  Thirdly, it 
may be necessary to determine the variation of irradiance and 
radiance with distance.  The necessary measurements are normally 
complex.  The spectrum of an arc lamp, a gas discharge lamp, or a 
fluorescent lamp consists of both line structure and a continuum.  
Significant errors can be introduced in the representation of the 
spectrum and in the weighting of the spectrum against a biological- 
or a safety-action spectrum, if the fraction of energy in each line 
is not properly added to the continuum. 

6.2.  Measurement Instrumentation

    The radiometric instruments of interest in this discussion 
generally consist of detectors that produce a voltage, a current, a 
resistance change, or an electronic charge, one of which is 
measured by a sensitive electronic meter.  The type of read-out 
meter for the radiometric instrument is not normally of great 
concern and seldom determines the selection of the instrument.  
There are both advantages and disadvantages associated with each 
type of detector and each has certain characteristics that may be 
useful for measuring a specific level of optical radiation in a 
wavelength range of interest.  No single detector is best for 
measuring all wavelengths and radiant powers of optical radiation.  
A very sensitive detector can be readily damaged or its response 
distorted by a high-powered laser beam, whereas a detector designed 
to measure very high-powered laser radiation is normally 
insensitive to low-power radiation.  Narrow-band detectors have the 
advantage of being insensitive to extraneous radiation, though 
their usefulness is limited to measurement of lasers operating in 
that particular spectral band. 

    In many countries, national physical standards laboratories 
exist, which offer calibration services for some radiometric 
instruments.  Examples are:  Laboratoire National d'Essais (France);  
National Physical Laboratory (United Kingdom); Assistance Committee 
for Measures and Standards (USSR); National Bureau of Standards 
(USA); and Physikalisch-Technische Bundesanstalt (Federal Republic 
of Germany). 

6.2.1.  Thermal detectors

    Thermal detectors are mainly used for measuring the output 
power or energy of lasers and total irradiances from broad-band 
optical sources, in particular infrared sources.  Some of these 
detectors are particularly useful for absolute measurements. 

    Thermopiles, bolometers, disc calorimeters, and pyroelectric 
detectors are characterized by a relatively flat response as a 
function of wavelength.  The spectral response of these detectors 
is dictated by the "black" absorbers that are normally used to coat 
the detector's metal or crystalline substrate.  As optical energy 
falls on the detector, the temperature increases.  The temperature 
rise in the substrate is then converted into an electrical voltage 
or current. Because of the thermal mass of the metal, the time 
required to heat or cool the detector element limits the response 
time of the instrument.  The absorber in a disc calorimeter may be 
a black painted disc or a glass volume absorber (James, ed. 1976).  
In recent years, the response time of thermopiles has been 
shortened by using thin-film techniques.  Low powers (typically 
0.01 - 100 mW) can be measured and the response time reduced.  
Pyroelectric detectors measure the rate of temperature change in a 
crystalline material rather than the final temperature elevation in 
a metal.  Radiometric calorimeters can be used to measure cw 
radiation over the power range from 1 mW to over 1 kW, depending on 
the detector and wavelength.  Pulsed radiant energy can be measured 
over the range from 10 mJ to 10 kJ. 

    The detector may be covered by a window, which limits spectral 
response.  Quartz windows are necessary for UVR, but there is 
probably no window material that is universally flat from 200 to 
20 000 nm. 

6.2.2.  Quantum detectors

    Quantum detectors operate normally at room temperature and 
offer by far the most sensitive means of measuring optical 
radiation, therefore, their principle use is in spectroradiometers 
and detectors required for the measurement of lower powers and 
irradiances or for temporal resolution of pulses.   The spectral 
sensitivity of photoemissive detectors depends on the photocathode 
material used in vacuum photodiodes or photomultiplier tubes, or 
in the characteristics of (doped) silicon.  All detectors that 
operate by means of the photoelectric effect have a characteristic 
cutoff wavelength.  At wavelengths greater than this cutoff, 
photons are largely ineffective in producing photoelectrons and the 
resulting photocurrent. 

    Because of the strong spectral dependence of the photodiode, 
these instruments are often not direct reading and the meter 
reading may have to be multiplied by one of several calibration 
factors. 

    Simple silicon-detector instruments can be quite useful, when 
the natural spectral response of silicon (approx 200 - 1100 nm) is 
changed by an appropriate input filter to yield a flat spectral 
response from 450 to 950 nm (Marshall, 1980). 

6.2.3.  Detectors to resolve short pulses

    A variety of techniques have been developed to resolve the 
temporal behaviour of short laser pulses.  An ultrafast 
oscilloscope with a solid-state silicon detector or biplanar vacuum 
photodiode is most often used to display the pulse shape of a 
Q-switched (1.0 - 100 ns) pulse.  For temporal domains of less than 
1 ns, streak cameras and some non-linear optical techniques are 
used to resolve the structure of a train of mode-locked laser 
pulses. 

    The techniques used in photometry and radiometry are far too 
numerous and complex to detail here (see for example:  Grum & 
Becherer, ed. 1979; Le Bodo, 1976; or Sliney & Wolbarsht, 1980).  
Differences in measured values for the same source from each of two 
different laboratories may arise through a problem of "geometry" or 
incorrect allowance for extraneous light. 

6.2.4.  Safety meters

    At present, inexpensive, portable radiometric measuring 
instruments that have been designed specifically for the risk 
analysis of a great variety of lasers, are not available.  Indeed, 
it is unlikely that such instruments will be made in the future, 
because of the great variation in exposure criteria for different 
wavelengths and different exposure durations.  The same holds true 
for non-laser optical source survey instruments.  However, some 
relatively expensive, microprocessor-based instruments have been 
developed to cover a wide range of measurements.  Simple 
instruments are possible for the purpose of measuring one type of 
laser or optical source. 

    Because of the great interest in photometry, there are many 
satisfactory photometers that measure both luminance and 
illuminance and follow the CIE photopic function Vlambda quite 
well.  This is not always the case for the other weighting 
functions.  Direct reading UVR instruments are a case in point.  
The principal difficulty in developing a suitable UVR instrument is 
the rejection of unwanted wavelengths.   It is difficult to measure 
only the UV-B and UV-C radiation with sufficient sensitivity, while 
still rejecting all of the UV-A and visible light. 

6.2.5.  Spectroradiometers

    As well as broad-band measurements, it is often necessary to 
measure the spectrum of a conventional source.  A grating or prism 
monochromator is used to resolve the spectrum. 

    An important part of measuring the spectral distribution of a 
broad-band lamp source or an arc process is the specification of 
the desired bandwidth, and the intervals at which data will be 
recorded.  One of the most useful approaches is to scan through a 
spectrum and record the detector output in analogue fashion on an 
X-Y recorder, rather than to record digitized data. 

    An X-Y recorder can indicate the bandwidth of the monochromator 
and may quite often indicate the regions of the spectrum in which 
problems may arise from stray light or extraneous signals. 

    The required bandwidth and sampling interval are determined by 
comparing the so-called "slit function" of the monochromator system 
with the need for sufficient spectral resolution to make possible 
accurate weighting against action spectra used in risk analysis. 

    Two common difficulties are encountered in obtaining an 
accurate radiometric description of a broad-band, extended source.  
The first problem is to achieve adequate rejection of unwanted 
wavelengths from the passband of the monochromator (e.g., rejecting 
"stray light").  The second is the proper definition of the actual 
or effective source size of an arc or a discharge lamp.  Several 
other special problem areas (such as background isolation, 
wavelength calibration, and the proper separation of line and 
continuous values) are also encountered in specific situations. 

6.3.  Biological Weighting of Spectroradiometric Data

    Many of the calculations that are useful in risk analyses 
require weighting of the measured spectrum against a biological 
action spectrum.  There are several, including the erythemal and 
the photokeratitic action spectra, the photopic response V lambda 
of the eye, and the retinal photochemical injury action spectrum B 
lambda (Ham, et al., 1976; ACGIH, 1981; Sliney & Wolbarsht, 1980) 
(Fig. 9). 

FIGURE 9

7.  BIOLOGICAL EFFECTS

    For biological effects to take place, some of the incident 
radiation must be absorbed.  This is known as Draper's Law (Smith, 
1977). 

    The primary effects can be related to two general mechanisms, 
thermal and photochemical injury.  In some special instances (as 
with ps pulses), non-linear effects related to the direct electric 
field of photons may be important.  The observable biological 
effects are the result of secondary events.  Under certain 
circumstances, these effects can be changed in size and direction.  
A number of these modifying factors are treated in the text such as 
pigmentation, increased body temperature, and photosensitizing 
substances. 

7.1.  Thermal Injury

    Thermal injury mechanisms all require that sufficient radiant 
energy is absorbed in a tissue, sufficiently fast, to create a 
substantial increase above normal tissue temperature (typically 
10 - 25 °C for short periods of a min or less).  There is no 
dependence on photon energy, though energy must be absorbed.  Heat 
conduction away from an irradiated area is of great importance.  
Thus, the presence of blood vessels and the size of the irradiated 
volume, as well as spectral absorption, influence the threshold of 
injury (Marshall, 1970). 

    For every short exposure, changes of state may occur so rapidly 
that micro-explosions (Hanson & Fine, 1968) or thermo-mechanical 
effects may become important (Vos, 1966a; Ham et al., 1970). 

7.2.  Photochemical Injury

    Significant adverse effects have now been shown to result 
initially from a photochemical reaction rather than through a 
thermal damage mechanism.  A photochemical reaction takes place 
when single photons have sufficient quantum energy to convert 
individual molecules to one or more different chemical molecules.  
A photochemical injury mechanism is demonstrated, when a 
reciprocity relation between irradiance (dose-rate) and exposure 
duration exists.  That is, a constant radiant exposure (dose) is 
required to elicit the response over a wide variation of exposure 
durations, up to durations at which biological repair comes into 
play.  An additional characteristic of any photochemical reaction 
is a rather steep drop-off of the action spectrum in the long 
wavelength end.  The yield of the photochemical reaction products 
is propotional to the photon flux and each photon must have the 
amount of energy required for the reaction.  At the long-wavelength 
end of the induced response (action spectrum), the energy of a 
single photon coupled with available thermal energy is generally 
insufficient to induce an effect. 

    Most photochemical effects of radiation are still not 
understood in detail.  The relative spectral effectiveness of 
radiation in eliciting any particular biological effect is referred 
to by photobiologists as an "action spectrum".  The steep slopes of 
many ultraviolet action spectra demonstrate the importance of not 
routinely extrapolating biological data concerning injury occurring 
at one wavelength to another wavelength, and of not assuming that 
any smooth curve does not have fine structures. 

    There are some instances where both photochemical and thermal 
effects contribute to the final biological effect.  In general, 
they will enhance one another. 

7.3.  Threshold of Injury

    All thermal injury has a macroscopically apparent threshold.  
Individual photons in the long-wavelength range do not have 
sufficient energy, normally, to cause more than temporary 
biological change at the molecular level.  Acute pathological 
changes can only be demonstrated, when a sufficient thermal photon 
flux exists to cause temperature rises so rapid that normal heat 
dissipation and molecular repair are overwhelmed.  In the case of 
photochemical injury, individual photons may alter or damage an 
individual molecule.  However, it has been shown that many critical 
biomolecules have repair mechanisms to correct such damage (Smith, 
1978).  For very high photon flux densities the repair processes 
may be overwhelmed and macroscopic damage will be apparent.  
Occupational exposure limits can thus be set for any type of 
radiation in which the reciprocal relation of progressively lower 
power levels and longer exposure duration seems to show a marked 
deviation from linearity (non-reciprocity).  At this irradiance 
level, any increases in the exposure duration may not be followed 
by pathological changes in the exposed tissues.  This would not be 
accepted as a true threshold of injury by some investigators, since 
it could always be argued that a repair mechanism could fail.  
Thus, it must be admitted that there may be some finite, albeit 
extremely slight, risk of injury or delayed effects in a small 
population (Sliney & Wolbarsht, 1980). 

7.3.1.  Means of determining thresholds of injury

    There are several different criteria that have been used in 
studying potential injury in tissue (Beatrice & Velez, 1978).  
Examples are: 

(a) direct observation of irradiated tissue at low
    magnification
      (i)  without special techniques
     (ii)  with special visualization techniques such as
           fundoscopy or fluorescein angiography (eye)
           (Borland et al., 1978);

(b) histology
      (i)  light microscopy
     (ii)  electron microscopy (EM)
    (iii)  histochemical studies;

(c) biochemical studies;

(d) electrophysiological tests (e.g., the eye) (Court et al., 
    1978);

(e) functional studies;

(f) epidemiological studies (i.e., skin cancer).

    In setting exposure limits, all of these studies must be taken 
into account.  In studies of skin reaction to optical radiation, 
the first criteria of direct observation of erythema mainly has 
been used, though histological and histochemical techniques have 
been used in a few studies.  The threshold criteria just listed 
will be discussed briefly, as they apply to studies of injury from 
optical radiation. 

8.  EFFECTS OF OPTICAL RADIATION ON THE EYE

    The attendant hazards of optical radiation vary greatly 
depending on the type of the source and its application.  Generally, 
the effects of laser radiation are not different from the effects 
of optical radiation from a conventional source with the same 
wavelength, exposure duration, and given irradiance. 

    The effects of optical radiation on the eye vary significantly 
with wavelength.  For this reason, the subject will be discussed in 
three sections.  First, the effects of UVR, which are generally 
photochemical, on the lens and cornea will be considered.  The main 
discussion will relate to the retinal hazard region (the visible 
and IR-A) where the eye is particularly vulnerable to injury, 
because of its imaging characteristics.  Finally, IR effects on the 
anterior structures of the eye will be discussed. 

    There are many end points that can be used in establishing 
injury.  Damage to the retina of the eye from visible radiation can 
appear as an altered light reflex or a white patch on the retina, 
visible during ophthalmoscopic examination.  This criterion has 
been used in most injury studies.  The end point could also be any 
histologically-defined injury seen with the light microscope; or, 
it could be consistently observed ultrastructural changes only 
visible with an electron microscope (transmission or scanning EM). 
Histochemical techniques can also serve to document an end point 
for injury.  There is also the detection of functional alterations 
in sensory (behavioural) responses of task-oriented animal 
subjects, e.g., visual acuity, colour vision, dark adaptation.  
Often, these functional changes can also be detected by 
electrophysiological recordings of altered neural function within 
the visual system.  A problem arises, when different investigators 
define the "threshold" for any one of these end points in different 
ways.  The most meaningful thresholds for health criteria are those 
related to a persistent functional decrement. 

8.1.  Anatomy and Physiology of the Human Eye

    In the human eye, light passes through the various ocular 
structures (Fig. 10) to fall on the retina, where it triggers a 
photochemical process that evokes the neural impulses that lead to 
vision.  The light first passes through the structures in the 
anterior portion of the eye - the cornea, the aqueous humor in the 
anterior chamber, the pupil (and sometimes the iris), the somewhat 
pliable crystalline lens, then into the posterior part, the 
vitreous humor and the numerous layers of the retina.  Only the 
three structures of the eye that are critical in relation to the 
subject of optical radiation hazards will be discussed, i.e., the 
cornea, the lens, and the retina. 

8.1.1.  The cornea

    The cornea and the conjuctiva of the eye are exposed directly 
to the environmental elements.  These structures are protected from 
drying by the tear film, which is 6 - 10 µ in thickness. 

FIGURE 10

    The cornea and conjuctiva are tissues rich in sensory receptors 
and nerve endings serving as triggers of protective reflexes for 
mechanical and thermal agents. 

    As the epithelial cells must survive a harsh environment, they 
have a very short life span of approximately 2 - 5 days.  If the 
cell death rate increases (as in photokerititis) and replacement is 
not in step with the loss, small erosions will develop that elicit 
a pain sensation. 

    The corneal stroma is built in a very regular fashion, which 
accounts for its transparency.  If this regularity is distorted, 
i.e., by an oedema, the cornea will be less transparent or even 
opaque.  A disturbance of the epithelium, or, more important still, 
of the inner cell layer of the cornea (the endothelium) will result 
in an oedema.  Any scar formation will also alter the regular 
construction and hence will result in an opaque cornea.  Thus 
serious visual handicap will be the result of irreversible corneal 
damage. 

8.1.2.  The lens

    The lens is a tissue built up from cells that progressively 
deform to produce the lens fibres.  These are arranged in an onion-
like way and are covered with an elastic capsule.  The capsule is 
attached through fine ligaments to the cilary muscle which alters 
the shape of the lens. 

    As new cells are formed, older cells become fibres and are 
progressively displaced towards the centre of the lens.  The more 
superficial fibres form the cortex while the central fibres 
constitute the nucleus. 

    The lens fibres are transparent, ribbon-like cells, each of 
which runs completely around from the front of the lens to the 
back.  Injury to any single fibre will, in time, extend throughout 
the entire fibre cell and will be more apparent in the thicker 
posterior part of the lens (posterior subcapsular cataract). 

    The lens, like the cornea, is not optically homogeneous but it 
is transparent in the visible range of the spectrum.  However, 
increasing absorption of short-wavelength light occurs with age 
(Wolbarsht et al., 1977).  The transparancy is the result of a 
precise relation of the various minute, optically well ordered, 
constituents.  A disturbance of the cell elements or the fibre will 
result in the hydration of the fibre.  Damage to the lens disturbs 
this relation resulting in increased light scattering.  The lens 
becomes milky (a cataract). 

8.1.3.  The retina and choroid

    The retina is divided into two major components - a pigmented 
monolayer - the retinal pigmented epithelium (RPE) and a 
multilayered lamina of neural cells called the neural retina.  The 
light-sensitive cells are adjacent to the RPE.  There are two types 
of photoreceptor cells:  rods and cones, named according to the 
shape of the distal (away from the synaptic end) extension of the 
photoreceptor cells.  Light entering the eye must first pass 
through all of the neural retina before striking the receptor 
cells.  In the retina, there are probably 120 million rods 
approximately 60 µm long and 2 µm in diameter and 6 million cones 
approximately 50 µm long and 3 - 5 µm in diameter.  These receptor 
cells are interconnected by other specialized cells.  Rods are 
mainly concerned with vision at low light levels and are 
predominant in the periphery of the retina.  Cones are responsible 
for colour vision and high acuity visual tasks.  They are most 
concentrated in a specialized central region of the retina, the 
fovea.  The fovea is responsible for central vision used for 
reading.  The anatomical aspects of the retinal pigment epithelium 
(RPE) and adjacent layers are of particular importance in a study 
of retinal injury from light sources. The outer segments of the 
rods and cones (the light-sensitive section of the cell) are 
immediately anterior to the RPE. 

