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Home , Contrast (vision), Glare (vision)

Beyond 20/20

Contrast Sensitivity, Glare, and Quality of Vision Susan Stenson, MD Denis Fisk

Background An understanding of what constitutes good or normal the traditional criterion employed for determining vision is fundamental to the eye care practitioner when surgery is indicated (with 20/50 or less whose goal is to provide the best vision care to his/her best corrected acuity being the usual cut-off point for patients. Traditionally, performance on the Snellen recommending surgery), many practitioners have chart has been used to measure how well a patient been forced to deal with patients with better than sees, with 20/20 vision being the gold standard for 20/50 vision, but with significant visual disabilities what is considered normal. But life is not lived in a associated with their , whom they suspect refraction lane and an individual’s visual demands successful surgery would benefit. Because of this, any extend beyond the boundaries of the phoroptor. A number of practical and ethical questions could arise: “perfect” 20/20 vision measured in the doctor’s office Who should decide when cataract surgery is indicat- may be far from perfect in the real world. Even in the ed: the cataract surgeon, the cataract patient, or the same individual, what is generally considered to government? When is cataract surgery really neces- be good vision may deteriorate into not-good- sary? Is 20/50 Snellen acuity a reasonable cut-off enough vision under specific work or environmental point to define visual disability? And, if so, why are conditions. some individuals with 20/50 (or even worse) visual Perhaps in no other area has the inadequacy of acuity satisfied with their vision, while others with Snellen acuity as the sole determinant of visual better vision are not? Are there more reliable ways to performance been recognized as in the evaluation of measure functional and to determine cataracts. Cataracts are the most common cause of visual disability than using standard Snellen acuity? blindness and visual disability worldwide. Cataract While non-traditional, non-Snellen modalities to surgery is the most frequently performed operation in assess visual function have existed for some time, the United States, with an estimated 2.5 million their use has been largely confined to the area of procedures done annually. Advances in surgical vision research. Thoughtful practitioners began techniques and visual rehabilitation of patients adapting these modalities for use in clinical practice. undergoing cataract surgery have greatly increased sensitivity and glare testing were employed the pool of mild-to-moderate cataract patients who as adjuncts to standard Snellen acuity in assessing the are potential candidates for this operation. As might need for cataract surgery. Quality of vision (as deter- be expected, this has led to concerns on the parts of mined by contrast sensitivity and glare testing) medical and consumer watchdog groups—as well as became as important a criterion as quantity of vision the federal government, who is largely responsible for (as measured by Snellen acuity) in deciding whether picking up the tab for cataract surgery under the cataract extraction was indicated. Medicare program—regarding the possibility of Not surprisingly, accusations were then made that unnecessary surgery. While Snellen acuity has been these non-traditional arbiters of visual function were being abused by over-eager cataract surgeons to justi- 20/20: The Problem With Snellen Vision fy operations when the Snellen acuity appeared ade- Snellen Acuity One of the problems with standardized Snellen acuity quate, with reports of 20/20 vision cataracts being What Is Snellen Acuity? is precisely that it is standardized. In real-world view- performed in some cases. The Snellen chart has become something of an oph- ing, conditions are not standardized. They vary and The situation became a public health issue when in thalmic icon when it comes to measuring vision. And these variations can cause a normal 20/20 vision 1989 the American Academy of con- 20/20, in addition to being considered synonymous measured under conditions of high illumination in the vened a panel of medical and research experts to with perfect vision, has evolved into a term unto absence of glare to deteriorate to a far less than 20/20 determine how to reliably test for acceptable vision itself, an integral part of our vocabulary. But what is