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Spectral Transmittance of Intraocular Lenses and Retinal Damage from Intense Light Sources
SPECTRAL TRANSMITTANCE O F INTRAOCULAR L E N S E S AND RETINAL DAMAGE FROM I N T E N S E L I G H T SOURCES M A R T I N A. M A I N S T E R ,
M.D.
Temple, Texas
The special transmittance of preretinal ocular media determines the amount of radiation that reaches the retina from a light source. In the visible and nearinfrared regions of the spectrum, there is little difference between the spectral transmittances of the individual ocular media; and a substantial amount of the light incident on the cornea reaches the retina. 1,2 In the near-ultraviolet region of the spectrum, however, the spectral trans mittance of the crystalline lens differs markedly from that of other ocular me dia. 1 While the other ocular media remain largely transparent, the lens transmits lit tle incident light and acts as an effective protective filter for the retina. A variety of intense near-ultraviolet light sources are now available; the sensi tivity of the intact eye to these sources has been previously reported. 3 ~ s Because the aphakic eye does not have the protection of the crystalline lens, it is more suscepti ble than the normal eye to thermal retinal damage from the absorption of light in the retinal pigment epithelium. To deter mine the potential sensitivity of the pseudophakic eye to intense near-ultraviolet sources, the spectral transmittance of the intraocular lens must be known. In this study I used measurements of intraocular lens transmittances to analyze the suscep tibility of the pseudophakic eye to retinal damage from intense light sources.
From the Department of Ophthalmology, Scott and White Clinic, Temple, Texas. Reprint requests to Martin A. Mainster, M.D., Department of Ophthalmology, Scott and White Clinic, Temple, TX 76501.
METHODS
Polymethylmethacrylate has been used almost exclusively in the fabrication of intraocular lenses. 6 I used a Beckman DB-G double-beam grating spectrophotometer to measure spectral transmittance from 300 nm to 700 nm for clear polyme thylmethacrylate intraocular lenses of the iris-suture, iris-plane, and anterior cham ber types. Cuvettes were fabricated for mounting the acrylic specimens in a sam ple beam masked by an aperture smaller than the lens to be tested. An identical aperture was placed in the reference beam. Transmittance was recorded as the ratio of the intensities of the sample and reference beams at the photomultiplier of the spectrophotometer. I recorded results of measurements, as well as the spectral transmittances of the cornea and crystal line lens as reported by Boettner and Wolter 1 and the infrared transmittance of clear polymethylmethacrylate as pre sented by the United States of America Standards Institute 7 (Figure). RESULTS
The lowest wavelength at which ultra violet spectral transmittance is significant is approximately 300 nm for cornea (also for aqueous humor and vitreous humor 1 ), 330 nm for polymethylmethacrylate intra ocular lens, and 400 nm for crystalline lens (Figure). Below 300 nm, corneal transmittance for ultraviolet light is negli gible and the retina is shielded. Above 400 nm, polymethylmethacrylate lenses and preretinal ocular media are largely transparent, and a majority of incident radiation reaches the retina. Between 300
AMERICAN JOURNAL OF OPHTHALMOLOGY 85:167-170, 1978
167
AMERICAN JOURNAL OF OPHTHALMOLOGY
168
I i i r | i i i | i I i | i i i |
FEBRUARY 1978
7 i 1 i 1 i 1 i 1 ' 1 ' 1
-
100 c
*
a.
t S S S i i S =
u
o
60
E
o k.
- f\ / j
~ If !2
to
~
0) u a to
1
/
o-
'*3
l/\
W
' —"
Part A
1 i i i 1 i i i 1 i i i 1 i i i 1
300
400
500
600
700
. * _
' !
20 "
-
:'3
■ 1 1 1
40
t—
~a
\,'2
iy^ i \ M
80
03
— -
Part B 1 1 1 1 1 1 1 1 1 1 1 11
700
900
1100
1300
Wavelength, NM Figure (Mainster). A, Near-ultraviolet and visible transmittance measured for clear polymethylmethacrylate intraocular lenses (curve 2). No significant differences are present between the spectral transmittance characteristics of iris-suture, iris plane, and anterior chamber lenses. For comparison, the data of Boettner and Wolter1 are included for the spectral transmittance of cornea (curve 1) and crystalline lens (curve 3). B, near-infrared transmittance for cornea (curve 1) and crystalline lens (curve 3), as well as for clear polymethylmethacrylate (curve 2).