    There are very small protrusions (microvilli) of the RPE that 
extend upward around the outer segments (Fig. 11).  As the cones do 
not extend as near to the RPE as the rods, the microvilli extend 
out further to the cone outer segments.  It is known that the RPE 
plays a critical role in the retinal metabolism and photochemistry, 
hence proper functioning of the REP is essential for normal vision.  
The outer segments of both rods and cones contain a stack of coin-
like membranes on which their visual pigments are oriented.  These 
discs are in a continuous state of flux.  In rods, 10 - 30 new 
discs are made each day, while a similar number are phagocytized by 
the RPE cells (Fig. 11); the rate of shedding is greatest in the 
early morning.  The life span of a disc is two weeks.  The cone 
lamellar membranes do not form complete discs and do not appear to 
shed in the same manner as in the rods.  The renewal activity of 
the cones is slower than that of the rods and it occurs at night. 

    Bruch's membrane separates the RPE from the blood supply in the 
capillary layer of the choriocapillaries which is the innermost 
layer of the choroid, and is where the smallest vessels are found.  
The choroid is an extremely vascular spongy tissue with many 
pigmented cells scattered throughout it.  The thickness is 
variable; the average is about 250 µm. The blood vessels 
progressively increase in size towards the scleral surface.  It has 
been suggested by Ernest & Potts (1971) that the primary function 
of the blood in the choroid is to keep the eye warm and at a 
uniform temperature.  Because of this extreme vascularity, heat 
from laser exposures introduced in this region under steady-state 
conditions will do little to elevate the temperature unless high 
power levels are used. 

FIGURE 11

8.2.  Spectral Properties of the Eye

    To understand the biological effects of different optical 
spectral bands on different ocular structures, it is first 
necessary to consider the relative spectral absorption of the 
different ocular media.  Fig. 12 shows the spectral absorption for 
each of the media. 

    Essentially all incident optical radiation at very short 
wavelengths in the ultraviolet and long wavelengths in the infrared 
is absorbed in the cornea.  Clearly an ocular structure cannot be 
damaged unless optical radiation is absorbed.  In some instances, 
particularly in the UV region, less than 1% of the total incident 
radiation absorbed in a structure can be significant, if the 
radiation contains critically effective wavelengths. 

FIGURE 12

8.3.  Injury to the Anterior Portion of the Eye

    The anterior structures of the eye are the cornea, conjunctiva, 
aqueous humor, iris, and lens.  The cornea, aqueous humor, and lens 
are part of the optical pathway and, as such, must be transparent 
to light.  Loss of transparency is serious.  Because of the rapid 
turnover of corneal epithelial cells, damage limited to this outer 
layer can be expected to be temporary.  Indeed, injury to this 
tissue by exposure to UV-B and UV-C, as occurs in a particular 
keratoconjuntivitis, the ultraviolet photokeratitis or 
photoophthalmia (known also as "arc eye", or "welder's flash"), 
seldom lasts more than one or two days.  Unless deeper tissues of 
the cornea are also affected, surface epithelium injuries are 
rarely permanent. 

    Near-ultraviolet and near-infrared radiation (UV-A, IR-A, and 
possibly IR-B) are strongly absorbed in the lens of the eye.  
Damage to this structure is of great concern in that the lens has a 
very slow turnover of cells.  A one-day exposure may result in 
effects that will not become evident for many years.  This is 
probably the case of glass-blower's or steel puddler's cataract and 
in cataracts caused by ionizing radiation.  Long-term exposure may 
also result in delayed effects (Tengroth et al., 1980). 

8.3.1.  Effects on the cornea

    UV-B and UV-C radiation are absorbed in the cornea and 
conjunctiva and sufficiently high doses will cause kerato-
conjunctivitis.  The initial effect of UVR exposure is damage to, 
or destruction of, the epithelial cells.  Under normal conditions, 
the corneal epithelial layer is completely replaced in a matter of 
a day or so.  After exposure, there is a latent period, generally 
shorter than 12 h, which varies inversely with the exposure dose.  
Healing of the corneal and conjuctival epithelium takes 1 - 2 days.  
In severe exposures, damage to Bowman's membrane and the stroma may 
occur and is commonly followed by scar formation (usually of a 
milky appearance) and sometimes by invasion of the entire cornea by 
blood vessels.  Some limited recovery of moderate damage may occur 
in months or years. 

    The action spectrum and threshold dose for ultraviolet 
keratoconjunctivitis (Fig. 13) have been generally agreed on by 
several groups of investigators. 

FIGURE 13

    The reciprocity or irradiance and exposure duration probably 
hold for time periods similar to those for ultraviolet erythema 
(reddening) of the skin.  Specifically, it matters little whether 
radiant exposure of the cornea occurs in 1 µs or in 2 h.  The 
product of the irradiance and the duration of exposure required for 
the same effect is a constant for periods up to several h. 

    The cornea is quite transparent in the IR-A.  In the IR-B, 
there are some fairly narrow water absorption bands at 1430 nm and 
1959 nm.  Above 2000 nm, absorption is very high, making the cornea 
very susceptible to far-infrared radiation.  Thus, as might be 
expected, the threshold for damage corresponds to the absorption 
bands.  Radiation in the IR-C band can induce a burn on the cornea 
similar to that on the skin. 

    The nerve endings of the cornea are quite sensitive to all 
temperature elevations and an elevation of 10 °C causes a pain 
response.  With full-face exposure, a temperature rise can be felt 
before corneal pain appears. 

    Infrared lasers such as CO2 lasers (10.69 µm), HF, and DF 
lasers (2.7 - 4.0 µm) or CO lasers (5 µm), having cw output 
irradiances of the order of 10 W/cm2 or more can produce corneal 
lesions by delivering at least 0.5 - 10 J/cm2, before the blink 
reflex is operative (Fig. 14). 

FIGURE 14

8.3.2.  UVR lenticular effects 

    The lens has much the same sensitivity to UVR as the cornea.  
With exposure to UV-A, there is substantial transmission in the 
cornea and high absorption in the lens (Fig. 12). 

    Acute exposures of the order of 105 to 3 x 106 J/m2 (10 to
300 J/cm2) to radiation in the 320 - 400 nm region cause corneal
opacities, but only exposure to radiation in the UV-B region 
appears to be strongly effective in causing lenticular opacities 
under acute exposure conditions (Pitts et al., 1977; Zuchlich & 
Kurtin, 1977) (Fig. 15).  The lenticular opacity may only last for 
a few days at low exposures. 

FIGURE 15

    On the other hand, since long-term exposure to UVR at ambient 
outdoor levels may be associated with a reduction in the ability of 
the lens to transmit short-wavelength light (Lerman, 1980b) and 
with senile cataracts (Weale, 1982), the effects of UVR on the lens 
may be cummulative.  In addition, there is experimental evidence 
that UV-A radiation can induce photosensitized oxidation of the 
ocular lens (Zigler & Goosey, 1981). 

8.3.3.  Infrared cataract

    As explained earlier, the ocular media absorb an increasing 
amount of the radiant energy incident upon the cornea for 
increasing wavelengths in the near infrared (IR-A).  For infrared 
wavelengths greater than 1400 nm (IR-B and IR-C), the cornea and 
aqueous absorb essentially all of the incident radiation, and, 
beyond 1900 nm, the cornea is considered to be the sole absorber 
(Fig. 12).  This absorbed energy may be conducted to interior 
structures of the eyes elevating the temperature of the lens, as 
well as that of the cornea itself.  Heating of the iris by 
absorption of visible and near-infrared radiation is considered to 
play a role in the development of opacities in the lens, at least 
for short exposure times (Goldmann, 1933).  Hence, it would be 
expected that the spectral absorption and reflectance of the iris 
would determine the action spectrum for this effect, for such 
exposure times.  However, radiation of the lens alone, at lower 
levels for longer exposures induces cataracts in the part of the 
lens not covered by the iris (Vogt, 1919; Wolbarsht, 1978b).  Most 
IR-B and IR-C lasers cause damage only to the cornea (Andeev et 
al., 1978; Stuck et al., 1981). 

8.4.  Retinal Injury

    The retina is particularly vulnerable to visible and near-
infrared radiation, a spectral domain known as the retinal hazard 
region.  In most real situations, the refractive power of the 
cornea and lens leads to a dramatic increase in irradiance between 
the cornea and the retina.  When an object is viewed directly, the 
light forms an image on the fovea, the centre of the macula, which, 
in man, is approximately 0.25 mm in diameter.  The typical result 
of a retinal injury is a blind spot (i.e., a scotoma) within the 
irradiated area.  A peripheral scotoma, unless very large, may go 
unnoticed.  However, if the scotoma results from a lesion located 
in the fovea, severe visual handicap results.  A central scotoma 
could result from looking directly at a hazardous source.  The size 
of the scotoma depends on the irradiance relative to threshold, the 
angular extent of the source, and the extent of accommodation. 

    Laser lesions of the retina resulting from exposures to light 
cause many alterations in structure that can only be seen 
histologically (Marshall, 1973).  Fig. 16 shows two typical lesions 
in the macula of a human being. 

    The subjective loss of vision associated with laser-induced 
retinal injury may be immediate or may develop progressively over a 
period of hours or days (Gabel & Birngruber, 1981).  The extent to 
which visual function recovers depends on the size of the initial 
injury and the types of retinal cells involved, as well as the 
presence and extent of retinal haemorrhages and their subsequent 
degree of resorption (Boldry, et al., 1981).  Retinal neurons do 
not undergo cell division so repair by cell replacement does not 
occur.  In small lesions, recovery may result from either growth of 
new parts of damaged cells such as photoreceptor outer segments, or 
by migration of viable cells into the damaged site.  Severe damage 
is irreversible and visual loss is permanent. 

FIGURE 16

    Only an extremely small fraction of UVR incident on the cornea 
reaches the retina.  Nevertheless, the fraction of a percent of 
UV-A that reaches the retina can have adverse effects, as has been 
demonstrated by Zuchlich & Taboada (1978).  Retinal lesions were 
seen following exposure to a He-Cd laser at 325 nm.  Because of the 
strong scattering of UV-A in the ocular media, it is difficult to 
estimate the retinal exposure.  Young rhesus monkeys were used, 
hence, the transmission of the ocular media was still relatively 
high (perhaps 1%), whereas, in adult human beings this transmission 
is much less. 

    Aphakics (persons with the lens removed) are special exceptions 
to this rule and would be expected to be extremely susceptible to 
UV-A injury of the retina, particularly as their corrective lenses 
will usually transmit UV-A radiation very well. 

8.4.1.  Determining the retinal exposure

    The optical properties of the eye play an important role in 
determining retinal injury.  Such factors as the image quality, 
pupil size, spectral absorption, and scattering by the cornea, 
aqueous, lens, and vitreous, as well as the spectral reflectance of 
the fundus and absorption and scattering in the various retinal 
layers, must be known for a definitive description of retinal 
exposure.  These factors will be considered separately. 

8.4.1.1.  Pupil size

    The limiting aperture of the eye determines the amount of 
radiant energy entering the eye, and thus reaching the retina.  It 
is therefore proportional to the area of the pupil.  For the 
normal, dark-adapted eye, pupil sizes range from approximately 7 to 
8 mm;  for outdoor daylight, the normal pupil constricts to 
approximately 1.6 - 2.0 mm.  The ratio of areas between a 2-mm and 
an 8-mm pupil is:  1:16; hence, a 2-mm pupil permits the entry of 
one-sixteenth of the light admitted by a 8-mm pupil.  The angle 
subtended by the source also plays a role; thus a light source of a 
given size and luminance causes a different pupil size, depending 
on the viewing distance (i.e., the image area on the retina), and 
the luminance of the surrounding field. 

    The pupilomotor reflex will constrict the pupil on exposure to 
a bright light source within a period of the order of 20 ms 
(Davson, ed., 1962).  Some medications and drugs will create an 
abnormal pupil size.  Therefore, in a large population, the pupil 
size may vary greatly under the same environmental exposure 
conditions. 

8.4.1.2.  Spectral transmission of the ocular media and spectral 
absorption by the retina and choroid

    The transmission of the ocular media between 300 nm and 1400 nm 
has been studied by several investigators and results vary 
(Geeraets & Berry, 1968; Gabel et al., 1976).  Fig. 17 shows a 
representative spectral transmission of the human ocular media. 

FIGURE 17

    There is considerable individual variability in these spectral 
quantities and this must be remembered when using the data in an 
accurate calculation.  The other factor that must not be forgotten 
is the shift of the short-wavelength cutoff at 390 - 410 nm with 
age.  The lens transmits less UV-A and blue light with increasing 
age as it yellows.  With the above factors taken into account, it 
is possible to multiply the spectral absorption data of the retina 
and choroid by the spectral transmission data for the ocular media 
to arrive at an estimate of the absorbed spectral exposure dose in 
the retina and choroid, relative to the spectral radiant exposure 
at the cornea.  This spectral effectiveness curve for retinal 
thermal injury is applicable, at least, to exposure durations of 
less than approximately 10 s (Fig. 17). 

8.4.1.3.  Optical image quality

    The retinal image size can be calculated for most extended 
sources by geometrical optics.  As shown in Fig. 18, the angle 
subtended by an extended source defines the image size.  The 
effective focal length of the relaxed normal eye fe is approx. 
1.7 cm.  With fe known, the retinal image size dr can be 
calculated, if the viewing distance r and the dimension of the 
light source DL are known: 

    dr = DLfe/r                                       Equation (12)

    From this, the quantitative relation can be obtained of retinal 
irradiance Er to source radiance L (or retinal illuminance to 
source luminance) for small angles: 

    Er = (pi x dp2 L x tau)/4fe2

       = 0.27 dp2 x L x tau(for dp in cm)             Equation (13)

where dp is the pupillary diameter and tau is the spectral 
transmission of the ocular media. 

    From equation 13, it is possible to define a permissible 
radiance (luminance) from a permissible retinal irradiance for any 
source of known radiance or luminance, without taking into account 
the viewing angle or viewing distance. 

    Each source point has a corresponding image point, hence, the 
retinal irradiance in an incremental area of the image is related 
to the radiance of a corresponding incremental area of the source 
(Hartman & Kleman, 1980). 

8.4.1.4.  Small images

    Equation 13 breaks down for very small images (or for very 
small hot spots in an image), where the source or source element in 
question subtends an angle of less than 10 minutes-of-arc (i.e., 
image size is less than 50 µm). 

FIGURE 18

    The diffraction theory can be applied to estimate the minimal 
retinal image size for viewing a point source such as a laser. 
However, scattering in the ocular media and corneal aberations 
limits this image diameter to approximately 10 - 20 µm (Sliney, 
1971). 

    It would be expected that, in the case of small-sized images, 
the retinal hazard would increase in proportion to the area of the 
pupil.  Therefore, night-time viewing of a point source would be 
expected to be far more dangerous than day-time viewing; however, 
this is not the case, since the image blur increases with larger 
pupil sizes.  It is worthwhile noting that the increase in 
irradiance from cornea to retina, when the relaxed eye views a 
point source, is about 105 (100 000 times) (Sliney & Freasier, 
1973).  Almost any directly viewed laser can appear as a point 
source. 

8.4.1.5.  Retinal pigment epithelium (RPE) absorption

    As previously indicated, visible and near-infrared radiation is 
transmitted through the ocular media and is absorbed principally in 
the retina.  The radiation passes through the neural layers of the 
retina before reaching the RPE and choroid.  The visual pigments in 
the rods and cones absorb only a small fraction of light to 
initiate the visual response, perhaps only 5% of the total energy 
entering the eye.  The RPE absorbs a substantial fraction of the 
light (about 50% in the green) and is optically the most absorbent 
layer.  As the absorption takes place in a highly concentrated 
layer of melanin granules approximately 3 - 6 µm thick, the greatest 

temperature rise occurs in this layer (Bergquist et al., 1978; 
Birngruber, 1978).  The actual size, shape, distribution, and 
physical characteristics of individual melanin granules become 
quite important for a thermal model adequate to describe the 
behaviour of this layer, during very short pulse exposures 
(Wolbarsht, et al., 1980).  The granules may be heated to 
incandescence during Q-switched exposures, and this incandescence 
can be seen, if viewed during a 1064-nm (neodymium-YAG) laser 
experiment (Mueller & Ham, 1980, private communication). 

8.4.2.  Chorioretinal thermal injury

    The retinal injury mechanism is considered to be largely 
thermal for accidental exposures from arc lamps, cw lasers, or 
optically aided viewing of the sun, for durations of the order of 
1 ms to ~10 s.  Since injury appears to result principally from 
protein denaturation and enzyme inactivation, the variation in 
retinal temperature during and following the insult must be 
considered (Beatrice & Velez, 1975).  Several efforts to develop 
mathematical models for light absorption, heat flow, and the rate-
process injury mechanisms within the complex structure of the 
retina have been moderately successful, over periods of exposure 
lasting from 1 ms to 10 s (Vos, 1966a; Mainster et al., 1970; 
Allen, 1980). 

    The tissue surrounding the absorption site can much more 
readily conduct away the absorbed heat for images, 10 - 50 µm in 
diameter, than it can for large images of the order of 1000 µm 
(1 mm).  Indeed, retinal injury thresholds for the same time range 
of 0.1 - 10 s are very closely related to image size, as would be 
expected from calculations of heat flow in the retina.  For 
example, exposure to irradiances of 10-100 kW/m2 (i.e., 1-10 W/cm2)
results in a minimal retinal injury threshold for a 1000-µm image, 
whereas an irradiance of 10 MW/m2 (1 kW/cm2) is required to produce 
the same type of threshold lesion in a  20-µm image (Ham et al., 
1970). 

