Ophthalmic Procedures Assessment*
Contrast Sensitivity and Glare Testing in the Evaluation of Anterior Segment Disease AMERICAN ACADEMY OF OPHTHALMOLOGY
*The purpose of the Committee on Ophthalmic Procedures Assessment is to evaluate on a scientific basis new and existing ophthalmic tests, devices, and procedures for their safety, efficacy, clinical effectiveness and appropriate uses. Evaluations include examination of available literature, epidemiological analyses when appropriate, and compilation of opinions from recognized experts and other interested parties. After appropriate review by all contributors, including legal counsel, assessments are submitted to the Academy's Board of Directors for consideration as official Academy policy.
Clinical applications of contrast sensitivity and glare testing have dramatically increased over the past two de cades. The main purpose of this document is to describe briefly the clinical uses of these tests and to explore the issue of test standardization. Especially important is the extent to which these tests distinguish, or fail to distin guish, between anterior segment and posterior segment disease. Emphasis will be placed on the clinical evaluation of visual disability from early cataract. A reliable measure of this disability is needed to help the clinician determine the patient's potential benefit from cataract surgery. There is a great deal of confusion in the clinical liter ature concerning contrast sensitivity and glare testing. The two terms are sometimes used synonymously. However, contrast sensitivity is a measure of the amount ofcontrast required to detect or recognize a target, and glare sensi tivity (as it is used here) refers to the change in visual function caused by the presence of a glare (light) source in another part of the visual field. Light from the glare source is scattered within the eye by inhomogeneities or opacities in the ocular media, and this scattered light re duces the contrast of images on the retina. Quite often, the visual function measured in a glare test is contrast Prepared by the Committee on Ophthalmic Procedures Assessment and approved by the Academy's Board of Directors on September 17, 1989.
sensitivity, hence the confusion. Alternatively, glare sen sitivity can be measured with a different type of vision test such as acuity or color matching.
CONTRAST SENSITIVITY TESTING Contrast sensitivity has been widely promoted as an adjunct or even replacement for visual acuity in clinical vision testing. Visual acuity measures the eye's ability to resolve fine detail at high contrast but does not adequately describe one's ability to see large, low contrast patterns such as faces or nearby objects. For example, it has been reported that in several disorders such as cerebral lesions, 1•2 retrobulbar optic neuritis from multiple sclerosis, 3 glau coma,4·5·6 and various forms of retinopathy, 4·7•8 contrast sensitivity may be markedly reduced despite near normal visual acuity. Contrast sensitivity testing thus provides a different and sensitive means for following the progression of these diseases and monitoring the effects of treatment. In a normal healthy human eye, however, contrast sen sitivity and visual acuity typically vary in the same fashion. For example, reduced visual acuity due to ametropia causes a predictable reduction of contrast sensitivity,9• 10 so that contrast sensitivity for an individual patient is much more meaningful after it has been normalized rel ative to that patient's visual acuity. 1233
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Contrast sensitivity tests have employed a wide variety of stimuli including disks, bipartite fields (a black-white edge), gratings, and letters. 11 Following the pioneering work of Schade 12 and Campbell and Robson, 13 most re searchers have used patterns consisting of alternating light and dark bars called "sine-wave gratings". Sine-wave gratings can vary in spatial frequency (light-dark cycles per degree), contrast, luminance, field size, and attenua tion at the edge of the field. By measuring the lowest de tectable contrast across a range of spatial frequencies, one derives a contrast sensitivity function , or CSF, of the pa tient. Several investigators have studied the relationship between the patient's CSF and performance of everyday visual tasks such as reading or target identification. There is controversy about the importance of the CSF for pre dicting visual performance of normal subjects, such as military pilots. 14• 15 •16 •17 Nevertheless, it has been shown that the CSF is important for predicting reading speed 18 and mobility performance 19 in low-vision patients. CONTRAST SENSITIVITY TESTING AND CATARACT EVALUATION
Contrast sensitivity has been used in the clinic for re fraction , vision screening and diagnosis, and in the eval uation of the effectiveness of medical treatment, surgical treatment, and devices such as contact lenses or intra ocular lenses. Recently it has been proposed that contrast sensitivity be used to quantify vision loss for purposes of considering treatment. One potential application of con trast sensitivity is for recommending or justifying early cataract surgery. Cataracts of all types are thought to increase intraocular light scatter. One effect of increased scatter is a reduction in retinal image contrast which will result in reduced con trast sensitivity. The effect of scatter on the shape of the CSF will depend on the exact nature of the intraocular scatter. If there is wide-angle scatter, the CSF will be de pressed across all spatial frequencies. However, if there is narrow-angle scatter, the CSF will be depressed only at high spatial frequencies. Indeed, Hess and Woo 20 have reported both types ofCSF loss in patients with cataracts. They suggest that early cataracts cause high frequency loss, while more mature cataracts produce CSF losses at both high and low spatial frequencies. While the magni tude of the high frequency loss should be predictable from acuity measures, the magnitude of the low frequency loss is not closely related to visual acuityY Results such as these have led to the suggestion that the CSF be used to set new standard indications for cat aract surgery. There is a fundamental problem with this recommendation. There is no pattern of CSF loss that is specific for cataracts. CSFs may be reduced by refractive blur and by retinal or neurological disorders in which the ocular media are entirely clear. In fact , a study of contrast sensitivity in diabetic patients with retinopathy and cataracts7 could not distinguish between CSF losses en countered in patients with cataracts and no retinopathy and patients with retinopathy and clear lenses. The types of CSFs measured in patients with macular disease 22 or 1234
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cerebrallesions23 can be similar to the CSFs measured in cataract patients, although detailed analyses with targets of different sizes24 or at different retinal eccentricities2 5 may help distinguish between various causes of the loss. A similar problem arises when visual acuity is used to set standard indications for cataract surgery. Visual acuity loss is not specific for cataracts. Only if the remainder of the visual system can be determined to be normal can the full amount of visual acuity loss (or contrast sensitivity loss) be attributed to the cataract. In order to determine whether the fundamental limitation on visual acuity is optical (lens opacification) or non-optical (e.g. retinal dis ease), several methods have been developed to assess "po tential visual acuity." 26 Similar methods could be applied to contrast sensitivity testing, such as using a small ap erture or laser interferometer to project the contrast sen sitivity target onto the retina through a clear area in the cataract. In summary, contrast sensitivity testing in isolation from other tests is not sufficient to be used as the standard indication for cataract surgery. However, it may be useful as part of a battery of vision tests for helping to establish the nature and amount of vision loss due to lens opaci fication.
