Ophthalmic laser safety

Introduction

A variety of laser systems is used in ophthalmic applications. In each application, there are potential laser hazards to both the patient and the clinician. In most applications these potential hazards are minimal, but it is well to remember that, if a laser is used for altering tissue by photocoagulation, photodisruption or laser ablation, the exposures, if misdirected, are almost by definition potentially hazardous to other biological tissues as well. In the special terminology of laser safety, virtually every surgical laser is known as ‘Class 4’ and will be considered by safety specialists as being very hazardous. Therefore, it is important for ophthalmologists who use lasers to be familiar with laser safety guidance and terminology, in order to have a balanced view of what might be perceived as a very hazardous laser and what procedures require serious attention in order to take appropriate precautions.

Safety standards worldwide group laser products into at least four different safety categories of risk, known as ‘hazard classes’. These range from Class 1 products, which pose no hazards, to Class 2, which are visible-wavelength lasers that are no more dangerous than a bright lamp, Class 3, which are significant hazards to the eye, and Class 4, which are hazardous to both skin and eye and are readily capable of cutting or photocoagulating biological tissue. From a physical standpoint, lasers owe their particular usefulness in most applications to their extraordinarily high brightness, and this factor, known technically as ‘radiance’ also leads

to their significant hazard (Fig. 1). A laser light source is millions of times brighter than an ordinary incandescent lamp, and even brighter than an arc lamp or the sun. Although, laser light is generally monochromatic and has spatial and temporal coherence, quite unlike any light from a conventional light source, these physical characteristics contribute little to the hazard. Coherence and monochromaticity can be important in some diagnostic applications, but are not very significant in terms of the surgical value or laser hazards.

Laser parameters

As noted above, it is the high radiance of lasers that leads to their significant ocular hazards, but this radiometric quantity is seldom actually specified, and is only indirectly used in laser safety assessments. Instead, the more familiar laser output parameters that determine the safety or hazard classification are: the wavelength, pulse duration and energy, if pulsed; and wavelength and power, if continuous wave (cw). The accessible emission limits (AELs) that determine the laser safety classes also vary with spectral band, e.g., ultraviolet (100400 nm), visible (400-780 nm) and infrared (780 nm-1 mm), since the biological risk varies with wavelength. Laser wavelength determines how effectively light is absorbed in the target tissue and how effectively light penetrates overlying media to reach a tissue target (transmission).1,2

Pulse durations can range from tens of femtoseconds (10-15 seconds) to milliseconds (msec). From

the point-of-view of laser safety, lasers that emit for any period greater than 0.25 seconds are considered to be cw.3 Power (in watts) describes the rate at which energy (in joules) is emitted from a laser or delivered to a target tissue (power = energy/time, in watts = joules/sec). Irradiance is the incident laser power per unit area delivered to a

target surface (irradiance = power/area, in watts/ cm²). If the backscattered irradiance is added and a cosine factor ignored, this is referred to as ‘fluence rate’ in watts/cm² (Fig. 2). Irradiance is sometimes referred to as ‘power density’ at the target surface (but this is technically incorrect). And, for pulsed lasers, it is often useful to describe laser exposures

in terms of radiant exposure rather than irradiance. Radiant exposure is the energy divided by the exposed surface area (radiant exposure = energy/area, in joules/cm²). Fluence is widely used as a synonym for radiant exposure, but technically it should be reserved for situations in which an exposed surface encounters both forward and back-scattered light.3 The concepts of fluence and fluence rate are important in dosimetry for photodynamic therapy, where multiple scattering and diffusion in the target tissue are of great importance for redistributing and homogenizing the incident light.