    For short-pulse durations, the reason for the spot-size 
dependence of the threshold is not clear (Frisch et al., 1971).  
The expectation, when exposure durations are of the order of 1 ms, 
is that injury will take place before there is significant heat 
flow.  Indeed, the variation of threshold with pulse duration 
itself is also rather puzzling, especially the increase in 
threshold for 20-ns, Q-switched pulses over those of 1-µs duration 
(Hansen & Fine, 1968; Harlen, 1978; Anderson, 1980b).  Obviously, 
the Q-switched pulse energy is being dissipated in some mechanism 
such as a mechanical displacement of tissue (an acoustic 
transient), which does not contribute to the normal thermal injury 
process that determines the minimum threshold. 

    In studies conducted by Beatrice & Lund (1979) with image 
distributions that were non-circular, such as line images on the 
retina, circular lesions were produced after long-term exposures.  
This would be predicted by thermal injury and heat flow 
calculations.  Very short, Q-switched exposures with similar line 

image distributions produced lesions that were very elongated, as, 
again, would be predicted by a simple thermal model or injury.  It 
might be thought that the contribution of acoustic transients from 
such a short exposure would have been to enlarge the lesion.  
Photoacoustic effects should certainly make a considerable 
contribution;  but whether the acoustic transient adds to, or 
subtracts from the injury mechanism is not yet clear. 

    For short-term exposures (< 1 µs) resulting from Q-switched 
lasers, exploding wires, super-radiant light, and mode-locked 
lasers, the exposure thresholds of injury are lowest.  Although it 
is believed that, for a Q-switched exposure, the injury mechanism 
is largely thermal, the effect of acoustic transients due to rapid 
heating and thermal expansion in the immediate vicinity of the 
absorption site (individual melanin granules) may play a role.  For 
still shorter durations of exposure, direct electric field effects, 
Raman and Brillouin scattering, and multiphoton absorption could 
play a role in the damage mechanism (Greisemann & Marti, 1978; 
Rockerolles, 1978). 

8.4.3.  Location of retinal burns

    As previously explained, the different regions of the retina 
play different roles in vision.  Thus, the significance of 
functional loss of all or part of any one of these regions, because 
of retinal injury, varies.  The loss of foveal vision seriously 
reduces visual performance.  Limited peripheral loss could be 
unnoticed, subjectively (Kaufman, 1970; Holzinger et al., 1978). 

8.5.  Photochemical Retinal Injury

8.5.1.  Very long-term exposure

    The human retina is normally subjected to irradiances below 
1 W/m2 (10-4 W/cm2), as shown in Fig. 19, except for occasional 
momentary exposure to the sun, welding arcs, and similar bright 
sources.  The retinal images resulting from viewing such sources 
are often quite small (for example 0.15 mm for the sun) and the 
duration of exposure is normally limited to the duration of the 
blink reflex (0.15 - 0.2 s).  Natural aversion to bright light 
usually limits further retinal exposures above 10-4 W/cm2.  Until 
recently, few studies of adverse retinal effects existed for the 
irradiance range of 10-4-1 W/cm2.  Studies in this range have 
generally centred on flash blindness effects following light 
exposures lasting up to 1 s. 

    Exposure of large areas of the retina to moderately high 
luminance light of the order of 105 cd/m2 (~100 µW/cm2, i.e., 
1 W/m2 at the retina) for durations of one to several hours has 
been investigated in experimental animals.  Generally, the light 
sources employed in these studies were fluorescent lamps.  A 
thermally-enhanced photochemical mechanism of injury or a 
phototoxic effect appears to be most likely (Kuwabara, 1970; Noell 
& Albrecht, 1971).  More recently, similar fluorescent light 
exposure of rhesus monkeys with dilated pupils showed that this 

effect was not limited to nocturnal animals but could conceivably 
be related to very lengthy direct-viewing by man (Sykes, et al., 
1981).  Collectively, the results of these studies suggest that 
abnormally high environmental levels of retinal illumination cause 
retinal degeneration for any species.  This is particularly marked 
in albinos (LaVail, 1980).  The effects are most dramatic when the 
normal, diurnal cycle of light and dark is eliminated by constant 
illumination (Williams & Baker, 1980).  The levels and durations of 
these retinal exposures exceed those encountered during normal 
human behaviour.  Only rarely have investigators reported that 
continual exposure to high luminance levels in the natural 
environment (or work environment) has elicited significant 
functional changes in the human retina (Livingston, 1932; Smith, 
1944; Peckham & Harley, 1951; Medvedovskaja, 1970; Roger, 1973). 

FIGURE 19

    It has been shown that short-wavelength exposures are most 
effective in inducing photochemical retinal injury (Ham et al., 
1976, 1978; Lawwill et al., 1977).  The adverse effect is normally 
centralized in the pigment layer and adjacent outer receptor layer.  
Other retinal layers may be affected, but this generally follows 
changes in the RPE.  The effect appears first where pigment is 
located. 

    Results of studies of photochemical retinal injury in 
experimental rhesus monkeys (Ham et al., 1978; Moon et al., 1978) 
agree with clinical experience reviewed by Sliney (1978) concerning 
the development of eclipse-burn photoretinitis after staring at the 
sun.  Until recently, it was generally accepted that solar retinal 
injury was permanent; however Hatfield (1970) and Penner & McNair 
(1966) reported significant numbers of patients who recovered 
within 30 days. 

    Repeated exposure of large retinal areas in trained monkeys to 
incoherent light at retinal irradiances just above those 
experienced in a bright natural outdoor environment, showed a 
permanent decrease in functional sensitivity to blue light 
(Harwerth & Sperling, 1975).  Exposure of the monkeys to narrow 
bands of wavelengths from the green to the red elicited a similar, 
but not lasting, reaction.  These types of studies, repeated by 
Zwick & Beatrice (1978), showed more dramatic changes with coherent 
light having a speckle pattern.  Prolonged erythropsia (red vision) 
in aphakics has also been reported following exposure to large-
area, high-luminance sources with large amounts of ultraviolet such 
as snow fields. 

    Follow-up studies conducted during the 1940s showed the effects 
on vision of the prolonged exposure of beach lifeguards to bright 
outdoor environments.  Temporary reductions in sensitivity for both 
daylight and night vision lasting for periods of several days were 
reported (Peckham & Harley, 1951). 

    There is growing concern among some investigators, who have 
studied the adverse retinal effects of intense light sources, that 
life-long exposure to light plays a role in retinal aging 
(Marshall, 1978).  Certain age-degenerative retinal effects may be 
light-initiated.  This opinion is prompted by the strong similarity 
between histological and ultrastructural changes in aged retinae 
and those in retinae exposed to intense light sources including 
disorganization of the outer receptors; depigmentation of the RPE, 
and a decrease in the total number of receptors.  Clearly, 
considerably more research will be required before this opinion can 
be confirmed. 

8.6.  Flash Blindness

    Flash blindness is a normal physiological process, being a 
transitory loss of visual function.  This phenomenon is a function 
of the preadaptive state of the subject, the nature of the visual 
task (i.e., task luminance, position in the visual field, and 
acuity), and the luminance of the stimulating source.  The 
mechanism is complex and not fully understood, as it involves both 
biochemical and neural processes.  The recovery time is a measure 
of the degree of flash blindness. 

    Flash blindness has been intensively investigated, both in 
predicting functional loss that might be expected from a given 
exposure and in the design of protective devices.  The response 
times of present-day protective devices range from a few ms down to 

several µs.  Even within this long delay, great difficulty has been 
encountered in producing a device that can attenuate the luminance 
of the source by a factor of more than 1000. 

    In dealing with the data on flash blindness and attempting to 
relate it to retinal mechanisms, it is well to keep in mind the 
ideas originally expressed by Brown (1973), who showed that 
recovery from flash blindness did not involve a single, simple 
mechanism.  It involves both dark adaptation and neurophysiological 
effects not usually seen in studies of dark adaption. 

8.7.  Discomfort Glare

    If a very bright light source is suddenly switched on after the 
eye has adapted to a luminance much lower than that of the source, 
the viewer experiences discomfort, blinks, and tends to turn the 
head or eyes.  This effect is much more dramatic when the ambient 
luminance is low.  Out-of-doors, in full daylight, the luminance of 
a large extended source required to elicit this phenomenon is of 
the order of 100 000 cd/m2 (10 cd/cm2 or 29 000 foot-lamberts); 
however, the luminance of this source must be far greater to elicit 
a response, if the source subtends an angle of less than about 4.5 
mrad (0.25 degree). 

8.8.  Flashing Lights

    Classical studies of vision suggest that a strobe operating at 
10 Hz can appear subjectively brighter than a cw source of the same 
peak brightness, due to the Bartley Effect for flashing light 
sources.  Similarly, the brightness of a single pulse appears 
brighter than it really is; this phenomenon is termed the Broca-
Sulzer Effect (Cornsweet, 1966). 

    Strobe light sources produce little annoyance during daylight 
hours.  However, at night, if a strobe illuminates an area that 
would otherwise be dark, at a frequency of 5 - 10 Hz, the static 
objects illuminated by the strobe appear to float around, because 
of loss of a fixation point during off-periods of the light. 

    The medical literature suggests that epileptic seizures in 
susceptible individuals represent the only well-documented health 
hazard from exposure to low-frequency intermittent light.  The most 
sensitive frequency range is from 8 to l6 Hz.  Various estimates 
indicate that approximately 1.0% of the epileptic population, which 
itself constitutes less than 1.0% of the general population, will 
experience these "flicker-induced" seizures. 

9.  THE SKIN

9.1.  Anatomy

    The skin is normally of less interest than the eye as far as 
optical radiation hazards are concerned.  However, under certain 
conditions, it may be more vulnerable than the eye, which can be 
protected by the blink reflex or by eye protectors. 

    The outermost layer, the stratum corneum or horny layer, 
consists of flattened, epidermal cells.  They originate in the 
germinative layer (a single cell layer of collumnar germinative 
basal cells) at the bottom of the epidermis, grow, and are 
gradually pushed outward until they die and are flattened to form a 
protective layer over the living cells. 

    The stratum corneum is approximately 10 - 20 µm thick over most 
parts of the body, except the soles of the feet and the palms of 
the hand, where it is much thicker (500 - 600 µm). 

    The epidermis is relatively uniform in thickness throughout the 
body (50 - 150 µm) and can be separated into a number of layers 
with the growing cells in the lowest level toward the basal 
membrane. 

    Melanocytes, specialized cells that produce melanin pigment 
granules, are located in the basal layer of the epidermis.  The 
melanocytes send out dendritic processes that interdigitate within 
the keratinocytes.  Melanosomes (the pigment granules) are then 
transferred into the keratinocytes.  The pigment is thus 
distributed throughout the epidermis and stratum corneum by 
migration of the keratinocytes. 

    The dermis, or corium, is thicker than the epidermis, and 
consists largely of connective tissue which gives the skin its 
elasticity and supportive strength.  The sweat glands extend into 
the corium. 

9.2.  Body Heat Regulation

    The skin plays a major role in the thermoregulatory system of 
the human body.  At incident irradiances less than those that cause 
thermal skin burns, the body can be subjected to heat stress.  
Several physical means are employed by the skin for cooling 
(conductive, convective, and radiative cooling). 

    The sweat glands produce sweat, which permits evaporative 
cooling as well as conductive cooling.  The reflectance of human 
skin in the far infrared is very low with a correspondingly high 
emissivity for wavelengths in the 8 - 13 µm infrared region.  The 
high emissivity at these wavelengths permits highly efficient 
radiative cooling of the skin at body temperature. 

    Comparison of the reflectance spectrum of the human skin with 
the solar emission spectrum shows a striking similarity.  The skin 
reflects largely in the visible part and near-infrared parts of the 
spectrum, where solar radiation is greatest, and absorbs heavily in 
the ultraviolet and far infrared, where there is very little solar 
radiation.  Any good radiator of infrared energy (a black body) 
must have very high absorption, hence very low total reflection.  
The skin does indeed have low reflectance in the far infrared, and 
seems well adapted to the natural environment.  The skin both 
reflects direct solar radiation and reradiates internally generated 
infrared radiation with the greatest possible efficiency.  However, 
the human body is less capable of reflecting the infrared radiation 
from man-made sources such as fire, or more specifically from 
molten steel in a steel mill (Hardy, 1968; Sliney & Freasier, 1973; 
Stolwijk, 1980). 

9.3.  Optical Properties

    The stratum corneum strongly absorbs actinic UVR, which causes 
sunburn.  This layer also strongly absorbs far-infrared radiation.  
Melanin granules are small (1 µm diameter) and not only protect the 
dermis by absorption of UVR, but also by scattering optical 
radiation.  Melanin scatters rather than absorbs radiation in the 
near-infrared region.  For this and other reasons, near-infrared 
radiation penetrates deeply into the tissue.  Since the index of 
refraction of the stratum corneum is about 1.5, the Fresnel 
reflective component is somewhat similar to that of glass (Fig. 2).  
Optical radiation incident on the skin at grazing angles of 
incidence is hardly absorbed at all.  The relative effectiveness of 
optical radiation in penetrating the epidermis (and dermis) varies 
approximately as the cosine of the angle of incidence.  Since light 
penetrates the outermost layers of the skin, undergoes multiple 
scattering, and some light is scattered back out of the skin, the 
skin has an appearance that cannot easily be duplicated by a non-
translucent surface (Anderson & Parrish, 1981). 

9.4.  Penetration Depth and Reflection

    Studies of the effects of pulsed visible and IR laser radiation 
on pig skin, performed in the 1960s, provided single-wavelength 
thresholds for injury for very short-term exposures of the order of 
1 ms and 20 ns.  For such short durations of exposure, the 
influence of heat flow from the absorbing site is not a major 
factor.  For this reason, these studies aptly showed that the 
threshold of injury depended on the reflectance of the skin and the 
depth of penetration of the optical radiation into the skin (Fig. 
20). 

    The reflectance of the skin also plays a role in determining 
how much radiation can effectively be absorbed.  The skin's 
spectral reflectance varies with pigmentation and is significant 
only in the visible and near-infrared spectrum.  The skin's 
reflectance at wavelengths of less than 310 nm and above 2.5 µm is 
less than 5%. 

FIGURE 20

    It must be remembered that small amounts of optical radiation 
penetrate deeply into the body, where it may react with 
photosensitive cells.  This may give rise to physiological 
reactions of great importance, such as circadian rhythms and annual 
rhythms.  Artificial constant irradiation may suppress the 
circadian rhythms, giving rise to health problems. 

9.4.1.  Injury to the skin

    Laser radiation injury to the skin is normally considered less 
important than injury to the eye, despite the fact that injury 
thresholds for the skin and eye are comparable, except in the 
retinal hazard region (400 - 1400 nm).  In the IR-C and UV-C 
spectral regions, where optical radiation is not focused on the 
retina, skin injury thresholds are approximately the same as 
corneal injury thresholds.  The probability of exposure is greater 
for the skin than for the eye, because of the skin's greater 
surface area, and yet injury to the eye is still considered to be 
of greater significance.  For a comparable area of tissue 
destruction, the functional losses associated with the eye are more 
debilitating than with the skin.  Threshold injuries resulting from 
the short-term (i.e., less than 10 s) exposure of the skin to far-
infrared (IR-C) and UV-C radiation are also very superficial and 
may only involve changes to the outer dead layer - the "horny 
layer" - of the skin cells.  A temporary injury to the skin may be 
painful, if sufficiently severe, but eventually it will heal, often 
without any sign of the injury.  Burns (thermal injuries) to larger 
areas of skin are far more serious, as they may lead to serious 
loss of body fluids, toxaemia, and systematic infections. 

    Thermal injury of the skin has been the subject of many studies 
in this century.  Hardy et al. (1956) found that severe pain could 
always be induced in human skin tissue, when the tissue temperature 
was elevated to 45 °C.  This temperature also corresponds to an 
injury threshold, if the exposure to optical radiation lasts for 
many seconds. 

    Skin injury resulting from momentary but very intense exposures 
to optical radiation are generally termed "flash burns".  Flash 
burns of the skin following exposure to optical radiation in 
industry are rare.  Most conventional sources such as open-arc 
processes and industrial furnaces do not create significant 
irradiances in work areas where skin injury could occur 
sufficiently fast to preclude a natural protective reaction to the 
intense heat.  The flash burns that do occur are more often the 
result of conductive heating of the skin by exceedingly hot gases 
or steam.  Though the eye is protected from most flash burns, the 
eye lid may be injured with resulting complications in vision.  The 
threshold depends on the area of irradiated tissue.  Heat 
conduction in tissue is far more efficient for small than for 
larger irradiated areas (e.g., 1 mm2) and exposure to higher levels 
of irradiance would be possible before injury occurred.  With 
extensive irradiation, injury would occur at a far lower level of 
irradiance.  Hazardous exposure of large areas of skin is unlikely 
to be encountered in the normal work environment, as the heat alone 
from a source that could produce heat stress due to elevation of 
deep body temperature, would require protective measures at lower 
irradiance levels. 

    The threshold of injury obviously depends also on the duration 
of exposure.  The previously mentioned thresholds are for just one 
exposure duration of 0.5 s.  For exposures lasting less than 0.5 s 
the irradiance required for an injury would significantly increase 
as the duration of exposure decreased. 

    Studies of the process of thermal injury in skin show that the 
longer the length of exposure, the lower the temperature required 
to coagulate proteins and destroy tissue by elevated temperature.  
Fig. 21 shows this time dependence of threshold for white-light, 
arc-source burns (upper curve), and for far infrared laser 
radiation (lower curve).  The explanation for these threshold 
differences lies in the fact that thermal injury depends on energy 
absorbed per unit volume (or mass) to produce a critical 
temperature elevation.  Skin reflectance and penetration greatly 
influence this absorption.  Skin disorders are common and may 
change the reaction of the skin to irradiation.  Some disorders may 
be aggravated while, in others, healing may be enhanced by optical 
radiation. 

FIGURE 21

9.4.2.  The sensation of warmth and heat flow

    The sensation of warmth, resulting from the absorption of 
radiant energy, normally provides adequate warning for avoiding 
action to prevent thermal injury of the skin from almost all 
sources except the nuclear fireball and some high-powered, far-
infrared lasers.  The spot size dependence of this sensation is 
illustrated by irradiating human skin with a beam of CO2-laser 
radiation at 10.6 µm.  An irradiance of 0.1 W/cm2 produces a 
definite sensation of warmth for beam diameters larger than 1 cm.  
On the other hand, one-tenth this level (0.01 W/cm2) can readily be 
sensed, if the whole body or a larger portion of the body is 
exposed.  The dependence on the size of the irradiated area results 
from conduction of heat away from the absorbing area, thus limitng 
surface temperature rise, the sensation  of heat being a function 
of temperature rise.  As noted previously, the skin temperature 
elevation required to elicit persistent pain (as well as thermal 
injury after several s) is approximately 45 °C (Hardy et al., 1956; 
Hardy, 1968). 