nm and 400 nm, the cornea (as well as the aqueous humor and vitreous humor) is largely transparent and the amount of light reaching the retina depends on len ticular spectral characteristics. In the nor mal eye, the crystalline lens is effectively opaque in this range of wavelengths and the retina is shielded. In the pseudophakic eye, the retina has normal protection only below 330 nm because the polyme thylmethacrylate lens has appreciable transmittance at longer wavelengths. The clear polymethylmethacrylate in traocular lenses tested in this study were fabricated from Plexiglass polymethyl methacrylate (Rohm and Haas Corpora tion.) 8 Perspex CQ (Imperial Chemical Industries) is a second polymethylmetha crylate commonly used in intraocular lens construction, but has appreciable trans mittance at wavelengths as low as 280 nm
(at 300 nm the transmittance is still ap proximately 80%). 9 Thus, the ultraviolet radiation protection of the pseudophakic eye with a Perspex CQ implant is essen tially the same as that of an aphakic eye, with the cornea providing the lower wavelength limit for ultraviolet absorp tion. DISCUSSION
In the range of light exposures that are neither long enough to produce photic retinopathy, 10,11 nor short and intense enough to produce acoustic transients and shock waves in the retina, 12 retinal damage from intense light sources such as continuous wave and pulsed lasers (nonq-switched) is caused primarily by light absorption in the retinal pigment epithe lium and subsequent temperature in creases in adjacent tissues. 13 ' 14 Since
VOL. 85, NO. 2
INTRAOCULAR LENS TRANSMITTANCE
absorption coefficients for the pigment epithelium are greater in the near-ultr aviolet than they are in the visible, 2,13,15 thermal damage will occur if sufficient near-ultraviolet light reaches the retina. The occurrence of this damage, even in eyes with intact crystalline lenses, has been demonstrated in experiments with the krypton-ion and argon-ion ultraviolet lasers. 3 The data in the figure permit the fol lowing quantitative conclusions to be drawn regarding the sensitivity of the pseudophakic eye to intense ultraviolet radiation: 1. Because the amount of nearultraviolet light reaching the retina de pends on the spectral transmittance of the lens, retinal temperature increases are also proportional to lens transmittance and the relationship between thermal ret inal damage in the normal eye and in the pseudophakic eye is readily established. For example, at 350 nm (krypton-ion la ser) the ratio of polymethylmethacrylate lens to crystalline lens transmittance is approximately 90:3 (Figure). Thus, at this wavelength, only 3 % of the source radi ance required to produce a threshold reti nal burn in a normal eye should be re quired to produce an equivalent lesion in a pseudophakic eye. 2. Analysis of the data (Figure) reveals no appreciable difference in the visible and near-infrared transmittances of poly methylmethacrylate and crystalline lens es. Thus, the thresholds for thermal reti nal damage at a given wavelength should be similar for the pseudophakic and the intact eye in these spectral regions. The recent clinical findings of Poole and Galin 16 support this conclusion. 3. Retinal temperature rise is propor tional to thermal source strength, which in turn is proportional to the product of the pigment epithelial absorption coeffi cient ( a j and the preretinal ocular trans mittance (Te).2'13 Ocular transmittance
169
for the pseudophakic eye is proportional to ocular transmittance for the intact eye (Te) multiplied by the ratio of the trans mittance of the polymethylmethacrylate lens (Tp) to the transmittance of the crys talline lens (Tc). Thus, for exposures in the millisecond range or shorter, the ratio of retinal temperature rise (v) in the pseu dophakic eye at 500 nm to temperature rise in the pseudophakic eye at 350 nm is v (at 500 nm) A (at 500 nm) v (at 350 nm) = A (at 350 nm) ' where ,4 ° c U l • Te*(TpITc), where Tp and Tc are given (Figure) (at 500 nm, T„ and Tc are 0.92 and 0.94, respec tively; at 350 nm, Tp and Tc are 0.84 and 0.02, respectively), and where Te and a, are given 2 (at 500 nm, Te and a, are 0.45 and 900, respectively; at 350 nm, Te and <*! are 0.015 and 7000, respectively). Thus, only 9% of the source radiance required to produce a threshold retinal burn in a pseudophakic eye with an argon laser (at approximately 500 nm) should be required to produce an equivalent ther mal lesion in a pseudophakic eye with a krypton-ion laser (at approximately 350 nm). Both the increased sensitivity of the pseudophakic eye to retinal damage from intense near-ultraviolet light sources and the chromatic aberration introduced by clear polymethylmethacrylate transmit tance of natural background nearultraviolet radiation can be eliminated by fabricating intraocular lenses from acryl ics with ultraviolet absorbing characteris tics more like those of crystalline lens. 7 - 9 While ultraviolet absorbing tints have been useful in spectacle lenses prescribed for aphakic eyes, 17 visual results with current clear polymethylmethacrylate in traocular lenses are excellent. 