GLARE TESTING Glare testing, on the other hand, is a promising method for isolating vision loss due to intraocular light scatter. 27 "Glare" can refer to a variety of phenomena. Most com monly these are divided into discomfort and disability glare. Discomfort glare refers to the photophobic sensation one experiences when the overall illumination is too bright, for example when leaving a darkened movie theater on a sunlit afternoon. Any disorder which alters the dy namics of light and dark adaptation, or reduces the light level at which the photoreceptors become saturated (e.g. retinitis pigmentosa) may cause problems due to discom fort glare. Specific tests have been developed to evaluate adaptation and saturation, but since these generally eval uate retinal and neurological function, they play no role in the evaluation of anterior segment disease. Disability glare refers to the reduced visibility of a target due to the presence of a light source elsewhere in the field. A common example is the reduced visibility of roadway markers in the presence ofoncoming headlights. Disability glare is the specific type of glare caused by light scattered by the ocular media and is thus of primary interest to us in the evaluation of cataracts. GLARE TESTING AND CATARACT EVALUATION
Any disorder which increases intraocular light scatter may cause problems due to disability glare. Glare tests have been used in a variety of anterior segment disorders such as keratoconus,28 corneal edema, 29 cataract,30 and capsular opacification.3 1 They have been used to evaluate the optical quality of intraocular lenses 32 and to assess refractive surgeryY
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Another possible source of increased glare sensitivity is intraocular light scatter at the retinal level due to mac ular edema. Unless the edematous tissue is perfectly clear and of the same refractive index as the surrounding fluid and tissue, one might expect it to scatter light converging to the retinal image. However, since the scattering centers are very close to the image plane, the effect on the image may be negligible. In a preliminary study of glare sensi tivity in diabetic patients with macular edema, glare sen sitivity was found to be more closely related to the clarity of the crystalline lens (if the patient was phakic) than to the amount of edema. 34 There have been anecdotes, how ever, of false positive glare testing results attributed to macular disease such as cystoid macular edema. The mechanism is unclear. Perhaps if there is already com promised foveal function from disease at the retinal level, the small amount of light normally scattered from the anterior segment, or the larger amount scattered from a cataract, could cause an exaggerated glare effect. Further study is necessary to clarify this possibility. Alternatively, ifthe patient with foveal disease should inadvertently look at the glare source, a photostress phenomenon (photo receptor bleaching) might cause a false positive glare result. In general, however, disability glare tests are considerably more specific for anterior segment disorders than are sim ple contrast sensitivity tests without the addition of a glare source. Not only are glare tests more specific than contrast sen sitivity for anterior segment disorders, they are also more sensitive to such disorders. Experimentally induced cor neal edema has only a small effect on contrast sensitivity but increases glare sensitivity almost three-fold. 29 In a large study of 196 normal, cataractous, and aphakic patients, glare sensitivity measured with a 20/400 Landolt C dif ferentiated the three groups much more decisively than did contrast sensitivity testing without glare. 30 Even though glare tests are specific and sensitive to an terior segment optical disorders, the question still remains: how does glare sensitivity, as measured in the laboratory or clinical setting, relate to everyday visual problems as sociated with glare? The data are still quite sparse on this subject. Glare testing of cataract patients can predict the reduction in visual acuity out-of-doors when facing the sun 30 or in direct overhead sunlight. 35 •36•37 Glare sensitivity has been shown to be correlated with lens opacity mea sured with fundus photography and slit-lamp examina tion, and to correspond to patient complaints. 38 For driv ers with normal visual acuity, glare sensitivity measure ments were loosely correlated with simulated nighttime driving performance, and corresponded to subjective complaints about glare from oncoming headlights. 39 Nevertheless, additional research is required to quantify the relationship between glare sensitivity and overall visual performance. Perhaps, for example, sensitivity of move ment detection, before and after adding a glare source, is more relevant to "real world" performance requirements than either acuity- or contrast sensitivity-based measures. Reliable measures of visual performance have not yet been identified sufficiently to allow correlation with results from any of the commercially-available glare testers. More re
search in this area appears needed before using glare testing to establish standard indications for cataract surgery. GLARE TESTING STANDARDS
Our understanding of disability glare is hampered, in part, by the wide variety of glare test configurations used. Unlike contrast sensitivity testing which has a rich history in the basic vision literature, there have been few inves tigations of glare testing methodology. Most glare tests assess the effects of a glare light on contrast sensitivity, either by measuring a CSF38 or by obtaining a contrast threshold for the detection of optotype targets in the pres ence or absence of glare. 40 Visual acuity has also been used in glare tests, 41 as have increment thresholds for the detection of disks42 or small spots of light. 33 Contrast detection tasks have been used more fre quently than acuity-based measures to determine glare sensitivity. This is not necessarily because the information obtained is clinically more useful; the main attraction of a contrast sensitivity measure is simplicity of analysis. Disability glare can be modeled in terms of a veiling lu minance scattered onto the test target from the glare source. This veiling luminance is added to both the bright and dark bars of a grating pattern, or to the target and background if optotypes are used. In either case, the veiling luminance effectively reduces image contrast. Since the amount of scattered glare light will determine the mag nitude of the contrast reduction, it makes sense to measure contrast sensitivity in order to assess disability glare. Visual acuity can also be used to measure disability glare because acuity is affected by reduced contrast of the acuity targets. For a standard letter recognition test with normal observers, however, acuity is related to contrast by a square-root function. That is, ifthe contrast is reduced by a factor of 2, acuity will be reduced by 2 or a factor of 1.4. 43 Factors that reduce contrast, such as glare, will have a relatively weak effect on acuity. In a study of glare sen sitivity with simulated ocular turbidity, the glare light caused contrast sensitivity to decline rapidly, while visual acuity remained near normal. 40 Therefore, tests based on contrast sensitivity measures should be more sensitive to disability glare than tests based on acuity. The sensitivity of acuity-based glare tests can be increased, however, by either reducing the contrast of the acuity targets41 •44 or increasing the intensity of the glare source (although high intensity glare sources may introduce confounding vari ables such as pupillary constriction and photoreceptor bleaching). Further research is necessary to determine whether or not contrast sensitivity-based measures provide different disability glare information from acuity-based tests. In addition to differences in the visual function mea sured, glare tests vary in the type of glare source used. Most of the early studies used point-source glare. This type of glare source requires simple instrumentation (usually a single light bulb), provides a good simulation of environmental glare sources such as automobile head lights, and is amenable to simple models of veiling lu minance. However, small glare sources require a relatively 1235