9.4.3.  Thermal injury threshold for the skin

    Fig. 22 presents data reported by Rockwell & Goldman (1974), 
which illustrate that for some wavelengths, the threshold depends 
on skin pigmentation.  In the far-infrared, however, all tissue 
absorbs heavily, not just melanin pigment granules. 

FIGURE 22

9.4.4.  Delayed effects

    The possibility of adverse effects from repeated or long-term 
laser irradiation of the skin is normally discounted, if scarring 
does not occur (Goldman et al., 1971).  Only UVR has been shown to 
cause long-term, delayed effects.  These effects are accelerated 
skin aging and skin cancer.  It is difficult to quantitatively 
evaluate the role of UVR in the induction of skin cancer.  For 
solar radiation, the high-risk wavelengths are around 310 nm.  Some 
attempts to calculate the dose-effect relationship have now been 
made (de Gruijl & van der Leun, 1980).  At present, laser safety 
standards for exposure of the skin do attempt to take into account 
all of these adverse effects. 

9.4.5.  Ambient environment and heat stress

    The temperature of the ambient environment can play a role in 
adding to or subtracting from, the temperature rise through 
continuous exposure of the skin to optical radiation, particularly 
if full-profile exposure is possible.  Though full-profile exposure 
to laser radiation is uncommon, it is not impossible.  Just as 
full-profile exposure to far-infrared radiation from furnaces can 
cause heat stress, so also can exposure to far-infrared laser 
radiation.  Radiant absorption is, however, only one factor in 
defining heat stress (WHO Scientific Group ..., 1969; Stolwijk, 
1980).  Increased body temperature due to fever or other causes 
generally lowers the irradiance threshold of the effects of optical 
radiation. 

    Man in his natural environment is already exposed to 
significant optical radiation out-of-doors.  The sun's irradiance 
on a clear day may vary from 0.5 to 1.1 kW/m2 (50 to 110 mW/cm2) at 
midday.  The human body is well designed to reflect direct solar 
radiation.  In the far-infrared region of the spectrum, the skin's 
low reflectance and high emissivity make it possible for the body 
to both radiate and strongly absorb 10-µm radiation.  The ambient 
radiant exitance of surrounding structures and the ground may vary 
from 10 to 400 W/m2 (1 to 40 mW/cm2). 

9.4.6.  UVR effects on the skin

    UVR gives rise to acute and delayed visible effects.  The acute 
effects are erythema (skin reddening), thickening of the stratum 
corneum and pigmentation through melanogenesis (Van der Leun, 
1965).  The delayed effects are accelerated skin aging and 
carcinogenesis (Urbach, 1980). 

    Threshold levels for the acute effects from UV-A are generally 
of the order of 1000 times greater than those of UV-B or UV-C.  The 
effects depend, to a large extent, on skin pigmentation at the time 
of exposure.  These subjects have been dealt with in detail in the 
WHO Environmental Health Criteria document on ultraviolet radiation 
(WHO, 1980). 

9.4.7.  Photosensitization

    Light-induced damage to the skin in the presence of certain 
chemicals (photosensitizers) may be considered phototoxic if an 
allergic mechanism is not ivolved.  It can occur in any type of 
skin exposed to UVR of the proper wavelength and looks like a 
normal erythema.  In some cases the reaction may be delayed, but in 
general it will appear immediately after exposure.  A number of 
systemic photosensitizers have been identified and examples are 
given in Table 6. 

9.4.8.  Photoallergy

    Photoallergy is an acquired altered capacity of the skin to 
react to light (and UVR) alone or in the presence of a 
photosensitizer.  This subject has been treated in detail in WHO 
(1979). 

Table 6.  Systemic photosensitizers:  Chemicals that induce 
photosensitivitya
-------------------------------------------------------------------
Uses                                 Name
-------------------------------------------------------------------
Antibacterial                        nalidixic acid

Anticonvulsant                       carbamazepine

Antimycotic                          griseofulvin
-------------------------------------------------------------------

Table 6.  (contd.)
-------------------------------------------------------------------
Uses                                 Name
-------------------------------------------------------------------

Artificial sweeteners                cyclamates, calcium
                                     cyclamate sodium
                                     cyclohexylsulfamate

Broad spectrum antibiotic            antibiotics

Chemotherapeutic, antibacterial      sulfonamides

Diuretics, antihypertensive          chlorthiazides

Hypoglycaemic or antidiabetic drugs  sulfonylurea

In vitiligo for sun tolerance        furocoumarins
and increased pigment formation

Laxative                             triacetyldiphenolisatin

Oral contraceptives                  estrogens and progesterones

Tranquillizer, nematode infestation  phenothiazines
control urinary antiseptic,
antihistamine

Tranquilizer, psychotropic           chlordiazepoxide
-------------------------------------------------------------------
a   Adapted from: Fitzpatrick et al. (1974).


10.  LASER SAFETY STANDARDS: RATIONALE AND CURRENT STANDARDS

10.1.  Introduction

    Laser safety standards may take several forms.  The standard 
may be simply a list of guidelines concerning laser operation or 
equipment design with no mention of exposure limits or a list of 
personnel exposure limits (ELs) or product emission limits.  Today, 
most safety standards incorporate all of these aspects, to some 
extent.  This section explains the scientific and philosophical 
problems encountered in the development of today's standards.  The 
distinction between occupational exposure standards and equipment 
performance standards will also be discussed. 

    Exposure limits may be applied in three general categories of 
standards, i.e., occupational safety and health standards, 
environmental quality standards and equipment performance 
standards.  ELs for laser radiation for general population and 
occupational exposure were developed in many countries during the 
late 1960s and throughout the 1970s.  A general consensus for many 
of these limits can be found in the 1982 draft international 
standard of the International Electrotechnical Commission (IEC, in 
preparation). 

10.2.  Laser Hazard Classification

    It was recognized during the early development of laser safety 
standards that some form of risk classification was necessary.  
This resulted from many complaints from research scientists in the 
1960s that they were being needlessly constrained in their use of 
small He-Ne lasers by safety specialists, who were attempting to 
apply guidelines originally drafted for high-power ruby and 
neodymium lasers. 

    Most recent laser safety standards therefore include a hazard 
classification scheme to simplify risk evaluation on which to base 
control measures (Harlen, 1978). 

    The safety procedures necessary for any laser operation vary 
according to three aspects:  (a) the laser hazard classification; 
(b) the environment in which the laser is to used, and (c) the 
people operating or within the vicinity of the laser beam.  Hazard 
classification schemes differ only slightly, depending on which 
standard is being followed; and a brief explanation of the most 
commonly used hazard classification system follows. 

    Class 1 lasers are the lowest powered lasers.  This group is 
normally limited to certain gallium-arsenide lasers or enclosed 
lasers.  These lasers are not considered hazardous, even if the 
output laser beam can be collected by 80-mm collecting optics and 
concentrated into the pupil of the eye.  An infrared or ultraviolet 
laser is Class 1 if the radiation concentrated on the skin or eye 
will not cause injury within the maximum exposure duration possible 
during one day of laser operation.  Most lasers are not Class 1, 
however, when they are incorporated into consumer or office machine 

equipment, the resulting system may become Class 1.  If a Class 1 
system contains a more dangerous laser, the access panel to it must 
be interlocked or contain a warning to alert the user of the 
hazardous laser radiation that may be encountered, when the panel 
is removed. 

    Class 2 lasers, often termed "low-power" or "low-risk" laser 
systems, are those that are only hazardous if the viewer overcomes 
the natural aversion response to bright light and stares 
continuously into the source - an unlikely event.  This could just 
as readily occur by forcing oneself to stare at the sun for more 
than a minute or to stare into a film projector source for several 
minutes.  This hazard, though rare, is as real as eclipse 
blindness, hence Class 2 lasers should have a caution label affixed 
to indicate that purposeful staring into the laser should be 
avoided.  Since the aversion response only occurs for light, the 
Class 2 category is limited to the visible spectrum from 400 to 
700 nm. 

    Class 3 "moderate-risk" or "medium-power" laser systems are 
those that can cause eye injury within the natural aversion 
response time, i.e., during the blink reflex (0.25 s).  Class 3 
lasers do not cause serious skin injury or hazardous diffuse 
reflections under normal use.  However, these must have danger 
labels and the safety precautions required are often considerable. 

    Class 4 laser systems are the highest powered lasers and 
present the greatest potential for injury or combustion of 
flammable materials.  They may also cause diffuse reflections that 
are hazardous to view or induce serious skin injury from direct 
exposure.  More restrictive control measures and additional 
warnings are necessary (Clevet & Mayer, 1980). 

    Fig. 23 summarizes the most typical classification
scheme.  Some standards further refine the above
classification scheme to include special subclasses referred
to as Class 2a, Class 3a, and Class 4a.  Relaxed restrictions
may apply to these subclasses.

FIGURE 23

11.  EXPOSURE LIMITS

11.1.  Rationale

    Exposure limits for lasers and optical radiation cover a wide 
range of wavelengths and exposure conditions, and biological 
effects may apply to both the eye and skin.  For this reason, no 
single rationale can apply to all of the specific ELs.  It is first 
important to distinguish between acute and chronic (or delayed) 
effects.  For acute effects, thresholds exist and statistical 
(probit analysis) techniques can indicate this threshold with a 
degree of uncertainty.  The method of assessing the acute effect 
may not always be the most sensitive, but is often chosen for 
reasons of simplicity and repeatability.  However, this approach is 
feasible only because the relationship between this assessment 
threshold and the onset of irreversible damage is known from more 
rigorous studies using the most sensitive techniques for damage 
assessment.  While a threshold for most chronic effects can be 
expected on theoretical grounds, this threshold can best be 
estimated from careful evaluation of epidemiological data.  ELs are 
set by considering both types of effects and the degree of 
uncertainty in thresholds. 

    Scientists working in the field of ionizing radiation are used 
to the problems of cumulative doses and total lifetime exposure.  
In describing the adverse biological effects of optical radiation 
at wavelengths greater than 320 nm, few scientists would argue that 
a linear hypothesis applies with total integration of the lifetime 
exposure. 

    Optical radiation is usually absorbed in a thin layer of tissue 
and its effects are thermal in nature, except for the ultraviolet 
and visible photochemical processes.  For both of these acute 
effects, there is a definite threshold; that is, an exposure level 
exists below which no adverse change will occur and no real risk 
exists.  Of course, the threshold can vary with the individual and 
with environmental conditions.  However, if the safety level is set 
well below these variations, then the exposure conditions are not 
hazardous. 

    To establish a rationale for developing exposure limits from 
the biological data requires careful analysis of the spread of the 
empirical data.  These include the variables influencing potential 
for injury in exposed individuals, the increase in severity of 
injury for suprathreshold exposure doses, and the degree of repair 
of injury. 

    The accuracy of available measuring instruments and the desire 
for simplicity in expressing the limits have also influenced the 
exposure limits.  It is difficult to inter-relate all these 
factors; however, most specialists agree on the final limits, even 
though they may have derived them in different ways. 

    Separate high risk occupational limits - in contrast to 
exposure limits for the general population - have not been 
developed.  Unlike ionizing radiation, there has been little debate 
as to whether a threshold of injury actually exists.  However, 
there can be a debate concerning the exact "threshold values" for 
specific wavelengths and exposure durations.  UV-B and UV-C laser 
radiation could, in theory, have delayed effects with no real 
threshold, but has nevertheless been treated like longer wavelength 
laser radiation.  Long-term effects of low-level exposure have been 
indicted as a possible contributing agents in senile degenerative 
processes in the retina (Marshall, 1978; Young, 1981).  If true, a 
simple threshold for these effects probably does not exist. 

11.2.  Assessment of the "Safety Factor"

    It is very difficult to decide on the margin that should be 
introduced, to account for individual variation in experimental 
error, in deriving the ELs.  This margin is sometimes loosely 
termed the safety factor; however, this is not correct.  The 
threshold of injury is actually the result of considering the 
probit analysis of many data points. 

    The most reliable statistical method for describing any 
biological threshold is probit analysis.  One point on the curve, 
the 50% damage probability, is often assigned a special 
significance.  This point is known as the ED50 and is the exposure 
dose required, for example, to produce an ophthalmoscopically 
visible (or an otherwise measurable) lesion in 50% of the exposures 
in a group of animals or in a single animal, where several 
exposures have taken place.  The ED50 point has in the past been 
termed a threshold point by some investigators, though clearly the 
use of the term threshold in describing a 50% probability of injury 
seems rather inappropriate.  In toxicological studies of the 
effects of chemicals on biological systems, the term threshold has 
often been used to define a 10% probability of a biological 
response. 

    Probit analysis is a powerful tool in determining safety 
information but was not originally applied, e.g., to retinal damage 
from laser exposure.  In the early studies on retinal damage, the 
experimental design was usually such as to facilitate calculations 
of only the ED50 point. 

    When the data are plotted on probit paper, a line results that 
can easily be extrapolated to the clinical damage probability.  
Indeed, studies should be designed to give the slope with maximum 
precision.  For this, two points at low and high probability, e.g., 
the 20% and 80% probability points are more important than the ED50 
point.  Once this information is available together with the 
criterion for injury and the accepted degree of safety, it is 
possible to determine the exposure limit.  Present-day laser ELs 
are typically a factor of 5 - 20 below an ED50 for observable acute 
injury, this ratio varying as a result of functional-loss, 
histological, and suprathreshold-severity studies. 

11.3.  Environmental Considerations

    Though laboratory studies of the adverse effects of optical 
radiation provide the basic insight into thresholds, locations, and 
mechanisms of tissue injury, it is difficult to extrapolate these 
findings to protection standards.  Of special importance, is the 
separation of acute temporary effects from those leading to delayed 
permanent detrimental effects. 

    In this regard, it will also be necessary to take the actual 
exposure conditions of man into consideration - both from natural 
environmental sources or from artificial sources.  It must be 
remembered, for instance, that terrestrial solar radiation changes, 
both in total irradiance and in spectral distribution, throughout 
the day.  Furthermore, the direction of illumination is of great 
importance because of the reflection from the cornea, as seen in 
Fig. 24. 

FIGURE 24

11.4.  Limiting Apertures

    One difficulty in developing ELs is the specification of the 
limiting aperture over which the values must be either measured or 
calculated.  For the skin, where no self-focusing effect takes 
place, the smallest feasible aperture is most desirable.  
Unfortunately, the smaller the aperture, the higher the sensitivity 

required for the measuring instrument and the greater the 
inaccuracy that will result from calibration problems associated 
with diffraction and other optical effects.  Since various 
biological effects are influenced differently by the size of an 
incident beam, the limiting aperture varies for different 
conditions. 

11.4.1.  The 1-mm aperture

    A 1-mm aperture has been typically considered the smallest 
practical aperture size for specifying ELs.  Under continuous 
exposure conditions, heat flow and scattering within the layers of 
the skin tend to eliminate any adverse effects from hot spots 
smaller than 1 mm in diameter. 

11.4.2.  The 11-mm aperture

    Wavelengths greater than 0.1 mm present a further difficulty.  
At these far-infrared, submillimetre wavelengths, a 1-mm aperture 
creates significant diffraction effects and calibration becomes a 
problem.  Hot spots predicted by physical optics are larger than at 
shorter wavelengths.  For these reasons, a 1-cm square, or 11-mm 
diameter (1 cm2) circular aperture has typically been chosen as the 
limiting aperture for wavelengths between 0.1 mm and 1 mm. 

11.4.3.  The 7-mm aperture

    For ocular ELs in the "retinal hazard region", from 
approximately 400 nm to 1400 nm, the averaging (sampling) aperture 
is determined by the pupil of the eye.  A pupil size of 7-mm has 
been decided as typical, though not without a great deal of debate. 

11.4.4.  The 80-mm aperture

    A still larger measuring aperture of 80 mm is conventionally 
used for power and energy measurements to account for intrabeam 
viewing conditions with optical telescopes or binoculars. 

11.5.  Spectral Dependence of Exposure Limits

    Eye and skin injury thresholds vary considerably with 
wavelength.  To establish the spectral dependence of ELs, it is 
generally accepted that the biological data can only be followed 
approximately.  The ELs have been adjusted for variation in 
wavelength, but do not precisely follow the empirical biological 
data.  Fig. 25 shows an example of theoretical variation of 
susceptibility modified to provide a spectral correction factor, 
useful for calculating the EL. 

    Fig. 25 provides the reciprocal of the product of the relative 
spectral transmission of the ocular media with the retinal 
absorption (Fig. 17).  This indicates the relative effectiveness of 
different wavelengths for causing retinal thermal injury.  However, 
this curve still does not show the relative spectral hazard to the 
lens of the eye in the near infrared.  Also plotted in this graph 

is the spectral modification factor used for pulsed retinal 
exposure limits in the American National Standards Institute 
standard (ANSI, 1980).  This modification factor is used for other 
limits for protection against thermal injury.  Because of variation 
of threshold with image size and variation of image size with 
wavelength, a further increase in the ratio between IR-A and 
visible ELs is given in the ANSI Z-136.1 standard for IR-A 
wavelengths between 1050 nm and 1400 nm, but only for durations of 
exposure of less than 0.05 ms. 

FIGURE 25

11.6.  Repetitively Pulsed Laser Exposure

    The values in the present standards for repetitive ocular 
exposure have been based on limited data and developed from purely 
empirical extrapolations.  The cumulative effect of repetitive 
pulses was considered to be a function of the duration of the 
individual pulse in a pulse train.  For short pulses (duration less 
than 10 µs), the EL for a single pulse was multiplied by a 
correction factor to provide a reduced exposure on a per-pulse 
basis.  This value was then compared with the EL values for the 
total energy and for a total exposure of the duration of the entire 
train of exposures, to determine which limit would apply.  For a 

train of pulses, where the individual pulse duration exceeded 10 
ms, a criterion based on total on-time Tt of the train of pulses 
was applied to each individual pulse.  This resulted in a reduced 
EL for each pulse.  However, all of these approaches were based 
largely on studies in which the eye of the rhesus monkey was 
exposed to single pulses of "minimal image size". 

