- Email: [email protected]
Reply: Blue-blocking intraocular lenses and pseudophakic scotopic sensitivity
LETTERS
Schwiegerling concludes the results section of his paper by noting that ‘‘[f]or the AcrySof Natural IOL, there is nearly a 52% increase in scotopic sensitivity relative to a young phakic individual.’’1 A reasonable reader would expect older AcrySof Natural pseudophakes to have substantially better scotopic sensitivity than when they were young. Is that believable considering that patients still have visual problems in dim environments after cataract surgery with IOLs that do not block any of the blue light needed for scotopic and mesopic vision? This exaggerated ‘‘52% increase in scotopic sensitivity’’ is listed for electronic circulation in his abstract and readers must quote it to obtain their continuing medical education (CME) credit from ASCRS for the January 2006 issue of this journal. Schwiegerling presumably used his artificial, blue-shifted scotopic sensitivity curve in Figure 1 to make his ‘‘52%’’1 calculation. However, if calculations are performed relative to Boettner and Wolter’s 4.5-year-old crystalline lens12 using Griswold and Stark’s4 original data,7 theoretical scotopic sensitivity improvements are roughly 10% and 3% for 20 D and 30 D AcrySof Natural IOLs, respectively.7 Even these numbers are far too high, however, because they fail to consider significant age-related decreases in rod photoreceptor population13 and pupil diameter14 that contribute to the substantial reduction in the scotopic sensitivity of older eyes relative to younger ones.2 Despite Schwiegerling’s admonition to readers about the hazards of using ‘‘incongruous data,’’1 he fails to consider the fact that if the scotopic performance of typical older pseudophakes is to be compared to young phakic individuals, reduced pupil diameter alone decreases the retinal illumination of older adults by 50%.14 In summary, Schwiegerling acknowledges that blue-blocking IOLs reduce scotopic sensitivity, but he uses a blue-shifted aphakic scotopic sensitivity curve that is not valid for a human eye. The basis for his paper is that the 10.9% difference between his current and our 2003 results is ‘‘enormous.’’ That assertion is untenable considering the 4 log unit range of human scotopic vision.8 Schwiegerling freely and repeatedly uses terms such as ‘‘erroneous,’’ ‘‘problem,’’ ‘‘incongruous,’’ ‘‘misconception,’’ ‘‘overstated,’’ ‘‘mismatched,’’ and ‘‘discrepancy’’ to criticize our manuscript,1 suggesting that our BJO article2 is inaccurate and by inference that we are careless and/or incompetent. Readers are encouraged to review our BJO article2; compare its scientific content, tone, and balance with Schwiegerling’s report1; and draw their own conclusions about the validity of Schwiegerling’s criticisms and its selection as required reading for CME credit for the January 2006 issue of JCRS. MARTIN A. MAINSTER, PHD, MD, FRCOPHTH Kansas City, Kansas, USA
REFERENCES 1. Schwiegerling J. Blue-light-absorbing lenses and their effect on scotopic vision. J Cataract Refract Surg 2006; 32:141–144 2. Mainster MA, Sparrow JR. How much blue light should an IOL transmit? [perspective]. Br J Ophthalmol 2003; 87:1523–1529 3. Wyszecki G, Stiles WS. Color Science; Concepts and Methods, Quantitative Data and Formulae,. 2nd ed. New York, NY, John Wiley & Sons, 1982 4. Griswold MS, Stark WS. Scotopic spectral sensitivity of phakic and aphakic observers extending into the near ultraviolet. Vision Res 1992; 32:1739–1743
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5. Brown PK, Wald G. Visual pigments in single rods and cones of the human retina; direct measurements reveal mechanisms of human night and color vision. Science 1964; 144:45–52 6. Wald G. Human vision and the spectrum. Science 1945; 101:653–658 7. Mainster MA. Violet and blue light blocking intraocular lenses: photoprotection versus photoreception [perspective]. Br J Ophthalmol 2006; 90:784–792 8. Werner JS. Night vision in the elderly: consequences for seeing through a ‘‘blue filtering’’ intraocular lens. Br J Ophthalmol 2005; 89:1518–1521 9. Davison JA, Patel AS. Light normalizing intraocular lenses. Int Ophthalmol Clin 2005; 45(1):55–106 10. Marshall J, Cionni RJ, Davison J, et al. Clinical results of the blue-light filtering AcrySof Natural foldable acrylic intraocular lens. J Cataract Refract Surg 2005; 31:2319–2323 11. Berson EL. Light deprivation and retinitis pigmentosa. Vision Res 1980; 20:1179–1184 12. Boettner EA, Wolter JR. Transmission of the ocular media. Invest Ophthalmol 1962; 1:776–783 13. Curcio CA, Millican CL, Allen KA, Kalina RE. Aging of the human photoreceptor mosaic: evidence for selective vulnerability of rods in central retina. Invest Ophthalmol Vis Sci 1993; 34:3278–3296 14. Charman WN. Age, lens transmittance, and the possible effects of light on melatonin suppression. Ophthalmic Physiol Opt 2003; 23:181–187