functional vision when illumination is reduced or and determine visual disability. The result was an Snellen acuity? What does 20/20 actually mean? glare conditions arise. Simply put, the Snellen chart is Ophthalmic Procedures Assessment, which addressed Snellen acuity is all about spatial resolution—the in black and white, while the real world exists in the role and value of tests other than the standard spatial resolution capacity of the central , more shades of gray. It is these shades of gray that need to Snellen acuity—specifically contrast sensitivity and accurately. Measuring visual acuity indirectly assesses be addressed in discussing quality-of-vision issues. glare testing—in assessing visual function in anterior the spatial resolution capacity of the central retina. segment diseases, particularly in cataracts. The report The higher the spatial resolution, the better the concluded that “…while it is premature to establish vision. The theory behind letter acuity is directly Figure 2. Michaelson formula. definitive guidelines for supplemental tests to visual related to spatial resolution capacity as measured Contrast Sensitivity acuity in assessing the overall visual disability from with . A consists of spatially repeating What Is Contrast Sensitivity? immature cataracts, contrast sensitivity or low-con- and dark bars. One cycle of a grating consists of trast visual acuity, measured before and after adding a If Snellen acuity measures how well the eyes see in black one light and one dark bar, and when each bar has a glare source, is probably sufficiently specific and sen- and white, contrast sensitivity acuity measures how well width of 30 minutes of minarc, the grating has a spa- sitive.” It went on to suggest that “…glare tests may the eye can discriminate the various shades of gray. tial frequency of 1 cycle per degree (cpd). The Snellen be of help in adding to the objective assessment of the Contrast is a measure of relative distribution of lighter chart presents optotypes of gradually decreasing size impact on visual disability of anterior segment dis- and darker parts of a visual stimulus. It is defined by the and correspondingly increasing cpd. The smaller the ease.” Michaelson formula, which relates the magnitude of the optotype (or the narrower the equivalent grating or Resulting clinical interest in the use of contrast sen- difference in light intensity between the light and dark the higher the cpd), the better the acuity. In a 20/20 sitivity and glare testing to assess quality of vision has areas to the overall of the stimulus: (Lmax- eye, the equivalent of a 30 cpd grating can be led to the increasing use of these tests in other ocu- Lmin)/(Lmax+Lmin), where Lmax is the luminance of resolved. In a 20/200 eye, resolution decreases to 3 lar—and systemic—diseases to determine how vari- the light bars and Lmin is the luminance of the dark cpd (Figure 1). ous disorders might affect visual function. It has also bars (Figure 2). With decreasing contrast, the luminance served to make the eye care practitioner more aware In more practical terms, an individual with 20/20 visual acuity is able to recognize letters that are difference in the grating is reduced until, at some level, that quantity of vision may not necessarily equate the luminance difference is too small to be perceived. with quality of vision, and to help explain the patient approximately 1/3-inch tall on a Snellen chart from a This point represents the contrast threshold. Contrast who consistently tests 20/20 in the practitioner’s distance of 20 feet. When vision is less than 20/20, the Figure 3. Contrast and spatial frequency. thresholds are normally related to spatial frequency by office but remains dissatisfied with his/her vision. denominator of the fraction indicates the equivalent distance at which a normally sighted observer can the contrast sensitivity function (CSF) (Figure 3). to bottom. Subjects read the letters from top to bot- It is important for the eye doctor to realize that tom and the smallest identifiable letter is recorded for there is more to testing—and correcting—vision than identify the letters. With 20/200 visual acuity, for How Is Contrast Sensitivity Function Measured? each chart. Using a nomogram supplied with the using the Snellen chart. Discrepancies between the example, the observer would have to be at 20 feet to Contrast sensitivity function can be measured clinically charts, a line is drawn between these two acuity meas- quantity and quality of vision may signal the