Laser Eye Effects (1968)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

CHAPTER I I I LASER FUNCTI0NAL EFFECTS Harry G. Sperling* INTR0DUCTI0N Little can be said directly of the effects of pulsed laser exposure on visual function because the unique combination of very narrow waveband and very brief, high energy flash has not often been studied. In addition to the few pulsed laser studies, there are a small number of experiments showing the effects of monochromatic light exposure on some aspects of visual function, which will be directly applicable to CW laser problems, and a fair-sized literature on the visual effects of flash exposure to stimulus fields of varying intensity. In the following, we will attempt to summarize these results and draw what tentative conclusions are possible for the present problem. It is clear at the outset that in some areas the best that can be hoped for will be to arrive at questions and inferences which will serve as hypotheses for future research. We will divide the problem of laser functional effects into psycho- physical effects and physiological effects based on differences in measure- ment technique. These, of course, will often be expected to show very much the same results, but due to the differing experiences and backgrounds of those interested in vision, the two types of effects are traditionally separated. Another classification might be in terms of reversible and ir- reversible change. Clearly, the laser has properties which differ from other light sources In the quality, quantity, and rate of application of energy. The high energy level of laser radiation and, more important, its very high rate of emission have been shown to affect tissue in special ways. When these properties are combined with a narrow waveband in the visible spectrum, it is reasonable to expect effects of both a thermal and photochemical nature, such as selective absorption by pigments, which will differ quantitatively, if not qualitatively, from those produced by other light sources. 0ne may also reasonably predict that the special property of coherency will produce new effects related to the hypothesized wave-guide function of visual re- ceptors and may serve as a potent tool for testing that hypothesis. Thus, the laser offers great promise as a research tool. 0n the other hand, there is no evidence to date that adapting the eye to laser light below energy levels which produce pathological change will show any results different from those which would be obtained by adapting the eye to an incoherent source with the same wave-length distribution and time rate of application of energy. *Department of Neural Sciences, University of Texas Graduate School of Biomedical Sciences and Department of 0phthalmology, Baylor University of Medicine, Houston, Texas 77025. 57

0n the question of the reversible psychophysica1 effects of laser exposure, we wi11 discuss the following topics: recovery time following different intensities, durations and stimulus sizes below the burn thresh- old, shifts in response to wavelength following exposure, after-image effects, and the relation between effect to level of pre-adaptation. For practical reasons, we will be primarily concerned with cone vision since the rods become less sensitive than the cones with relatively low levels of adapting intensity and recover much more slowly. Functional impairment resulting from gross damage (retinal burns) may cause distortions, loss of visual acuity, scotoma in the visual fields and wavelength shifts in the color matching mechanisms; some of these will be considered below under the appropriate headings. REC0VERY TIME STUDIES The topic of intensity-duration-area relationships embraces much of the fundamental research in visual science. Unfortunately, most experi- mental results have dealt either with functional relationships at absolute threshold or increment threshold against relatively low intensity back- grounds. Also, effects within the first minute after flash exposure have rarely been studied. 0nly with recent emphasis on the applied problems arising from exposure to nuclear fireballs have the effects of more intense levels been explored. With consideration of seriousness of very brief loss of sight to a low-flying tactical aviator, more attention is also being paid to the study of visual response immediately after exposure. In order to predict laser effects on vision in various tactical mili- tary situations as well as those of accidental exposure in the laboratory, we would like to know what the recovery times are for various relevant visual tasks as a function of different physical stimulus parameters. 0nly with adequate knowledge of these can adequate safety standards, operational and training procedures, and protective devices be prescribed. In general, the studies using flashes of heterochromatic (white) light show that log recovery time is increased as a function of increased total energy in the adapting flash. The typical experiment has used acuity targets (e.g. Snellen letters, grids, Landolt rings) to measure vision, although the detection of light at threshold has also been used. Severin et a I.' show that recovery time accelerates slightly as a function of in- creased adapting luminance for flashes of less than 5 lambert-seconds (low flash intensity). 0ver a middle range of up to somewhere between 100 and 500 lambert-seconds (approx. 7.0 log troland-seconds), there is a nearly linear increase of log-seconds recovery time as a function of log intensity of flash. Beyond a flash intensity in that region, there is a leveling-off or deceleration of the recovery time which is presumed to indicate that maximum possible bleaching of the photo-receptive chemicals has occurred. The data of Miller2 in Figure 1 show the levcllng-off of recovery time clearing in the region of 1 x 10**^ troland-seconds. The data of Metcalf and Horn^, Chisum and HH1 , and Hill and ChlsunP more or 58