6 In view of the limited usage of potentially hazardous ultraviolet sources, testing a new class of acrylics is not justified by this unlikely hazard. Nonetheless, current clear poly-
170
AMERICAN JOURNAL OF OPHTHALMOLOGY
methylmethacrylate intraocular lenses do not provide the same level of protection against near-ultraviolet light as does the original crystalline lens. This must be considered in formulating permissible ex posure levels for such radiation, and in counseling pseudophakic patients about possible occupational or environmental hazards. SUMMARY
I measured the spectral transmittance of clear polymethylmethacrylate intraoc ular lenses in the 300- to 700-nm range. The near-ultraviolet transmittance of the polymethylmethacrylate lens was signifi cantly greater than that of the crystalline lens. Therefore, the pseudophakic eye is more susceptible to retinal damage from intense near-ultraviolet light sources than the normal eye. Retinal thermal response of the pseudophakic eye was compared with that of the normal eye for nearultraviolet radiation, and retinal thermal response to near-ultraviolet radiation was compared with that to visible light for the pseudophakic eye. Additionally, because there was no significant difference be tween polymethylmethacrylate and crys talline lens in visible and near-infrared transmittance, thresholds for thermal reti nal damage at a given wavelength are similar for the pseudophakic and the in tact eye in these spectral regions. REFERENCES
1. Boettner, E. A., and Wolter, J. R.: Transmission of the ocular media. Invest. Ophthalmol. 1:776, 1962.
FEBRUARY 1978
2. White, T. J., Mainster, M. A., Wilson, P. W., and Tips, J. H.: Chorioretinal temperature increases from solar observation. Bull. Math. Biophys. 33:1, 1971. 3. Zuclich, J. A., and Connolly, J. S.: Ocular damage induced by near-ultraviolet radiation. In vest. Ophthalmol. 15:760, 1976. 4. Ham, W. T., Mueller, H. A., and Sliney, D. H.: Retinal sensitivity to damage from short wavelength light. Nature 260:153, 1976. 5. Ebbers, R. W., and Sears, D.: Ocular effects of a 325 nm laser. Am. J. Optom. Physiol. Optics 52:216, 1975. 6. Jaffe, N. S.: Cataract Surgery and its Complica tions, 2nd ed. St. Louis, C. V. Mosby, 1976, pp. 99-113. 7. United States of America Standards Institute: The Spectral-Transmissive Properties of Plastics for Use in Eye Protection. New York, 1955. 8. Rohm and Haas Company: Plexiglass design and fabrication data. Report PL-612c. Philadelphia, 1974. 9. Imperial Chemical Industries, Plastics Divi sion: Perspex CQ. Technical Data Sheet PX. TD 232, 2nd ed. Welwyn Garden, England, 1970. 10. Naidoff, M. A., and Sliney, D. H.: Retinal injury from a welding arc. Am. J. Ophthalmol. 77:663, 1974. 11. Gibbons, W. D., and Allen, R. G.: Retinal damage from long-term exposure to laser radiation. Invest. Ophthalmol. Visual Sci. 16:521, 1977. 12. Goldman, A. I., Ham, W. T., and Mueller, H. A.: Ocular damage thresholds and mechanisms for ultrashort pulses of both visible and infrared laser radiation in the rhesus monkey. Exp. Eye Res. 24:45, 1977. 13. Mainster, M. A., White, T. J., and Wilson, P. W.: Retinal temperature increases produced by intense light sources. J. Opt. Soc. Am. 60:264, 1970. 14. Priebe, L. A., Cain, C. P., and Welch, A. J.: Temperature rise required for production of mini mal lesions in the macaca mulatta retina. Am. J. Ophthalmol. 79:405, 1975. 15. Geeraets, W. J., and Berry, E. R.: Ocular spectral characteristics as related to hazards from lasers and other light sources. Am. J. Ophthalmol. 66:15, 1968. 16. Poole, T. A., and Galin, M. A.: Argon laser photocoagulation of the posterior segment in pseudophakia. Am. J. Ophthalmol. 83:185, 1977. 17. Davis, J. K.: Spectacle lenses. In Duane, T. D. (ed.): Clinical Ophthalmology, vol. 1. New York, Harper and Row, 1976, p. 33.