Ocular hazards

Normally, the eye is well adapted to protecting itself against optical radiation (ultraviolet, visible, and infrared radiant energy) from the natural environment. The brow ridge and upper lids greatly shield the eyes from overhead solar radiation,4 and the aversion response with blink reflex and eye movements limit direct viewing of the sun and welding arcs to a fraction of a second, and therefore preclude photoretinitis.5-7 However, when a patient is anesthetized, the normal aversion responses to bright light or heat may be greatly reduced – or absent – in the cornea and skin. The normal aversion response to a heat sensation of exposed skin will normally limit potentially hazardous thermal exposure of the skin if the injury threshold is not reached within 0.2-0.3 seconds.8 Furthermore, some body movements (e.g., eye movement) will not limit exposure duration, and injury that would normally not occur can result to the tissues. Vascularized tissues can carry away excess heat and limit the possibility of thermal injury of tissue (e.g., the choroid), but this is a limited capability that can be readily overtaxed by laser exposure or if vascular flow is impaired. During ophthalmic examination, a cooperative patient may willingly be exposed to extremely discomforting light in order to cooperate with the examining clinician.9,10

It is generally well accepted that the eye is more susceptible to optical hazards than the skin. Although both skin and eye are susceptible to injury from lasers and other intense optical sources, protection of the eye is central because of the potential for loss of vision. Therefore, it is important to recognize any potential hazards of intense light sources used in diagnosis and surgery.

Optical radiation hazards

There are at least nine separate types of hazards to the eye and skin from lasers and other intense optical radiation sources, and protective measures must be chosen with an understanding of each of these. One or more of the following effects may pose a potential hazard, depending upon the laser wave-

length and the temporal and geometrical characteristics of the exposure:5,11,12

(a)ultraviolet photokeratoconjunctivitis (also known as ‘welder’s flash’ or simply ‘photokeratitis’, one aspect of ‘snow-blindness’ from wavelengths of ~180-400 nm);13

(b)ultraviolet cataract (~295-325 nm – and perhaps to 400 nm). This is expected only from chronic exposure under normal conditions;13-15

(c)ultraviolet erythema (~200-400 nm);

(d)skin cancers arising from chronic exposure to ultraviolet radiation, particularly from UV-B (280-315 nm), but also demonstrated for UV- A (315-400 nm);16,17

(e)thermal injury to the retina (400-1400 nm). Normally this type of injury is only possible from lasers, a focused, very intense xenon-arc source, or the nuclear fireball. A local burn of the retina results in a blind spot (scotoma). Because of heat conduction from the retinal image, a very intense exposure delivered within seconds normally is required to cause a retinal coagulation, otherwise surrounding tissue conducts the heat away from the retinal image. This leads to an image-size dependence of retinal thermal injury;2,5

(f)blue-light photochemical injury to the retina (principally 400-550 nm blue light);18 ‘blue light’ photoretinitis, e.g., solar retinitis and welder’s maculopathy, which may lead to a permanent scotoma. Prior to conclusive theoretical work19 and animal experiments only two decades ago,20 solar retinitis was thought to be a thermal injury mechanism. Unlike thermal injury, there is no image-size dependence as with (e) above;

(g)near-infrared thermal hazards to the lens (approximately 800-3000 nm) with a potential for industrial heat cataract.21,22 The average corneal exposure from infrared radiation in sunlight is of the order of 10 W/m². In comparison, glass and steel workers exposed to infrared irradiances of the order of 0.8-3 kW/ m² daily for ten to 15 years have reportedly developed lenticular opacities.5 These spectral bands include IR-A (700-1400 nm) and IR-B (1.4-3.0 µm);

(h)thermal injury of the cornea and conjunctiva (approximately 1400 nm to 1 mm). This type of injury is almost exclusively limited to pulsed, or very brief, laser radiation exposure;5

(i)thermal injury of the skin (approximately 400 nm to 1 mm). This type of injury rarely occurs from conventional optical sources, since the aversion to thermal pain (occurring at a temperature of 45°C or greater) will normally limit exposure to a few seconds. Laser-induced thermal injury is possible from most Class 4 lasers.