(/I •o u o> O U V tc 01 o Q, x - Miller (32): 16.2' test letter; x represents decreased flash durations; 0 represents decreased luminance for 1.4 msec duration. E- Miller: 16.2' test letter; flash duration kept constant at 1.5 msec. 2.0 - 1.5 1.0 .5 28.2 0.0076mL OllmL 0.063mL O a O.lSmL n 0.43mL ..^ r»282mL 6.6 6.8 7.0 7.2 7.^ 7.6 7.8 8.0 Log Flash Energy (troland seconds) Flash Energy (lambert seconds) 1782 Figure 1. Recovery time as a function of flash energy and target luminance. The flattening out above 7.4 log troland-seconds (450 L-sec.) is clear, (from Czef e_t a_l . ref. 8). 59

less confirm Miller's findings in this regard. Above the region of 7 to 8 log troland-seconds (approx. 1 x 1CT 1ambert-seconds), there is some evi- dence that recovery time again increases. Figure 2 shows data from White- side and Bazarnik*3 which illustrates this. As shown in Miller's data in Figure 1, increased target luminance reduces recovery time. In Figure 3, this relationship shows that for three flash intensities over a range of letter-target luminances ranging from approximately 0.1 to 100 mi 11ilamberts, recovery time decreases as a negatively accelerated function from almost 100 seconds for a 3 x 10' trld. second flash and .1 millilambert target to only 2 or 3 seconds for a 100 ml. target. Brown7 hypothesizes the relationship of flash energy, target luminance, and recovery time over a broad range of values to be as shown in Figure 4. He shows the positively accelerating recovery times for the lower flash energies and the approximately linear relation- ship over the middle range to 1 x 107 troland-seconds, which levels off over a range of about one log unit of adapting intensity and then accel- erates toward an asymptote representing irreversible injury. This hypo- thetical family of functions fairly well summarizes the findings on the relationships between flash intensity, target luminance, target acuity, and recovery time for white light flashes of from .04 msec duration to 1 sec. Brown? and Czeh& have proposed predictive equations to relate recovery time to flash energy. The recovery time-flash energy relationship has been totally unex- plored for narrow-band spectral stimuli, such as those produced by lasers. DURATI0N OF FLASH A topic of great importance to the laser problem is the relationship between intensity and duration for a constant visual effect. This is the question of efficiency of light action as a function of flash duration. A number of studies of this relationship have been performed at the ab- solute threshold of seeing. Even those, with one exception, have not been performed over a range of narrow spectral wavebands with foveal vision. Graham and Margaria^ and Karn10 have shown that for peripheral rods and foveal cones the dark adapted absolute threshold is determined by total energy (I x t = c) up to flashes of 0.1 second's duration for stimuli. They show an interaction with stimulus size such that as the retinal area stimulated becomes larger, the range of duration over which total energy determines the threshold becomes smaller. The "critical duration" beyond which there is a transition from total energy constant to intensity alone constant at threshold, ranges from 0.1 seconds for 2 min. diameter stimuli to .045 seconds for 45 min. diameter stimuli in the fovea. Brindley'' has extended the intensity-time relationship at threshold to very short durations and found constancy down to 1 microsecond. Recently, Sperling and Jolltff*"* have shown that for cone vision there is an appreciable wavelength dependency of the range over which total energy is constant at threshold. Stimuli from the short wave end of the visible spectrum summate over longer duration flashes than those composed of the 60