M.D.
Temple, Texas
The special transmittance of preretinal ocular media determines the amount of radiation that reaches the retina from a light source. In the visible and nearinfrared regions of the spectrum, there is little difference between the spectral transmittances of the individual ocular media; and a substantial amount of the light incident on the cornea reaches the retina. 1,2 In the near-ultraviolet region of the spectrum, however, the spectral trans mittance of the crystalline lens differs markedly from that of other ocular me dia. 1 While the other ocular media remain largely transparent, the lens transmits lit tle incident light and acts as an effective protective filter for the retina. A variety of intense near-ultraviolet light sources are now available; the sensi tivity of the intact eye to these sources has been previously reported. 3 ~ s Because the aphakic eye does not have the protection of the crystalline lens, it is more suscepti ble than the normal eye to thermal retinal damage from the absorption of light in the retinal pigment epithelium. To deter mine the potential sensitivity of the pseudophakic eye to intense near-ultraviolet sources, the spectral transmittance of the intraocular lens must be known. In this study I used measurements of intraocular lens transmittances to analyze the suscep tibility of the pseudophakic eye to retinal damage from intense light sources.
From the Department of Ophthalmology, Scott and White Clinic, Temple, Texas. Reprint requests to Martin A. Mainster, M.D., Department of Ophthalmology, Scott and White Clinic, Temple, TX 76501.
METHODS
Polymethylmethacrylate has been used almost exclusively in the fabrication of intraocular lenses. 6 I used a Beckman DB-G double-beam grating spectrophotometer to measure spectral transmittance from 300 nm to 700 nm for clear polyme thylmethacrylate intraocular lenses of the iris-suture, iris-plane, and anterior cham ber types. Cuvettes were fabricated for mounting the acrylic specimens in a sam ple beam masked by an aperture smaller than the lens to be tested. An identical aperture was placed in the reference beam. Transmittance was recorded as the ratio of the intensities of the sample and reference beams at the photomultiplier of the spectrophotometer. I recorded results of measurements, as well as the spectral transmittances of the cornea and crystal line lens as reported by Boettner and Wolter 1 and the infrared transmittance of clear polymethylmethacrylate as pre sented by the United States of America Standards Institute 7 (Figure). RESULTS
The lowest wavelength at which ultra violet spectral transmittance is significant is approximately 300 nm for cornea (also for aqueous humor and vitreous humor 1 ), 330 nm for polymethylmethacrylate intra ocular lens, and 400 nm for crystalline lens (Figure). Below 300 nm, corneal transmittance for ultraviolet light is negli gible and the retina is shielded. Above 400 nm, polymethylmethacrylate lenses and preretinal ocular media are largely transparent, and a majority of incident radiation reaches the retina. Between 300
AMERICAN JOURNAL OF OPHTHALMOLOGY 85:167-170, 1978
167
AMERICAN JOURNAL OF OPHTHALMOLOGY
168
I i i r | i i i | i I i | i i i |
FEBRUARY 1978
7 i 1 i 1 i 1 i 1 ' 1 ' 1
-
100 c
*
a.
t S S S i i S =
u
o
60
E
o k.
- f\ / j
~ If !2
to
~
0) u a to
1
/
o-
'*3
l/\
W
' —"
Part A
1 i i i 1 i i i 1 i i i 1 i i i 1
300
400
500
600
700
. * _
' !
20 "
-
:'3
■ 1 1 1
40
t—
~a
\,'2
iy^ i \ M
80
03
— -
Part B 1 1 1 1 1 1 1 1 1 1 1 11
700
900
1100
1300
Wavelength, NM Figure (Mainster). A, Near-ultraviolet and visible transmittance measured for clear polymethylmethacrylate intraocular lenses (curve 2). No significant differences are present between the spectral transmittance characteristics of iris-suture, iris plane, and anterior chamber lenses. For comparison, the data of Boettner and Wolter1 are included for the spectral transmittance of cornea (curve 1) and crystalline lens (curve 3). B, near-infrared transmittance for cornea (curve 1) and crystalline lens (curve 3), as well as for clear polymethylmethacrylate (curve 2).