    More recent studies (Greiss et al., 1980) suggest that the 
pulse thresholds add as a function of N for small image sizes, 
but not for large image sizes.  The letter N refers to the number 
of pulses in the train. 

11.7.  Restrictions for Special Applications (Class 3a)

    The low risk of Class 2 lasers differs little from the lack of 
risk of Class 1 lasers, in practice.  Class 2 lasers emit a power 
of 0.4 µW - 1 mW, a light level difficult to stare into because of 
the aversion response.  The risk increases significantly, when the 
eye is unable to protect itself, as occurs when a visible laser 
beam irradiance exceeds 2.5 mW/cm2 (i.e., a total of 1 mW entering 
the 7-mm pupil of the eye by either unaided or optically-aided 
viewing).  The laser classification denotes risk, when the laser is 
viewed under worse-case conditions.  In practice, if worst-case 
conditions are seldom experienced, further relaxations can be 
applied for certain limited applications.  An example of this is 
the 5 mW limit applied to the total power for surveying/ alignment 
of lasers.  This recognizes that "moderate-risk" (Class 3) lasers 
are sometimes needed in this application but that the benefit in 
this application outweighs the moderate risk of the 1-5 mW visible 
laser group (US DHEW, 1979). 

11.8.  Present Standards of Exposure

11.8.1.  Laser standards

    A number of national and international standards have been 
promulgated that show only minor differences, some of which may be 
partly resolved, when future editions of the American National 
Standards Institute (ANSI), British Standards Institute (BSI), 
GOST, and the International Electrotechnical Commission (IEC) 
appear. 

    Tables 7-11 present the most recent set of occupational ELs, 
those promulgated by the ANSI Z-136.1 Standard (ANSI, 1980) and the 
threshold limit values (TLVs) of the American Conference of 
Governmental Industrial Hygienists (ACGIH, 1981), those given in 
the draft International Electrotechnical Commission standard (IEC, 
in preparation), and the values mentioned as "best available" in 
Suess, ed. (1982). 

11.8.1.1.  Exposure limits

    The tables and figures presented in this section are from the 
ACGIH booklet  Threshold limit values for chemical substances and 
 physical agents in the workroom environment with intended changes 

 for 1981 (ACGIH, 1981).  For this criteria document, the term 
"exposure limit" is used. 

    While the concept of an EL (or TLV) is that neither the general 
population nor workers should be intentionally exposed above the 
limit, accidental over-exposure may not always result in injury.  
It is helpful to quote the TLV preamble given by ACGIH:  "The 
threshold limit values are for exposure to laser radiation under 
conditions to which nearly all workers may be exposed without 
adverse effects.  The values should be used as guides in the 
control of exposures and should not be regarded as fine lines 
between safe and dangerous levels.  They are based on the best 
available information from experimental studies". 

11.8.1.2.  Repetitively pulsed lasers

    Since the additive effects of multiple pulses are not fully 
understood, caution must be used in the evaluation of such 
exposures.  The exposure limits for irradiance or radiant exposure 
in multiple pulse trains have the following limitations: 

(a)  The exposure from any single pulse in the train is limited to 
     the exposure limit for a single comparable pulse;

(b)  The average irradiance for a group of pulses is limited to the 
     EL (as given in Tables 7, 8, and 9) of a single pulse of the 
     same duration as the entire pulse group;

(c)  When the instantaneous Pulse Repetition Frequency (PRF) of any 
     pulses within a train exceeds 1, the EL, applicable to each 
     pulse, is reduced by a factor (Cp), as shown in Fig. 26 for 
     pulse durations of less than 10-5 s.  For pulses of greater 
     duration, the EL of a pulse in the train is found by dividing 
     the EL of a longer pulse of duration Nt by N, where N is the 
     number of pulses in the train, t is the duration of a single 
     pulse in the train, and the EL of Nt is the exposure limit of 
     one pulse having a duration equal to Nt s.  The "pulse" 
     duration Nt is known as the TOTP (total on time pulse), Tt in 
     the ANSI standard.  For a short group of N pulses, the reduced 
     EL will not be less than the single pulse EL divided by N.

    Repeated exposures at repetition rates of less than 1 Hz should 
be considered additive over a 24-h period. 

FIGURE 26

Table 7.  Exposure limits for direct ocular exposures (intrabeam viewing) 
from a laser beam
-------------------------------------------------------------------------
Spectral  Wavelength          Exposure time (t)    Exposure limits
region                        seconds (s)
-------------------------------------------------------------------------
UVC       200 nm to 280 nm    10-9 to 3 x 104      3 mJ/cm2
UVB       280 nm to 302 nm    10-9 to 3 x 104      3 mJ/cm2
          303 nm              10-9 to 3 x 104      4 mJ/cm2
          304 nm              10-9 to 3 x 104      6 mJ/cm2
          305 nm              10-9 to 3 x 104      10 mJ/cm2
          306 nm              10-9 to 3 x 104      16 mJ/cm2 )
          307 nm              10-9 to 3 x 104      25 mJ/cm2 )
          308 nm              10-9 to 3 x 104      40 mJ/cm2 ) not to
          309 nm              10-9 to 3 x 104      63 mJ/cm2 ) exceed
          310 nm              10-9 to 3 x 104      100 mJ/cm2) 0.56t¨
          311 nm              10-9 to 3 x 104      160 mJ/cm2) J/cm2
          312 nm              10-9 to 3 x 104      250 mJ/cm2)
          313 nm              10-9 to 3 x 104      400 mJ/cm2)
          314 nm              10-9 to 3 x 104      630 mJ/cm2)
          315 nm              10-9 to 3 x 104      1.0  J/cm2)
          315 nm to 400 nm    10-9 to 10           0.56t1/4 J/cm2
UVA       315 nm to 400 nm    10 to 103            1.0 J/cm2
          315 nm to 400 nm    103 to 3 x 104       1.0 mW/cm2
-------------------------------------------------------------------------

Table 7 (contd.)
-------------------------------------------------------------------------
Spectral  Wavelength          Exposure time (t)    Exposure limits
region                        seconds (s)
-------------------------------------------------------------------------
Light     400 nm to 700 nm    10-9 to 1.8 x 10-5   5 x 10-7J/cm2
          400 nm to 700 nm    1.8 x 10-5 to 10     1.8(t/4 ´t) mJ/cm2
          400 nm to 549 nm    10 to 104            10 mJ/cm2
          500 nm to 700 nm    10 to T1             1.8(t/4 ´t) mJ/cm2
          550 nm to 700 nm    T1 to 104            10CB mJ/cm2
          400 nm to 700 nm    104 to 3 x 104       CB µW/cm2
IR-A      700 nm to 1049 nm   10-9 to 1.8 x 10-5   5 CA x 10-7 J/cm2
          700 nm to 1049 nm   1.8 x 10-5 to 103    1.8CA(t/4 ´/t) mJ/cm2
          1050 nm to 1400 nm  10-9 to 5 x 10-4     5 x 10-6 J/cm2
          1050 nm to 1400 nm  5 x 10-4 to 103      9(t/4 ´/t) mJ/cm2
          700 nm to 1400 nm   103 to 3 x 104       320 CA µW/cm2
IR-B & C  1.4 µm to 103 µm    10-9 to 10-7         10-2 J/cm2
          1.4 µm to 103 µm    10-7 to 10           0.56 4 ´/t) J/cm2
          1.4 µm to 103 µm    10 to 3 x 104        0.1 W/cm2
-------------------------------------------------------------------------

The formula for Correction Factor CA (Fig. 25) is

CA = 1 for wavelength (lambda) of 400 nm - 700 nm;
CA = 10(0.002[lambda-700 nm]) for 700 nm < lambda < 1050 nm; and
CA = 5 for 1050 < lambda < 1400 nm.
CB = 1 for lambda = 400 - 550 nm;
CB = 10(0.015[lambda-550]) for lambda = 550 - 700 nm.
T1 = 10 s for g = 400 - 550 nm; T1 = 10 x 10(0.02[lambda-550]) for 
     lambda = 550 - 700 nm.

For lambda = 1.5 to 1.6 xi m increase EL by 100 for periods of less 
than 1 µs.

Table 8.  Exposure limits for viewing a diffuse reflection of a
laser beam or an extended source laser
-------------------------------------------------------------------------
Spectral  Exposure limits    Wave-length      Exposure time (t) (s)
region
-------------------------------------------------------------------------
UV        200 nm to 400 nm   10-9 to 3 x 104  Same as Table 7
Light     400 nm to 700 nm   10-9 to 10       10 3 ´t J/(cm2 x sr)
          400 nm to 549 nm   10 to 104        21 J/(cm2 x sr)
          550 nm to 700 nm   10 to T1         3.83 (t/ 4 ´t) J/(cm2 x sr)
          550 nm to 700 nm   T1 to 104        21/CB J/(cm2 x sr)
          400 nm to 700 nm   104 to 3 x 104   2.1/CB x 10-3 W/(cm2 x sr)
IR-A      700 nm to 1400 nm  10-9 to 10       10CA 3 ´t J/(cm2 x sr)
          700 nm to 1400 nm  10 to 103        3.83CA(t/4 ´t)J/(cm2 x sr)
          700 nm to 1400 nm  103 to 3 x 104   0.64CA W/(cm2 x sr)
IR-B & C  1.4 µm to 1 mm     10-9 to 3 x 104  Same as Table 7
-------------------------------------------------------------------------
CA, CB, and T1 are the same as in footnote to Table 7.

Table 9.  Exposure limits for skin exposure from a laser beam
---------------------------------------------------------------
Spectral  Wave-length        Exposure time    Exposure limits
region                       (t) (s)
---------------------------------------------------------------
UV        200 nm to 400 nm   10-9 to 3x104    Same as Table 7
Light &   400 nm to 1400 nm  10-9 to 10-7     2CA x 10-2 J/cm2
IR-A      400 nm to 1400 nm  10-7 to 10       1.1CA 4´t J/cm2
IR-B & C  1.4 µm to 1 mm     10-9 to 3 x 104  Same as Table 6
---------------------------------------------------------------
CA = 1.0 for lambda = 400 - 700 nm; see Fig. 25 for value at 
     greater wavelengths.

NOTE:  To aid in the determination of ELs for exposure 
       durations requiring calculations of fractional powers, 
       Fig. 25 may be used.
Table 10.  Additivity of effects on eye and skin from different spectral regionsa
---------------------------------------------------------------------------------
                  UV-C and UV-B  UV-A        Visible and IR-A  IR-B and IR-C
Spectral region   200-315 nm     315-400 nm  400-1400 nm       1400-106 nm
---------------------------------------------------------------------------------
UV-C and UV-B     eye
200-314 nm        skin

UV-A                             eye                           eye
315 nm                           skin        skin              skin

Visible and IR-A                             eye
400-1400 nm                      skin        skin              skin

IR-B and IR-C                    eye                           eye
1400-106 nm                      skin        skin              skin
---------------------------------------------------------------------------------
a Some synergism is expected when 2 spectral bands illuminate the same tissue 
  simultaneously.  Exact formulae to treat these additive effects have not been 
  developed for most standards.
Table 11.  Selected values of the minimum angle of an extended 
source that may be used for applying extended source ELs
--------------------------------
Exposure duration  Angle alpha       
(s)                (mrad)
--------------------------------
10-9               8.0a 
10-8               5.4a 
10-7               3.7a 
10-6               2.5a 
10-5               1.7a 
10-4               2.2  
10-3               3.6
10-2               5.7
10-1               9.2
--------------------------------

Table 11 (contd.)
--------------------------------
Exposure duration  Angle alpha       
(s)                (mrad)
--------------------------------
1.0                15
10                 24
102                24
103                24
104                24
--------------------------------
a For exposure durations of less 
  than 0.05 ms alphamin is less
  for lambda = 1050 to 1400 nm.

11.8.1.3.  Extended source laser exposure

    The ELs for "extended sources" apply to sources that subtend an 
angle greater than alphamin (Table 11), which varies with exposure 
duration (t).  This angle is not the beam divergence of the source.  
Limits expressed as either radiance or integrated radiance may be 
averaged over an angle as great as alphamin or sampled over a 
source area as small as 1 mm in diameter. 

    Table 8 should be used to calculate the EL (as a brightness) 
for an extended source such as a holographic display or a screen 
illuminated by a static or scanning laser beam.  The values in 
Table 8 apply to viewing a diffuse reflection from a laser beam, 
where a truly extensive retinal image occurs.  Some laser devices 
are intentionally designed as diffuse sources (e.g., beacons) to 
radiate monochromatic optical power and still remain Class 1.  The 
extended source ELs of Table 8 apply to the direct output of the 
laser system if the source is diffuse.  As a further example, a 
low-quality semiconductor diode laser or a semiconductor laser 
diode array may be "extended".  In this case, the average radiance 
of the diode array might be applied against the extended source EL. 
It must be emphasized that, in almost all instances, a laser source 
is still a "point source" within definitions used by the standards. 

    To aid in determining when extended-source ELs are applicable, 
the concept of alphamin was invented.  The value of alphamin is a 
linear angle expressed in mrads and is the minimum viewing angle at 
which extended-source ELs apply.  For viewing distances beyond the 
location where the source angle subtends an angle less than 
alphamin, the source is considered from a safety standpoint to be a 
"point source" and the intrabeam viewing criteria of Table 7 apply. 
Because the extended source ELs and the point source ELs do not 
vary in exactly the same way as a function of pulse duration (or 
exposure duration) (t), this limiting angle alphamin varies with 
exposure duration (Table 11).  Indeed alphamin is nothing more than: 

alphamin = (4/pi) (EL [point source]/EL [extended source])´ Equation (14)

11.8.1.4.  Restriction on ELs

    The ELs were developed for conditions of occupational exposure 
and the underlying assumption is that nearly all workers may be 
exposed to the levels without adverse effects.  However, some 
photosensitive individuals may experience adverse effects at lower 
levels for wavelengths of less than 500 nm. 

11.8.2.  Standards for non-laser sources

11.8.2.1.  Introduction

    The most commonly occurring hazardous effects from arcs and 
high-intensity lamps are ultraviolet erythema and photokeratitis. 
Retinal injury from such sources is seldom recognized though it is 
not unheard of.  Considering that much was known about optical 
radiation hazards prior to the development of the laser, it seems 
somewhat surprising that ELs and safety standards for lamps and 
arcs did not exist prior to laser safety standards.  Standards were 
developed empirically for eye protective filters for welders, but 
were not based on ELs. 

    Bright sources emitting cw light elicit a normal aversion or 
pain response that serves to protect the eye and skin from injury.  
Visual comfort has often been used as an approximate hazard index.  
Eye protection baffles and other controls have been provided on 
this basis.  The determination of shade number for welding goggles 
is one example.  The present standards for welding goggle 
specifications were simply based on a comfort index for viewing the 
arc. Since UVR and infrared radiation were considered to be of no 
value in viewing welding arcs, they were deliberately filtered out. 
UV and IR filtration factors exceeding those for light were 
specified as the best that could be achieved with readily available 
glass materials. 

    Quantitative guidance is often sought with regard to both eye 
and skin safety in relation to new sources of radiation.  Though 
several safety limits for optical radiation have been proposed in 
the literature within recent years, it is only for the ultraviolet 
spectral region that there have been any widely accepted limits, 
but even these have provoked controversy. 

    ELs applicable to broad-band sources such as open-arc 
processes, arc lamps, incandescent lamps, and gas discharge lamps 
may differ considerably from laser ELs for two main reasons.  The 
first is that the source normally emits in a broad spectral band. 
Therefore, effects due to narrow wavelength absorption or 
coherence, which are potentially of concern with laser exposure, 
are not likely to have a substantial impact on the hazards from a 
broad-band source.  All of the composite optical spectral bands for 
conventional sources must be evaluated separately.  For instance, 
ultraviolet hazard criteria differ completely from light hazard 
criteria. 

    The second major difference between laser and non-laser health 
criteria results from the fact that most hazardous laser exposures 

result from viewing a point source, whereas hazardous lamps and 
arcs are usually extended sources.  In the development of a new 
lamp, any unwanted ultraviolet radiation should be filtered out by 
the choice of an appropriately thick glass envelope, based on 
computation, and assessments of both acute and chronic risks and 
actual UVR measurements.  In the past, manufacturers have watched 
for acute effects in people exposed to prototype lamps. 

    The radiance from a conventional source is generally physically 
limited compared with that of a laser source.  The exposure of an 
individual from a lamp source is seldom likely to exceed that under 
normal operating conditions.  Laser output powers can change 
enormously with slight changes in the laser cavity. 

11.8.2.2.  UVR Criteria

    As previously noted, the health hazards associated with UV-B + C 
exposure of the eye and skin are often considered separately from 
those associated with UV-A. 

    In the development of health criteria for industrial UVR 
exposure, the prime consideration must be ELs that would prevent 
unwanted acute and chronic effects.  At the same time, simplicity 
of measurement and application are important. If a single 
instrument having a spectral response weighted against the envelope 
action spectrum for UV-B and UV-C injury were developed, then a 
direct measurement could be made of the UVR risk.  Without a 
spectrally integrating instrument, the spectral irradiance from the 
source of interest can be measured at the point of greatest concern 
(normally the nearest point of access).  This spectral irradiance 
Elambda is then weighted by the ACGIH envelope action curve Slambda 
(Fig. 27, Table 12 and 13) for wavelengths of less than 320 nm. 

FIGURE 27

Table 12.  Relative spectral 
effectiveness for selected wavelengths
--------------------------------------
                       Relative
                       spectral
Wavelength   Elambda   effectiveness
(nm)         (mJ/cm2)  Slambda
--------------------------------------
200          100       0.03
210          40        0.075
220          25        0.12
230          16        0.19
240          10        0.30
250          7.0       0.43
254          6.0       0.5
260          4.6       0.65
270          3.0       1.0
280          3.4       0.88
290          4.7       0.64
300          10        0.30
305          50        0.06
310          200       0.015
315          1000      0.003
--------------------------------------

Table 13.  ACGIH ultraviolet exposure limits
--------------------------------------------
Duration of exposure   Effective irradiance,
per day                Eeff (µW/cm2)
--------------------------------------------
8 h                    0.1
4 h                    0.2
2 h                    0.4
1 h                    0.8
30 min                 1.7
15 min                 3.3
10 min                 5
5 min                  10
1 min                  50
30 s                   100
10 s                   300
1 s                    3000
0.5 s                  6000
0.1 s                  30 000
--------------------------------------------

    All the preceding ELs for ultraviolet energy apply to sources 
that subtend an angle less than 80°.  Sources that subtend a 
greater angle need to be measured only over an angle of 80°.