Type of laser

Argon-fluoride Xenon-chloride Argon ion Copper vapor Helium-neon Gold vapor Krypton ion Neodymium:YAG (primary λ)

Neodymium:YAG laser (secondary λ)

Pulsed Nd:YAG (1.44 µm) Pulsed holmium

cw holmium

cw carbon monoxide

Carbon dioxide

Principal wavelength(s)

MPE (eye)

3.0mJ/cm² over 8 hours 40 mJ/cm² over 8 hours

3.2 mW/cm² for 0.1 seconds2.5 mW/cm² for 0.25 seconds1.8 mW/cm² for 1.0 seconds1.0 mW/cm² for 10 seconds

5.0µJ/cm² for 1 nsec to 50 µsec no MPE for t < 1 nsec

5 mW/cm² for t >10 seconds

40 µJ/cm² for 1 nsec to 50 µsec

40 mW/cm² for > 10 seconds

0.1J/cm² for 1 nsec to 1 msec

100 mW/cm² for 10 seconds to 8 hours,limited area

10 mW/cm² for t > 10 secondsfor most of body (skin surface)

same as 2.1 and 5 µm wavelengths above

The importance of wavelength and time of exposure

Thermal injuries (e) and (h) above are generally limited to very brief exposure durations, and eye protection is designed to prevent these acute injuries; the laser-tissue interaction mechanism is put to use in (e) retinal photocoagulation. However, photochemical injuries such as (a) and (c) are possible from low dose rates spread over minutes or even hours as a result of the Bunsen-Roscoe law. Most photochemical effects are limited to a narrow range of wavelengths known as an ‘action spectrum’, whereas a thermal effect can occur at any wavelength in the spectrum.

Still other photochemical interaction mechanisms besides (f) exist that can produce retinal injury, but these are generally thought to be only theoretical in the case of human exposure. Noell42, Lawwill43, Kremers and van Norren44, and others described retinal damage from light produced by relatively low-brightness fluorescent lamps, and these mechanisms were only detectable for exposure durations extending beyond two hours and repeated for several days. These effects are clearly related to excessive over-stimulation of the photoreceptors.

Safety standards

Laser safety standards for medical applications exist in many countries (including Australia, Great Britain, Germany, and the USA). The basic guidance is very similar in all these standards. In addition, the IEC has a technical note (IEC 60825- 8-1999-11) on the safe use of medical lasers. In the USA, there are two safety standards which apply to

medical lasers: the American National Standard for the Safe Use of Lasers in the Health Care Environment, ANSI Z136.3-1996, which is a voluntary, consensus standard for users,23 and a Federal Regulation that applies to laser manufacturers (21 CFR1040), issued by the Food and Drug Administration (FDA).24 The latter regulatory standard imposes certain labelling, informational, and design performance requirements upon the manufacturer and all laser products (medical or non-medical) must meet these. Examples of the FDA performance requirements are the laser emission indicator and the key switch.

Although many occupational safety standards and guidelines (e.g., the ANSI Standard in the USA) are ‘voluntary’, the guidelines can have a legal impact if labor, insurance or health-service regulations refer to guidelines when questions arise as to the adequacy of the safety in a facility. If an accident were to occur, these types of standards would surely be referred to in any litigation. For an ophthalmic laser facility, most standards basically require that one person be given safety oversight responsibility, and that person is usually termed the ‘laser safety officer’, or simply, the LSO; in the UK, this is the ‘laser safety advisor’. The guidelines also recommend that the laser operator be trained in the safe use of the laser device, that a laser safety warning sign be placed at the entrance to the laser treatment room, and that persons assisting wear laser eye protection within the range of potentially hazardous exposure. This region – where diffuse reflections and stray beams could be hazardous – is termed the ‘nominal ocular hazard area’ (NOHA) in some international standards, or the ‘nominal hazard zone’ (NHZ) in the USA.23 A

key element in all laser standards is the risk assignment of the laser to a ‘hazard class’. Class 1 products may be thought of as ‘eye-safe’ lasers; Class 2 is a 1-mW (or less) visible laser (e.g., an aiming beam); Class 3 is a significant eye hazard (e.g., an Nd:YAG photodisruptor); and Class 4 is a skin hazard as well (e.g., all photocoagulators capable of exceeding 500 mW total average power). Hazard control measures are assigned to each class by the ANSI standard.3,5,23

Occupational exposure limits

Guidelines for limiting human exposure (both eye and skin) to laser radiation have been issued by the International Commission on Non-Ionizing Ra-

diation Protection (INCIRP) and by other national and international standardizing groups following the ICNIRP guidelines.1,2,25-29 Table 1 lists permissible occupational maximum permissible exposure (MPE) limits for some of the commonly used surgical lasers.