Reply:
In my recent JCRS article,1 I sought to fix 2 problems associated with an analysis of blue-light filtering IOLs performed by Mainster and Sparrow in their 2003 BJO paper.2 The criticisms in my paper are fair and in no way suggest that the researchers are careless or incompetent. However, the nature of science and progress is to examine the current state of knowledge and reassess the current dogma when new and better techniques and information become available. The readership and their patients certainly deserve this type of reflection to provide informed care. Furthermore, I did not fail to cite Mainster’s presentations at the CSCRS, ASCRS, and ESCRS meetings since my manuscript was submitted before these events took place, as is clearly shown by the acceptance date. In my article, I fixed 2 problems in the Mainster and Sparrow paper and reanalyzed the effect of the lenses on scotopic vision. With these corrections, the conclusions in the Mainster and Sparrow paper were completely reversed. Instead of a marked loss in scotopic performance, as suggested in the paper, a potential increase in photons available in the scotopic regime was found. I consider a gain versus a loss an enormous difference. The 2 problems in the original paper were that the incorrect transmission data were used for the AcrySof Natural material and the incorrect scotopic response curve was used for assessing pseudophakic patients. The AcrySof Natural IOL transmission data were measured with the IOL in air. There is roughly a 9% loss in transmission across the visible spectrum due to surface reflections when IOLs are measured in air. When IOLs are immersed in saline or aqueous humor, these surface reflections are reduced to roughly 1%. The 25% loss in scotopic vision that was a conclusion in the Mainster and Sparrow paper decreases to 14% when the proper IOL transmission characteristics are used. This change is relatively small when considering the dynamic range of scotopic vision, as pointed out in Mainster’s letter. However, Mainster
J CATARACT REFRACT SURG - VOL 32, SEPTEMBER 2006
LETTERS
suggests that this is the end of the story and downplays the role of the second correction I made. The major changes in scotopic performance occur when using the correct scotopic response curve. In the Mainster and Sparrow paper, the CIE 1951 standard scotopic response curve was used to determine the effects of blue-light filtering IOLs on scotopic vision. This is the incorrect function to use in this analysis since the crystalline lens is removed in cataract surgery. The scotopic response of the eye is controlled primarily by 2 factors: the spectral response of rhodopsin, the photosensitive material contained in rods, and the transmission of the ocular media. The crystalline lens, even in a young eye, absorbs a high percentage of the short wavelengths. When this lens is removed, there is an enormous increase in the amount of light entering the eye. Consequently, to properly assess scotopic vision in a pseudophakic patient, an aphakic scotopic response curve is needed to account for the removal of the crystalline lens. Unfortunately, neither the CIE nor any other organization has defined a standard aphakic scotopic response curve. Such a curve is unlikely to be defined due to the relative rarity of aphakes. To define the required response curve, the literature must be examined to extract this information. In reviewing the literature, I found only one suitable source for defining the aphakic scotopic response curve. In their 1992 paper, Griswold and Stark3 measured absolute scotopic response in 5 phakic and 3 aphakic individuals. Due to the scarcity of subjects, these measurements are noisy, but they are all that is available to define the aphakic response curve. In Mainster’s letter, he adamantly stands by the choice of using the CIE phakic scotopic response curve because it ‘‘is a recognized standard.’’ However, improper use of a standard does not make the methodology correct. Furthermore, Mainster criticizes my use and analysis of the Griswold and Stark data. Yet, in his letter, Mainster fails to mention that he uses the same Griswold and Stark data in his recently published article on violet- and blue-blocking IOLs.3 The only difference between our use of these data is that I chose to fit the noisy data by the technique described below and Mainster chose to use the noisy data. Since Griswold and Stark measured only 3 aphakic eyes, their data are noisy. I am confident this variation is noise since the same magnitude variation is seen in the phakic eye measurements, which should match the smooth CIE 1951 scotopic response curve. I digitized the Griswold and Stark data and performed a least-squares fit to a 2nd-order polynomial (R2 Z 98.6% for the aphakic subjects and R2 Z 96.8% for the phakic subjects). To verify the technique, the fit to the phakic data was compared with the CIE 1951 scotopic response curve. These curves match remarkably closely, both with a peak wavelength of lmax of 505 nm. The peak wavelength for the aphakic curve is lmax Z 490 nm. Mainster criticizes my analysis because this peak wavelength is shifted from the peak rod response of 505 nm found by Wald4 and Brown and Wald.5 However, more recent measurements of the rod sensitivity find peaks shifted more toward the blue. Dartnall et al.6 found a peak sensitivity of 496.3 G 2.3 nm, while Kraft et al.7 found a peak around 495 nm. This shift is expected since the peak of the phakic scotopic response curve (as measured through the crystalline lens) is 505 nm. When the lens, which absorbs at the short wavelengths, is removed or bypassed, the peak response of rods must shift toward the blue. While the fit I performed to the Griswold and Stark data is slightly more toward the blue than expected, it is certainly more representative of what is occurring in the pseudophakic eye than the CIE phakic scotopic luminous efficiency curve. It is also
certainly more representative of the noisy data Mainster uses in his current analysis.3 I indeed used this fitted aphakic scotopic sensitivity curve to determine the effects of pseudophakia on scotopic vision. It is unclear how Mainster made his calculations, but they appear to have been made ‘‘relative to Boettner and Wolter’s 4.5-year-old crystalline lens.’’ This makes no sense in this case since there is no crystalline lens in pseudophakia. If the areas under the Griswold and Stark’s aphakic and phakic scotopic sensitivity curves are integrated, there is twice as much light entering the aphakic eye than a 20- to 30-year-old phakic eye. Placing a 20 D AcrySof Natural IOL into these eyes reduces this gain but still leads to a 52% gain in the amount of light entering the eye. The Griswold and Stark aphakic measurements were performed in two 43-year-old eyes and one 60-year-old eye. Although dark-adapted pupil sizes are not given, the mean values of dark-adapted pupil diameters in a European population of these ages are 5.9 to 6.5 mm.8 By age 70, this mean pupil size decreases to 5.4 mm. This corresponds to a reduction in pupil area of 16% to 31%. A gain in the amount of light entering the eye is therefore still expected at age 70 and can be projected for older ages. Consequently, the limiting factor on scotopic performance is photoreceptor density and health, not the IOL. Mainster does not seem to accept that there is an enormous increase in the amount of light entering the eye when the crystalline lens is removed since he insists on using a phakic scotopic sensitivity curve or the transmission of a 4.5-year-old crystalline lens in making calculations in subjects without a crystalline lens. More light in the scotopic regime enters the eye in our pseudophakic patients than enters the 20- to 30-year-old eye. The early criticism of the AcrySof Natural IOL, such as loss of scotopic performance and shifts in color vision, have evaporated. These IOLs have been successfully implanted worldwide, providing recipients with high-quality vision. The potential benefits of these IOLs to retinal health are far more difficult to demonstrate due to many confounding factors such as diet, smoking, exposure and genetics, and further work is necessary to determine the retinal benefits of these IOLs.dJim Schwiegerling, PhD
REFERENCES 1. Schwiegerling J. Blue-light-absorbing lenses and their effect on scotopic vision. J Cataract Refract Surg 2006; 32:141–144 2. Mainster MA, Sparrow JR. How much blue light should an IOL transmit? [perspective]. Br J Ophthalmol 2003; 87:1523–1529 3. Mainster MA. Violet and blue light blocking intraocular lenses: photoprotection versus photoreception [perspective]. Br J Ophthalmol 2006; 90:784–792 4. Wald G. Human vision and the spectrum. Science 1945; 101:653–658 5. Brown PK, Wald G. Visual pigments in single rods and cones of the human retina. Direct measurements reveal mechanisms of human night and color vision. Science 1964; 144:45–52 6. Dartnall H JA, Bowmaker JK, Mollon JD. Human visual pigments: microspectrophotometric results from the eyes of seven persons. Proc R Soc Lond B Biol Sci 1983; 220:115–130 7. Kraft TW, Schneeweis DM, Schnapf JL. Visual transduction in human rod photoreceptors. J Physiol 1993; 464:747–765 8. Said FS, Sawires WS. Age dependence of changes in pupil diameter in the dark. Optica Acta 1972; 19:359–361 See editors’ note about these letters on following page.