possibil- identify the same letter that a 20/20 sighted observer using special charts (eg, Peli-Robson and Regan low- ures. Contrast deficits are indicated if the slope of the ity of underlying disease. And even in the normal eye, could identify at 200 feet. contrast acuity charts). line is steeper than normal. the quality of vision and the visual experience may be With the Peli-Robson chart, letter optotypes are pre- affected by non-ocular factors—specifically environ- sented at a fundamental spatial frequency of 0.5 cpd. Why Is Contrast Sensitivity Important? mental conditions related to light exposure and mod- The chart consists of two groups of three letters per The world is a visually complex place. Objects vary in ulation—that may be addressed by the judicious use many dimensions, including size, brightness, and con- of spectacle lens treatments to provide the patient row. The contrast of each letter group decreases from trast. Standard Snellen visual acuity measurements with the best quantity and quality of vision possible 90% at the top of the chart to 0.5% at the bottom. only provide information about high contrast resolu- under diverse circumstances. A meticulously per- Subjects are required to read the letters from top to tion (ie, the smallest, high contrast object that can be formed refraction is the surest way for the practition- bottom until two of three letters are named incorrectly. er to provide 20/20 vision to the ametropic patient The Regan low contrast acuity test consists of two seen). Contrast sensitivity testing helps provide when ocular health allows. The appropriate use of charts of letter optotypes. The contrasts of the letters important additional information about the visual spectacle lens treatments enables the practitioner to are 96% and 11%. All of the letters on a single chart world. This includes information about the visibility go beyond 20/20 with his/her patient and provide the have the same contrast and decrease in size from top of objects that vary in size, contrast, and orientation. maximum quantity and quality in vision correction. Figure 1. Comparison of Snellen acuity and contrast acuity. 2 3 Contrast sensitivity losses can occur at high, low, and ticularly diabetic retinopathy—the eye care practition- broad spatial frequencies. Various ocular and systemic er must be aware that visual complaints may some- diseases can affect contrast sensitivity functions in dif- times point to a diagnosis of unsuspected diabetes and ferent ways and at different frequencies (Figures 4-6). It screen patients accordingly. This is especially impor- is especially important in patients presenting with nor- tant since diabetic retinopathy produces 12000-14000 mal Snellen acuity but with persistent visual complaints cases of potentially preventable or treatable blindness to consider evaluating contrast sensitivity to rule out each year. possible contributing ocular—or even systemic— In many of the previously mentioned conditions, disease that might be affecting the quality of vision. Snellen acuity will be affected as well, indicating to The value of contrast sensitivity testing in assessing the clinician that ocular health has been compro- visual impairment in cataract patients has already been mised. Occasionally, however, significant ocular dis- discussed. However, the usefulness of contrast sensitiv- ease may not be manifested by changes in Snellen acu- Figure 4. High spatial frequency loss and eye disease. ity, along with glare testing, in determining the need for ity, and it is only after contrast sensitivity is found to Figure 9. . cataract surgery with mild-to-moderate anatomical be affected that more careful clinical evaluation cataracts cannot be overemphasized. With most reveals the presence of such sight-threatening disor- types of cataracts, a broad spatial frequency loss moderate-to-severe , ders as optic neuritis, RP, or . is encountered. cataracts, and advanced chronic Contrast Sensitivity and Normal Eyes High spatial frequency losses can be produced by open-angle glaucoma with wide- optical or non-optical abnormalities (Figure 4). spread field defects (Figure 11). The real world is not black and white. It is the various Conditions affecting the optical quality of the eye that Diabetic retinopathy will often shades of gray encountered in everyday life that make Figure 10. produce broad spatial frequency loss contrast sensitivity crucial in determining how well may lead to high spatial frequency losses include Retinitis pigmen- refractive