Log Recovery Time (seconds) 1 K> 3" 3 rt ••< 01 Q. (B -hO. -« rt —•«< A r» ill (O ftS2 — n oi i w> n> 3 — (B (B 3- Q.- (B (B ^ ~1 •O (6 (B ft 3 n I D. O ft i/> < S§3 Sir, 'Ji -I -I TJ § "S 3 rt rt O (B (B N 3 A rt rt 10 OJ *R M b T" N) o 0 x i o -h O -I —• O (B -t < O (B (B O A (B «/> ~\ —• •< Q. (B •I \rt> rt « 3 A A Ul (0 U3 |rt (B — ft 3 t» n ""r t V> (B I ft A < M f»~ (B •Q. (B • ft T> •n ft m 3 (B •1 to 01 cr (B VI A O O CL M * V 5 u ro Ul < — . — . X N A w 8 rt 01 «B 3* ^ 3 Qj A rt A — • n • ft •o j^ • ^ (0 A f A r~ rt (B 3 •1 rt H ft in ft 3 •t ID ^3 (O * (B A CL 2, rt -o r~ 3" 3- 1 A \ C 01 U3 — A 1 V • ' S \ I * J2; O < cr A c O i/> T (B (B rt •y « o x C rt (O • O ro Ul \ \\\ \ \\\ t V > \ 1 O O uj ^ UJ CD a> ro 61

Log Recovery Time (seconds) 1 o • un vn -I J A 3- 00 O rt O 3" < (DO 3 — •< _ 01 rt rt y o 1 -tl C A 0) O. 0) TJ 0> T| VI Q> «£> I u> O) A CL O. n> c 3 Vn O duration. VO O r> 0) A A 3- 3 A rt O 0* 00<JO VA> «o i/i (/< rt i/i c Ar t rt 3 rt I/I n a* O n O O -h O v-n ID -h (B — rt Ot l/i QJ 3" O C CL — • C rt z 3 r- O rt 01 3 3 0 X Z I/I V — o rt rt — ^3 "O fD Q. -I rt rt I/I I/I /—v Ol rt rt V*) rt 3 3 M — rt rt «—' V> I/I •• H§ sT o -h O C -h -1 — < O) A <fi l/i 3" -h rt O 3 - i rt n O IO 01 *—> 3 — O. • rt 10 oi o ua Tt 5T UD A 3 | A 3 Ol n 01 rt -i n —• g| T3 O Ol rt ^o — or t rt O rt -h A CL m 3 A -1 IO rt O 01 3 CL I/I A n o Q. I/I Ui 31 rt — A O 3 -i A • rt -5? -h^ -i 1 A § i'rt O A 3- N 3 A rt vi 3-- Q |rt 3 15. 00 b 62

Recovery Time •n f 1 1 1 1 1 I I '"I • Q) rt rt rt 3-3-01 — rt UQ IQ CT CD 3" rt O -O O rt rt (t CO -• 01 rt O 3 — rt C Oi O> 3 < O O CD CD C O • • — C A ~\ 01 .0 i/i n C CO rt -i in o —- • 9 C C 3033 (DO* •-• 3 —• 3 rt O1 OO O) • rt »— 3 ro • n Q. O 3 M in 3" 3 IO rt ft _.g 3" 3 A O A rt Wl 3 A b -1 IO — re o o £ rt in C rt 01 o> o> n O rt 3 s- S -O o 3- rt — Ol <<i/!—' C C -i rt n » — c a* o — o -i -i c ft n •< n n 3 W «rt C I Oi n -. 3 3 < rt Q. C 9 3" n rt 3" rt -*» 3- O — O rt Oi 3* O) in -O V) — A ft (fj 3 O 3 3" » C « -I T < (O < m ai •< n c Q) X fD CO 3 —. T3 Q. 3 to A — n 01 O ui 3 ft Irreversible Injury