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Schwiegerling concludes the results section of his paper by noting that ‘‘[f]or the AcrySof Natural IOL, there is nearly a 52% increase in scotopic sensitivity relative to a young phakic individual.’’1 A reasonable reader would expect older AcrySof Natural pseudophakes to have substantially better scotopic sensitivity than when they were young. Is that believable considering that patients still have visual problems in dim environments after cataract surgery with IOLs that do not block any of the blue light needed for scotopic and mesopic vision? This exaggerated ‘‘52% increase in scotopic sensitivity’’ is listed for electronic circulation in his abstract and readers must quote it to obtain their continuing medical education (CME) credit from ASCRS for the January 2006 issue of this journal. Schwiegerling presumably used his artificial, blue-shifted scotopic sensitivity curve in Figure 1 to make his ‘‘52%’’1 calculation. However, if calculations are performed relative to Boettner and Wolter’s 4.5-year-old crystalline lens12 using Griswold and Stark’s4 original data,7 theoretical scotopic sensitivity improvements are roughly 10% and 3% for 20 D and 30 D AcrySof Natural IOLs, respectively.7 Even these numbers are far too high, however, because they fail to consider significant age-related decreases in rod photoreceptor population13 and pupil diameter14 that contribute to the substantial reduction in the scotopic sensitivity of older eyes relative to younger ones.2 Despite Schwiegerling’s admonition to readers about the hazards of using ‘‘incongruous data,’’1 he fails to consider the fact that if the scotopic performance of typical older pseudophakes is to be compared to young phakic individuals, reduced pupil diameter alone decreases the retinal illumination of older adults by 50%.14 In summary, Schwiegerling acknowledges that blue-blocking IOLs reduce scotopic sensitivity, but he uses a blue-shifted aphakic scotopic sensitivity curve that is not valid for a human eye. The basis for his paper is that the 10.9% difference between his current and our 2003 results is ‘‘enormous.’’ That assertion is untenable considering the 4 log unit range of human scotopic vision.8 Schwiegerling freely and repeatedly uses terms such as ‘‘erroneous,’’ ‘‘problem,’’ ‘‘incongruous,’’ ‘‘misconception,’’ ‘‘overstated,’’ ‘‘mismatched,’’ and ‘‘discrepancy’’ to criticize our manuscript,1 suggesting that our BJO article2 is inaccurate and by inference that we are careless and/or incompetent. Readers are encouraged to review our BJO article2; compare its scientific content, tone, and balance with Schwiegerling’s report1; and draw their own conclusions about the validity of Schwiegerling’s criticisms and its selection as required reading for CME credit for the January 2006 issue of JCRS. MARTIN A. MAINSTER, PHD, MD, FRCOPHTH Kansas City, Kansas, USA
REFERENCES 1. Schwiegerling J. Blue-light-absorbing lenses and their effect on scotopic vision. J Cataract Refract Surg 2006; 32:141–144 2. Mainster MA, Sparrow JR. How much blue light should an IOL transmit? [perspective]. Br J Ophthalmol 2003; 87:1523–1529 3. Wyszecki G, Stiles WS. Color Science; Concepts and Methods, Quantitative Data and Formulae,. 2nd ed. New York, NY, John Wiley & Sons, 1982 4. Griswold MS, Stark WS. Scotopic spectral sensitivity of phakic and aphakic observers extending into the near ultraviolet. Vision Res 1992; 32:1739–1743
1404
5. Brown PK, Wald G. Visual pigments in single rods and cones of the human retina; direct measurements reveal mechanisms of human night and color vision. Science 1964; 144:45–52 6. Wald G. Human vision and the spectrum. Science 1945; 101:653–658 7. Mainster MA. Violet and blue light blocking intraocular lenses: photoprotection versus photoreception [perspective]. Br J Ophthalmol 2006; 90:784–792 8. Werner JS. Night vision in the elderly: consequences for seeing through a ‘‘blue filtering’’ intraocular lens. Br J Ophthalmol 2005; 89:1518–1521 9. Davison JA, Patel AS. Light normalizing intraocular lenses. Int Ophthalmol Clin 2005; 45(1):55–106 10. Marshall J, Cionni RJ, Davison J, et al. Clinical results of the blue-light filtering AcrySof Natural foldable acrylic intraocular lens. J Cataract Refract Surg 2005; 31:2319–2323 11. Berson EL. Light deprivation and retinitis pigmentosa. Vision Res 1980; 20:1179–1184 12. Boettner EA, Wolter JR. Transmission of the ocular media. Invest Ophthalmol 1962; 1:776–783 13. Curcio CA, Millican CL, Allen KA, Kalina RE. Aging of the human photoreceptor mosaic: evidence for selective vulnerability of rods in central retina. Invest Ophthalmol Vis Sci 1993; 34:3278–3296 14. Charman WN. Age, lens transmittance, and the possible effects of light on melatonin suppression. Ophthalmic Physiol Opt 2003; 23:181–187