Reflections and probability of exposure

An examination of laser accident records indicates that the source of accidental ocular exposure is most frequently a reflected beam. Figure 3 illustrates the types of mirror-like (specular) laser beam reflections that can occur from the flat or curved surfaces, which are characteristic of contact lenses

Fig. 4. Tissue interaction.

or of the metallic instruments used in some surgical procedures. Skin injury of the hand holding an instrument is also possible. Normally, the collimated beam is considered the most hazardous type of reflection, but, at very close range, a diverging beam may pose a greater likelihood of striking the

eye.3,8,9

A number of steps can be taken to minimize the potential hazards to both the patient and the surgical staff. Preventive measures will depend upon the type of laser. Since laser wavelengths in the ultraviolet and infrared spectral regions are invisible, the presence of hazardous secondary beams could go unnoticed. This added hazard resulting from an infrared laser beam’s lack of visibility is common to the 2.1 µm holmium or the 1064 nm Nd:YAG laser. In contrast, the argon and the second-har- monic Nd:YAG (sometimes referred to as the ‘KTP’) lasers emit highly visible, blue-green (488, 514.5, and 532 nm) beams and, in some respects, pose a lesser potential hazard.

Most current surgical lasers, such as the Nd: YAG, holmium, diode or argon, are cw, or nearly so. In contrast, the single-pulse ophthalmic laser photodisruptors or some excimer ablative lasers emit very short pulses. The biological effects and potential hazards from high-peak, power-pulsed lasers are quite different from those of cw lasers. This is particularly true of lasers operating in the retinal hazard region of the visible (400-760 nm) and near-infrared spectrum (IR-A: 760-780 to 1400 nm), as shown in Figure 4. The severity of retinal lesions from a visible or near-infrared (IR-A) cw laser is normally considered to be far less than from a Q-switched laser. Another major factor that influences the potential hazard is the degree of beam collimation. Almost all surgical lasers are focused,

thereby limiting the hazardous area (referred to as the ‘nominal hazard zone’ in IEC 60825-1 and ANSI Z136.1-2000.1 An exception is the highly collimated beam from many lasers with articulated aims, which may remain hazardous at some distance from the instrument.8

Reflections are most serious from flat mirror-like (specular) surfaces – characteristic of many metallic surgical instruments. Many surgical instruments now have black anodized or sandblasted, roughened surfaces to reduce (but not eliminate) potentially hazardous reflections. The strong curvature and surface roughening spread the reflected energy and greatly reduce the reflection hazard. The surface roughening is generally more effective than the black (ebonized) surface, since the beam is diffused. However, in some cases, combining a special black surface with roughening provides increased protection, and adding a black polymer finish, has been shown by experiment measurements to offer the greatest protection at the CO2 wavelength – despite initial scepticism by investigators.9 However, other groups argue against blackening the surface, since the instrument will become hotter than without for visible wavelengths. Therefore, the use of the special blackened surfaces must be approached with caution for each application.

It should be noted that both the surface finish and reflectance seen in the visible spectrum do not indicate those qualities in the invisible, far-infrared spectrum. In fact, a roughened surface that appears to be quite dull and diffuse at a shorter, visible, or IR-A wavelengths, will always be more specular at far-infrared wavelengths (e.g., at 10.6 µm). This results from the fact that the relative size of the microscopic structure of the surface relative to the incident wavelength determines whether the beam

Fig. 5. The beam irradiance decreases rapidly with distance from a bare fiber.

is reflected as a specular or diffuse reflection.3,9,30 A specularly reflected beam with only 1% of the initial beam’s power can still be quite hazardous. Hence, the rougher the surface of an instrument likely to intercept the beam, the safer the reflection. For example, even a 1% reflection of a 40-W laser beam is 400 mW! It is somewhat surprising that there have been few cases reported of eye injuries to residents and other persons observing Nd:YAG laser surgery without eye protectors. Hazardous specular reflections from a laser beam emerging from an endoscopic optical fiber are limited in extent because the beam rapidly diverges as

shown in Figure 5.