longer wavelengths. As seen in Figure 5, for 2 second's flash duration, a t>50 nm red stimulus requires twice the energy to reach threshold as a ^60 nm blue. These curves have been equated for the shortest duration flash. At the opposite end of the intensity range from absolute threshold phenomena, there is a decided variation in the energy required to reach burn threshold as a function of flash duration. Here, high energy density pulses of shorter durations are more efficient than the same amount of energy delivered in a longer pulse in producing burns (see Chapter ll), presumably because they give less time for heat conduction away from the burn site. Between these two extremes of threshold of seeing and burn threshold, little is known from psychophysical results. For purely photochemical results on the bleaching efficiency of different duration flashes, as summarized by Brown': Campbell and Rushton^3 found total energy determined amount of bleaching up to kS seconds. For equivalent total amounts of bleaching energy, equal amounts of rhodopsin were bleached during exposures up to this duration. The minimum time investigated was 300 msec. Some evidence obtained with the same technique has been presented which would lead to the conclusion that the effects of adapting flashes of very short duration might not be as severe as the effects of longer flashes of the same total energy. Hagins found it impossible to bleach more than 50 percent of the rhodopsin of the rabbit retina with flashes of less than a msec duration no matter how high the luminance. If the same amount of energy was distributed between two flashes separated by 1 or 2 seconds, it was possible to bleach up to 75 percent of the rhodopsin. Dowiing and Hubbard'5 have explained this result in terms of underlying photochemical processes. A portion of certain unstable intermedi- ate products of bleaching is isomerized back into photosensitive forms by light itself. With prolonged exposure, these are again bleached, a lesser portion has reached the maximum possible for the luminance used. Thus, complete bleaching requires both light energy and time. According to these results, when an adapting flash is of the order of 1 msec duration or less, bleaching cannot be as great as that which will occur for the same or even lesser amounts of light energy spread out more in time. 0ne would pre- dict that early psychophysical thresholds, meausred after a short adapting flash, would not be elevated as much as those after a longer duration. Similarly, dark adaptation following a short flash would not be equivalent to dark adaptation following longer exposure to a low luminance which results in the same amount of bleaching. A part of the dark adaptation process, hydrolysis of all-trans bleaching products, can occur during exposure to light. Hence, when a longer duration adapting light is extinguished, the process of recovery is at a more advanced stage than is the case after exposure to a short flash, even though photosensitive pig- ment concentrations and initial thresholds are the same. Complete recovery would take longer after the shorter flash. 6k

R.S. A M C.J. IX) I- .10 i 10 i.o B.p. .10 AVERAGE to to •00 -1—1_ US A C.J. S ft 1^ 4VERAOE R.S. C.J. A /A •,' .•' L AVERAGE .001 .01 (o) o.i T(SECONDS) (b) 1.0 .001 .01 _J L- 1 1 1.0 (0 Figure 5. Relative IT values of three subjects and the subjects' average for foveal presentation. Each curve is pinned to unity at the shortest duration (0.0028 sec) to aid in the comparison of the results with two spectral stimuli. Figure 5(a) represents data with 4.5' diam stimuli and dark surround; Figure 5(b) 45' diam stimuli with a light surround (138 trolands). Data taken with the 650-mjj stimuli are represented by a dashed line with crosses and with the J+50-mp stimuli by a dotted line with open circles. Ranges of threshold determinations are included on all individ- ual curves as brackets. A model based on Hart line's single- receptor results (solid line) is superposed on the averages. (from Sperling and Jolliffe ref. 12) 65