errors, mild cortical or nuclear cataracts, tosa. (Slide cour- (Figure 12). The incidence of dia- even a normally sighted individual truly sees. If the and various corneal disorders (eg, edema, irregulari- tesy of Dr. Irwin betes mellitus is assuming epidemic properly prescribed modern spectacle lens defines the ties, or opacities) (Figures 7 and 8). Among the non- Siegel). proportions in the United States at black and white for the wearer, spectacle lens treat- optical abnormalities leading to high spatial frequency this time, with an estimated 17 mil- ments—such as photochromics and anti-reflection losses are mild amblyopia, macular disease, chronic coatings—may serve to fill in the gray in between. open-angle glaucoma with moderate visual field loss, And, although contrast sensitivity testing may appear and retinitis pigmentosa (RP) in its early stages to be primarily a laboratory tool, the fact is that meas- Figure 5. Low spatial frequency loss and eye disease. (Figures 9 and 10). uring contrast sensitivity acuity clinically is one way to go beyond the simple quantification of vision and gain important information about quality of vision. Recent research studied the effects of various spec- tacle lens tints at different levels of transmittance on Figure 11. Extensive visual field loss in an advanced case of contrast sensitivity acuity in normal subjects and in chronic open-angle glaucoma. those with incipient senile cataracts. The impetus for the research was the realization that while spectacle lion Americans lens tints are important in attenuating excessive light currently affected; exposure and promoting visual comfort by decreas- of these, roughly ing illumination when necessary, the very decrease in 5.9 million are illumination that is produced might adversely affect undiagnosed. An contrast sensitivity. The aims of the study were to average of 5 years determine: 1) if contrast sensitivity function was Figure 7. Moderate cataract. may elapse before affected by spectacle lens tints and 2) whether there the onset of type were differences in how various tints affected con- Figure 6. Broad spatial frequency loss and eye disease. II diabetes and its trast sensitivity. Selective loss at low spatial clinical recogni- Results demonstrated that all spectacle lens tints Contrast Sensitivity and Ocular Disease frequency (Figure 5) is most com- tion. Because of tested (gray, brown, yellow, green, purple, and blue) Perhaps the most important determinant of contrast monly seen with optic nerve dis- the strong associ- produced an increase in contrast thresholds under sensitivity is the health of the eye. A corollary of this ease, especially in cases of optic Figure 8. Irregular ation between glare conditions. There were definite differences in the corneal surface and is that abnormalities in contrast sensitivity may point neuritis. Figure 12. Diabetic retinopathy. (Slide diabetes and ocu- amount of increase with the various tints, however, opacification in a courtesy of Dr. Carol Lee). lar disease—par- and these differences varied between the normal and to the possibility of underlying ocular disease and Broad spatial frequency loss case of granular alert the clinician to the need for additional testing. (Figure 6) is characteristic of corneal dystrophy. 4 5 the cataractous eyes. In the latter, brown or yellow and constriction of the pupil. Often the affected indi- macular edema (Figures 13 and 14). An area of great tints caused the least change in thresholds, while in vidual will try to avoid the glare by shielding the eyes interest relating to glare sensitivity at this time is in the Figure 13. Severe the former, purple and gray tints were preferable. or turning the head in another direction. corneal edema in a post-operative refractive surgery patient. Both with These differences are probably related to changes in case of congenital older techniques (radial keratotomy and photorefrac- Disabling Glare clarity and transmission characteristics of the normal glaucoma. tive keratoplasty or PRK) and newer techniques versus the cataractous lens. Disabling or veiling glare is when the level of light (LASIK and LASEK), a variety of qualitative vision increases to 10000 lumens or more and it produces a issues may arise, even in the face of an excellent quan- glare that can actually interfere with or block vision. titative (20/20) result (Figure 15). These include prob- This type of glare causes objects to appear to have Figure 14. lems with night vision, distortion, ghost