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CHAPTER IV PERS0NNEL PR0TECTI0N FR0M LASERS AND A DISCUSSI0N 0F EYE PR0TECTIVE DEVICES A. E. Jones* The laser has decidedly found its place in military field operations as well as in many scientific laboratories. The laser is an increasingly used tool for the scientist, a therapeutic device for the physician, a ruler for precise measurement of length, a stimulus for the imagination of fiction writers, a production device for microwelding and drilling, and a very pre- cise range finder for the soldier. The quick acceptance of lasers and their rapid application to a variety of problems have placed many people in a potentially hazardous environment. Moreover, since the first ruby laser was developed by Maiman'^ in I960, there has not been enough time to evaluate all of the medical hazards that might be present in the unique radiation of lasers. Rapid development of new laser materials in the laboratory and by the laser industry has provided laser users with highly effective wavelengths of adequate power to destroy and alter biological systems and tissues. Hazards that exist from lasers include some unique conditions other than the radiation itself. Solid state pulsed lasers are frequently cooled with liquid nitrogen. Extremely serious "burns" can be produced by small amounts of liquid nitrogen, and the havoc that would result in a closed space if a few pounds of this coolant were spilled on the floor can be imagined. The electrical requirements of the laser create potential environmental hazard. High power CW devices can demand power supplies producing voltages on the order of 100 KV. Pulsed solid state laser require stored charges of several hundred thousand joules. In addition to the hazard of fatal electric shock, conditions can exist in these devices which produce x-rays and become potential radiation hazards. The power is usually transmitted some distance from the power source to the laser head by cables that can be both an elec- trical and mechanical hazard. Exploding components are another hazard associated with laser operation. Flashtube explosion is an occasional, if not frequent, occurrence. Most of the flying glass is confined within the cavity when a flashtube explodes, but in some systems, the end caps of the tubes are not confined and blow out with considerable force. Gas lasers can present a special hazard when brok- en if the gases used are toxic, for example, cyanide compounds. *Honeywell, Inc., Systems and Research Center, 23^5 Walnut Street, St. Paul, Minnesota 55113 75
The organ most susceptible to severe laser injury continues to be the eye. While the laser beam is the primary hazard, the eye can also be dam- aged by the flashtube discharge and the light plume produced by laser im- pact on a target. The use of protective goggles is necessary in a laser environment but not sufficient to guarantee safety. Ideally, goggles should be totally opaque to the laser wavelength being used but preserve visual sensitivity throughout most of the visible spectrum. They should be light- weight and well-fitted but provide adequate ventilation for use over long periods of time. Finally, goggles should protect the wearer from the harm- ful brilliance of the plume. A discussion of the types of eye protective devices available and current development trends has been provided by Rob Roy MacGregor in the next section. Many people who might be accidentally exposed to laser radiation will not be provided with eye protective devices. Considerable care must be taken in the preparation of spaces where lasers will be used. Spaces in which lasers are used should be clearly marked with distinctive warnings. Moreover, the laser beam must be contained within the marked spaces. The laser beam is capable of producing ocular damage at extreme distances. The characteristics that make the laser a suitable instrument for ranging, in- herently make the instrument hazardous to a far field observer. These con- siderations require that the laser be so arranged in the space that the beam cannot go out an open door or window unless it is intentional. The space where lasers will be used should be painted with a flat paint of low reflec- tance. Where possible, draping the target area with fire-proofed black material will help to contain the glare from the plume and reflected laser radiation. A good procedure used at the Lawrence Radiation Laboratory" re- stricts the path of laser beams to heights less than five feet or greater than seven feet so that the beam will not be at eye level. 0ur knowledge of potential ophthalmological hazards that exists from lasers must be extended as laser technology produces higher power devices, both CW and pulsed. The hazards from normally pulsed ruby devices appear to be primarily thermal events and are most severe in pigmented tissues, i.e., iris, pigment epithelium and choroid. Ruby energy absorbed in the iris can produce synechia, and heat conduction from the iris can be an ade- quate stimulus for cataract formation. Normally pulsed ruby directed to the fundus is absorbed by the pigment epithelium and choroid. If the retinal dose is on the order of 0.8 J/CM^, visible lesions are produced. If the energy density is sufficient to produce choroidal involvement, preretinal and vitreal hemorrhages occur with disastrous effects on vision (see the discussion of "Retinal injury from laser and light exposure" by Dr. Geeraets i n thi s volume). Direct viewing of laser impact or the output face of a laser is seldom required. Although laser impacts from pulsed and Q-switched devices pro- duce dramatic and drastic effects, the time sequence of the events is so short that little or nothing can be seen directly. High speed photography with framing rates in excess of 2,000/sec. is sometimes to slow to clearly elaborate the formation and travel of shock waves and other phenomena. For metal working and other applications, indirect viewing is a good solution 7b