Brindley has made some observations of after images which may be related to Hagin's findings. After images induced by flash luminances in excess of 3 x 10^ m-candles were of the same appearance for all luminances so long as the total flash energy was presented in a short interval. The after image following a single flash was comparable to the after image of two succes- sive flashes of the same luminance if the two were separated by only 250p sec. If the two flashes were separated by k msec or more, however, there was a clear difference in the after images. The additional energy of the second flash apparently had no effect unless it irradiated the retina at an interval of several milli- seconds after the first flash. j The findings of Hill and Chisum (op. cit.) may be an illustra- tion of this kind of effect. Their curve of recovery time versus adapting flash energy indicates a more rapid recovery from a short adapting flash (165/u sec. at 1/3 amplitude) than from a long a- dapting flash (9.8 msec at 1/3 amplitude) of the same total energy. When the energy is distributed over a longer period, it is appar- ently more effective in reducing sensitivity. The experiment of Hill and Chisum is the only experiment in which extensive data on functional visual effects to high intensity adapting flashes have been obtained for flash durations both below and above a duration of from 1 to k msec. This is the range of durations below which Hagins and Brindley found a reduction in the effectiveness of a given amount of stimulus energy for the bleaching of rhodopsin and for the production of an after image. WAVELENGTH SPECIFICITY 0F THE ADAPTING FLASH \ Little effort has been expended to study the adapting effects of in- tense narrow spectral bands in the visible spectrum, and no effort has been directed toward study of visual effects of adaptation to ultraviolet or infrared radiation. Burchl7 created effects similar to hereditary color blindness in human subjects with "intense color adaption". Recovery times of up to 2 hours were reported. The exact intensities used are not known, but Burch utilized the sun as a source and a 2 inch focal length lens of unspecified diameter. He used prolonged stimulation and employed broadband color filters. Apparently successive after images in addition to lowered sensitivity were found. Either of these two effects would have great practical significance for tasks utilizing vision performed after intense adaptation. Auerbach and Wald'° demonstrated very sizable changes in spectral sensitivity after exposure to broadband filtered red, orange-red, orange, yellow, green, blue, and white wavebands of intensities between 500 and 6,000 lumens/crrr. Maximum dark-adapted sensitivity in the periphery of the eye (served by the rods) required up to 30 minutes to recover. The cone receptors required between 5 to 10 minutes to recover their original sensitivity. Recovery time to 50% of initial sensitivity (a crude measure of the rate of recovery) was 1 to 2 minutes. The reduction in sensivity ( 66

for the cones ranged from 2.5 to 3.5 log intensity units. The studies of Brindley'" and Cornsweet e_t a_[.20 also employed intense spectral stimuli. Brindley's study showed that the rod (rhodopsin) and red and blue cone mechanisms are relatively more photolabile than the green cone mechanism. Cornsweet's study also demonstrated this effect, as did a more recent study by Weale^'. These data imply that after more detailed study, eye protective devices might be specified which would provide visi- bility in the green region on the spectrum, and at the same time afford protection from intensities causing temporary or permanent flash blindness. Sidley e_t aj.. * and Sperling e_t *!. have performed experiments in which they used very narrow-band stimuli for adaptation (2-10 nm wide) instead of the wider spectral bands (15~50 nm) which workers had previous- ly found necessary to obtain high intensities. These studies have been especially concerned with the hitherto unexplained submaxima in the foveal sensitivity function. Early studies (Figure 6) show that there is a main peak in the function at 550 nm and smaller humps at 560-590 and 590-670 nm. Narrow-band red adaptation at 690 nm greatly reduces the hump in the red (590-670 nm), while 580 nm adaptation reduces the peak in the yellow region (560-670 nm) without eliminating the red hump. 509 nm (green) has a slight effect on the main peak, but retains both yellow and red humps. To obtain better separation of these components, more intense adapting stimuli were used, and in order to maintain a constant control condition, they were superimposed on a constant white light background of 3000 tro- lands. In these studies?^, data were obtained on two rhesus monkeys (Figure 7). The sensitivity data obtained with the white background light alone (Figure 7, ml white and m3 white) were obtained in three separate blocks on each of the two monkeys. In all cases, they show an exaggerated peak in the blue at about 440-50 nm, the green at 530-40 nm, and in the orange to red at above 600 nm. The effects of adding intense narrow-band spectral lines from the red, green and blue parts of the spectrum are shown below the white light control curves for each monkey in Figure 7. Ten thousand trolands of a red, green or blue wavelength were added. Clearly, red and green intense spectral light serves to reduce sensitivity very selectively over the region of the nearest peak as seen in Figure 7A, B, C and D. Blue adaptation has a somewhat different effect, reducing sensitivity over the entire spectrum in addition to removing the peak in the blue region. Apparently, also, intense blue light reveals a peak in the 570-90 nm region, which does not show under the white light plus red, white plus green conditions. The apparent difference between intense blue adaptation and red and green is discussed in a theoretical context by Sper 1 ing es£ a_K24 por our present purposes, these results demonstrate that the eye's sensitivity in other spectral regions may be relatively preserved after an intense narrow-band flash, whether it emanates from a laser or a filtered incoherent source. It appears that the effect of adding an intense spectral light to the field of a 1 i ght-adapted eye is to lower sensitivity over the region of the lobe or peak in that part of the spectrum between 2x and l00x more than in the region of the adjacent peak, thus altering the shape of the spectral sensitivity function in a predictable way. These results conclusively demonstrate that calculations of the effect of intense mono- chromatic light exposure on vision (such as attempted recently by Zaret b7