images, Glare lower contrast than they would were there no glare. It Pseudophakic cystoid monocular diplopia, and glare. Many of these are occurs because the eye is not a perfect optical system macular edema attributable to corneal haze and surface irregularities What Is Glare? (Slide courtesy of due to inhomogeneities in the optical media that lead after surgery, producing increased incident light scatter Glare is the loss in visual performance or visibility, or Dr. Carol Lee). to light scattering which, in turn, reduces visual acuity with resultant Discomforting or even Disabling glare. the annoyance or discomfort, produced by a lumi- and raises the differential light threshold. Disabling nance in the visual field greater than the illuminance to glare tends to become more problematic in the elderly, which the eyes are adapted. Luminance is defined in as the decreasing transparency of the crystalline lens terms of the lumen: a unit of measurement of the that comes with age leads to incipient cataract forma- Figure 15. Corneal haze after LASIK (Slide Photophobia amount of light incident on a surface. The higher the tion. luminance, the brighter the surface. courtesy of Dr. Wilson Ko). What Is Photophobia? Optimal is in the range of 1000-1400 Blinding Glare Photophobia is a symptom, not a disease. Photophobia, lumens. Examples of typical environmental lumi- Blinding glare results from incident light reflecting as its name implies, is “a fear of light.” It is not the same nances include: from smooth shiny surfaces such as water and snow, as glare, although individuals who are photophobic and becoming plane polarized. It can block vision tend to be more bothered by the effects of glare than Indoor, artificial light 400 lumens to the extent that the wearer becomes visually those who are not. With glare, it is the amount of light Sunny day, shady side of street 1000-1400 lumens compromised. How Does Glare Affect the Normal Eye? or how it is presented that produces the problem. With Sunny day, sunny side of street 3500 lumens The normal cornea, lens, and vitreous scatter 10%- photophobia, it is not necessarily the amount or the Concrete highway 6000-8000 lumens How Does Glare Differ From Contrast Sensitivity? 20% of incident light. Glare is caused by light scatter presentation that is the problem, it is simply the light Beach or ski slopes 10000-12000 lumens There exists considerable confusion about the and is influenced by the dynamics of light-to-dark-to- difference between contrast sensitivity and glare. This itself. light and by retinal photoreceptor satura- Photophobia, or light sensitivity, is one of the most Glare may come directly from a light source (eg, is because glare is used in testing contrast sensitivity. tion. In the normal eye, increasing levels of glare will common complaints made to the eye doctor. It should facing toward the sun) or be reflected. There are four Simply put, contrast sensitivity is about differentiat- increase baseline incident light scatter and adversely be separated into two categories: pathological photo- types of glare: Distracting glare, Discomforting glare, ing the various shades of gray. Glare, on the other affect contrast sensitivity. The result is visual discom- phobia, where there is demonstrable ocular disease Disabling glare, and Blinding Glare. hand, relates to how it becomes more difficult to fort and fatigue. differentiate those various shades of gray when illumi- to account for the light sensitivity; and non-patholog- A frequent complaint in normally sighted individuals Distracting Glare nance is excessive. Contrast sensitivity tests measure ical photophobia, where there is no obvious ocular relates to difficulties encountered with glare during Distracting glare results from light being reflected the amount of contrast necessary to recognize a abnormality to explain the symptom. night driving. Studies have demonstrated that in sub- from the surface of an optical medium. Wherever the target. Glare sensitivity tests measure the change in jects with healthy eyes, glare sensitivity correlates well Pathological Photophobia incident light moves from one optical medium to visual function that results from a glare source in with simulated nighttime driving performance. A variety of eye diseases may lead to pathological another (eg, from air to glass) some of the incident another part of the field of vision. photophobia. This type of photophobia can be truly light is reflected. This results in reflections from the How Does Glare Affect the