Reply:
In my recent JCRS article,1 I sought to fix 2 problems associated with an analysis of blue-light filtering IOLs performed by Mainster and Sparrow in their 2003 BJO paper.2 The criticisms in my paper are fair and in no way suggest that the researchers are careless or incompetent. However, the nature of science and progress is to examine the current state of knowledge and reassess the current dogma when new and better techniques and information become available. The readership and their patients certainly deserve this type of reflection to provide informed care. Furthermore, I did not fail to cite Mainster’s presentations at the CSCRS, ASCRS, and ESCRS meetings since my manuscript was submitted before these events took place, as is clearly shown by the acceptance date. In my article, I fixed 2 problems in the Mainster and Sparrow paper and reanalyzed the effect of the lenses on scotopic vision. With these corrections, the conclusions in the Mainster and Sparrow paper were completely reversed. Instead of a marked loss in scotopic performance, as suggested in the paper, a potential increase in photons available in the scotopic regime was found. I consider a gain versus a loss an enormous difference. The 2 problems in the original paper were that the incorrect transmission data were used for the AcrySof Natural material and the incorrect scotopic response curve was used for assessing pseudophakic patients. The AcrySof Natural IOL transmission data were measured with the IOL in air. There is roughly a 9% loss in transmission across the visible spectrum due to surface reflections when IOLs are measured in air. When IOLs are immersed in saline or aqueous humor, these surface reflections are reduced to roughly 1%. The 25% loss in scotopic vision that was a conclusion in the Mainster and Sparrow paper decreases to 14% when the proper IOL transmission characteristics are used. This change is relatively small when considering the dynamic range of scotopic vision, as pointed out in Mainster’s letter. However, Mainster
J CATARACT REFRACT SURG - VOL 32, SEPTEMBER 2006
LETTERS
suggests that this is the end of the story and downplays the role of the second correction I made. The major changes in scotopic performance occur when using the correct scotopic response curve. In the Mainster and Sparrow paper, the CIE 1951 standard scotopic response curve was used to determine the effects of blue-light filtering IOLs on scotopic vision. This is the incorrect function to use in this analysis since the crystalline lens is removed in cataract surgery. The scotopic response of the eye is controlled primarily by 2 factors: the spectral response of rhodopsin, the photosensitive material contained in rods, and the transmission of the ocular media. The crystalline lens, even in a young eye, absorbs a high percentage of the short wavelengths. When this lens is removed, there is an enormous increase in the amount of light entering the eye. Consequently, to properly assess scotopic vision in a pseudophakic patient, an aphakic scotopic response curve is needed to account for the removal of the crystalline lens. Unfortunately, neither the CIE nor any other organization has defined a standard aphakic scotopic response curve. Such a curve is unlikely to be defined due to the relative rarity of aphakes. To define the required response curve, the literature must be examined to extract this information. In reviewing the literature, I found only one suitable source for defining the aphakic scotopic response curve. In their 1992 paper, Griswold and Stark3 measured absolute scotopic response in 5 phakic and 3 aphakic individuals. Due to the scarcity of subjects, these measurements are noisy, but they are all that is available to define the aphakic response curve. In Mainster’s letter, he adamantly stands by the choice of using the CIE phakic scotopic response curve because it ‘‘is a recognized standard.’’ However, improper use of a standard does not make the methodology correct. Furthermore, Mainster criticizes my use and analysis of the Griswold and Stark data. Yet, in his letter, Mainster fails to mention that he uses the same Griswold and Stark data in his recently published article on violet- and blue-blocking IOLs.3 The only difference between our use of these data is that I chose to fit the noisy data by the technique described below and Mainster chose to use the noisy data. Since Griswold and Stark measured only 3 aphakic eyes, their data are noisy. I am confident this variation is noise since the same magnitude variation is seen in the phakic eye measurements, which should match the smooth CIE 1951 scotopic response curve. I digitized the Griswold and Stark