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high luminous intensity to produce the desired effect and may draw the observer's fixation away from the target to the glare light. 38 This may result in an afterimage which interferes with the visual function measurement. Extended glare sources, such as a bright annulus produced by a circular fluorescent tube or a bright rectangular back ground, cause fewer afterimage problems and are said to be better accepted by patients. 40 Intensities of the glare sources also vary widely from study to study. The Miller Nadler glare tester, the first such device commercially available in the United States, projects a dark Landolt C on a bright background. 45 At the other extreme, Applegate used a mesopic source for his laboratory glare tester.33 Two studies have systematically varied glare intensity. Abrahamsson and Sjostrand 38 report that glare sensitivity of cataract patients decreased as the luminance of an an nular glare source was reduced from 200 cd/m2 to 2 cd/ m 2 • However, Applegate 33 found that lower glare inten sities increased glare sensitivity in patients after radial keratotomy (RK). He argues that lower intensities are preferable for evaluating headlight glare problems in RK patients because pupillary dilation is then comparable to that obtained during nighttime driving conditions. It is also important to recognize that if the glare intensity is too high, a false disability can be diagnosed. The normal cornea, lens, and vitreous combine to scatter between 10 to 20 percent of the light incident on the eye. Thus a very bright glare source can reduce contrast sensitivity in a normal eye. 46 Regardless of the visual function test or glare source chosen, there are several basic principles of clinical vision test design that should be followed: l . The test should be "criterion-free" . Although the patient's subjective responses should not be ignored, the objective test results should not be affected by whether the subject is more or less cautious in his or her responses. For example, a test which allows the patient to decide if the target is visible or not will produce one result with an individual who is very cautious and demands that the target be clearly discernible before venturing a response, and another result with a patient who is willing to guess at anything. Forced-choice procedures circumvent this problem. A standard eye chart uses a forced-choice pro cedure when the patient is asked to identify the letter (make a "forced choice" ) and the examiner determines whether the answer is correct or incorrect. If the patient is allowed to answer that he or she cannot see the letter, then the test is not forced-choice. Forced-choice tests yield more reliable results, especially with unpracticed observ ers, than criterion-dependent tests.47 •34 2. The test targets should follow a uniform progression. For acuity targets this means that letter size should de crease in equal logarithmic steps. For contrast sensitivity, target contrast should decline in equal logarithmic steps. U niforrn step size is important for making quantitative comparisons, such as comparing glare sensitivity before and after cataract surgery across patients who may have different overall sensitivities. Surprisingly, many Snellen acuity charts do not follow a uniform progression ofletter sizes, although the ETDRS acuity chart, 48 which is quickly 1236
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becoming a clinical standard, adheres to equal logarithmic steps. 3. To insure high test-retest reliability, it is necessary to have several trials at each level of difficulty, while test efficiency dictates that the number of trials be kept to a minimum. An often overlooked factor for determining reliability is the criterion used by the examiner to score the test. For example, must the patient identify all letters correctly on the 20/20 line or will one or two errors be acceptable to earn a 20/20 acuity score? A detailed study of the effects of trial number and scoring criterion on test efficiency and reliability indicates that requiring two cor rect responses out of three trials provides a reasonable compromise. 49 In summary, while it is premature to establish definitive guidelines for supplemental tests to visual acuity in as sessing the overall visual disability from immature cata racts, several recommendations emerge. Contrast sensi tivity, by itself, is not specific enough for this purpose. However, contrast sensitivity or low-contrast visual acuity, measured before and after adding a glare source, is prob ably sufficiently specific and sensitive. While contrast sen sitivity is more sensitive to glare than visual acuity, the sensitivity of acuity-based glare tests can be improved by reducing the contrast of the acuity targets or, along with some confounding variables, by increasing the intensity of the glare source. The optimal glare source for reliable and reproducible testing, however, has not yet been iden tified. Glare tests, like all visual function tests, should ad here to test procedures which maximize test reliability and interpretability. At this time there is no universally accepted standard for glare disability testing and no agreement on how the results of a glare disability test should be used in the eval uation of patients for medical or surgical therapy. The presently available glare tests may be of help in adding to the objective assessment of the impact on visual disability of anterior segment disease. Glare testing should not be considered mandatory in the preoperative assessment of patients, and if used, should not be used as the sole cri terion for surgical intervention. Glare testing may be used as an adjunct to the clinical history, to the ophthalmologic examination, and to other tests of visual function to pre dict the objective visual acuity outcome of surgery. This predicted objective result should be carefully considered in terms of its likely effect on the individual patient's quality of life, and weighed against the known risks of surgical intervention.
ACKNOWLEDGMENTS Original draft by: Gary S. Rubin, Ph.D.; Reviewed by: Ivan Bodis-Wollner, M.D., John D. Bullock, M.D., Jack T. Holladay, M.D., Gary F. Jaffe, M.D., Douglas Koch, M.D., Philip Lempert, M.D., Mark J. Mannis, M.D., Michael F. Marmor, M.D., Samuel Masket, M.D., David Miller, M.D., Albert C. Neumann, M.D., David M. Regan, D.Sc., Frederick A. Richburg, M.D., Charles B. Slonim, M.D., Joseph W. Weiss, M.D., Jonathan Wirtschafter, M.D.; Edited by: David L. Guyton, M.D., Gary S. Rubin, Ph.D.; Approved by: Board of Directors, September 17, 1989.