nm and 400 nm, the cornea (as well as the aqueous humor and vitreous humor) is largely transparent and the amount of light reaching the retina depends on len ticular spectral characteristics. In the nor mal eye, the crystalline lens is effectively opaque in this range of wavelengths and the retina is shielded. In the pseudophakic eye, the retina has normal protection only below 330 nm because the polyme thylmethacrylate lens has appreciable transmittance at longer wavelengths. The clear polymethylmethacrylate in traocular lenses tested in this study were fabricated from Plexiglass polymethyl methacrylate (Rohm and Haas Corpora tion.) 8 Perspex CQ (Imperial Chemical Industries) is a second polymethylmetha crylate commonly used in intraocular lens construction, but has appreciable trans mittance at wavelengths as low as 280 nm
(at 300 nm the transmittance is still ap proximately 80%). 9 Thus, the ultraviolet radiation protection of the pseudophakic eye with a Perspex CQ implant is essen tially the same as that of an aphakic eye, with the cornea providing the lower wavelength limit for ultraviolet absorp tion. DISCUSSION
In the range of light exposures that are neither long enough to produce photic retinopathy, 10,11 nor short and intense enough to produce acoustic transients and shock waves in the retina, 12 retinal damage from intense light sources such as continuous wave and pulsed lasers (nonq-switched) is caused primarily by light absorption in the retinal pigment epithe lium and subsequent temperature in creases in adjacent tissues. 13 ' 14 Since
VOL. 85, NO. 2
INTRAOCULAR LENS TRANSMITTANCE
absorption coefficients for the pigment epithelium are greater in the near-ultr aviolet than they are in the visible, 2,13,15 thermal damage will occur if sufficient near-ultraviolet light reaches the retina. The occurrence of this damage, even in eyes with intact crystalline lenses, has been demonstrated in experiments with the krypton-ion and argon-ion ultraviolet lasers. 3 The data in the figure permit the fol lowing quantitative conclusions to be drawn regarding the sensitivity of the pseudophakic eye to intense ultraviolet radiation: 1. Because the amount of nearultraviolet light reaching the retina de pends on the spectral transmittance of the lens, retinal temperature increases are also proportional to lens transmittance and the relationship between thermal ret inal damage in the normal eye and in the pseudophakic eye is readily established. For example, at 350 nm (krypton-ion la ser) the ratio of polymethylmethacrylate lens to crystalline lens transmittance is approximately 90:3 (Figure). Thus, at this wavelength, only 3 % of the source radi ance required to produce a threshold reti nal burn in a normal eye should be re quired to produce an equivalent lesion in a pseudophakic eye. 2. Analysis of the data (Figure) reveals no appreciable difference in the visible and near-infrared transmittances of poly methylmethacrylate and crystalline lens es. Thus, the thresholds for thermal reti nal damage at a given wavelength should be similar for the pseudophakic and the intact eye in these spectral regions. The recent clinical findings of Poole and Galin 16 support this conclusion. 3. Retinal temperature rise is propor tional to thermal source strength, which in turn is proportional to the product of the pigment epithelial absorption coeffi cient ( a j and the preretinal ocular trans mittance (Te).2'13 Ocular transmittance
169
for the pseudophakic eye is proportional to ocular transmittance for the intact eye (Te) multiplied by the ratio of the trans mittance of the polymethylmethacrylate lens (Tp) to the transmittance of the crys talline lens (Tc). Thus, for exposures in the millisecond range or shorter, the ratio of retinal temperature rise (v) in the pseu dophakic eye at 500 nm to temperature rise in the pseudophakic eye at 350 nm is v (at 500 nm) A (at 500 nm) v (at 350 nm) = A (at 350 nm) ' where ,4 ° c U l • Te*(TpITc), where Tp and Tc are given (Figure) (at 500 nm, T„ and Tc are 0.92 and 0.94, respec tively; at 350 nm, Tp and Tc are 0.84 and 0.02, respectively), and where Te and a, are given 2 (at 500 nm, Te and a, are 0.45 and 900, respectively; at 350 nm, Te and <*! are 0.015 and 7000, respectively). Thus, only 9% of the source radiance required to produce a threshold retinal burn in a pseudophakic eye with an argon laser (at approximately 500 nm) should be required to produce an equivalent ther mal lesion in a pseudophakic eye with a krypton-ion laser (at approximately 350 nm). Both the increased sensitivity of the pseudophakic eye to retinal damage from intense near-ultraviolet light sources and the chromatic aberration introduced by clear polymethylmethacrylate transmit tance of natural background nearultraviolet radiation can be eliminated by fabricating intraocular lenses from acryl ics with ultraviolet absorbing characteris tics more like those of crystalline lens. 7 - 9 While ultraviolet absorbing tints have been useful in spectacle lenses prescribed for aphakic eyes, 17 visual results with current clear polymethylmethacrylate in traocular lenses are excellent. 