Background

An understanding of what constitutes good or normalvision is fundamental to the eye care practitionerwhose goal is to provide the best vision care to his/herpatients. Traditionally, performance on the Snellenchart has been used to measure how well a patientsees, with 20/20 vision being the gold standard forwhat is considered normal. But life is not lived in arefraction lane and an individual’s visual demandsextend beyond the boundaries of the phoroptor. A“perfect” 20/20 vision measured in the doctor’s officemay be far from perfect in the real world. Even in thesame individual, what is generally considered to be good vision may deteriorate into not-good-enough vision under specific work or environmentalconditions.

Perhaps in no other area has the inadequacy ofSnellen acuity as the sole determinant of visual performance been recognized as in the evaluation ofcataracts. Cataracts are the most common cause ofblindness and visual disability worldwide. Cataractsurgery is the most frequently performed operation inthe United States, with an estimated 2.5 million procedures done annually. Advances in surgical techniques and visual rehabilitation of patients undergoing cataract surgery have greatly increasedthe pool of mild-to-moderate cataract patients whoare potential candidates for this operation. As mightbe expected, this has led to concerns on the parts ofmedical and consumer watchdog groups—as well asthe federal government, who is largely responsible forpicking up the tab for cataract surgery under theMedicare program—regarding the possibility ofunnecessary surgery. While Snellen acuity has been

the traditional criterion employed for determiningwhen cataract surgery is indicated (with 20/50 or lessbest corrected acuity being the usual cut-off point forrecommending surgery), many practitioners havebeen forced to deal with patients with better than20/50 vision, but with significant visual disabilitiesassociated with their cataracts, whom they suspectsuccessful surgery would benefit. Because of this, anynumber of practical and ethical questions could arise:Who should decide when cataract surgery is indicat-ed: the cataract surgeon, the cataract patient, or thegovernment? When is cataract surgery really neces-sary? Is 20/50 Snellen acuity a reasonable cut-offpoint to define visual disability? And, if so, why aresome individuals with 20/50 (or even worse) visualacuity satisfied with their vision, while others withbetter vision are not? Are there more reliable ways tomeasure functional visual acuity and to determinevisual disability than using standard Snellen acuity?

While non-traditional, non-Snellen modalities toassess visual function have existed for some time,their use has been largely confined to the area ofvision research. Thoughtful practitioners beganadapting these modalities for use in clinical practice.Contrast sensitivity and glare testing were employedas adjuncts to standard Snellen acuity in assessing theneed for cataract surgery. Quality of vision (as deter-mined by contrast sensitivity and glare testing)became as important a criterion as quantity of vision(as measured by Snellen acuity) in deciding whethercataract extraction was indicated.

Not surprisingly, accusations were then made thatthese non-traditional arbiters of visual function were

Susan Stenson, MDDenis Fisk

Beyond 20/20

Contrast Sensitivity, Glare, and Quality of Vision

20/20: The Problem With Snellen Vision

One of the problems with standardized Snellen acuityis precisely that it is standardized. In real-world view-ing, conditions are not standardized. They vary andthese variations can cause a normal 20/20 visionmeasured under conditions of high illumination in theabsence of glare to deteriorate to a far less than 20/20functional vision when illumination is reduced orglare conditions arise. Simply put, the Snellen chart isin black and white, while the real world exists inshades of gray. It is these shades of gray that need tobe addressed in discussing quality-of-vision issues.

Contrast SensitivityWhat Is Contrast Sensitivity?

If Snellen acuity measures how well the eyes see in blackand white, contrast sensitivity acuity measures how wellthe eye can discriminate the various shades of gray.Contrast is a measure of relative distribution of lighterand darker parts of a visual stimulus. It is defined by theMichaelson formula, which relates the magnitude of thedifference in light intensity between the light and darkareas to the overall luminance of the stimulus: (Lmax-Lmin)/(Lmax+Lmin), where Lmax is the luminance ofthe light bars and Lmin is the luminance of the darkbars (Figure 2). With decreasing contrast, the luminancedifference in the grating is reduced until, at some level,the luminance difference is too small to be perceived.This point represents the contrast threshold. Contrastthresholds are normally related to spatial frequency bythe contrast sensitivity function (CSF) (Figure 3).

How Is Contrast Sensitivity Function Measured?

Contrast sensitivity function can be measured clinicallyusing special charts (eg, Peli-Robson and Regan low-contrast acuity charts).

With the Peli-Robson chart, letter optotypes are pre-sented at a fundamental spatial frequency of 0.5 cpd.The chart consists of two groups of three letters perrow. The contrast of each letter group decreases from90% at the top of the chart to 0.5% at the bottom.Subjects are required to read the letters from top tobottom until two of three letters are named incorrectly.