Abnormal Eye? How Is Glare Tested? disabling, even incapacitating at times, serving to lens surface or in the presence of halos around bright Since glare depends on light scatter in the ocular Glare sensitivity may be tested using contrast detec- compound the basic visual deficit caused by the at night. Distracting glare can represent an media, any abnormality or inhomogeneity in the ocu- tion tasks or by acuity-based measures. The majority underlying disease. annoyance to the viewer and lead to eye fatigue. lar media that further increases intraocular light scat- of glare tests in current use assess the effects of glare ter will increase glare sensitivity. It is generally accept- Discomforting Glare on contrast sensitivity by measuring contrast thresh- ed that glare tests are more specific for anterior seg- olds in the presence or absence of glare, since veiling Discomforting glare may result from direct or reflected ment disorders than simple contrast sensitivity tests. luminance reduces image contrast. Acuity-based tests glare. It ranges from 3000 lumens up to about 10000 For example, it has been shown that corneal edema rely on the fact that acuity is affected by changing the lumens, at which point the glare becomes produces only minimal effects on contrast sensitivity, contrast of the acuity targets. Targets used in glare disabling. Even mild degrees of discomforting glare but can lead to a threefold increase in glare sensitivity. testing may be point sources or extended-glare produce ocular discomfort, often manifested by Among the ocular diseases which affect glare sensi- Figure 16 a & b. Ocular albinism: (a) Iris tran- sources. Subjects generally are more comfortable with sillumination; (b) Albinoid fundus (Slides cour- symptoms of asthenopia or fatigue. The unprotected tivity are corneal edema, irregularity, and opacifica- the latter. tesy of Dr. Irwin Siegel). eye will respond to discomforting glare by squinting tion; cataract and aftercataract; vitreous syneresis; and

6 7 Probably the most commonly recognized ocular condition associated with photophobia is albinism (Figures 16a and b). The characteristic white-blond Figure 17. Corneal crys- hair, brows, and lashes; pale talline deposition in a complexion; light blue (almost case of rosacea keratitis. transparent) irides; hypopig- mented fundi; nystagmus; and poor vision usually point Figure 22. Photochromic chemical reaction. to the diagnosis in individuals with this genetic disease. Certain rare metabolic disorders where crystalline of the tint. The darker the tint, the more the light is lenses allow light to work best for the eye and permit deposits accumulate in the cornea may also produce filtered, and the greater the shift towards the scotopic the eye to function at its peak under various levels of pathological photophobia (eg, cystinosis). Secondary Figure 20. Contrast sensitivity and light conditions. side, with a resulting increase in contrast sensitivity illumination (Figure 23). crystalline or lipid deposition can occur in the cornea as threshold and a corresponding decrease in CSF. Tints, Another useful spectacle lens treatment is the anti- a result of chronic inflammato- Optical Solutions then, should be expected to adversely affect vision. reflection (AR) coating to reduce Distracting glare. ry disease (Figure 17). Transient All of the subjects discussed so far share one common But the real-life, real-vision situation is significantly Antireflection coatings function by reflecting light. irregularities or defects in the element: a problem with light—too much, not enough, more complicated. Glare is intimately related to con- The reflected light from the lens coating interferes corneal surface can induce tem- the wrong kind, and difficulties with presentation. trast sensitivity function (Figure 21). Excessive glare with the light being reflected from the lens substrate porary disease-based photopho- Actually, they share a second element: the possibility of decreases CSF. Spectacle lens treatments that decrease or underlying layer. These coatings do not effectively bia. These clinical manifesta- Figure 18. employing available spectacle lens treatments to correct glare should improve contrast sensitivity. So do filter out or attenuate non-glare light stimuli and tions are most commonly Keratoconjunctivitis or alleviate the problem. these t r eatments help or hinder the visual therefore do not shift the photopic-scotopic curves. encountered in traumatic sicca: Rose Bengal A variety of spectacle lens