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REFERENCES 1. Bodis-Wollner I. Visual acuity and contrast sensitivity in patients with cerebral lesions. Science 1972; 178:769-71. 2. Bodis-Wollner I, Diamond SP. The measurement of spatial contrast sensitivity in cases of blurred vision associated with cerebral lesions. Brain 1976; 99:695-710. 3. Regan 0, Silver R, Murray TJ. Visual acuity and contrast sensitivity in multiple sclerosis-hidden visual loss: an auxiliary diagnostic test. Brain 1977; 100:563-79. 4. Regan 0, Neima D. low contrast letter charts in diabetic retinopathy, ocular hypertension, glaucoma, and Parkinson's disease. Br J Ophthalmol1984; 69:885-9. 5. Ross JE, Bron AJ, Clarke DO. Contrast sensitivity and visual disability in chronic simple glaucoma. Br J Ophthalmol1984; 68:821-7. 6 . Atkin A, Bodis-Wollner I, Wolkstein M, Moss A, Podos SM. Abnor malities of central contrast sensitivity in glaucoma. Am J Ophthalmol 1979; 88:205-11. 7. Howes SC, Caelli T, Mitchell P. Contrast sensitivity in diabetics with retinopathy and cataract. Aust J Ophthalmol1982; 10:173-8. 8. Wolkstein M, Atkin A, Bodis-Wollner I. Contrast sensitivity in retinal disease. Ophthalmology 1980; 87:1140-9. 9. Campbell FW, Green DG. Optical and retinal factors affecting visual resolution. J Physiol1965; 181 :576-93. 10. Marmor MF, Gawande A. Effect of visual blur on contrast sensitivity. Ophthalmology 1988; 95:139-43. 11. Hecht G, Hendley CD, Frank S, Schlaer S. Contrast discrimination charts for demonstrating the effects of anoxia on vision. J Opt Soc Am 1949; 39:922-3. 12. Schade OH. Optical and photoelectric analog of the eye. J Opt Soc Am 1956; 46:721-39. 13. Campbell FW, Robson JG. Application of Fourier analysis to the visibility of gratings. J Physiol1968; 197:551-66. 14. Ginsburg AP, Easterly J, Evans OW. Contrast sensitivity predicts target detection field performance of pilots. Proceedings of the Human Fac tors Society· 27th Annual Meeting 1983; 269-73. 15. O 'Neal MR, Miller RE. Further investigation of contrast sensitivity and visual acuity in pilot detection of aircraft. Noninvasive Assessment of the Visual System, 1988 Technical Digest Series, Vol. 3, Washington, D.C., Optical Society of America, 1988; 134-7. 16. Kruk R, Regan 0, Beverley Kl, Langridge T. Correlations between visual test results and flying performance on the advanced simulator for pilot training (ASPT). Aviat Space Environ Med 1981; 52:455-60. 17. Kruk R, Regan D. Visual test results compared with flying performance in telemetry tracked aircraft. Aviat Space Environ Med 1983; 54:906 11 . 18. Rubin GS, Legge GE. The psychophysics of reading . VI. The role of contrast in low vision. Vision Res 1989; 29:79-91 . 19. Marron JA, Bailey ll. Visual factors and orientation-mobility perfor mance. Am J Optom Physiol Opt 1982; 59:413-26. 20. Hess R, Woo G. Vision through cataracts. Invest Ophthalmol Vis Sci 1978; 17:428-35. 21. Lempert P, Hopcroft M, Lempert Y. Evaluation of posterior subcapsular cataracts with spatial contrast acuity. Ophthalmology 1987; 94(S):14 18. 22. Sjostrand J, Frisen l. Contrast sensitivity in macular disease: a pre liminary report. Acta Ophthalmol1977; 55:507-14. 23. Bodis-Wollner I. Differences in low and high spatial frequency vulner abilities in ocular and cerebral lesions. In Maffei L (ed). Pathophysiology of the Visual System. The Hague, Doc W Junk, 1981 ; 195-204. 24. Hyvarinen L, Laurinen P, Rovamo J. Contrast sensitivity in evaluation of visual impairment due to diabetes. Acta Ophthalmol1983; 61:94 101.
25. Lundh BL. Central and peripheral contrast sensitivity for static and dynamic sinusoidal gratings in glaucoma. Acta Ophthalmol1985; 63: 487-92. 26. Guyton Dl. Preoperative visual acuity evaluation. lnt Ophthalmol Clin 1987; 27:140-8. 27. Van den Berg TJTP. Importance of pathological intraocular light scatter for visual disability. Doc Ophthalmol1986; 61:327-33. 28. Hess RF, Carney LG. Vision through an abnormal cornea; a pilot study of the relationship between visual loss from corneal distortion, corneal edema, keratoconus and some allied corneal pathology. Invest Ophthalmol Vis Sci 1979; 18:476-83. 29. Carney LG, Jacobs RJ. Mechanisms of visual loss in corneal edema. Arch Ophthalmol1984; 102:1068-71 . 30. Hirsch RP, Nadler MP, Miller D. Glare measurement as a predictor of outdoor vision among cataract patients. Ann Ophthalmol 1984; 16: 965-8. 31. Knighton RW, Slornovic AR, Parrish RK. Glare measurements before and after neodymium-VAG laser posterior capsulotomy. Am J Ophthalmol1985; 100:708-13. 32. Nadler OJ, Jaffe NS, Clayman HM, et al. Glare disability in eyes with intraocular lenses. Am J Ophthalmol1984; 97:43-7. 33. Applegate RA, Trick LR, Meade DL, Hartstein J. Radial keratotomy increases the effects of disability glare: initial results. Ann Ophthalrnol 1987; 19:293-7. 34. Rubin GS, Sunness JS. Assessing visual function in patients with macular edema. Noninvasive Assessment of the Visual System, 1988 Technical Digest Series, Vol. 3, Washington , D.C., Optical Society of America, 1988; 140-3. 35. Holladay JT, Trujillo J, Prager TC, Ruiz RS. Brightness acuity test and outdoor visual acuity in cataract patients. J Cataract Refract Surg 1987; 13:67-9. 36. Neumann AC, McCarty GR, Steedle TO, et al. The relationship between cataract type and glare disability as measured by the Miller-Nadler Glare Tester. J Cataract Refract Surg 1988; 14:40-5. 37 . Neumann AC, McCarty GR, Locke J, Cobb B. Glare disability devices for cataractous eyes: a consumer's guide. J Cataract Refract Surg 1988; 14:212-6. 38. Abrahammson M, Sjostrand J. Impairment of contrast sensitivity func tion (est) as a measure of disability glare. Invest Ophthalmol Vis Sci 1986; 27:1131-6. 39. Pulling NH, WolfE, Sturgis SP, Vaillancourt DR, Dolliver JJ. Headlight glare resistance and driver age. Human Factors 1980; 22:103-12. 40. Miller 0, Jernigan ME, Molnar S, et al. Laboratory evaluation of a clinical glare tester. Arch Ophthalmol1972; 87:324-32. 41. Lempert P, Hopcraft M, Lempert Y. Evaluation of posterior subcapsular cataracts. Ophthalmology 1987; 94:14-18. 42. Wolf E. Glare and age. Arch Ophthalmol1960; 64:502-54. 43. Legge GE, Rubin GS, Luebker A. Psychophysics of reading . V. The role of contrast in normal vision . Vis Res 1987; 27:1165-77. 44. Regan D. Low-contrast letter charts and sinewave grating tests in ophthalmological and neurological disorders. Clin Vision Sci 1988; 2: 235-50. 45. LeClaire J, Nadler MP, Weiss S, Miller D. A new glare tester for clinical testing - results comparing normal subjects and variously corrected aphakic patients. Arch Ophthalmol1982; 100:153-8. 46. Miller 0, Benedek S. Intraocular Light Scattering. Springfield, IL, CC Thomas, 1973; 38. 47 . Higgins KE, Jaffe MJ, Coletta NJ, et al. Spatial contrast sensitivity. Importance of controlling the patient's visibility criterion. Arch Ophthal mol1984; 102:1035-41 . 48. Ferris F, Kassof A, Bresnick A, et al. New visual acuity charts for clinical research. Am J Ophthalmol 1982; 94:91-6. 49. Pelli DG, Robson JG, Wilkins AJ . The design of a new letter chart for measuring contrast sensitivity. Clin Vision Sci 1988; 2:187-99.