400 700 Figure 6. Relative spectral sensitivity of the fovea for different con- ditions of very narrow-band adaptation. A: 5~3 troland white; B: 800 troland white; C: 5-3 troland 690 mjj red; D: 72 troland 580 nyj yellow; and E: 27.7 troland 509 mp green. Superimposed on A, as shown by crosses and dashed line, are the results ob- tained by Sperling and Lewis with completely dark surround. b8

«J "> 5 1.5 1.0 0.5 -0.5 500 600 700 500 600 700 WAVE LENGTH (nm) 500 600 700 Figure 7. Log relative spectral sensitivity for white-light and white-plus-spectral-light conditions for two monkeys. Intense narrow-band 650 -nm red added to white is com- pared with white alone (a,b); 520-nm green plus white with white alone (c,d); and 463-nm blue plus white with white alone (e,f) (from Sper 1 i ng e_t a_[ ref. 2k).

and Grosof28 to compare flash blindness and burn thresholds) must not be based on a shape-invariant function such as the CIE photopic luminosity curve. Aside from differential absorption of different wavelengths in the rods and cones, the different parts of the retina have different absorp- tion peaks. The high intensity narrow wavelength radiation from differ- ent lasers might be expected to cause irreversible damage in different layers of the retina as the wavelength is changed. For example, Wolbarsht £t a_K29 found that the ruby laser (69^3 Angstroms) caused greater damage of the pigment epithelium and adjacent tissue, whereas the neodymium laser (10,600 Angstroms) appears to damage the photoreceptor and neural layers at energy levels in which no histological damage could be seen in the pig- ment epithelium. We would expect to cause functional damage of large areas of the retina at 10,600 Angstroms from burns on or near the optic disc involving the overlying nerve fibers. Ruby and other lasers in the visible part of the spectrum would be expected to cause little of such damage. VISUAL ACUITY MEASUREMENTS Since the fovea has not only the most acute color vision but also the highest spatial resolution (acuity), the majority of noticeable reversible and irreversible changes may be expected from laser exposure in this region. When the chromatic adaptation is at a level sufficient to cause deviations from normal color vision, Brindley has found that there is also a degradation of visual acuity. As Rathkey^0 and Yarczower e_t £].. have shown, there is a marked permanent degradation of visual acuity following large laser lesions of the fovea. In this respect, much of the data already available from previous studies on retinal burns re- sulting from fireballs or sun viewing would be appropriate. However, the laser with its higher power density and narrower wavelength may cause lesions with special features not found in other forms of damage. The high power density could possibly cause smaller burns in the fovea1 area, and the narrower band wavelengths could cause selective destruction of single photoreceptor types with a permanent change, for example, in color vision, in a portion of the retina. ERG AND LASER EXP0SURE The electroretinogram is relatively easy to record and yields a quick assessment of the functional state of the retina. However, the typical ERG is a response of the whole retina and is elicited as much by stray light from entopic scatter as it is by direct stimulation of the retina. This was demonstrated by Asher32 wno restricted the stimulus to the blind spot and recorded normal ERGs. Detection of a small scotoma or chorio- retinal burn with the electroretinogram is difficult but probably can be done with precise stimulus control and with computer averaging of a large number of responses. 70