Most invisible beam surgical lasers and pulsed lasers have a visible alignment beam. Infrared lasers usually make use of a low-power coaxial HeNe (632.8 nm) or diode (e.g., 635 nm) red laser. Where feasible, it is desirable for this alignment beam to be 1 mW or less, since the maximum cw, visible laser beam power that can safely enter the eye within the aversion response (i.e., within the blink reflex, etc., of 0.25 seconds) is 1 mW. This type of laser is then classified as Class 2, and poses a very low risk to the user.

Patient safety

Most laser safety regulations do not apply to the exposure of the patient at the target site for surgery. However, accidental exposure to the patient from misdirection of the laser beam should be of concern, and can result in injury of eye and skin.3 Ignition of drapes can be particularly hazardous to the patient, who is under anesthesia and unable to warn the operating-room (OR) staff of the sensa-

tion of heat. Details of accidents are often not published because of litigation, but anecdotal reports indicate that misfiring of a laser when not in use, or undetected breakage of optical fibers, has led to fires from the ignition of surgical drapes, with serious injury to patients. Procedural methods, such as the use of the standby switch or proper placement of the laser foot-switch, can reduce the number of such accidents, but never completely eliminate them. Preparations for extinguishing fires or the moistening of drapes must always be part of the OR safety standing operating procedure (SOP).

Accidental injury to the eye is of particular concern when lasers are used for non-ophthalmic procedures near the eyes. Where exposure of the eye itself is not intended, special eye shields are available for patient protection, such as that shown in Figure 6.

Safety of the surgeon

Normally, the surgeon views the target issue through the optics of an endoscope, operating microscope, colposcope, slit-lamp biomicroscope, etc., and the reflections are safely attenuated within the optics. Under such indirect viewing conditions, the surgeon or laser operator is not normally highly susceptible to injury, due to the proper design of the laser instrument. However, if the laser is accidentally actuated when the surgeon is not looking through the viewing optics, he or she will be just as much at risk as any other person in the room. Additionally, with hand-held laser delivery systems, it should be remembered that the surgeon’s hand is the closest to the laser target and therefore it is closest to potentially hazardous reflections from adjacent surgical instruments (e.g., metal retractors).

Safety of the surgical staff

Nurses, surgical assistants, and other assisting staff are potentially exposed to misdirected laser beams. Lasers have been accidentally initiated when the beam delivery system was directed other than at the patient, a foot switch was accidentally pressed, or similar errors have occurred, and the beam directed at a person. In panretinal photocoagulation, an unexpected eye movement has led to an unin-

tentional exposure of the macula. Accidental firing of a laser has also occurred because of confusion created by having more than one foot switch positioned nearby. Some safety guidelines recommend that the laser foot switch be covered and clearly identified. Assistants are potentially exposed to secondary reflections from contact lenses (and in some special procedures, from reflections from surgical instruments), whereas the surgeon’s eyes are protected by filtration in the viewing optics. Reflections from the cornea or from the contact lens used in ophthalmic surgery have been shown to be potentially hazardous to assistants or bystanders in line of view of the contact lens to a distance of 1-2 m. The operating microscope used in laser microsurgery by a number of specialties would protect the eyes of the surgeon if properly designed, whereas, assistants, and bystanders may be exposed to potentially hazardous reflections from surgical instruments inserted into the beam path.

Safety of other bystanders

Bystanders in the surgical facility or outpatient laser facility who are present to observe or to calm the patient (e.g., a relative) may be susceptible to exposure from reflected laser beams in the same manner as a surgical assistant or nurse. In addition, because of lack of training or knowledge about the laser surgical procedure, bystanders may be at greater risk by inadvertently placing themselves in a dangerous position. Those individuals should always be provided with laser eye protectors.