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Contrast Sensitivity and Glare Testing in the Evaluation of Anterior Segment Disease AMERICAN ACADEMY OF OPHTHALMOLOGY
*The purpose of the Committee on Ophthalmic Procedures Assessment is to evaluate on a scientific basis new and existing ophthalmic tests, devices, and procedures for their safety, efficacy, clinical effectiveness and appropriate uses. Evaluations include examination of available literature, epidemiological analyses when appropriate, and compilation of opinions from recognized experts and other interested parties. After appropriate review by all contributors, including legal counsel, assessments are submitted to the Academy's Board of Directors for consideration as official Academy policy.
Clinical applications of contrast sensitivity and glare testing have dramatically increased over the past two de cades. The main purpose of this document is to describe briefly the clinical uses of these tests and to explore the issue of test standardization. Especially important is the extent to which these tests distinguish, or fail to distin guish, between anterior segment and posterior segment disease. Emphasis will be placed on the clinical evaluation of visual disability from early cataract. A reliable measure of this disability is needed to help the clinician determine the patient's potential benefit from cataract surgery. There is a great deal of confusion in the clinical liter ature concerning contrast sensitivity and glare testing. The two terms are sometimes used synonymously. However, contrast sensitivity is a measure of the amount ofcontrast required to detect or recognize a target, and glare sensi tivity (as it is used here) refers to the change in visual function caused by the presence of a glare (light) source in another part of the visual field. Light from the glare source is scattered within the eye by inhomogeneities or opacities in the ocular media, and this scattered light re duces the contrast of images on the retina. Quite often, the visual function measured in a glare test is contrast Prepared by the Committee on Ophthalmic Procedures Assessment and approved by the Academy's Board of Directors on September 17, 1989.
sensitivity, hence the confusion. Alternatively, glare sen sitivity can be measured with a different type of vision test such as acuity or color matching.
CONTRAST SENSITIVITY TESTING Contrast sensitivity has been widely promoted as an adjunct or even replacement for visual acuity in clinical vision testing. Visual acuity measures the eye's ability to resolve fine detail at high contrast but does not adequately describe one's ability to see large, low contrast patterns such as faces or nearby objects. For example, it has been reported that in several disorders such as cerebral lesions, 1•2 retrobulbar optic neuritis from multiple sclerosis, 3 glau coma,4·5·6 and various forms of retinopathy, 4·7•8 contrast sensitivity may be markedly reduced despite near normal visual acuity. Contrast sensitivity testing thus provides a different and sensitive means for following the progression of these diseases and monitoring the effects of treatment. In a normal healthy human eye, however, contrast sen sitivity and visual acuity typically vary in the same fashion. For example, reduced visual acuity due to ametropia causes a predictable reduction of contrast sensitivity,9• 10 so that contrast sensitivity for an individual patient is much more meaningful after it has been normalized rel ative to that patient's visual acuity. 1233
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Contrast sensitivity tests have employed a wide variety of stimuli including disks, bipartite fields (a black-white edge), gratings, and letters. 11 Following the pioneering work of Schade 12 and Campbell and Robson, 13 most re searchers have used patterns consisting of alternating light and dark bars called "sine-wave gratings". Sine-wave gratings can vary in spatial frequency (light-dark cycles per degree), contrast, luminance, field size, and attenua tion at the edge of the field. By measuring the lowest de tectable contrast across a range of spatial frequencies, one derives a contrast sensitivity function , or CSF, of the pa tient. Several investigators have studied the relationship between the patient's CSF and performance of everyday visual tasks such as reading or target identification. There is controversy about the importance of the CSF for pre dicting visual performance of normal subjects, such as military pilots. 14• 15 •16 •17 Nevertheless, it has been shown that the CSF is important for predicting reading speed 18 and mobility performance 19 in low-vision patients. CONTRAST SENSITIVITY TESTING AND CATARACT EVALUATION
Contrast sensitivity has been used in the clinic for re fraction , vision screening and diagnosis, and in the eval uation of the effectiveness of medical treatment, surgical treatment, and devices such as contact lenses or intra ocular lenses. Recently it has been proposed that contrast sensitivity be used to quantify vision loss for purposes of considering treatment. One potential application of con trast sensitivity is for recommending or justifying early cataract surgery. Cataracts of all types are thought to increase intraocular light scatter. One effect of increased scatter is a reduction in retinal image contrast which will result in reduced con trast sensitivity. The effect of scatter on the shape of the CSF will depend on the exact nature of the intraocular scatter. If there is wide-angle scatter, the CSF will be de pressed across all spatial frequencies. However, if there is narrow-angle scatter, the CSF will be depressed only at high spatial frequencies. Indeed, Hess and Woo 20 have reported both types ofCSF loss in patients with cataracts. They suggest that early cataracts cause high frequency loss, while more mature cataracts produce CSF losses at both high and low spatial frequencies. While the magni tude of the high frequency loss should be predictable from acuity measures, the magnitude of the low frequency loss is not closely related to visual acuityY Results such as these have led to the suggestion that the CSF be used to set new standard indications for cat aract surgery. There is a fundamental problem with this recommendation. There is no pattern of CSF loss that is specific for cataracts. CSFs may be reduced by refractive blur and by retinal or neurological disorders in which the ocular media are entirely clear. In fact , a study of contrast sensitivity in diabetic patients with retinopathy and cataracts7 could not distinguish between CSF losses en countered in patients with cataracts and no retinopathy and patients with retinopathy and clear lenses. The types of CSFs measured in patients with macular disease 22 or 1234
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cerebrallesions23 can be similar to the CSFs measured in cataract patients, although detailed analyses with targets of different sizes24 or at different retinal eccentricities2 5 may help distinguish between various causes of the loss. A similar problem arises when visual acuity is used to set standard indications for cataract surgery. Visual acuity loss is not specific for cataracts. Only if the remainder of the visual system can be determined to be normal can the full amount of visual acuity loss (or contrast sensitivity loss) be attributed to the cataract. In order to determine whether the fundamental limitation on visual acuity is optical (lens opacification) or non-optical (e.g. retinal dis ease), several methods have been developed to assess "po tential visual acuity." 26 Similar methods could be applied to contrast sensitivity testing, such as using a small ap erture or laser interferometer to project the contrast sen sitivity target onto the retina through a clear area in the cataract. In summary, contrast sensitivity testing in isolation from other tests is not sufficient to be used as the standard indication for cataract surgery. However, it may be useful as part of a battery of vision tests for helping to establish the nature and amount of vision loss due to lens opaci fication.