6 In view of the limited usage of potentially hazardous ultraviolet sources, testing a new class of acrylics is not justified by this unlikely hazard. Nonetheless, current clear poly-
170
AMERICAN JOURNAL OF OPHTHALMOLOGY
methylmethacrylate intraocular lenses do not provide the same level of protection against near-ultraviolet light as does the original crystalline lens. This must be considered in formulating permissible ex posure levels for such radiation, and in counseling pseudophakic patients about possible occupational or environmental hazards. SUMMARY
I measured the spectral transmittance of clear polymethylmethacrylate intraoc ular lenses in the 300- to 700-nm range. The near-ultraviolet transmittance of the polymethylmethacrylate lens was signifi cantly greater than that of the crystalline lens. Therefore, the pseudophakic eye is more susceptible to retinal damage from intense near-ultraviolet light sources than the normal eye. Retinal thermal response of the pseudophakic eye was compared with that of the normal eye for nearultraviolet radiation, and retinal thermal response to near-ultraviolet radiation was compared with that to visible light for the pseudophakic eye. Additionally, because there was no significant difference be tween polymethylmethacrylate and crys talline lens in visible and near-infrared transmittance, thresholds for thermal reti nal damage at a given wavelength are similar for the pseudophakic and the in tact eye in these spectral regions. REFERENCES
1. Boettner, E. A., and Wolter, J. R.: Transmission of the ocular media. Invest. Ophthalmol. 1:776, 1962.
FEBRUARY 1978
2. White, T. J., Mainster, M. A., Wilson, P. W., and Tips, J. H.: Chorioretinal temperature increases from solar observation. Bull. Math. Biophys. 33:1, 1971. 3. Zuclich, J. A., and Connolly, J. S.: Ocular damage induced by near-ultraviolet radiation. In vest. Ophthalmol. 15:760, 1976. 4. Ham, W. T., Mueller, H. A., and Sliney, D. H.: Retinal sensitivity to damage from short wavelength light. Nature 260:153, 1976. 5. Ebbers, R. W., and Sears, D.: Ocular effects of a 325 nm laser. Am. J. Optom. Physiol. Optics 52:216, 1975. 6. Jaffe, N. S.: Cataract Surgery and its Complica tions, 2nd ed. St. Louis, C. V. Mosby, 1976, pp. 99-113. 7. United States of America Standards Institute: The Spectral-Transmissive Properties of Plastics for Use in Eye Protection. New York, 1955. 8. Rohm and Haas Company: Plexiglass design and fabrication data. Report PL-612c. Philadelphia, 1974. 9. Imperial Chemical Industries, Plastics Divi sion: Perspex CQ. Technical Data Sheet PX. TD 232, 2nd ed. Welwyn Garden, England, 1970. 10. Naidoff, M. A., and Sliney, D. H.: Retinal injury from a welding arc. Am. J. Ophthalmol. 77:663, 1974. 11. Gibbons, W. D., and Allen, R. G.: Retinal damage from long-term exposure to laser radiation. Invest. Ophthalmol. Visual Sci. 16:521, 1977. 12. Goldman, A. I., Ham, W. T., and Mueller, H. A.: Ocular damage thresholds and mechanisms for ultrashort pulses of both visible and infrared laser radiation in the rhesus monkey. Exp. Eye Res. 24:45, 1977. 13. Mainster, M. A., White, T. J., and Wilson, P. W.: Retinal temperature increases produced by intense light sources. J. Opt. Soc. Am. 60:264, 1970. 14. Priebe, L. A., Cain, C. P., and Welch, A. J.: Temperature rise required for production of mini mal lesions in the macaca mulatta retina. Am. J. Ophthalmol. 79:405, 1975. 15. Geeraets, W. J., and Berry, E. R.: Ocular spectral characteristics as related to hazards from lasers and other light sources. Am. J. Ophthalmol. 66:15, 1968. 16. Poole, T. A., and Galin, M. A.: Argon laser photocoagulation of the posterior segment in pseudophakia. Am. J. Ophthalmol. 83:185, 1977. 17. Davis, J. K.: Spectacle lenses. In Duane, T. D. (ed.): Clinical Ophthalmology, vol. 1. New York, Harper and Row, 1976, p. 33.