Service personnel

Service personnel are particularly susceptible to laser injury since they often gain access to collimated laser beams from the laser cavity itself or from opening up the beam delivery optics to gain access to collimated laser beams prior to the beam focusing optics or fiber-optic beam delivery system. Once the laser beam leaves the delivery system and comes rapidly into a focus, it then diverges again, or if emerging from a fiber, it also rapidly diverges. The zone where the beam is concentrated to a level sufficient to pose severe hazard to the eyes or skin (the NHZ) is normally a limited zone of 1-2 m near the beam focal point. However, a collimated laser beam, as the raw beam for most laser cavities, or a specular reflection from a turning mirror or Brewster window in the laser console may be emitted from the laser cabinet (protective housing) when the service person gains access. Several serious eye injuries have occurred to service personnel exposed to secondary, collimated, invisible 1063 nm Nd:YAG laser beams when the service personnel gained access to the laser cavity.

Protection

Photocoagulators

Laser photocoagulators with slit-lamp delivery systems provide a fixed beam with limited directional movement. Ideally, to minimize the chance of anyone intercepting the primary or reflected beam, the laser delivery system is directed at a wall to terminate the beam in case a patient is not in position, reflections back from the contact lens are terminated at the opposite wall and not directed toward the door (Fig. 3). Cw argon, krypton, and diode laser photocoagulators may have a collimated beam emerging from the laser (as with an articulated-arm delivery system), and the open beam can in theory be hazardous for some distance greater than the clinical treatment room. However, with most devices, the beam emerges as a diverging beam and the hazardous distance may be only 2-3 m when a patient is not in the beam. The reflection from any contact lens will produce potentially hazardous secondary, specularly reflected beams as shown in Figure 3 to a distance of 0.3-2 m, depending upon the laser system. Therefore, it is important that persons observing or holding the patient be given laser eye protectors.5 The operator of the laser is protected by a laser safety filter that is permanently built into the viewing optics. This may be fixed as in the case of the diode laser, or part of a shutter system as in the case of argon and krypton lasers. There have been some instances in the past of filter shutter failure, but modern photocoagulators have effectively eliminated this potential. The visible reflection of the aiming beam between therapeutic exposures has been measured, and is below permissible exposure limits.31 Of particular note is the potential hazard of attaching auxiliary viewing optics, unless laser safety filters are installed.

The laser photocoagulator system employing an indirect ophthalmoscopic delivery system poses more problems than the slit-lamp delivery systems. The beam can be directed anywhere, and a momentary misfiring can be directed away from the patient. Hence, laser eye protectors are mandatory for any ancillary personnel or visitors in the treatment room. A warning light or sign should be displayed during laser use.

Photodisruptors

Nd:YAG photodisruptors have very large convergence angles, such that the hazard distance along the primary beam is only 1-2 m from the focus if the patient were not in the beam.32 With the patient in position during treatment, the specular reflections from the contact lens are potentially hazardous, at least out to a distance of 0.3-1 m, as shown in Figure 3.32 Therefore, it is important that persons observing or holding the patient be given laser eye protectors. The operator of the laser is pro-

tected by a laser safety filter built into the viewing optics. As with the photocoagulator, the photodisruptor should be positioned so that neither the direct beam nor the reflections from the contact lens can be directed toward the doorway to the treatment room.

Excimer laser photoablators

As with slit-lamp delivery systems, the fixed delivery system in ArF excimer lasers used in corneal ablative procedures has a stable beam path, and the nominal ocular hazard area is very limited. In fact, there is virtually no reflection hazard during the procedure and the need for eye protection for persons in the treatment room can be reasonably questioned. The ocular hazard in the room is only theoretical, but from a legal standpoint, it may be advisable to offer visitors clear plastic visitor’s goggles. Generally, the greatest safety concerns with excimer lasers relate to the safe handling of the excimer laser gasses, such as fluorine, and not the direct laser beam.33 Fortunately, today, ArF excimer lasers are well designed and only premixed gasses are used.