GLARE TESTING Glare testing, on the other hand, is a promising method for isolating vision loss due to intraocular light scatter. 27 "Glare" can refer to a variety of phenomena. Most com monly these are divided into discomfort and disability glare. Discomfort glare refers to the photophobic sensation one experiences when the overall illumination is too bright, for example when leaving a darkened movie theater on a sunlit afternoon. Any disorder which alters the dy namics of light and dark adaptation, or reduces the light level at which the photoreceptors become saturated (e.g. retinitis pigmentosa) may cause problems due to discom fort glare. Specific tests have been developed to evaluate adaptation and saturation, but since these generally eval uate retinal and neurological function, they play no role in the evaluation of anterior segment disease. Disability glare refers to the reduced visibility of a target due to the presence of a light source elsewhere in the field. A common example is the reduced visibility of roadway markers in the presence ofoncoming headlights. Disability glare is the specific type of glare caused by light scattered by the ocular media and is thus of primary interest to us in the evaluation of cataracts. GLARE TESTING AND CATARACT EVALUATION
Any disorder which increases intraocular light scatter may cause problems due to disability glare. Glare tests have been used in a variety of anterior segment disorders such as keratoconus,28 corneal edema, 29 cataract,30 and capsular opacification.3 1 They have been used to evaluate the optical quality of intraocular lenses 32 and to assess refractive surgeryY
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Another possible source of increased glare sensitivity is intraocular light scatter at the retinal level due to mac ular edema. Unless the edematous tissue is perfectly clear and of the same refractive index as the surrounding fluid and tissue, one might expect it to scatter light converging to the retinal image. However, since the scattering centers are very close to the image plane, the effect on the image may be negligible. In a preliminary study of glare sensi tivity in diabetic patients with macular edema, glare sen sitivity was found to be more closely related to the clarity of the crystalline lens (if the patient was phakic) than to the amount of edema. 34 There have been anecdotes, how ever, of false positive glare testing results attributed to macular disease such as cystoid macular edema. The mechanism is unclear. Perhaps if there is already com promised foveal function from disease at the retinal level, the small amount of light normally scattered from the anterior segment, or the larger amount scattered from a cataract, could cause an exaggerated glare effect. Further study is necessary to clarify this possibility. Alternatively, ifthe patient with foveal disease should inadvertently look at the glare source, a photostress phenomenon (photo receptor bleaching) might cause a false positive glare result. In general, however, disability glare tests are considerably more specific for anterior segment disorders than are sim ple contrast sensitivity tests without the addition of a glare source. Not only are glare tests more specific than contrast sen sitivity for anterior segment disorders, they are also more sensitive to such disorders. Experimentally induced cor neal edema has only a small effect on contrast sensitivity but increases glare sensitivity almost three-fold. 29 In a large study of 196 normal, cataractous, and aphakic patients, glare sensitivity measured with a 20/400 Landolt C dif ferentiated the three groups much more decisively than did contrast sensitivity testing without glare. 30 Even though glare tests are specific and sensitive to an terior segment optical disorders, the question still remains: how does glare sensitivity, as measured in the laboratory or clinical setting, relate to everyday visual problems as sociated with glare? The data are still quite sparse on this subject. Glare testing of cataract patients can predict the reduction in visual acuity out-of-doors when facing the sun 30 or in direct overhead sunlight. 35 •36•37 Glare sensitivity has been shown to be correlated with lens opacity mea sured with fundus photography and slit-lamp examina tion, and to correspond to patient complaints. 38 For driv ers with normal visual acuity, glare sensitivity measure ments were loosely correlated with simulated nighttime driving performance, and corresponded to subjective complaints about glare from oncoming headlights. 39 Nevertheless, additional research is required to quantify the relationship between glare sensitivity and overall visual performance. Perhaps, for example, sensitivity of move ment detection, before and after adding a glare source, is more relevant to "real world" performance requirements than either acuity- or contrast sensitivity-based measures. Reliable measures of visual performance have not yet been identified sufficiently to allow correlation with results from any of the commercially-available glare testers. More re
search in this area appears needed before using glare testing to establish standard indications for cataract surgery. GLARE TESTING STANDARDS
Our understanding of disability glare is hampered, in part, by the wide variety of glare test configurations used. Unlike contrast sensitivity testing which has a rich history in the basic vision literature, there have been few inves tigations of glare testing methodology. Most glare tests assess the effects of a glare light on contrast sensitivity, either by measuring a CSF38 or by obtaining a contrast threshold for the detection of optotype targets in the pres ence or absence of glare. 40 Visual acuity has also been used in glare tests, 41 as have increment thresholds for the detection of disks42 or small spots of light. 33 Contrast detection tasks have been used more fre quently than acuity-based measures to determine glare sensitivity. This is not necessarily because the information obtained is clinically more useful; the main attraction of a contrast sensitivity measure is simplicity of analysis. Disability glare can be modeled in terms of a veiling lu minance scattered onto the test target from the glare source. This veiling luminance is added to both the bright and dark bars of a grating pattern, or to the target and background if optotypes are used. In either case, the veiling luminance effectively reduces image contrast. Since the amount of scattered glare light will determine the mag nitude of the contrast reduction, it makes sense to measure contrast sensitivity in order to assess disability glare. Visual acuity can also be used to measure disability glare because acuity is affected by reduced contrast of the acuity targets. For a standard letter recognition test with normal observers, however, acuity is related to contrast by a square-root function. That is, ifthe contrast is reduced by a factor of 2, acuity will be reduced by 2 or a factor of 1.4. 43 Factors that reduce contrast, such as glare, will have a relatively weak effect on acuity. In a study of glare sen sitivity with simulated ocular turbidity, the glare light caused contrast sensitivity to decline rapidly, while visual acuity remained near normal. 40 Therefore, tests based on contrast sensitivity measures should be more sensitive to disability glare than tests based on acuity. The sensitivity of acuity-based glare tests can be increased, however, by either reducing the contrast of the acuity targets41 •44 or increasing the intensity of the glare source (although high intensity glare sources may introduce confounding vari ables such as pupillary constriction and photoreceptor bleaching). Further research is necessary to determine whether or not contrast sensitivity-based measures provide different disability glare information from acuity-based tests. In addition to differences in the visual function mea sured, glare tests vary in the type of glare source used. Most of the early studies used point-source glare. This type of glare source requires simple instrumentation (usually a single light bulb), provides a good simulation of environmental glare sources such as automobile head lights, and is amenable to simple models of veiling lu minance. However, small glare sources require a relatively 1235