Armington, e_t ajk" first demonstrated that if the stimulus spot is superimposed on a large background field, the effect of stray light out- side the geometrical image of the stimulus is greatly reduced. Brindley and Westheimer3^ found that a background luminance of 8.5 cd/m2 was suffi- cient to eliminate the effect of light scattered from the geometrical image of the stimulus, "and to establish electroretinographic perimetry as a technique available for the investigation of local disorders of the retina." Extreme care must be taken in the collection and interpretation of the electroretinogram if valid conclusions about the functional state of the retina are to be drawn. Several electroretinographic investigations of retinae with grossly visible lesions have found no difference between normal and damaged eyes. Ponte'-* has compared the ERG of 16 subjects with focal solar mascular injury with five normal subjects and found no differ- ences in the photopic or scotopic components of the ERG. Jacobson et al.36 produced focal macular destruction in monkeys with a white light photo- coagulator and were unable to demonstrate any loss of photopic ERG function. However, McNeer et aJL37 were able to demonstrate a significant de- crease in the amplitude of the b wave in rabbits if a retinal area of approximately kOmnf- was exposed with a number of "subthreshold" coagula- tions. The retinal energy density required to produce detectable changes in the ERG was about 50% of the retinal dose required to produce a visible retinal lesion. Jones e_t a_l.^ have recently demonstrated that a single ruby laser pulse of large retinal subtense (approximately 1 cm2) at a retinal energy density of 0.2 J/cnr produces a significant decrease in the implicit time of the b wave and depresses the third oscillatory potential of the x wave in the Mangabey monkey. These changes parallel the kinds of changes seen in the ERG of the protanope as reported by Rendahl39. The value of the ERG as a diagnostic device to determine if a person has received a damaging retinal dose of intense spectral radiation is somewhat doubtful. If the exposure is to undiffused laser light in the far field, the retinal image diameter of exposure will be a diffraction limited image of 20-40^i. If retinal damage cannot be seen in a careful eye examination, the likelihood of detecting any damage with electro- retinographic studies is extremely remote. 0n the other hand, if a large retinal area is exposed at subthreshold energy densities through multiple exposures or diffused light, significant changes in the waveform of the ERG may be detectable. SUMMARY Although surely incomplete, the above survey (of intense light action on the retina) demonstrates that a great deal of the evidence needed to predict laser functional effects is missing. 71

Little has been done on the effects of narrow-band spectral light and nothing for intensities in the laser region. - - No attempt has been made to include the variables of adapting wave- length and intensity in studies of recovery to a useful acuity threshold. - - No study has been reported which provides a quantitative relationship between flash duration and adapting wavelength for a functional criterion such as threshold or supra-threshold light detection or threshold acuity. - - No work has been done on functional loss in the range of intensities between where photopigment bleaching approaches 100% and where gross burn lesions are observable. This range varies from approximately 1 to 3 log units of energy depending on the adapting and test wavelengths, duration of the flash, and retinal image size. It is, therefore, quite important that data be obtained on these variables, since a two log unit range may well be the difference between feasibility and impossibility with regard to eye protective devices. REFERENCES 1. Severin, S. L., Adler, A. V., Newton, N. L. and Culver, J. F. Photostress and Flashblindness in Aerospace 0perations. School of Aerospace Medicine, Brooks AFB, Texas. Report No. USAF SAM- TDR-63-67, September 1963, 15 p. (AD-600 402). 2. Miller, N. D. Visual Recovery from Brief Exposure to Very High Luminance Levels. Final Report, Part I, on Contract AF 33(657) -9229, May1965, 74 p. (AD-450 072). 3. Metcalf, R. D. and Horn, R. E. Visual Recovery Times from High Intensity Flashes of Light. WADC TR-58232, 0ctober 1958, 10 p. (AD-205-543). 4. Chisum, G. T. and Hill, J. H. Flashblindness Recovery Time Follow- ing Exposure to High Intensity Short Duration Flashes. Aviation Medical Acceleration Laboratory, NADC-MA-6142, November 1961, 13 p. (AD-272 285). 5. Hill, J. H. and Chisum, G. T. Aerospace Medicine, 33., 958-964, (August 1962). 6. Whiteside, T.C.D. and Bazarnik, K. The Dazzle Effect of an Atomic Explosion at Night. Flying Personnel Research Committee, Air Min- istry, Farnborough, England. FPRC 787, 18 p., May 1952. 7. Brown, J. L. J. Human Factors Soc., 6, 503-516, (1964). 8. Czeh, R. S. e_t a_K A Mathematical Model of Flashbl indness. USAF School of Aviation Medicine, 0ctober 1965. 72