    ACGIH recommended values:  The threshold limit values for 
occupational exposure to UVR incident on skin or eye, where 
irradiance values are known and exposure time is controlled, are as 

follows: 

1.  For the near ultraviolet spectral region (320-400 nm), 
    total irradiance incident on the unprotected skin or eye
    should not exceed 1 mW/cm2 for periods greater than 103 s
    (approximately 16 min) and for exposure times less than
    103 s should not exceed one J/cm2.

2.  For the actinic ultraviolet spectral region (200-315 nm),
    radiant exposure incident on the unprotected skin or eye
    should not exceed the values given in Table 10, within an
    8-h period.

3.  To determine the effective irradiance of a broad-band
    source weighted against the peak of the spectral
    effectiveness curve (270 nm), the following weighting
    formula should be used:

    Eeff = SIGMA Elambda Slambda delta lambda   where:

    Eeff = effective irradiance relative to a monochromatic
           source at 270 nm in W/cm2 (J/s/cm2)

    Elambda = spectral irradiance in W/cm2/nm

    Slambda = relative spectral effectiveness (unitless)

    delta lambda = band width (nm)

4.  Permissible exposure time in seconds for exposure to
    actinic ultraviolet radiation incident on the unprotected
    skin or eye may be computed by dividing 0.003 J/cm2 by
    Eeff in W/cm2.  The exposure time may also be determined
    using Table 13, which provides exposure times
    corresponding to effective irradiances in µW/cm2.

    Conditioned (tanned) individuals can tolerate skin exposure in 
excess of the TLV without erythemal effects.  However, such 
conditioning may not protect persons against skin cancer. 

    For the UV-A, ACGIH considered it reasonable to propose a 
guideline for ocular exposure so low that no conceivable thermal or 
photochemical injury mechanisms were likely to be demonstrated. To 
prevent thermal injury, it was assumed that, for exposure durations 
of less than 100 s, the eye should be protected against exposures 
above 10 kJ/m2 (1 J/cm2).  Because of the lack of adverse effects 
reported in individuals working with ultraviolet "black light" 
sources at levels of 1 mW/cm2 (or above), it was presumed that a 
level of 1 mW/cm2, the approximate level of exposure of the eye to 
UVA-A in outdoor reflected sunlight, would be a reasonable upper 
limit for exposures lasting 1000 s or more.  The skin exposure limit 
could presumably be increased by a factor of 5 for the longer 
exposure durations.  To avoid thermal effects at very short 
exposure durations, the total UVR corneal irradiance was also 
limited to 1 W/cm2.  It is now known that photochemical effects 
occur in both the eye and skin and that total daily doses of 20 - 

100 J/cm2 cause acute corneal opacities (Pitts et al., 1977; 
Zuchlich & Kurtin, 1977) and skin erythema from the UV-A (Parrish 
et al., 1978).  Hence, these UV-A criteria must be applied with 
great caution for conditions of very long (exceeding 4 h) exposure. 

11.8.2.3.  Retinal health criteria

    Laser protection standards incorporate several simplifications 
that depend on the single-wavelength and point-source 
characteristics of the laser.  These standards may provide a too 
conservative estimate of the real risk, if laser criteria are 
applied to broad-band sources.  No official standards exist for the 
retinal risk evaluation of a broad-band source.  Tentative 
guidelines exist from ACGIH.  To use these guidelines, both a blue-
light hazard function B lambda and a retinal thermal injury 
function R lambda must be used.  The source spectrum may be weighted 
to indicate comparative levels of risk from the two types of 
retinal injury mechanisms.  Using equation 13 (section 8.4.1.3), in 
which the retinal irradiance Er is directly proportional to the 
radiance L of the source, the square of the pupil diameter dp, and 
the transmission t of the ocular media.  The retinal spectral 
irradiance distribution can be calculated from the spectral 
radiance distribution Llambda and knowledge of the spectral 
transmission of the ocular media taulambda.  In the absence of a 
radiance standard, this approach can be used to calculate retinal 
levels directly and to compare them directly with thresholds of 
injury (Sliney & Freasier, 1973).  However, the present approach is 
to establish ELs for the spectrally weighted radiances.  These 
safety weighting functions are given in Table 14.  Spectral factors 
weighted against the spectral radiance are then applied as shown in 
sections 11.8.2.4 and 11.8.2.5. 

Table 14.  Spectral weighting functions for assessing 
retinal risks from broad-band optical sourcesa
-------------------------------------------------------
             Blue-light            Thermal                 
Wavelength   hazard function       hazard function         
(nm)         Blambda               Rlambda                 
-------------------------------------------------------
400          0.10                  1.0                     
405          0.20                  2.0                     
410          0.40                  4.0                     
415          0.80                  8.0                     
420          0.90                  9.0                     
425          0.95                  9.5                     
430          0.98                  9.8                     
435          1.0                   10                      
440          1.0                   10                      
445          0.97                  9.7                     
450          0.94                  9.4                     
455          0.90                  9.0                     
460          0.80                  8.0                     
465          0.70                  7.0                     
470          0.62                  6.2                     
475          0.55                  5.5                     
-------------------------------------------------------

Table 14 (contd.)
-------------------------------------------------------
             Blue-light            Thermal                 
Wavelength   hazard function       hazard function         
(nm)         Blambda               Rlambda                 
-------------------------------------------------------

480          0.45                  4.5                     
485          0.40                  4.0                     
490          0.22                  2.2                     
495          0.16                  1.6                     
500-600      10[(450-lambda)/50]   1.0                     
600-700      0.001                 1.0                     
700-1049     0.001                 10[(700-lambda)/500]    
1050-1400    0.001                 0.2                     
-------------------------------------------------------
a From:  ACGIH (1981).

11.8.2.4.  Retinal thermal risk evaluation

    To protect against thermal retinal injury from short-term 
exposures, the spectral radiance of the lamp weighted against the 
function Rlambda (Table 14) should not exceed: 

1400
SIGMA Llambda Rlambda delta lambda L(Haz) = ´ t/(alpha t) W/(cm2 x sr)
400                                                     Equation (15)

where Llambda is given in W/(cm2 x sr), t is the viewing duration 
(or pulse duration if the lamp is pulse limited) which is limited 
to 1 ms - 10 s, and alpha is the angular subtense of the source in 
radians.  The angle alpha should be limited to approximately 0.1 
radian.  If the lamp is oblong, alpha refers to the longest 
dimension (1) that can be viewed.  For instance, at a viewing 
distance (r) of 500 cm from a tubular lamp 50 cm long, the viewing 
angle alpha is 1/r or 0.1 rad.  Spectral radiance (Llambda) 
measurements must be made at frequent wavelength intervals (delta 
lambda) to preclude serious error.  The delta lambda should be less 
than 5 nm in the UV and blue end of the visible spectrum. 

11.8.2.5.  Retinal blue-light risk evaluation

    To protect against retinal injury from blue-light exposure, the 
integrated spectral radiance of the lamp weighted with the blue-
light hazard function (Blambda of Table 14) should not exceed 100 
J/cm2 x sr for a duration of less that 104 or exceed 10 mW/(cm2 x 
sr) for t > 104 s: 
1400        
SIGMA Llambda x t x Blambda x delta lambda < 100 J/(cm2 x sr) for t < 104 s
400                                                             Equation (16)
                               or
1400        
SIGMA Llambda x Blambda x delta lambda < 10 W/(cm2 x sr) for t > 104 s
400                                                             Equation (17)

and for a point source (alpha < 11 mrad)

1400        
SIGMA Elambda x t x Blambda x delta lambda < H(Haz) = 10 mJ/cm2 for t < 104 s
400                                                             Equation (18)
                               or
1400        
SIGMA Elambda x Blambda x delta lambda < E(Haz) = 1 µW/cm2 for t > 104 s
400                                                             Equation (19)
    These levels assume a constricted pupil, as would occur with 
fixed viewing of any type of extended source with a radiance 
approaching the EL.  For a spectrally weighted source radiance (L) 
that exceeds 10 mW/(cm2 x sr) in the blue-light spectral region, 
the permissible exposure duration tmax in s is simply: 
                          1400    
tmax = 100 J/(cm2 x sr)  /  SIGMA Llambda x Blambda x delta lambda for t < 104 s
                          400                                   Equation (20)
    The extended-source limits are greater than the 198.  ELs for a 
440-nm laser radiation source given by either ANSI or ACGIH, which 
assume a 7-mm pupil rather than the 3-mm used for the broad-band 
source analysis. 

11.8.2.6.  IR-A risk analysis

    The proposed ACGIH EL also limited the IR-A and IR-B infrared 
radiation beyond 770 nm to 10 mW/cm2 to avoid possible 
cataractogenesis (the appearance of which may be delayed).  For an 
infrared heat lamp or other source that lacks a strong visual 
stimulus, the radiance for wavelengths between 700 and 1400 nm for 
long-term viewing should be limited to: 

1400                                                         
SIGMA Llambda x delta lambda < L(Haz) = [0.6/alpha] W/(cm2 x sr)
700                                                   Equation (21)

This limit is also based on a 7-mm pupil diameter.

NOTE:  Equations 16 to 21 are empirical and are not, strictly
       speaking, dimensionally correct.  To make these formulae
       correct, a dimensional correction factor must be inserted 
       into each formula.  It is, therefore, important to use only 
       the units specified.

11.8.3.  Infrared standards

    There are no established non-laser, infrared (IR) health 
standards.  However, the laser ELs can be applied for broad-band 
sources, if, in addition, the whole-body irradiation is evaluated.  
Even irradiances as low as 100 W/m2 (10 mW/cm2) can place an 
uncomfortable thermal load on the human body, especially when the 
irradiation is not confined to one side of the body and this 
radiant heat load occurs along with high ambient air temperatures. 

In contrast, the IR laser EL for periods exceeding 10 s is 1 kW/m2 
(100 mW/cm2), assuming that the total irradiated area of the skin 
or the eye will be small.  For laser exposure the irradiated area 
is generally small, but this is not so likely when the body is 
exposed to optical radiation from non-laser sources; and heat 
stress must be evaluated. 

    The determination of the wet-bulb-globe-temperature (WBGT) 
index requires a combination of a dry-bulb temperature with a wet-
bulb (WB) temperature (which involves humidity, air movement, etc.) 
and a black-globe (BG) temperature (which includes the radiant 
(predominantly infrared) contribution).  These three temperatures 
are weighted differently in two equations used for evaluating heat 
stress - one for outdoor workers exposed to sunlight, another for 
indoor workers exposed to infrared sources.  The nature of the 
skin's reflectance is such that much of the visible and IR-A are 
reflected, whereas IR-B and IR-C are almost totally absorbed.  The 
spectral reflectance of most clothing is somewhat similar to that 
of skin in the infrared.  Obviously the second formula would be 
used in any IR risk evaluation.  The ACGIH formula for indoor heat 
stress is: 

    WBGT = 0.7 WB + 0.3 GT                            Equation (22)

A heat-stress condition exists when this WBGT value exceeds 25 - 
30 °C depending on work load. 

    A major problem in any infrared safety standard concerned with 
wavelengths beyond 1.4 µm is ambient IR-C.  The black-body radiant 
exitance at 273 K (0 °C) is 32 mW/cm2; at 300 K (27.2 °C), it is 46 
mW/cm2.  A whole-body irradiance of 20 - 50 mW/cm2 from radiant 
warmers on a cold (0 °C) winter day is comfortable; but the same 
irradiance on a hot summer day could bring on heat stress. 
Therefore any IR safety standard should distinguish between all the 
IR bands, and IR-C limits would have to vary with ambient 
conditions. 

12.  RISK EVALUATION

    There are three broad areas of concern for any potentially 
hazardous optical source:  (a) the potential of the source for 
causing personal injury; (b) the environment in which the laser or 
optical source is used; and (c) the individuals who operate and 
those who are potentially at risk from exposure to the emitted 
optical radiation.  For both lasers and lamp systems, it is 
possible to develop a hazard classification scheme that would 
greatly assist the health and safety professional in evaluating the 
risk from an optical source in a particular environment (Anderson, 
1980a). 

    It is important to understand that the laser classification 
system was developed to aid the user in establishing a safety 
programme for a particular laser device to relieve the user and 
also the health and safety professional of the burden of detailed 
and often complex measurements or calculations.  The unique risks 
and control measures applicable to specific environments depend on 
the personnel potentially exposed and vary with each laser 
application.  However, fortunately, many of the protection measures 
depend entirely, or to a great extent, on the laser hazard 
classification. 

    Since the control measures required for Class 1 and Class 2 
laser systems are minimal or nonexistent for the user, it is the 
applications of Class 3 and Class 4 lasers that require careful 
study of the risks, and the development of detailed control 
measures.  There are several protective methods, which can apply to 
a Class 3 or a Class 4 laser product.  The total enclosure of the 
source is certainly the most desirable control measure.  However, 
since total enclosure with proper interlocks would result in a 
Class 1 laser product, there is normally a reason why a laser 
system was not originally designed as a Class 1 device.  There are 
a few instances where a specific enclosure must be developed for 
each application.  Where the enclosure approach is feasible, this 
solution is strongly recommended. 

    Where the laser beam is operated without being enclosed - 
either indoors or out-of-doors - the laser safety officer (a health 
or safety professional or other special trained individual) has 
great need of reference material and technical data.  These data 
include the reflective properties of materials found in the 
environment, attenuating properties of filters, windows, or other 
enclosures, and a working knowledge of several aspects of optical 
systems. 

    Several system-safety items should be considered for 
incorporation in laser system design, including: 

    beam attenuators;
    interlocks;
    manual switches;
    the enclosure;
    emission indicators;
    accidental laser firing;

    beam diffusers;
    multiple wavelengths;
    mode locking;
    beam hotspots.

12.1.  Laser Hazard Classification

    For the classification of a laser, the following variables 
concerned with output should be known:  (a) the wavelength or 
wavelength range; (b) the classification duration (i.e., in the 
ANSI standard:  how long is it possible for a person to be exposed 
to an applicable EL); (c) average power output (for cw or 
repetitively-pulsed lasers); and (d) total energy per pulse (or 
peak power, pulse duration, PRF, and emergent beam radiant 
exposures) for all pulsed lasers.  The laser source radiance or 
integrated radiance and the maximum viewing angular subtense is 
required, if the source is an extended laser source and is 
operating in the retinal hazard region (400 - 1400 nm). 

    If the laser is modified, after manufacture, in a way that 
could affect the hazard classification, then the user or the 
individual performing the modification should reclassify the 
modified system. 

12.2.  Environmental Considerations including Reflection and the 
Probability of Exposure

    Environmental considerations probably play a greater role in 
determining the control measures for Class 3 and Class 4 laser 
systems, and these can only be evaluated by the user.  These 
environmental considerations include the possibility of reflections 
(Gorodetski et al., 1968). 


12.2.1.  Reflections

    The three general types of reflection that may be encountered 
in many environments are shown in Fig. 28.  Diffuse reflections 
normally greatly reduce the hazards of the primary beam, though, 
for Class 4 visible and near-infrared lasers, hazardous diffuse 
reflections are likely (Komorova et al., 1978).  The dividing line 
between Class 3 and Class 4 visible and IR-A lasers is defined by 
diffusely reflective hazard conditions.  Reflections from flat 
mirrors produce substantial risks of hazardous exposure at 
considerable distances from the reflector as can be seen in 
Fig. 28.  Where random orientation of the reflectors can occur, the 
potentialy hazardous area can be quite large.  Curved surface 
specular reflections,on the other hand, are normally hazardous only 
at relatively short distances of the order of magnitude of the 
radius of curvature from the reflector surface.  Though specular 
surfaces are of greatest concern in open laboratory situations, 
they are not unheard of in the outdoor use of lasers and in medical 
applications. 


FIGURE 28

    Absolute values of spectral reflectance are relatively 
unimportant because such values vary only by a factor of 10 - 20, 
whereas the laser beam irradiance may exceed the applicable EL by 
orders of magnitude above this factor. 

    Most reported laser accidents have occurred, when the 
probability of exposure was very high.  No discussion of reflection 
hazards is therefore complete without the consideration of the 
probability of exposure.  It must be remembered that the underlying 
philosophy of the classification system is that control measures 
should increase with increasing risk of exposure as well as with 
increased severity of exposure. 

12.2.2.  Retroreflection

    Some materials exhibit a property known as retrodirective 
reflection.  The reflection does not obey either the law of regular 
(specular) reflection, or the cosine dependence of Lambertian 
reflection.  A collimated indigent beam may remain collimated and 
be redirected along the original axis of propagation, regardless of 
the angle of incidence at the retroflector.  Corner cubes and some 
specialized highway signs are examples of retroflectors. 

12.2.3.  Optically aided viewing

    Nearly all laser workers know that viewing a laser source with 
a telescope may substantially increase the risk.  This increase in 
risk is most dramatic for intra-beam viewing of a collimated 

source.  In this case, an increase in the power entering the eye is 
possible, because the diameter of the objective (Do) of the 
telescope or binocular is much larger than the pupil of the eye dp. 
The actual increase in risk (G) depends on whether the pupil of the 
eye is larger or smaller than the exit pupil (De) of the optical 
system, the spectral transmittance (taulambda) of the optical 
system at the laser wavelength(s), and the beam diameter relative 
to the objective (collecting aperture) diameter. 

    When a bright object, larger than a point source, is viewed 
through a well-designed optical system, the radiant power reaching 
the retina (visible and IR-A) is theoretically increased by the 
square of the instrument's magnifying power.  However, since there 
is a commensurate increase in the area of the retinal image, the 
retinal irradiance remains unchanged, except for a slight reduction 
because of reflective loss and the absorption of light in the 
optical system.  The retinal hazard in this case may increase in 
some instances, since the thermal retinal injury threshold 
decreases with increasing image size. 