to the problem of seeing what you are doing. In other applications, i.e., alignment of optical devices or laser surgery, direct viewing is almost required. For the case of direct viewing, a "Hazard Slide Rule for Direct Illumination" has been developed by Graham W. Flint of the Martin Company. The rationale of the rule and the parameters upon which it is based are discussed elsewhere3. 0nly a few of the basic considerations will be dis- cussed here. The resting eye will focus rays of light emanating from objects between the distance of 30 meters to infinity on the surface of the retina. A single point of light will present a retinal image diameter that is deter- mined by the resolving power of the eye. Images of distant point sources are diffraction-1imited images. Laser radiation in the visible region of the spectrum will be focused by the cornea and lens on the surface of the retina. There is some question about just how small the far field diffrac- tion-limited image is, but the consensus is some value between 7 and 20 u. in diameter. The hazard slide rule uses a fixed diffraction-1imited image diameter of 16 /n. A second fixed value incorporated in the slide rule is pupil diameter. The diameter of the pupil can vary from 1.5 to 7.5 millimeters. Since there is no way in which the actual pupil diameter can be specified other than on- the-spot measurement, the pupil is assumed to be maximally dilated. This assumption is conservative and the actual energy density incident on the retina will be overestimated. Each parameter that constitutes a variable is given a separate sliding scale. The variables considered are: 1) power in watts; 2) pulse length in seconds; 3) beam width in radius; k) range in meters; and 5) loss factor. These factors are self-explanatory with the possible exception of the loss factor. Atmospheric attenuations over the path of the laser beam must be considered in terms of the laser wavelength and the distance involved at the time the laser is used. These data are not always immediately available to the operator, and for the sake of field utility a medium value of 2 X 10~' CM~l is used. Provision is made for taking account of an additional loss factor if the true atmospheric attenuation factor over the laser beam path for the wavelength used is known. A second use of the loss factor involves the absorption of the laser wavelength by the ocular media. The slide rule assumes a transmission factor for the ocular media of 100%. If the laser wavelength is one which the ocular media absorb a known percentage, the additional loss factor can be included. A factor also can be used if the observer is wearing protective goggles, if the attenuation factor of the goggles for the specific wavelength of the laser in use is known. When all slides are correctly set, the safety factor for the specific set of conditions is indicated on a fixed scale at the bottom of the rule A safety factor of I indicates that a "threshold" retinal dose would result, meaning that damage would be produced. A safety factor of 10 is considered adequate, and the safe region of the scale is marked in green. Safety fac- tors between 10 and 1 are marked in pink, and factors less than 1 are marked in red. Instructions in the use of the slide rule and the assumptions upon 77
which it is based are printed on the back. The slide rule is available from the Martin Company, 0rlando Division, 0rlando, Florida. The laser hazard slide rule is a useful device that allows a novice to determine a safety factor for a given set of conditions, with a minimum amount of information. However, the information required, i.e., power of the device, pulse length, beam width, range and wavelength, is seldom avail- able for the devices found on the production line or in the field. This situation could easily be rectified by the manufacturer being required to put the information on a plaque on the device. It should be emphasized that the slide rule safety factors are intended only for direct viewing of the laser output. Even when the rule is used for appropriate conditions, some caution must be exercised in the use of the safety factor. A safety factor of ten or greater should be considered correct only for single exposures. The rule is based on data that are very carefully controlled and verified. These data indicate the energy densities required on the retina for a given time dura- tion to produce a visible burn in 50% of the exposures (Geeraets, this volume) Energy densities that are known to create visible lesions have a safety fac- tor of 1 on the slide rule. A safety factor of 10 has not been shown to be safe for multiple exposures, especially if they are closely spaced in time. The situation can arise in which a pulsed laser is operating at the rate of 100 or 200 pulses per second and have a safety factor of 10 for any single pulse. Until data on the effect of multiple exposures and latent effects are available, a factor of 10 should be considered safe only for single ex- posures. Finally, some consideration must be given to the laser wavelength in use. The absorption of the ocular media as a function of wavelength has been carefully determined by Ham and his co-workers*5. If we assume a thermal model of injury, the attenuation of the laser energy by absorption