Illuminators

Not all potential hazards stem from laser radiant energy. The illumination from operating microscopes can pose a potential hazard to the retina of the patient. Studies show that the tungsten filament of an operating microscope imaged on the retina can lead to photoretinitis. This risk is significant only in cataract surgery if the illuminators’ image is fixed on the macular area for a period exceeding at least 15 minutes. Other ophthalmic diagnostic lights are far less hazardous.

Conclusions

Ophthalmic lasers pose potential hazards which are well understood by the ophthalmologist. Fortunately, the laser beams emitted by most ophthalmic lasers are stable and fixed, so apart from the placement of specular (mirror-like) surfaces, such as contact lenses, in the beam path, the nominal ocular hazard area is extremely limited. The greatest safety problem is introduced by a non-fixed beam delivery system such as the indirect ophthalmoscope.

References

1.Boettner EA, Wolter JR: Transmission of the ocular media. Invest Ophthalmol Vis Sci 1:776-783, 1962

2.Mainster MA: Wavelength selection in macular photocoagulation: tissue optics, thermal effects and laser systems. Ophthalmology 93:952-958, 1986

3.Sliney DH, Trokel SL: Medical Lasers and Their Safe

Use. New York, NY: Springer Verlag 1993

4.Sliney DH: Eye protective techniques for bright light. Ophthalmology 90(8):937-944, 1983

5.Sliney DH. Wolbarsht ML: Safety with Lasers and Other Optical Sources. New York, NY: Plenum Publ Corp 1980

6.Gerathewohl SJ, Strughold H: Motoric response of the eyes when exposed to light flashes of high intensities and short durations. J Aviat Med 24:200-207, 1953

7.Sliney DH: Laser effects on vision and ocular exposure limits. Appl Occup Environ Hyg 11(4):313-319, 1996

8.Fisher KJ, Gehly J, Sliney DH: Surgical lighting: a review of ocular safety. J Illum Eng Soc (North Am) 20(1):28-31, 1991

9.Mainster MA, Ham WT, Delori FC: Potential retinal hazards: instrument and environmental light sources. Ophthalmology 90(8):927-931, 1983

10.Sliney DH, Wolbarsht ML: Safety standards and measurement techniques for high intensity light sources. Vision Res 20(12):1133-1141, 1980

11.World Health Organization (WHO): Environmental Health Criteria No 160, Ultraviolet Radiation. Joint publication of the United Nations Environmental Program, the International Radiation Protection Association and the World Health Organization. Geneva: WHO 1994

12.World Health Organization (WHO): Environmental Health Criteria No 23, Lasers and Optical Radiation. Joint publication of the United Nations Environmental Program, the International Radiation Protection Association and the World Health Organization. Geneva: WHO 1982

13.Pitts DG, Cullen AP, Hacker PD: Ocular effects of ultraviolet radiation from 295-365 nm. Invest Ophthalmol Vis Sci 16(10):932-939, 1977

14.Sliney DH: Physical factors in cataractogenesis: ambient ultraviolet radiation and temperature. Invest Ophthalmol Vis Sci 27(5):781-790, 1986

15.Taylor HR, West SK, Rosenthal FS, Munoz B, Newland HS, Abbey H, Emmett EA: Effect of ultraviolet radiation on cataract formation. New Engl J Med 319:1429-1433, 1988

16.Forbes PD, Davies PD: Factors that influence photocarcinogenesis. In: Parrish JA, Kripke ML, Morison WL (eds) Photoimmunology, Ch 7. New York, NY: Plenum Publ Corp 1982

17.Sterenborg HJCM, Van der Leun JC: Action spectra for tumorigenesis by ultraviolet radiation. In: Passchier WF, Bosnjakovic BFM (eds) Human Exposure to Ultraviolet Radiation: Risks and Regulations, pp 173-191. New York, NY: Excerpta Medica Division, Elsevier Science Publ 1987

18.Ham WT Jr, Mueller HA: The photopathology and nature of the blue-light and near-UV retinal lesion produced by lasers and other optical sources. In: Wolbarsht ML (ed) Laser Applications in Medicine and Biology, pp 191-246. New York, NY: Plenum Publ Corp 1989