13.  ACCIDENTAL INJURIES

    At present, retinal injuries with loss of sight following 
exposure to visible and IR-A laser radiation have been the most 
catastrophic of all effects from laser radiation (Boldry et al., 
1981; Zhokov, 1981).  Though the relatively high-powered, far-
infrared lasers such as the cw, carbon-dioxide laser, have caused 
numerous burns to the hands and clothes, these are considered 
inconsequential in comparison with the serious retinal injuries. 
One postal survey conducted several years ago (Rockwell & Goldman, 
1974) appeared to indicate that there had been at least 100 
injuries to the eye from laser radiation in the USA.  One 
subjective account in this regard was that of Decker (1977). 

    Despite such reports, surprisingly few serious injuries of the 
eye have been reported in the last 15 years in relation to exposure 
to pulsed lasers.  This low accident rate cannot be accounted for 
by assuming that the ocular ELs are too conservative.  The 
explanation is probably that accidental exposure of the eye to a 
collimated beam is normally an extremely remote possibility, if 
precautions are taken to keep the eye out of the beam.  One of the 
few situations in which the probability of hazardous exposure is 
great is during work in the laboratory and, each year, several 
retinal injuries are reported under these conditions.  However, it 
is difficult to ascertain the exposure conditions in sufficient 
detail for useful threshold or injury data to be derived.  Concern 
may also exist regarding the potential bias in an individual's 
description of the accidental exposure conditions to support a 
claim of compliance with safety measures.  It is not clear whether 
laser exposures are always detected by the exposed individual, and 
this may be a particular problem with IR lasers (Kasuba & Akifev, 
1977).  It is known that retinal injury outside the macula may have 
little or no effect on everyday visual performance and therefore 
may not be detected subjectively. 

14. CONTROL MEASURES

    Risk control guidelines are not mutually exclusive.  Following 
one or two guidelines may reduce the risk to such an extent that 
other recommended control measures, in that particular class, are 
no longer essential.  For example, if the beam path of a Class 4 
laser were enclosed, then it would hardly be necessary to remove 
all glass objects, or other specular surfaces near the beam path 
but outside the enclosure, nor would it seem necessary to wear eye 
protectors.  However, the eye protection might be necessary if the 
enclosure were being modified or during initial alignment.  The 
decision to use any particular set of controls depends on use 
conditions and whether general population exposure is likely. 

    Table 15 provides a brief list of the most commonly recommended 
laser control measures. 

Table 15.  Control measures for general population and occupational 
exposure
-------------------------------------------------------------------
I  Engineering control measures:

   protective housing enclosure and service panel requirements;
   interlocks on the protective housing;
   door interlocks and remote control connector;
   beam attenuator or beam shutter;
   key switch or padlock over aperture cover;
   filtered viewing optics and windows;
   emission delay (BRH);
   warning lights;
   emission indicators (audible or visible);
   enclosed area or room;
   beam enclosure;
   remote firing and/or monitoring;

II  Personal protective equipment:

    eyewear;
    clothing;
    gloves;

III  Administrative and procedural controls:

     laser safety officer;
     standard operating procedures (SOPs);
     limitations on use by class;
     education and training;
     maintenance and servicing manuals;
     marking of protective devices;
     warning signs and labels;
     entry limitations for visitors, etc.;
     accident procedures.
-------------------------------------------------------------------

    Because of the risk associated with exposure to Class 4 high-
risk lasers, the safety precautions associated with these laser 
installations indoors, generally include the installation of door 
interlocks to prevent exposure of unauthorized or transient 
personnel entering the laboratory, the use of baffles to terminate 
the primary and any secondary beams, and the use of safety eyewear 
by personnel within the interlocked facility (Klost, 1971). 

    At one time, it was recommended that ambient light levels 
should be sufficient to constrict pupils.  However, since a 
constricted pupil provides only a small safety factor, the 
requirement for good illumination, which remains in present safety 
standards, is related to good general visibility, as the wearing of 
eye protectors reduces visual capabilities.  Light-coloured, matt 
surfaces in the room minimize glare, and thus promote visibility. 

    In summary, the ability to analyse potential risks from any 
laser system is enhanced by a broad knowledge of optics, general 
laser technology, and the imaging process of the human eye.  A 
laser safety specialist should have a general background knowledge 
of optics with the basic knowledge necessary to perform a risk 
analysis.  It has been shown that the risk analysis depends on at 
least three aspects - the laser system and its potential hazards, 
the type of personnel who may be exposed, and, finally, the 
reflective materials and other optically important materials in the 
environment that can influence the risk analysis. 

15.  HAZARDS OF LAMP SOURCES AND PROJECTION SYSTEMS

    The optical radiation emitted by a conventional light source, 
either a bare lamp, a luminaire, or a projection system, can be 
evaluated using the previously mentioned tentative exposure limits 
or guidelines. 

    Prior to exhaustive measurements and safety calculations, it 
may be worthwhile to determine the need for a comprehensive risk 
evaluation.  Many categories of lamps or other types of light 
sources can be excluded from all or several of the evaluations.  
The following multi-step scheme (Sliney & Wolbarsht, 1980) may be 
useful in this regard. 

STEP 1 - Categorization of the lamp.  Certain hazards are specific 
for certain types of lamp or light source.  The following grouping 
is useful: 

    (a)  incandescent lamps and incandescent heating sources;

    (b)  low-pressure discharge lamps;

    (c)  fluorescent lamps;

    (d)  high-intensity discharge (HID) lamps;

    (e)  short-arc (compact arc) lamps;

    (f)  carbon arcs;

    (g)  solid-state sources (LEDs etc.);

    (h)  cathode-ray tubes (CRTs).

STEP 2 - Determination of the source envelope. Any glass between 
the actual source of radiation (e.g., the arc or tungsten filament) 
and the point of access can greatly influence the potential hazard. 
Soft (lime) glass of any reasonable thickness will greatly 
attenuate UV-B and UV-C radiation. 
                             
    (a)  Incandescent lamps,  other than quartz-halide lamps,
        normally have a sufficiently thick glass envelope to
        completely preclude a UVR hazard.  The blue-light
        hazard does not appear to be theoretically possible
        at black-body temperatures below 2000 K (Sliney &
        Wolbarsht, 1980), but most filaments operate at
        effective temperatures exceeding 2000 K.

    (b)  Low-pressure discharge lamps.  Low-pressure discharge
        lamps do not normally present a retinal hazard,
        because of the relatively low radiance.  Only lamps
        with quartz envelopes can transmit sufficient UV-B
        and UV-C to be of concern.  Of the common low-pressure
        lamps, only mercury lamps can create a severe UVR
        hazard.  Many may be quite hot to the touch.

    (c)  Fluorescent lamps.  Low-pressure tubular lamps in
        almost all cases have a thin glass envelope, but
        could often present a potential UVR hazard at the
        surface.  They do not represent a thermal retinal
        injury hazard and seldom a blue-light hazard.

    (d)  HID lamps.  These lamps may present both blue-light
        and thermal retinal hazards, and possible UVR
        hazards.  Since most lamp envelopes are glass, there
        is little UV-B leakage.  Nevertheless, the UV-B
        leakage may be of concern at very short distances.
        Quartz-mercury HID lamps require a UVR risk
        evaluation.  If the outer glass envelope of an HID
        lamp breaks, hazardous UV levels will be emitted.
        Governmental regulations in Canada and the USA
        require HID lamps to have a self-extinguishing
        feature to preclude this hazard, unless the packaging
        clearly warns against use without adequate shielding.

    (e)  Short-arc lamps.  Of all the electric lamp categories,
        this group will require the most extensive risk
        evaluation.  All potential hazards may be present
        (UV-B/C, UV-A, blue-light, retinal thermal injury,
        and skin thermal injury).  Because of the high
        temperature of the arc, a quartz envelope (which
        transmits UV-B and C) is characteristic.  These lamps
        are often used in UV photocuring processes in
        industry (Moss, 1980).

    (f)  Carbon arcs.  Where a glass lens or filter plate does
        not exist between the open arc and a point of access,
        the carbon arc, like the short-arc lamp, is
        potentially injurious.

    (g)  Solid-state lamps (e.g., LEDs).  The present solid-
        state lamps including LEDs, which emit visible
        radiation, do not present any health risk, regardless
        of the type of envelope.

    (h)  Cathode-ray tubes (CRTs).  Present CRTs emit optical
        radiation at levels that could pose a potential
        health hazard (Wolbarsht et al., 1980).

STEP 3 - The obtaining of available manufacturers' radiometric and 
photometric data and lamp descriptions.  Any radiometric or 
photometric specification may be of value either for calculation or 
for direct intercomparison with measurements.  Spectral data are 
most useful.  The dimensions of the emitting area of the lamp will 
be required for retinal hazard evaluation. 

STEP 4 - Comparison of lamp specifications with those of previously 
evaluated lamps.  From experience, it is often possible to complete 
the risk evaluation with this step. 

STEP 5 - Performance of detailed spectroradiometric measurements, 
when necessary.  In addition, where feasible, measurements of 
luminance, illuminance, and total irradiance should be performed. 
These will provide confirmation of the spectroradiometric 
measurements.  The pulse duration must be measured for a pulsed 
source. 

STEP 6 - Determination of the source dimensions. A photograph and 
microdensitometer scan of the negative may be necessary for a non-
uniform source.  The maximum angular subtense alpha of the source 
should be calculated at the point of human access or at 15 cm from 
the source, whichever is closer. 

STEP 7 - Estimation of the exposure and comparison with the 
exposure limits to determine the degree of risk. 

STEP 8 - Consideration of potentially hazardous failure modes.  For 
example, breakage of the outer envelope of some high-intensity 
discharge (HID) lamps can create a serious UVR hazard (Anderson, 
1980b). 

16.  PROJECTION OPTICS

    Broad-band sources involving projection optics are most 
difficult to evaluate.  Besides the problems encountered in 
evaluating exposed lamps, the projected beam and projected source 
size must be characterized.  When viewing a collimated light source 
from within the beam (other than a laser), a magnified view of the 
actual source will be seen.  The source is generally a high-
brightness lamp.  The brighter the lamp, the greater the maximal 
irradiance in the projected beam.  This is a consequence of the Law 
of Conservation of Brightness (Radiance) (see, for example, Kline, 
1970; or Sliney & Wolbarsht, 1980).  Some usually safe lamps become 
hazardous to view through projection optics, despite the fact that 
the optics cannot make the lamp brighter.  The risk increases 
because of the dependence of retinal injury on image size.  Besides 
the obvious projection sources - such as spotlights, searchlights, 
slide projectors, and film projectors - solar concentrators and 
other non-imaging light collectors may also require risk 
evaluation.  From the Law of Conservation of Radiance, it is 
possible to evaluate the retinal risks from projector systems. 
Collimating optics may consist of refracting elements (lenses), 
reflecting elements (curved mirrors), or both. 

17.  SAFETY GUIDELINES FOR HIGH-INTENSITY SOURCES

    Since lamp or arc sources may be hazardous from several 
aspects, it may be helpful to develop a safety classification 
scheme, similar to the one applied to laser products.  The 
following scheme of Sliney & Wolbarsht (1980) illustrates this 
approach to evaluate the retinal risks from projector systems. 

    Both lamps and total lighting systems could be included.  The 
categories could be as follows: 

    Safety Group 1:   Safe sources.  These lamps are considered
    safe to view throughout the day.  No warning label would be
    required.  Examples:  a frosted 15-W filament lamp or a
    TV-display, cathode-ray tube.

    Safety Group 2:   Low-risk sources.  These lamps are safe
    for momentary 0.25-s, unintentional viewing.  Examples:
    most spotlights and film-projector lamp bulbs.  A caution
    label should be required on the lamp base, and possibly on
    the projection system itself.  No ultraviolet or infrared
    hazard would exist at distances of more than 10 cm from
    the lamp or projected beam.

    Safety Group 3:   Moderate-risk sources.  These lamps would
    be unsafe to view at close range, even momentarily.
    Presumably, skin injury could also occur from ultraviolet
    radiation as from germicidal lamps, sun lamps, and high
    intensity UV-A lamps.  A danger-label, clearly visible on
    the equipment, could be required.  A common lamp that might
    fit into this category would be a 600-1000-W tungsten-
    halogen lamp without a Fresnel lens, such as is used for a
    home cine film spot lamp.  The emergent beam irradiance is
    far in excess of that required to ignite paper within half
    a metre of the source.  Obviously, the basis for the
    determination of a hazard classification would differ
    according to whether the hazard classification criteria
    were based on retinal or skin injury.  Each measurement for
    classification would be for a specified accessible
    approach distance, using a standard aperture and solid
    angle of acceptance.  The minimum approach distance could
    vary with application.  Other examples that might be
    included in this category are some very high intensity,
    short-pulse, laser flash tubes, and 20 kW xenon-arc
    searchlights.

    Safety Group 4:   High-risk sources.  These sources would
    cause skin burns and/or erythema within a standardized
    period of exposure (e.g., within 10 s) at a standardized
    distance at which the effective UV irradiance would exceed
    3 W/m2 (0.3 mW/cm2), or the total irradiance across the
    entire spectrum would exceed 2 kW/m2 (0.2 W/cm2). Examples
    of such sources are an open carbon-arc spotlight or an
    open 1-kW mercury lamp.  It may be that safety groups 3 and
    4 are so similar in degree of risk, that they could be
    combined.

18.  WELDING ARCS

    The most common high-intensity arc is probably the welding arc. 
These arcs vary in brightness and in UVR content, primarily 
according to the arc current, type of shielding gas, and the metals 
being welded. 

    The greatest population exposed to intense sources of optical 
radiation are welders and their assistants.  The American Welding 
Society estimated that there may be as many as 500 000 welders in 
the USA alone (Emmett & Horstman, 1976).  The two broad categories 
of welding equipment are gas (acetylene) welding equipment and 
electric-arc welding equipment.  A gas welding torch or cutting 
torch has a luminance not much greater than a candle flame, 
typically ranging from 10 to 200 kcd/m2 (1-20 cd/cm2), and the UVR 
emission is quite small.  The optical radiation hazards of such 
torches are virtually nonexistant.  Welding filter goggles used 
with such torches are to reduce glare, and are little darker than 
very dark sunglasses having a shade number of the order of 3 - 5 
(visual transmittance of 5 - 15 %).  On the other hand, electric 
welding arcs may be 1000 times brighter than gas torches and emit 
UVR at proportionately higher levels (Sutter et al., 1972). 

    Protective shields, curtains, screens for bystanders, and 
welders' goggles are the standard protective equipment used in 
welding (Mayer et al., 1979; Sliney et al., 1981).  Protective 
procedures and protective equipment for the welder have been 
developed empirically over the last three-quarters of a century. 
Only very recently have detailed measurements of the radiometric 
output of welding arcs been available.  When these measurements 
were carefully compared with exposure limits being developed for 
protection against bright light sources, it was shown that the 
empirically-developed protective equipment standards were adequate. 

19.  EYE AND SKIN PROTECTION

19.1.  Laser Safety Eyewear

    From a safety point of view, the most desirable laser hazard 
control measure is complete enclosure of the laser or laser system; 
however, this may not always be practical and laser eye protectors 
are generally the best alternative.  Though most industrial laser 
applications do not require the use of eye protectors, this is not 
always true for laser applications in the research laboratory.  Eye 
protectors provide the simplest solution to the laser safety 
problem for a constantly changing experimental arrangement.  Several 
factors play a role in determining whether eyewear is necessary in 
any situation.  At least three output parameters of the laser must 
be known:  maximum exposure duration, wavelength, and output power 
(or output irradiance, or radiant exposure, or energy), as well as 
the applicable safe corneal radiant exposure.  In addition, some 
knowledge of such environmental factors as ambient lighting and the 
nature of the laser operation may also be required. 

    Laser eye protection generally consists of a filter (often 
composed of several individual filter plates) which selectively 
attenuates at specific laser wavelengths, but elsewhere transmits 
as much visible radiation as possible (Swope & Koester, 1965; 
Schreibeis, 1968; Scherr et al., 1969; Swope, 1969, 1970; Straub, 
1970; Sliney, 1974).  Eyewear is available in several designs - 
spectacles, coverall types with opaque side-shields, and coverall 
types with somewhat transparent filter side-shields.  The selection 
of appropriate laser protective eyewear may be complex (Envall & 
Murray, 1979).  Active electronic imaging devices have also served 
an additional role, as eye protection. 

19.2.  Welders' Filters

    Eye protection filters, which were originally developed for 
welders, were based more on available materials than on knowledge 
of ocular protection requirements.  The first organized study of 
glass filter materials was carried out by Sir William Crookes 
(1914) in England.  Optical transmission characteristics are now 
standardized as "shades" and specified for particular applications 
(Coblentz et al., 1931; Stair, 1948; ANSI, 1978).  Though maximum 
transmittances for ultraviolet and infrared radiation are specified 
for each shade, the mean photopic visual transmittance tauv, or 
visual optical density Dv, has traditionally been used to define 
the shade number S: 

        S = (7/3) Dv + 1 = - ln tauv + 1               Equation (23)

or      Dv = (3/7) (S - 1)                             Equation (24)

where   Dv = -log10 tauv                               Equation (25)

19.3  Eye Protection for Furnace Radiation

    As well as protective clothing and equipment, many industrial 
methods now used probably reduce the level of glass furnace 
radiation to which the eye is exposed.  For instance, the openings 
to higher temperature furnaces are a great deal smaller than they 
have been in past years; this would reduce the total irradiance of 
the eye from infrared radiation.  There are sufficient data and 
cases of a very specific form of cataract in workers to suggest 
that infrared does, indeed, cause glass-blower's or furnaceman's 
cataract (Duke-Elder, 1972). 

19.4.  Eye Protection Filters for Solar Radiation

    Direct viewing of the sun, for whatever reason, requires 
protection against several different portions of the spectrum.  A 
yellowish or reddish filter generally protects against ultraviolet 
radiation.  Protection against intense visible rays should be 
weighted to filter out more of the blue-light than the rest of the 
visible spectrum.  It is generally found that a shade 12 or 13 
welder's filter is quite adequate to protect against ultraviolet 
radiation, infrared, and visible radiation.  The protection 
afforded against the IR, however, is far greater than necessary. 

    The use of darkened coloured slides is not advisable, since 
these slides (usually made by developing unexposed colour film) use 
organic dyes that transmit in the near-infrared (IR-A) spectral 
band. 