in the ocular media can be determined from Ham's*2 data and incorporated in the slide rule loss factor. Most of the data collected for white light and normally pulsed ruby and neodymium doped glass lasers support a thermal in- jury model. Lasers using Q-switching generate lesions in which the thermal effect is compounded by many effects, I.e., acoustic or shock waves generated by transformation within a closed cavity phase, among others. Nonlinear ef- fects may contribute to lesions produced by normally pulsed lasers also, but they may be very minimal and completely masked by the thermal events. The situation might arise in which a focused beam had high power density, but the target absorbed little of the energy because of the wavelength. This might allow the nonthermal effects to be seen in the production of tissue damage. Unlikely as this hypothesis is, it must be considered because phase transformation and other nonlinear effects have been reported by Fine, et a_K . phase transformation in a closed cavity like the eye has disas- trous consequences totally out of proportion to the radiant energy involved. ⢠Laser radiation with a wavelength around 5500 A will undergo appreci- able absorption by the photopigments contained in the outersegments of the receptors. With the energy densities possible in laser beams, it is likely that 100% bleaching of the photopigments can be achieved with very short 78
exposures, and this might result in damage to the photoreceptors themselves (see discussion of intensity-time relationships, page 35 'n tne third chapter) It has been clearly demonstrated that the radiant energy used so far to pro- duce retinal lesions is primarily absorbed in the melanin granules of the pigment epithelium and choroid. Heat generated at the absorption site coagu- lates the overlying retina. In the case in which a greater percentage of the energy is absorbed in the photopigments, enough heat might be generated in the outersegment to damage the cell. Ruby laser radiation at 69^3 A is an inefficient visual wavelength and should be selectively absorbed primarily by the red sensitive photopigment, although such effects have never been ob- served with lasers, pronounced differential adaptation effects have been shown to intense spectral lights (page k] in the third paper). Safety factors determined with the slide rule are definitely not appli- cable when the observer is using an optical device. Binoculars, telescopes, and other such instruments greatly increase the energy density of the retinal image without increasing the image size, at least in the far field case. The major concern in laser safety programs has been to establish some "threshold" value of retinal energy density and stay below it by an arbitrary factor. A threshold is a statistical value derived from some operational procedure. 0bviously, if a given retinal energy density produces a visible lesion in 50% of the cases, then some lower energy density will produce a visible lesion in 25% of the cases. The threshold, in this sense, is a value referred to a percentage. Since retinal lesions are irreversible and no med- ical treatment can restore the damage, a safety level must exclude a higher percentage than 50% of injuries. Ruby laser radiation impinging on parts of the eye other than the cornea have been shown to produce retinal damage, holes in the iris, hemorrhage and cataract. Moreover, it has been demonstrated that extremely high energy im- pacts in one eye can produce lesions in the other eye even though the eye was closed and covered with a black patch7. The most likely cause for this outcome is transmission of the radiation through the intervening tissues. This would imply that a very high energy impact on the face could produce retinal damage even though eye protective devices were used. 0n the lower end of the scale of intensities, Jones, e_t aj..^ have demonstrated that a retinal energy density of 0.2 J/cnr delivered in 1.5 ms to a retinal area of 1 cm2 produces statistically significant changes in the implicit time and the waveform of the electroretinogram (ERG). This experiment has recently been repeated for the third time with the same outcome. The changes have persisted for six months and are presumed to be irreversible. See the third paper for a more complete discussion of these topics. Several discussions of the hazards from lasers and personnel protection are available in the I i teraturel.2 ,^,5,9. Safety recommendations for per- sonnel protection from laser hazards really are comprised of some informa- tion and a lot of common sense. Workers who are unfamiliar with lasers and laser effects should be informed of the potential consequences of an acci- dental laser exposure to themselves and others. The individual responsible 79
for laser safety and laser-proofing the area should explain the procedures used, e.g. counting down. Each worker should have enough indoctrination so as to be able to recognize an unsafe condition, and he should be aware of the individual safety measures that he must observe both for his own protection and the protection of others. The responsibility for safe operation of laser devices must necessarily vary with the situation. The prime responsibility for protection against laser exposure must remain with the individual. In the field, range safety procedures should be followed. In the laboratory, the laser operator should determine that maximum safety conditions exist before the laser is fired. Fixing the responsibility for laser safety procedures is not the intent or even within the scope of this section. However, some individual should be responsible for safe operation of laser devices regardless of the unique use and situation in which the laser is being used. The primary recommendation is that people be provided with adequate information to use the laser safely. It is proposed that this be accom- plished by: 1. Explaining all of the hazards associated with laser devices. 