9. Graham, C. H. and Margaria, R. Am. J. Physiol., 113. 299, (1935). 10. Karn, H. W. J. Gen. Psychol., J4, 360, (1936). 11. Brindley, G. S. J. Physiol., 118, 135, (1952). 12. Sperling, H. G. and Jolliffe, C. L. J. 0pt. Soc. Amer., 55, 191, (1965). 13. Campbell, F. W. and Rushton, W. A. H. J. Physiol., 130, 989, (1955). 14. Hagins, W. A. Nature, 177. 989, (1956). 15. Dowling, J. E. and Hubbard, R. Nature, 199. 972, (1963). I6. Brindley, G. S. J. Physiol., 147. 194, (1959). 17. Burch, G. Phil. Trans., 1918. 1, (1898). 18. Auerbach, E. and Wald, G. Science, 120, 401 (1954) 19. Brindley, G. S. Physiology of the Retina and Visual Pathways (Edward Arnold, Ltd., London, p. 208, I960). 20. Cornsweet et a_[. J. Opt. Soc. Am., 48, 283, (1958). 21. Weale, R. A. Nature, 201. 661, (1964). 22. Sperling, H. G. Fed. Proc. Suppl. 14, 24, S-73, (1965). 23. SidleyetaM. Science, J50, 1837, (1965) 24. Sperling est aj_. J. Opt. Soc. Am. "Increment-Threshold Spectral Sensi- tivity of the Rhesus Monkey as a Function of the Spectral Composition of the Background". J. 0pt. Soc. Am (1967). 25. Brown, P. K. and Wald, G. Nature, 200:4901, 37-43, (1963). 26. Marks, W. B., Dobelle, W. H., and MacNichol, E. F., Jr. Science, J43:3611, 1181-1183, (1964). 27. Wald, G., Science, _I4£:3636, 1007-1016, (1964). 28. Zaret, M. M. and Grosof, G. M. Visual and Retinal Effects of Exposure to High Intensity Light Sources, AGARD Symposium 16-17 (March 1966). 29. Wolbarsht, M. L., Fligsten, K. E., and Hayes, J. R. Science, 150, 1453-1454, (1965). 30. Rathkey, A. S. AMA Arch. 0phthalmology, Jk, 346-348, (1965). 73

31. Yarczower, M., Wolbarsht, M. L., Galloway, W. D., Fligsten, K. E. and Malcolm, R. Science, 152. 1392, (1966). 32. Asher, H. J. Physiol., _n_2:40P, (1951). 33. Armington, J. C., Tepas, D. I., Kropfl, W. J., and Hengst, W. H. J. 0pt. Soc. Amer., 5J., 877-886, (1961). 34. Brindley, G. S. and Westheimer, G. J. Physiol., 179, 518-537 (1965). 35. Ponte, Francesco, Acta 0phthalmol., Supp. 70, 238-244. 36. Jacobson, J. H., Najac, H. T., Stephens, G., Kara, G. B. and Gesting, G. F. Amer. J. 0phthal., 50, 889/219, (I960). 37. McNeer, K., Ghosh, M., Geeraets, W. J. and Guerry, D. Mi. Acta 0phthal., Supp. 76, 94-100, (1964). 38. Jones, A. E., Bryan, A. H., and Adams, C. K. Laser Induced Changes in the Implicit Time and 0scillatory Potentials of the Mangabey ERG. In press. 39. Rendahl, I. Acta Physiol. Scand., 44. 189-202, (1958). 74