data and performed a least-squares fit to a 2nd-order polynomial (R2 Z 98.6% for the aphakic subjects and R2 Z 96.8% for the phakic subjects). To verify the technique, the fit to the phakic data was compared with the CIE 1951 scotopic response curve. These curves match remarkably closely, both with a peak wavelength of lmax of 505 nm. The peak wavelength for the aphakic curve is lmax Z 490 nm. Mainster criticizes my analysis because this peak wavelength is shifted from the peak rod response of 505 nm found by Wald4 and Brown and Wald.5 However, more recent measurements of the rod sensitivity find peaks shifted more toward the blue. Dartnall et al.6 found a peak sensitivity of 496.3 G 2.3 nm, while Kraft et al.7 found a peak around 495 nm. This shift is expected since the peak of the phakic scotopic response curve (as measured through the crystalline lens) is 505 nm. When the lens, which absorbs at the short wavelengths, is removed or bypassed, the peak response of rods must shift toward the blue. While the fit I performed to the Griswold and Stark data is slightly more toward the blue than expected, it is certainly more representative of what is occurring in the pseudophakic eye than the CIE phakic scotopic luminous efficiency curve. It is also
certainly more representative of the noisy data Mainster uses in his current analysis.3 I indeed used this fitted aphakic scotopic sensitivity curve to determine the effects of pseudophakia on scotopic vision. It is unclear how Mainster made his calculations, but they appear to have been made ‘‘relative to Boettner and Wolter’s 4.5-year-old crystalline lens.’’ This makes no sense in this case since there is no crystalline lens in pseudophakia. If the areas under the Griswold and Stark’s aphakic and phakic scotopic sensitivity curves are integrated, there is twice as much light entering the aphakic eye than a 20- to 30-year-old phakic eye. Placing a 20 D AcrySof Natural IOL into these eyes reduces this gain but still leads to a 52% gain in the amount of light entering the eye. The Griswold and Stark aphakic measurements were performed in two 43-year-old eyes and one 60-year-old eye. Although dark-adapted pupil sizes are not given, the mean values of dark-adapted pupil diameters in a European population of these ages are 5.9 to 6.5 mm.8 By age 70, this mean pupil size decreases to 5.4 mm. This corresponds to a reduction in pupil area of 16% to 31%. A gain in the amount of light entering the eye is therefore still expected at age 70 and can be projected for older ages. Consequently, the limiting factor on scotopic performance is photoreceptor density and health, not the IOL. Mainster does not seem to accept that there is an enormous increase in the amount of light entering the eye when the crystalline lens is removed since he insists on using a phakic scotopic sensitivity curve or the transmission of a 4.5-year-old crystalline lens in making calculations in subjects without a crystalline lens. More light in the scotopic regime enters the eye in our pseudophakic patients than enters the 20- to 30-year-old eye. The early criticism of the AcrySof Natural IOL, such as loss of scotopic performance and shifts in color vision, have evaporated. These IOLs have been successfully implanted worldwide, providing recipients with high-quality vision. The potential benefits of these IOLs to retinal health are far more difficult to demonstrate due to many confounding factors such as diet, smoking, exposure and genetics, and further work is necessary to determine the retinal benefits of these IOLs.dJim Schwiegerling, PhD
REFERENCES 1. Schwiegerling J. Blue-light-absorbing lenses and their effect on scotopic vision. J Cataract Refract Surg 2006; 32:141–144 2. Mainster MA, Sparrow JR. How much blue light should an IOL transmit? [perspective]. Br J Ophthalmol 2003; 87:1523–1529 3. Mainster MA. Violet and blue light blocking intraocular lenses: photoprotection versus photoreception [perspective]. Br J Ophthalmol 2006; 90:784–792 4. Wald G. Human vision and the spectrum. Science 1945; 101:653–658 5. Brown PK, Wald G. Visual pigments in single rods and cones of the human retina. Direct measurements reveal mechanisms of human night and color vision. Science 1964; 144:45–52 6. Dartnall H JA, Bowmaker JK, Mollon JD. Human visual pigments: microspectrophotometric results from the eyes of seven persons. Proc R Soc Lond B Biol Sci 1983; 220:115–130 7. Kraft TW, Schneeweis DM, Schnapf JL. Visual transduction in human rod photoreceptors. J Physiol 1993; 464:747–765 8. Said FS, Sawires WS. Age dependence of changes in pupil diameter in the dark. Optica Acta 1972; 19:359–361 See editors’ note about these letters on following page.
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