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PREFACE AND REC0MMENDATI0NS At the time of the I965 meeting of the Committee on Vision, I was asked to be chairman of a working group to study laser eye effects related to military and laboratory applications and to recommend safety measures, protective devices, and continuing research to support those areas. The results of the working group's efforts thus far are presented here under six topics: technical characteristics of lasers, laser-eye pathological effects, laser-eye functional effects, personnel protective measures, devices for eye protection and eye examination and treatment standards. The chairman's recommendations derived from the group's discussions and culled from the following sections are as follows: A. Research Recommendations 1. A program should be established to standardize on criteria of retinal damage from light exposure and on radiometric calibration pro- cedures. Data from the laboratories of Geeraets and Ham, of Fine and Zweng, to name four workers in this area, disagree by substantial amounts. It seems that the trouble lies with the interpretation of pathological materials to determine when threshold damage has occurred. There is also a strong possibility that these laboratories differ in their energy cali- brati ons. 2. It is highly possible that irreversible effects occur below the level of observable pathology, which may nevertheless be detectable by electron-microscopy or by enzyme stain techniques. It is recommended that programs be continued: (a) to identify and define the energy levels at which these microscopic changes may occur, (b) to relate these to amount of visual loss, and (c) to define criteria for standardizing these effects. 3. Controlled studies of long-term effects and the effects of repeated exposures on the cornea, iris, lens, ocular media, as well as the retina and choroid, should be undertaken. Data have been presented that indicate laser exposure has an impact on all of these structures. 4. The laser hazard slide rule should be verified for wavelengths other than ruby. B. Safety Recommendations 1. A standard sign should be adopted to clearly delineate laser spaces. 2. All personnel who will be in the vicinity of lasers and laser spaces should be indoctrinated on the hazards of laser equipment. 3. Industrial devices should be equipped for indirect viewing wherever possible.
k. A fail-safe interlock system should be developed for surgical and machining devices to preclude the possibility of accidental exposure of the operator and others in the operating room or laboratory. 5. Manufacturing standards should be established to insure that under normal conditions the user has a safe instrument. 6. Those first coming into laser work should receive an eye examina- tion such as described in Section VI to serve as a reference base for further periodic examinations and for examinations in the event of exposure. C. Safe Exposure Levels The most significant and also most difficult to deal with topic of this report is the question of what is the maximum permissible dosage of laser radiation of the different wavelength transitions that can safely be permitted to reach the eye. Those dealing with the application of lasers in the field or laboratory, in the government or institutional setting, are concerned with allowing operators to perform their assigned tasks with a minimum of interference, at the same time that hazards are minimized. Complaints have been voiced, particularly with regard to mili- tary field devices, that the provisional safety standards which have been set preclude the operation of the devices. It is hinted that in the face of this situation people are ignoring safety precautions in order to per- form their tasks. It is therefore urged, from all quarters, that reasonable safety standards are needed as a basis for sound operating procedures. The heart of this question is: what is the maximum safe dose? As will be seen in Chapter It of this volume, there is considerable evidence which is in fairly good agreement on the threshold dose of the ruby laser pulse that produces a minimally observable lesion, as viewed with an ophthalmoscope. From those data, the value 0.8 joules/Cnr on the retina has been stated and widely accepted as the minimum dose producing observable damage by that technique. -As seen in Chapter II, when giant-pulse laser radiation is used, the minimal ophthaImoscopic lesion is produced by .07 joules/Cm2 on the retina. The latter value appears to hold between 5 and 50 nanosecond pulses. These threshold levels have been projected to the cornea by com- puting the energy at the cornea which would produce the threshold retinal irradiance in a minimum sized retinal spot. Assuming a night-adapted pupil, these values are 1 X 10~6 joules per square centimeter in the plane of the cornea for the non-q-switched pulse and 1 X 10~7 joules per square centi- meter at the cornea for the giant pulse. As a provisional safety standard, the Armyl has added an additional safety factor of 10 in promulgating these values as allowable safe corneal irradiance levels. Beyond the results for ruby laser, as may be seen in Chapter II, re- sults thus far on neodymium laser pulses show that 5 to 6 times more energy than from ruby is required to arrive at the minimal retinal lesion with this transition.
For continuous wave gas lasers (helium-neon), there is no evidence on which to base a threshold number for minimum retina) damage. The Lawrence Radiation Laboratory^ Makes extensive use of continuous wave lasers in optical alignment applications, so they have formulated a safety standard based on allowing a maximum of 1 degree centigrade rise in temperature at the retina. They arrive at a figure of 1 X 10~6 Watts/Cm2 at the cornea, which they state as a tentative maximum safe level for ex- posure to continuous wave helium-neon lasers. The foregoing numerical results must be considered valid only for grossly observable thermal effects on the retina. As will be pointed out in Chapter Ml, the problems of flash blindness effects on the re- ceptor mechanisms of the eye as produced by laser exposure are totally different. In addition, between the extremes of retinal burn on the one hand and levels which we know produce completely recoverable flash blind- ness on the other, lies an almost totally unexplored range which we estimate to be from I to 3 log units of irradiance over which more subtle damage to retinal tissue, particularly the receptor cells, may be encount- ered. Continued vigorous research to obtain information on effects over this unknown range has been recommended above and is strongly urged. Finally, the Working Group has decided to call the reader's attention to the above cited threshold burn values which are the best available information on the maximum allowable irradiance at the cornea. It does so without recommending that these values be adopted as the basis for any long-term safety standards. It is our continuing, and much debated, view that too much evidence is missing to allow us to state maximum safe dose levels with confidence at this time. The chairman is grateful to the members of the Working Group for their efforts. H. G. Sperling REFERENCES 1. Sliney, 0. H. and Palmisano, W. A. The Evaluation of Laser Hazards. U. S. Army Environmental Hygiene Agency, Edgewood Arsenal, Maryland, May 1Q67. 2. University of California, Lawrence Radiation Laboratory, H. C. Manual, Part I, Procedure 842, Issured August 2, 1965.