19.White TJ, Mainster MA, Wilson PW, Tips JH: Chorioretinal temperature increases from solar observation. Bull Math Biophys 33:1-17, 1971

20.Ham WT Jr, Mueller HA, Sliney DH: Retinal sensitivity to damage from short wavelength light. Nature 260(5547): 153-155, 1976

21.Lydahl E: Infrared cataract. Acta Ophthalmol (Kbh) Suppl 166:1-63 (with six appendices), 1984

22.Pitts DG, Cullen AP: Determination of infrared radiation levels for acute ocular cataractogenesis. Graefe’s Arch Klin Exp Ophthalmol 217:285-297, 1981

23.American National Standards Institute (ANSI): Safe Use of Lasers in Health Care Facilities ANSI Standard Z136.3- 1996. New York, NY: ANSI 1996

24.US Food and Drug Administration (FDA): Laser Performance Standard, Title 21, Code Federal Regulations, Part

1040 (21CFR1040). Washington, DC: Government Printing Office 1986

25.Mainster MA, Sliney DH, Belcher CD III, Buzney SM: Laser photodisruptors: damage mechanisms, instrument design and safety. Ophthalmology 90:973-991, 1983

26.Overheim RD, Wagner DL: Light and Color. New York, NY: Wiley 1982

27.Minnaert M: Light and Color in the Outdoors. New York, NY: Springer-Verlag 1993

28.Meyer-Arendt JR: Introduction to Classical and Modern Optics, 2nd edn. Englewood Cliffs, NJ: Prentice-Hall 1984

29.Mainster MA: Ophthalmic applications of infrared lasers: thermal considerations. Invest Ophthalmol Vis Sci 18:414420, 1979

30.Bursell SE, Mainster MA, Sliney DH: Spectral properties of ocular pigments. (Unpublished data)

31.Sliney DH, Mainster MA: Potential laser hazards to the clinician during photocoagulation. Am J Ophthalmol 103:758-760, 1987

32.Wood RL, Sliney DH, Basye RA: Laser reflections from surgical instruments. Lasers Surg Med 12:675-678, 1992

33.Sliney DH: YAG laser safety. In: Trokel SL (ed) YAG Laser Ophthalmic Microsurgery, pp 67-84. Norwalk, NJ: Appleton-Century-Crofts 1983

34.Sliney DH: Safety of ophthalmic excimer lasers with an emphasis on compressed gasses. Refract Corneal Surg 7:308314, 1991

35.Byrnes GA, Antoszyk AN, Mazur DO, Kao TC, Miller SA: Photic maculopathy after extracapsular cataract sur-

gery: a prospective study. Ophthalmology 99(5):731-737, 1992

36.Calkins JL, Hochheimer BF, D’Anna SA: Potential hazards from specific ophthalmic devices. Vision Res 20:10391053, 1980

37.Marshall J: Light damage and ageing in the human macula. Research and Clinical Forums 7(3):27-43, 1986

38.Sliney DH, Armstrong BC: Radiometric evaluation of surgical microscope lights for hazards analyses. Appl Opt 25(12):1882-1889, 1986

39.Sliney DH, Campbell CE: Ophthalmic instrument safety standards. Laser Light Ophthalmol 6(4):207-215, 1994

40.James RH, Bostrom RG, Remark D, Sliney DH: Handheld ophthalmoscopes for hazards analysis: an evaluation. Appl Opt 27:5072-5076, 1988

41.Young RSL, Goldberg MF, Fishman GA: The minimum retinal irradiance required for viewing human fundi in indirect ophthalmoscopy. Invest Ophthalmol Vis Sci 20:701704, 1981

42.Noell WK: Effects of environmental lighting and dietary vitamin A on the vulnerability of the retina to light damage. Photochem Photobiol 24(4):717-723, 1979

43.Lawwill T, Crockett S, Currier G: Retinal damage secondary to chronic light exposure. Documenta Opthalmologica 44(2):379-402, 1977

44.Kremers JJM, van Norren D: Retinal damage in macaque after white light exposures lasting ten minutes to twelve hours. Invest Opthal Vis Sci 30(6):1032-1040, 1989