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high luminous intensity to produce the desired effect and may draw the observer's fixation away from the target to the glare light. 38 This may result in an afterimage which interferes with the visual function measurement. Extended glare sources, such as a bright annulus produced by a circular fluorescent tube or a bright rectangular back ground, cause fewer afterimage problems and are said to be better accepted by patients. 40 Intensities of the glare sources also vary widely from study to study. The Miller Nadler glare tester, the first such device commercially available in the United States, projects a dark Landolt C on a bright background. 45 At the other extreme, Applegate used a mesopic source for his laboratory glare tester.33 Two studies have systematically varied glare intensity. Abrahamsson and Sjostrand 38 report that glare sensitivity of cataract patients decreased as the luminance of an an nular glare source was reduced from 200 cd/m2 to 2 cd/ m 2 • However, Applegate 33 found that lower glare inten sities increased glare sensitivity in patients after radial keratotomy (RK). He argues that lower intensities are preferable for evaluating headlight glare problems in RK patients because pupillary dilation is then comparable to that obtained during nighttime driving conditions. It is also important to recognize that if the glare intensity is too high, a false disability can be diagnosed. The normal cornea, lens, and vitreous combine to scatter between 10 to 20 percent of the light incident on the eye. Thus a very bright glare source can reduce contrast sensitivity in a normal eye. 46 Regardless of the visual function test or glare source chosen, there are several basic principles of clinical vision test design that should be followed: l . The test should be "criterion-free" . Although the patient's subjective responses should not be ignored, the objective test results should not be affected by whether the subject is more or less cautious in his or her responses. For example, a test which allows the patient to decide if the target is visible or not will produce one result with an individual who is very cautious and demands that the target be clearly discernible before venturing a response, and another result with a patient who is willing to guess at anything. Forced-choice procedures circumvent this problem. A standard eye chart uses a forced-choice pro cedure when the patient is asked to identify the letter (make a "forced choice" ) and the examiner determines whether the answer is correct or incorrect. If the patient is allowed to answer that he or she cannot see the letter, then the test is not forced-choice. Forced-choice tests yield more reliable results, especially with unpracticed observ ers, than criterion-dependent tests.47 •34 2. The test targets should follow a uniform progression. For acuity targets this means that letter size should de crease in equal logarithmic steps. For contrast sensitivity, target contrast should decline in equal logarithmic steps. U niforrn step size is important for making quantitative comparisons, such as comparing glare sensitivity before and after cataract surgery across patients who may have different overall sensitivities. Surprisingly, many Snellen acuity charts do not follow a uniform progression ofletter sizes, although the ETDRS acuity chart, 48 which is quickly 1236
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becoming a clinical standard, adheres to equal logarithmic steps. 3. To insure high test-retest reliability, it is necessary to have several trials at each level of difficulty, while test efficiency dictates that the number of trials be kept to a minimum. An often overlooked factor for determining reliability is the criterion used by the examiner to score the test. For example, must the patient identify all letters correctly on the 20/20 line or will one or two errors be acceptable to earn a 20/20 acuity score? A detailed study of the effects of trial number and scoring criterion on test efficiency and reliability indicates that requiring two cor rect responses out of three trials provides a reasonable compromise. 49 In summary, while it is premature to establish definitive guidelines for supplemental tests to visual acuity in as sessing the overall visual disability from immature cata racts, several recommendations emerge. Contrast sensi tivity, by itself, is not specific enough for this purpose. However, contrast sensitivity or low-contrast visual acuity, measured before and after adding a glare source, is prob ably sufficiently specific and sensitive. While contrast sen sitivity is more sensitive to glare than visual acuity, the sensitivity of acuity-based glare tests can be improved by reducing the contrast of the acuity targets or, along with some confounding variables, by increasing the intensity of the glare source. The optimal glare source for reliable and reproducible testing, however, has not yet been iden tified. Glare tests, like all visual function tests, should ad here to test procedures which maximize test reliability and interpretability. At this time there is no universally accepted standard for glare disability testing and no agreement on how the results of a glare disability test should be used in the eval uation of patients for medical or surgical therapy. The presently available glare tests may be of help in adding to the objective assessment of the impact on visual disability of anterior segment disease. Glare testing should not be considered mandatory in the preoperative assessment of patients, and if used, should not be used as the sole cri terion for surgical intervention. Glare testing may be used as an adjunct to the clinical history, to the ophthalmologic examination, and to other tests of visual function to pre dict the objective visual acuity outcome of surgery. This predicted objective result should be carefully considered in terms of its likely effect on the individual patient's quality of life, and weighed against the known risks of surgical intervention.
ACKNOWLEDGMENTS Original draft by: Gary S. Rubin, Ph.D.; Reviewed by: Ivan Bodis-Wollner, M.D., John D. Bullock, M.D., Jack T. Holladay, M.D., Gary F. Jaffe, M.D., Douglas Koch, M.D., Philip Lempert, M.D., Mark J. Mannis, M.D., Michael F. Marmor, M.D., Samuel Masket, M.D., David Miller, M.D., Albert C. Neumann, M.D., David M. Regan, D.Sc., Frederick A. Richburg, M.D., Charles B. Slonim, M.D., Joseph W. Weiss, M.D., Jonathan Wirtschafter, M.D.; Edited by: David L. Guyton, M.D., Gary S. Rubin, Ph.D.; Approved by: Board of Directors, September 17, 1989.