19.5.  Skin Protecting Agents for UVR (Sunscreens)

    A number of topical, physical, and chemical screening agents 
have been developed that provide nearly total or partial filtration 
of ultraviolet radiation.  Since actinic UV-B and UV-C radiation 
are the most hazardous, efforts to develop topical agents have 
concentrated primarily on filtering out this type of radiation.  The 
chemical agents in these "sunscreens" include para-aminobenzoic 
acids (PABA) and its esters, salicylates, and cyanamates.  These 
materials are mixed in solution with substances that have good 
substantivity (i.e., adhere to the skin) (Dahling et al., 1970; 
Fitzpatrick et al., 1974). 

19.6.  Protective Garments

    Aluminized fabrics were greatly improved during various manned 
space programmes (Stoll & Chiantra, 1971).  Such fabrics, when used 
in thermal protective garments, have been shown to offer equivalent 
or superior reflective and mechanical properties compared with 
conventional aluminized asbestos garments (Wren et al., 1977). 
Aluminized rayon (basket weave) and certain aluminized cottons were 
shown to allow the least transmission of infrared radiation. 

20.  MEDICAL SURVEILLANCE (RATIONALE)

    In the past two decades, many employees working routinely with 
lasers have been subjected to preplacement, periodic, and end-of-
job eye examinations in order to obtain sufficient information 
concerning the risk of retinal damage.  Many of these studies 
indicated that periodic eye examinations rarely located hitherto 
unsuspected retinal damage.  In general, published reports of 
ophthalmic accidents have been those in which the acute over-
exposure was sufficient to subjectively alert the individual 
(Hathaway et al., 1977).  Thus, many authorities (Suess, ed., 1982) 
suggest that ophthalmic examinations are unnecessary for 
individuals routinely working with Class 1 and 2 lasers, and that, 
if requested, the examinations should be confined to those working 
with Class 3 or 4 systems only. 

    An examination is required within 24 - 48 h of any event in 
which the worker suspects, or knows, that the eye might have been 
exposed.  Laser lesions change in appearance and may even tend to 
disappear within the heterogeneous appearance of the fundus within 
a period of time, so that ophthalmic examinations, some time after 
exposure, may be difficult to interpret. 

21.  FORMAL TRAINING FOR LASER WORKERS

    It is necessary to establish a safety programme that assures 
the safe use of lasers and other radiation sources.  To assure 
knowledge of, and compliance with, applicable standards, a certain 
amount of formalized teaching is often necessary. 

    At work places, a specific individual should be assigned to 
maintain and enforce the safety programme (in some countries this 
individual is termed a laser safety officer (LSO)). 

    All workers occupied with, or working near, the radiation
source should be included in the teaching programme.

    The object is to make workers and work leaders aware of the 
risks of lasers, how to avoid the hazards, the proper use of 
protective devices, and how to realize when overexposure has taken 
place. 

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GLOSSARY

ACOUSTICO-OPTIC: involving the interaction of light and an acoustic 
wave.  Acoustico-optic devices such as Q switches and modulators 
are used in manipulating laser beams. 

ACTIVE MEDIUM: the atomic or molecular species which can provide 
gain for laser oscillation.  Also called laser medium, lasing 
medium or active material. 

ANGSTROM (A): a non-SI unit equal to 10-10 metre.  Its use as a 
unit of optical wavelength has largely been supplanted in recent 
years by the nanometre (10-9 metre). 

ARC LAMP: an electric lamp in which current passes through the 
ionized air between two electrodes, giving off light.  Applications 
include laser excitation. 

ATOMIC LASER: a gas laser in which the active material is an atomic 
species rather than a molecule. 

ATTENUATION: reduction in intensity that results when optical 
radiation travels through an absorbing or scattering medium.  In 
optical fibre, attenuation (in decibels) equals 10 log ( Po/ Pin), 
where  Po is the power at the output end of the fibre and  Pin is 
the power launched into the fibre. 

AVERAGE POWER: in a repetitively pulsed laser, the energy per pulse 
times the repetition rate.  When the energy per pulse is expressed 
in joules and the repetition rate in hertz, the average power is 
expressed in watts. 

BEAM DIAMETER: the distance between the two opposing points at 
which the irradiance or radiant exposure is a specified fraction 
(typically 1/e or 1/e2) of the irradiance or radiant exposure of 
the emitted radiation. 

BEAM DIVERGENCE: the increase in beam diameter with distance from 
the laser's exit aperture.  Measured in milliradians at specified 
points, usually where irradiance or radiant exposure is 1/e or 1/e2 
the maximum value, and expressed as the "full-angle" divergence. 

BOLOMETER: a type of detector which measures infrared radiation by 
the temperature-induced change of resistance in a metal foil 
exposed to the radiation and heated by it. 

BREWSTER ANGLE: the angle between an incident beam of light and a 
dielectric reflecting surface at which none of the light polarized 
in the plane of incidence is reflected.  Brewster's angle is 
tan-1 n2/ n1, where  n1, and  n2 are the indexes of refraction of the 
first and second media respectively.  A window mounted at 
Brewster's angle with respect to an incident beam is often used as 
a window in laser cavities. 

CALORIMETER: a type of detector that measures heat produced by 
absorption of radiation. 

CHEMICAL LASER: a type of laser in which the population inversion 
is produced directly by an "elementary" chemical reaction (a 
collision process in which one or more molecules undergo changes in 
their chemical bonds). 

COHERENCE: a fixed phase relationship among various points of an 
electromagnetic wave in space (spatial coherence) or in time 
(temporal coherence). 

COLLIMATOR: an optical device for converting a diverging or 
converging beam of light into a collimated or parallel beam, or for 
expanding or reducing the cross-sectional area of an incident 
collimated beam.  A target collimator projects a parallel beam from 
its own light source such that, viewed from any distance, the light 
source appears to be at infinity. 

CONTINUOUS WAVE (cw) LASER OPERATION: laser operation in which 
radiation is emitted continuously. 

CORE: the central region of an optical fibre.  The core must have a 
higher refractive index than the cladding for light to be 
transmitted through the fibre via total internal reflection 

CORNER CUBE: an optical component with three mutually perpendicular 
faces and a hypotenuse face.  Because light entering through the 
hypotenuse is totally internally reflected at each perpendicular 
face, the cube acts as a retroreflector.  Also known as corner 
reflector. 

CRYSTAL LASER: a type of laser in which the active medium is an 
atomic species in a crystal such as ruby, YAG (yttrium aluminium 
garnet), or YALO (yttrium aluminate). 

DETECTOR: see PHOTODETECTOR.

DIFFRACTION: deviation of light rays from the paths predicted by 
geometical optics. 

DIODE LASER: see SEMICONDUCTOR LASER.

DISPERSION: variation of the refractive index of an optical 
material with change in wavelength, as in a prism; in an optical 
fibre, the temporal spreading of a light pulse due to the fibre's 
different propagation speeds for different wavelengths and modes of 
light. Such spreading limits the fibre's information-carrying 
capacity of bandwidth. 

DIVERGENCE: see BEAM DIVERGENCE.

DYE LASER: a type of laser in which the active medium is an organic 
dye, generally in solution with the liquid either flowing or 
encapsulated within a cell.  Experimental solid and gas dye lasers 
also have been built.  Also called organic-dye, tunable-dye or 
liquid laser. 

ELECTRON-BEAM-SUSTAINED LASER: a molecular-gas laser in which the 
electrical discharge is sustained with a beam of high-energy 
electrons.  Usually injected transversely to the laser cavity's 
optical axis, the electron beam permits laser operation at 
pressures and cross-section-to-length ratios higher than possible 
with an unsustained discharge.  This technique is often used in 
commercial carbon-dioxide lasers with very high continuous-wave 
output power. 

ELECTRO-OPTIC: applying to modulators, Q switches and other beam-
manipulating devices in which operation relies on modification of a 
material's refractive indices by an applied electrical field.  In a 
Kerr cell the index change is proportional to the square of the 
electrical field, and the material is usually a liquid.  In a 
Pockels cell the material is a crystal whose index change is linear 
with the electric field. 

EMISSIVITY: ratio of radiant exitance of a thermal radiator to that 
of a full radiator (black body) at the same temperature - (ISO 
31/VI 1980). 

EXCIMER LASER: a laser in which the active medium is an excimer, a 
molecule which is chemically unstable except in its excited state.  
The term often is applied to lasers in which the active medium is a 
rare-gas halide (or monohalide) excimer such as KrF* or XeF*. 

GAS LASER: a type of laser in which the active medium is a gas.  
The category is subclassified according to the active medium into 
atomic (such as helium-neon), molecular (carbon dioxide, hydrogen 
cyanide and water vapour), ionic (argon, krypton, xenon, and the 
metal-vapour types such as helium-cadmium and helium-selenium), and 
excimer (typically rare-gas halides).  Loosely applied, "ion" means 
argon and krypton. 

GLASS LASER: a type of solid-state laser in which the active medium 
is a glass rod doped with rare-earth atoms, usually neodymium. 

HERTZ (Hz): the SI unit of frequency of periodic phenomena.  It 
replaces the non-SI unit "cycles per second".  The number of pulses 
per second that a laser can produce may be expressed in hertz. 

HOLOGRAM: a recording of the interference of coherent light 
reflected from an object with light direct from the same source or 
reflected from a mirror.  Illumination of the hologram reproduces 
the object's three-dimensional image. 

IMAGE CONVERTER: an electron tube which produces a visual replica 
of an image formed on its cathode by some form of electromagnetic 
radiation.  In an image converter camera, the image formed by the 
electron tube is focused on to photographic film for a permanent 
record. 

INFRARED: electromagnetic radiation with wavelength between 0.76 
micrometre and about 1 millimetre.  Wavelengths at the shorter end 
of this range are frequently called "near" infrared, and those 
longer than about 20 micrometres, "far" infrared. 

INTEGRATED OPTICS: devices in which several optical components are 
"integrated" on to a single substrate;  analogous to integrated 
electronic circuits.  Although still in the research phase, 
integrated optics has potential for use in optical signal 
processing and in fibre-optic communications. 

INTERFERENCE FILTER: an optical component which depends on 
interference in a series of thin films deposited on a substrate to 
limit transmission to a desired spectral band. 

ION LASER: a type in which the active element is an ionized gas, 
generally argon or krypton. 

IRRADIANCE ( E): radiant flux per unit area, expressed in watts per 
square centimetre. 

LASER: acronym for "light amplication by stimulated emission of 
radiation."  A device which generates or amplifies electromagnetic 
oscillations at wavelengths between the far infrared 
(submillimetre) and ultraviolet.  Like any electromagnetic 
oscillator, a laser oscillator consists of two basic elements:  an 
amplifying (active) medium and a regeneration or feedback device 
(resonant cavity).  A laser's amplifying medium can be a gas, 
semiconductor, dye solution, etc;  feedback is typically from two 
mirrors.  Distinctive properties of the electromagnetic 
oscillations produced include monochromaticity, high intensity, 
small beam divergence, and phase coherence.  As a description of a 
device, "laser" refers to the active medium plus all equipment 
necessary to produce the effect called lasing. 

LASER DIODE: see SEMICONDUCTOR LASER. 

LED: abbreviation of light-emitting diode.  A semiconductor 
emitting incoherent light into a broad field of view, used in low-
speed or short-haul fibre-optic links.  Most LEDs used in fibre-
optic applications emit in the near infrared. 

LIDAR: acronym for "light detection and ranging," a system 
employing a laser beam to gather ranging information as well as 
intelligence on reflection and scattering of light by clouds and 
atmospheric pollutants. 

LIQUID LASER: a type in which the active element is either an 
organic dye or an inorganic liquid.  See also DYE LASER. 

MULTIMODE: emission at several frequencies simultaneously, 
generally closely spaced, each frequency representing a different 
mode of laser oscillation inthe resonant cavity. 

Nd-GLASS: neodymium-doped glass, used in some solid state lasers.  
The neodymium atoms are the active medium. 

Nd-YAG: neodymium-doped yttrium-aluminium-garnet (YAG), a crystal 
which is used in some solid state lasers.  The neodymium atoms are 
the active medium. 

NEUTRAL DENSITY FILTER: a filter which reduces the intensity of 
light without affecting its spectral character. 

NONLINEAR EFFECTS: changes in a medium transmitting electromagnetic 
waves that are proportional to the second, third or higher powers 
of external electric field.  Nonlinear optical effects include 
harmonic generation and the electro-optic effect.  See electro-
optic. 

OPTICALLY PUMPED LASER: a laser whose active medium is excited by 
another light source to produce a population inversion.  For solid-
state and some dye lasers this source usually is an incoherent type 
such as a flash- or arc-lamp.  For gas and other dye lasers, 
coherent laser sources generally provide such optical pumping. 

PARAMETRIC OSCILLATOR: a nonlinear device, usually a crystal, which 
produces tunable laser oscillations at the sum or difference 
frequency of mixed laser beams.  Also called tunable parametric 
oscillator or optical parametric oscillator.  Loosely applied to 
the complete instrument containing the pump laser and the tuning 
crystal. 

PHOTODETECTOR: any device which detects light, generally producing 
an electronic signal with intensity proportional to that of the 
incident light. 

PHOTON: a massless "particle" of electromagnetic radiation, with 
energy equal to  hc/lambda where h is Planck's constant (6.6 x 10-34 
joule second) and  c/lambda is the frequency of the radiation (speed 
of light divided by wavelength). 

POLARIZER: an optical component which only transmits lightwaves 
that oscillate in a given plane. 

POPULATION INVERSION: a condition in which most atoms of a species 
are in an excited, metastable state.  Collision of a photon with 
such an atom causes the atom to relax to a lower energy state, and 
to emit a second photon, amplifying the light signal.  Population 
inversion is required for lasing to occur. 

PULSELENGTH: the duration of the burst of energy emitted by a 
pulsed or Q-switched laser. Expressed in seconds and usually 
measured at the half-power (half the full height of a voltage or 
current pulse).  Also called pulsewidth. 

PULSED LASER: a laser that emits light in pulses rather than 
continuously. 

PUMP: the energy source (such as flashlamp, electron beam or 
current supply) that drives the amplification in the active medium 
of a laser by creating a population inversion. 

PYROELECTRIC CRYSTAL: a type of crystal that shows electrical 
effects when its temperature is changed; these effects are used to 
detect infrared radiation. 

Q SWITCH:  essentially a "shutter" which prevents laser emission 
until opened.  Q stands for "quality factor" of the laser's 
resonant cavity.  "Active" Q switching is achieved with a rotating 
mirror or prism, Kerr or Pockels cell, or acoustico-optic device; 
"passive" Q switching is achieved with a saturable absorber such as 
a gas or dye.  In a pulsed laser a Q switch increases pulse power 
by shortening pulse duration while not significantly decreasing the 
energy; in a continuous wave laser the device provides shorter and 
more intense pulses at a higher repetition rate than could be 
achieved by pulsing the laser directly. 

RADIANCE ( L): At a point of a surface and in a given direction, 
the radiant intensity of an element of the surface, divided by the 
area of the orthogonal projection of this element on a plane 
perpendicular to the given direction (ISO 31/6-1980).  Expressed in 
watts per steradian square centimetre. 

RADIANT FLUX: the rate of flow of radiant energy, measured in 
watts. 

RADIOMETER: an instrument for measuring incident radiation in 
radiometric units (watts).  Radiometric measurements can be made at 
any wavelength, but the spectral range of a particular instrument 
may be limited to a narrow range. 

RADIOMETRIC UNITS: units defined for measurement of the intensity 
of electromagnetic radiation; the basic unit is the SI unit watt. 

RAMAN EFFECT: the appearance of additional weak lines in the 
spectrum of light that has been scattered by a transparent 
substance.  The extra lines result from rotational or vibrational 
transitions of the molecules in the scattering medium.  If the 
medium is illuminated with laser light of sufficient intensity, the 
emission at the Raman frequencies is amplified, exhibiting 
characteristics of stimulated emission (i.e., stimulated Raman 
effect.). 

REFLECTANCE: the ratio of wave energy reflected from a surface to 
the wave energy incident on a surface. 

SEMICONDUCTOR LASER: a type in which the active material is a 
semiconductor, either a diode or homogeneous.  Commercial 
types are generally diodes in which lasing occurs at the 
junction of n-type and p-type semiconductors, usually gallium- 
arsenide or gallium-aluminium-arsenide.  Homogeneous types are 
made of undoped semiconductor material and are pumped by an 
electron beam.

SOLID-STATE LASER: a type the active medium of which is an atomic 
species in a glass or crystal. The atomic species may be added to 
the glass or crystal, as neodymium is added to glass, or may be 
instrinic, as chromium is in ruby.  This term is generally not 
applied to semiconductor lasers. 

SUPER-RADIANT: applying to coherent optical amplification of 
spontaneous emission that occurs without relaxation processes.  
Commonly used to describe a laser whose gain is high enough to 
permit amplification without mirrors; examples are nitrogen and 
molecular hydrogen.  Beam quality of a super radiant laser is 
generally inferior to that of a laser with a complete optical 
cavity.  "Super-fluorescent" has been proposed as a more precise 
description of this type of laser. 

TEA LASER: acronym for transversely excited, atmospheric pressure 
laser.  A gas laser in which excitation of the active medium is 
transverse to the flow of the medium.  Because of shorter breakdown 
length, this type operates in a gas-pressure range higher than that 
for longitudinally excited gas lasers (but not necessarily 
atmospheric) and offers a potentially higher power output per unit 
volumes because of a greater density of lasing molecules. 

TUNABLE LASER: a laser or a parametric oscillator whose emission 
can be varied across a broad spectral range. 

ULTRAVIOLET: electromagnetic radiation with wavelengths between 
about 40 and 400 nanometres.  Radiation between 40 and 200 nm is 
termed "vacuum ultraviolet" because it is absorbed by air and 
travels only through a vacuum.  The "near" ultraviolet has 
wavelengths close to those of visible light; the "far" ultraviolet 
has shorter wavelengths. 

YAG: yttrium aluminium garnet, a crystal host which can be doped 
with an active laser medium, usually neodymium. 

YALO: yttrium aluminate (YA103), a crystal host doped with an 
active laser medium, usually neodymium. 

YLF: yttrium lithium fluoride, a crystal host which can be doped 
with an active laser ion, usually holmium. 



    See Also:
       Toxicological Abbreviations