treatments can be used experience? The answer lies in the balance—the By minimizing unwanted reflections while still maxi- corneal abrasions. In individu- staining. alone or in combination to moderate or modulate the balance between the amount of light and the mizing transmitted light, the coatings enhance the als with severe keratoconjunctivitis sicca and associated amount and quality of light presented to the eye. They amount of glare. Depending on the specific quality of vision while decreasing ocular discomfort corneal epithelial damage, photophobia is a frequent are helpful in alleviating the ocular discomfort resulting illumination under conditions of low-to-moderate light with glare. complaint (Figure 18). from excessive illumination, bothersome reflections, conditions, a fil- The combination of a photochromic lens with an AR Since the iris serves to control the amount of light enter- and glare, and, in so doing, serve to enhance both visu- ter or tint can coating would appear to be the ideal in achieving the ing the eye, any abnormality in the structure or integrity of al comfort and performance. But to work well, these either facilitate or proper balance between illumination and glare to maxi- the iris may lead to pathological photophobia. This can treatments must be used selectively and efficiently. compromise mize contrast sensitivity function, and offer visual com- develop primarily (eg, in con- Contrast sensitivity depends on light levels and is vision. This is fort and convenience under varying light conditions. genital aniridia or mesodermal generally optimal under photophobic (ie, daylight) con- why clear lenses One other important aspect of light and vision when dysgenesis [Figure 19]) or sec- ditions, with contrast thresholds increasing progressive- Figure 23. Photochromic lenses: and fixed-tint considering spectacle lens treatments is polarized light. ondarily, after damage to the iris ly as light dims (mesopic and scotopic conditions) UVR-mediated reaction. sunglasses are Ambient is unpolarized. With unpolarized as a result of surgery, trauma, or (Figure 20). Filters or tints artificially produce a shift inadequate and incomplete solutions to the light, the direction of vibration is random (ie, in all inflammation. Pharmacological Figure 19. Multiple iris from the photopic towards the scotopic state, with the light/vision equation. What should be ideal is not a directions). When light is reflected from a surface, it is dilation of the pupil will also defects in a case of meso- amount of this shift directly proportional to the density constant or fixed tint, but an as-needed, on-demand partially or completely plane polarized, with the plane dermal dysgenesis. cause temporary photophobia. type of light modulation that decreases incident light of polarization of the reflected light perpendicular to the Non-Pathological Photophobia when levels are too high or glare conditions arise yet plane of incidence of the light. Light incident on smooth allows sufficient light into the eye when levels are surfaces such as glass, concrete, or water produces Most individuals who complain of photophobia, how- lower and glare is not an issue. This is the primary bothersome polarized light (Blinding glare). This can be ever, fall into the category of non-pathological photo- advantage that photochromic (ie, “colored by light”) eliminated through the use of a polarizing lens oriented phobia. These are typically the same people who com- lenses offer to the wearer. These variable tint plain of having “sensitive eyes.” Although this has tra- lenses rely on ultraviolet radiation-induced ditionally been associated with fair-skinned, light-eyed chemical reactions to produce a darkening of the lens individuals, there does not appear to be any proven upon exposure to light, with a return to the clear state racial or ethnic bias in this regard, with many people of when the light stimulus is removed (Figure 22). African-American and Hispanic descent having sympto- Photochromic lenses are indeed “colored by light” matic non-pathological photophobia. In these individu- and, with newer designs that are essentially clear als, visual acuity is typically normal and there is no indoors or under low light conditions but darken to obvious ocular pathology to account for the light sensi- Figure 21. Effect of glare on contrast sensitivity. the level of the standard fixed-tint sunglass outdoors tivity described. or under conditions of intense incident light, these Figure 24. Polarized lenses.