2. Explaining the safety procedures which are in force. 3. Designating responsibility for safe operation of laser devices. 4. Explaining the individual safety measures that must be carried out. To ensure that accidental laser exposures are kept to a minimum, some care must be taken in the preparation of spaces in which laser devices will be used. Specific procedures are difficult to propose because of the di- versity of uses of lasers. For example, the use of flat black paint is recommended. However, few hospital administrators would be in favor of this color scheme for operating rooms. 0bviously, some procedures must be a compromise between several requirements. The laser group at the Children's Hospital Research Foundation in Cincinnati has obviated this difficulty to a degree by developing a fail-safe occluder for personnel in the room. This goggle device is triggered by the surgeon and completely occludes the eyes of the people in the area, if they are wearing the device, for the period of the laser burst and plume dissipation. The closure period is on the order of 0.5 sec. and has minimal interference with surgical procedures. No laser burst may occur until all goggles are actuated. The master switch which closes the goggles operates a mechanical shutter in the laser beam. This system, while limiting the operator's visual field, provides unobstruc- ted vision except for the period of the laser flash and plume dissipation. However, this system does not prevent direct or reflected exposure of other parts of the face. Laser spaces should be clearly marked with distinctive warning signs. A number of signs have been proposed for national and international use, 80
but at- present there is no standard warning device. Laser Focus (Vol. 2:23, p. 9-) -has recently discussed the warning signs developed in the interest of laser safety. We currently use a modification of the sign developed at Texas Instruments Inc. 0ur modification is primarily a change in color scheme since the sign, as supplied, was essentially unreadable by color anomalous observers while it was read with difficulty by normal observers at moderately low levels of illumination. Laser spaces should be provided with some sort of interlock doors to prevent a person from stepping into the room and being exposed. Such systems are frequently bothersome but well worth the bother. Laser devices as sup- plied by the manufacturer usually must be modified to be compatible with an interlock system. However, this is not a difficult thing to do. Where an interlock system is not feasible, access to laser spaces should be prevented without prior warning. Visual loss due to laser exposure will undoubtedly be considered a job- connected disability. Therefore, if possible, a thorough assessment of vision capabilities and existing ocular pathologies should be made before a person is introduced to the laser environment and follow-up ophthalmolog- ical examinations should be made at regular intervals. Such a procedure is recommended in the fourth paper, page of this volume. A person who may have received an ocular laser exposure should be seen by an ophthalmologist immediately. To summarize this discussion, there are some general rules that should be followed whether the laser device is used in a high school demonstration or as a production tool: (a) the hazards associated with lasers should be explained to the people who will be working around and with them; (b) special attention should be paid to the preparation of the spaces where lasers will be used and beam paths of the laser flash; (c) laser areas should be clearly demarked by distinctive warning signs; and (d) regular ophthalmologieal ex- aminations should be carried out. Protective goggles should be used at all times in the vicinity of active laser devices. A discussion of the types of eye protective devices avail- able and current research trends follows. REFERENCES 1. Daniels, R. G. and Goldstein, B. Lasers and Masers - health hazards and their control, Fed. Proc.. 1965, supp. 14. 2. Fine, S., Klein, E., Fine, B. S.t Litwin, M., Nowak, W., Hansen, W. P., Caron, J., and Forman, J. Mechanisms and Control of Laser Hazards and Management of Accidents. Monograph of Second National Conference on Laser Technology, 1965. 81
3. Flint, G. W. Derivation of Laser Hazard Criteria, Proceedings of the First Conference on Laser Safety, Flint, G. W. (Ed.)» Martin Co., 0rlando, Fla., 1966. k. Goldman, L. and Hornby, P. Personnel Protection from High Energy Lasers, Amer. Indust. Hygiene Assoc. J., 1965, 26, 553-557. 5. Goldman, L. and Hornby, P. The Design of a Medical Laser Laboratory, Arch. Environ. Health, 1965, j[0, b. Ham, W. T. Jr. 0cular Effects of Laser Radiation on Mammals, Pro- ceedings of the First Conference on Laser Safety, Flint, G.W. (Ed.), Martin Co., 0rlando, Fla., 1966. 7. Jones, A. E. and McCartney, A. J. Ruby Laser Effects on the Monkey Eye, Invest. 0phthal., 5., ^7^-^83, 1966. 8. Jones, A. E., Bryan, A. H., and Adams, C. K. Laser Induced Changes in the Implicit Time and 0scillatory Potentials of the Mangabey Electroretinogram (in press). 9. Lawrence Radiation Laboratory, Livermore, California. Laser Safety Standards, N. C. Manual, Part 1. 10. Maiman, T. H. Stimulated 0ptical Radiation in Ruby Masers, Nature, 187. ^93, I960. 82