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REFERENCES 1. Bodis-Wollner I. Visual acuity and contrast sensitivity in patients with cerebral lesions. Science 1972; 178:769-71. 2. Bodis-Wollner I, Diamond SP. The measurement of spatial contrast sensitivity in cases of blurred vision associated with cerebral lesions. Brain 1976; 99:695-710. 3. Regan 0, Silver R, Murray TJ. Visual acuity and contrast sensitivity in multiple sclerosis-hidden visual loss: an auxiliary diagnostic test. Brain 1977; 100:563-79. 4. Regan 0, Neima D. low contrast letter charts in diabetic retinopathy, ocular hypertension, glaucoma, and Parkinson's disease. Br J Ophthalmol1984; 69:885-9. 5. Ross JE, Bron AJ, Clarke DO. Contrast sensitivity and visual disability in chronic simple glaucoma. Br J Ophthalmol1984; 68:821-7. 6 . Atkin A, Bodis-Wollner I, Wolkstein M, Moss A, Podos SM. Abnor malities of central contrast sensitivity in glaucoma. Am J Ophthalmol 1979; 88:205-11. 7. Howes SC, Caelli T, Mitchell P. Contrast sensitivity in diabetics with retinopathy and cataract. Aust J Ophthalmol1982; 10:173-8. 8. Wolkstein M, Atkin A, Bodis-Wollner I. Contrast sensitivity in retinal disease. Ophthalmology 1980; 87:1140-9. 9. Campbell FW, Green DG. Optical and retinal factors affecting visual resolution. J Physiol1965; 181 :576-93. 10. Marmor MF, Gawande A. Effect of visual blur on contrast sensitivity. Ophthalmology 1988; 95:139-43. 11. Hecht G, Hendley CD, Frank S, Schlaer S. Contrast discrimination charts for demonstrating the effects of anoxia on vision. J Opt Soc Am 1949; 39:922-3. 12. Schade OH. Optical and photoelectric analog of the eye. J Opt Soc Am 1956; 46:721-39. 13. Campbell FW, Robson JG. Application of Fourier analysis to the visibility of gratings. J Physiol1968; 197:551-66. 14. Ginsburg AP, Easterly J, Evans OW. Contrast sensitivity predicts target detection field performance of pilots. Proceedings of the Human Fac tors Society· 27th Annual Meeting 1983; 269-73. 15. O 'Neal MR, Miller RE. Further investigation of contrast sensitivity and visual acuity in pilot detection of aircraft. Noninvasive Assessment of the Visual System, 1988 Technical Digest Series, Vol. 3, Washington, D.C., Optical Society of America, 1988; 134-7. 16. Kruk R, Regan 0, Beverley Kl, Langridge T. Correlations between visual test results and flying performance on the advanced simulator for pilot training (ASPT). Aviat Space Environ Med 1981; 52:455-60. 17. Kruk R, Regan D. Visual test results compared with flying performance in telemetry tracked aircraft. Aviat Space Environ Med 1983; 54:906 11 . 18. Rubin GS, Legge GE. The psychophysics of reading . VI. The role of contrast in low vision. Vision Res 1989; 29:79-91 . 19. Marron JA, Bailey ll. Visual factors and orientation-mobility perfor mance. Am J Optom Physiol Opt 1982; 59:413-26. 20. Hess R, Woo G. Vision through cataracts. Invest Ophthalmol Vis Sci 1978; 17:428-35. 21. Lempert P, Hopcroft M, Lempert Y. Evaluation of posterior subcapsular cataracts with spatial contrast acuity. Ophthalmology 1987; 94(S):14 18. 22. Sjostrand J, Frisen l. Contrast sensitivity in macular disease: a pre liminary report. Acta Ophthalmol1977; 55:507-14. 23. Bodis-Wollner I. Differences in low and high spatial frequency vulner abilities in ocular and cerebral lesions. In Maffei L (ed). Pathophysiology of the Visual System. The Hague, Doc W Junk, 1981 ; 195-204. 24. Hyvarinen L, Laurinen P, Rovamo J. Contrast sensitivity in evaluation of visual impairment due to diabetes. Acta Ophthalmol1983; 61:94 101.
25. Lundh BL. Central and peripheral contrast sensitivity for static and dynamic sinusoidal gratings in glaucoma. Acta Ophthalmol1985; 63: 487-92. 26. Guyton Dl. Preoperative visual acuity evaluation. lnt Ophthalmol Clin 1987; 27:140-8. 27. Van den Berg TJTP. Importance of pathological intraocular light scatter for visual disability. Doc Ophthalmol1986; 61:327-33. 28. Hess RF, Carney LG. Vision through an abnormal cornea; a pilot study of the relationship between visual loss from corneal distortion, corneal edema, keratoconus and some allied corneal pathology. Invest Ophthalmol Vis Sci 1979; 18:476-83. 29. Carney LG, Jacobs RJ. Mechanisms of visual loss in corneal edema. Arch Ophthalmol1984; 102:1068-71 . 30. Hirsch RP, Nadler MP, Miller D. Glare measurement as a predictor of outdoor vision among cataract patients. Ann Ophthalmol 1984; 16: 965-8. 31. Knighton RW, Slornovic AR, Parrish RK. Glare measurements before and after neodymium-VAG laser posterior capsulotomy. Am J Ophthalmol1985; 100:708-13. 32. Nadler OJ, Jaffe NS, Clayman HM, et al. Glare disability in eyes with intraocular lenses. Am J Ophthalmol1984; 97:43-7. 33. Applegate RA, Trick LR, Meade DL, Hartstein J. Radial keratotomy increases the effects of disability glare: initial results. Ann Ophthalrnol 1987; 19:293-7. 34. Rubin GS, Sunness JS. Assessing visual function in patients with macular edema. Noninvasive Assessment of the Visual System, 1988 Technical Digest Series, Vol. 3, Washington , D.C., Optical Society of America, 1988; 140-3. 35. Holladay JT, Trujillo J, Prager TC, Ruiz RS. Brightness acuity test and outdoor visual acuity in cataract patients. J Cataract Refract Surg 1987; 13:67-9. 36. Neumann AC, McCarty GR, Steedle TO, et al. The relationship between cataract type and glare disability as measured by the Miller-Nadler Glare Tester. J Cataract Refract Surg 1988; 14:40-5. 37 . Neumann AC, McCarty GR, Locke J, Cobb B. Glare disability devices for cataractous eyes: a consumer's guide. J Cataract Refract Surg 1988; 14:212-6. 38. Abrahammson M, Sjostrand J. Impairment of contrast sensitivity func tion (est) as a measure of disability glare. Invest Ophthalmol Vis Sci 1986; 27:1131-6. 39. Pulling NH, WolfE, Sturgis SP, Vaillancourt DR, Dolliver JJ. Headlight glare resistance and driver age. Human Factors 1980; 22:103-12. 40. Miller 0, Jernigan ME, Molnar S, et al. Laboratory evaluation of a clinical glare tester. Arch Ophthalmol1972; 87:324-32. 41. Lempert P, Hopcraft M, Lempert Y. Evaluation of posterior subcapsular cataracts. Ophthalmology 1987; 94:14-18. 42. Wolf E. Glare and age. Arch Ophthalmol1960; 64:502-54. 43. Legge GE, Rubin GS, Luebker A. Psychophysics of reading . V. The role of contrast in normal vision . Vis Res 1987; 27:1165-77. 44. Regan D. Low-contrast letter charts and sinewave grating tests in ophthalmological and neurological disorders. Clin Vision Sci 1988; 2: 235-50. 45. LeClaire J, Nadler MP, Weiss S, Miller D. A new glare tester for clinical testing - results comparing normal subjects and variously corrected aphakic patients. Arch Ophthalmol1982; 100:153-8. 46. Miller 0, Benedek S. Intraocular Light Scattering. Springfield, IL, CC Thomas, 1973; 38. 47 . Higgins KE, Jaffe MJ, Coletta NJ, et al. Spatial contrast sensitivity. Importance of controlling the patient's visibility criterion. Arch Ophthal mol1984; 102:1035-41 . 48. Ferris F, Kassof A, Bresnick A, et al. New visual acuity charts for clinical research. Am J Ophthalmol 1982; 94:91-6. 49. Pelli DG, Robson JG, Wilkins AJ . The design of a new letter chart for measuring contrast sensitivity. Clin Vision Sci 1988; 2:187-99.
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