8 9 with its vibration plane perpendicular to the reflected light gy has evolved to the point where there is a spectacle lens (Figure 24). Polarized lenses eliminate reflected glare, treatment or combination of treatments to meet the visu- improving the quality of vision and relieving visual discom- al requirements of most people under most circum- fort. This is especially important for individuals who work stances. It is the responsibility of the eye care practition- outdoors or enjoy water sports or skiing. er to assist the vision care consumer in choosing these The relationship between light and sight remains a treatments wisely. complicated one, but fortunately spectacle lens technolo-

References:

1. Stenson SM, ed. Light, Sight, and Photochromics. 14. Knighton RW, Stomovic AR, Parrish RK 2nd. Glare Pinellas Park, FL. Transitions Optical; 2002. measurements before and after neodymium-YAG laser 2. American Academy of Ophthalmology. Contrast sen- posterior capsulotomy. Am J Ophthalmol. sitivity and glare testing in the evaluation of anterior seg- 1985;100:708-713. ment disease. Ophthalmology. 1990;97:1233-1237. 15. Applegate RA, Trick LR, Meade DL, Hartstein J. 3. Ross JE, Bron AJ, Clarke DD. Contrast sensitivity and Radial keratotomy increases the effects of disability visual disability in chronic simple glaucoma. Br J glare: initial results. Ann Ophthalmol. 1987;19:293-297. Ophthalmol. 1984;68:821-827. 16. Neumann AC, McCarty GR, Steedle TO, Sanders 4. Atkin A, Bodis-Wollner I, Wolkstein M, Moss A, DR, Raanan MG. The relationship between cataract type Podos SM. Abnormalities of central contrast sensitivity and glare disability as measured by the Miller-Nadler in glaucoma. Am J Ophthalmol. 1979;88:205-211. Glare Tester. J Cataract Refract Surg. 1988;14:40-45. 5. Wolkstein M, Atkin a, Bodis-Wollner. Contrast sensitivi- 17. Abrahammson M, Sjostrand J. Impairment of con- ty in retinal disease. Ophthalmology. 1980;87:1140-1149. trast sensitivity function (CSF) as a measure of disability 6. Marron JA, Bailey IL. Visual factors and orientation- glare. Invest Ophthalmol Vis Sci. 1986;27:1131-1136. mobility performance. Am J Optom Physiol Opt. 18. Wolf E. Glare and age. Arch Ophthalmol. 1982;59:413-426. 1960;64:502-514. 7. Lempert P, Hopcroft M, Lempert Y. Evaluation of 19. Regan D. Low-contrast letter charts and sinewave posterior subcapsular cataracts: with spatial contrast grating tests in ophthalmological and neurological disor- acuity. Ophthalmology. 1987;(pt 2):14-18. ders. Clin Vision Sci. 1988;2:235-250. 8. Sjostrand J, Frisen L. Contrast sensitivity in macular 20. Pelli DG, Robson JG, Wilkins AJ. The design of a disease: a preliminary report. Acta Ophthalmol new letter chart for measuring contrast sensitivity. Clin (Copenh). 1977;55:507-514. Vision Sci. 1988;2:187-199. 9. Hyvarinen L, Laurinen P, Rovamo J. Contrast sensitiv- 21. Naidu S, Lee JE, Holopigian K, Seiple WH, ity in evaluation of visual impairment due to diabetes. Greenstein VC, Stenson SM. The effect of variably tinted Acta Ophthalmol (Copenh). 1983;61:94-101. spectacle lenses on visual performance in cataract sub- 10. Guyton DL. Preoperative visual acuity evaluation. jects. Eye Contact Lens. 2003;29:17-20. Int Ophthalmol Clin. 1987;27:140-148. 22. Lee JE, Stein JJ, Prevor MB, Seiple WH, Holopigian 11. van den Berg TJ. Importance of pathological intraoc- K, Greenstein VC, Stenson SM. Effect of variable tinted ular light scatter for visual disability. Doc Ophthalmol. spectacle lenses on visual performance in control sub- 1986;61:327-333. jects. CLAO J. 2002;28:80-82. 12. Hess RF, Carney LG. Vision through an abnormal 23. Fry GA, King VM. The pupillary response and dis- cornea: a pilot study of the relationship between visual comfort glare. J Illum Eng Soc. 1975;5:307-324. loss from corneal distortion, corneal edema, kerato- 24. Seiple WH. The clinical utility of spatial contrast conus, and some allied corneal pathology. Invest sensitivity testing. In: Tasman W, Jaeger EA, eds. Duane’s Ophthalmol Vis Sci. 1979;18:476-483. Foundations of Clinical Ophthalmology. Philadelphia, 13. Carney LG, Jacobs RJ. Mechanisms of visual loss in Pa: Lippincott; 1991. corneal edema. Arch Ophthalmol. 1984;102:1068-1071.

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