The Regan low contrast acuity test consists of twocharts of letter optotypes. The contrasts of the lettersare 96% and 11%. All of the letters on a single charthave the same contrast and decrease in size from top

to bottom. Subjects read the letters from top to bot-tom and the smallest identifiable letter is recorded foreach chart. Using a nomogram supplied with thecharts, a line is drawn between these two acuity meas-ures. Contrast deficits are indicated if the slope of theline is steeper than normal.

Why Is Contrast Sensitivity Important?

The world is a visually complex place. Objects vary inmany dimensions, including size, brightness, and con-trast. Standard Snellen visual acuity measurementsonly provide information about high contrast resolu-tion (ie, the smallest, high contrast object that can beseen). Contrast sensitivity testing helps provideimportant additional information about the visualworld. This includes information about the visibilityof objects that vary in size, contrast, and orientation.

being abused by over-eager cataract surgeons to justi-fy operations when the Snellen acuity appeared ade-quate, with reports of 20/20 vision cataracts beingperformed in some cases.

The situation became a public health issue when in1989 the American Academy of Ophthalmology con-vened a panel of medical and research experts todetermine how to reliably test for acceptable visionand determine visual disability. The result was anOphthalmic Procedures Assessment, which addressedthe role and value of tests other than the standardSnellen acuity—specifically contrast sensitivity andglare testing—in assessing visual function in anteriorsegment diseases, particularly in cataracts. The reportconcluded that “…while it is premature to establishdefinitive guidelines for supplemental tests to visualacuity in assessing the overall visual disability fromimmature cataracts, contrast sensitivity or low-con-trast visual acuity, measured before and after adding aglare source, is probably sufficiently specific and sen-sitive.” It went on to suggest that “…glare tests maybe of help in adding to the objective assessment of theimpact on visual disability of anterior segment dis-ease.”

Resulting clinical interest in the use of contrast sen-sitivity and glare testing to assess quality of vision hasled to the increasing use of these tests in other ocu-lar—and systemic—diseases to determine how vari-ous disorders might affect visual function. It has alsoserved to make the eye care practitioner more awarethat quantity of vision may not necessarily equatewith quality of vision, and to help explain the patientwho consistently tests 20/20 in the practitioner’soffice but remains dissatisfied with his/her vision.

It is important for the eye doctor to realize thatthere is more to testing—and correcting—vision thanusing the Snellen chart. Discrepancies between thequantity and quality of vision may signal the possibil-ity of underlying disease. And even in the normal eye,the quality of vision and the visual experience may beaffected by non-ocular factors—specifically environ-mental conditions related to light exposure and mod-ulation—that may be addressed by the judicious useof spectacle lens treatments to provide the patientwith the best quantity and quality of vision possibleunder diverse circumstances. A meticulously per-formed refraction is the surest way for the practition-er to provide 20/20 vision to the ametropic patientwhen ocular health allows. The appropriate use ofspectacle lens treatments enables the practitioner togo beyond 20/20 with his/her patient and provide themaximum quantity and quality in vision correction.

Snellen AcuityWhat Is Snellen Acuity?

The Snellen chart has become something of an oph-thalmic icon when it comes to measuring vision. And20/20, in addition to being considered synonymouswith perfect vision, has evolved into a term untoitself, an integral part of our vocabulary. But what isSnellen acuity? What does 20/20 actually mean?

Snellen acuity is all about spatial resolution—thespatial resolution capacity of the central retina, moreaccurately. Measuring visual acuity indirectly assessesthe spatial resolution capacity of the central retina.The higher the spatial resolution, the better thevision. The theory behind letter acuity is directlyrelated to spatial resolution capacity as measuredwith gratings. A grating consists of spatially repeatinglight and dark bars. One cycle of a grating consists ofone light and one dark bar, and when each bar has awidth of 30 minutes of minarc, the grating has a spa-tial frequency of 1 cycle per degree (cpd). The Snellenchart presents optotypes of gradually decreasing sizeand correspondingly increasing cpd. The smaller theoptotype (or the narrower the equivalent grating orthe higher the cpd), the better the acuity. In a 20/20eye, the equivalent of a 30 cpd grating can beresolved. In a 20/200 eye, resolution decreases to 3cpd (Figure 1).

In more practical terms, an individual with 20/20visual acuity is able to recognize letters that areapproximately 1/3-inch tall on a Snellen chart from adistance of 20 feet. When vision is less than 20/20, thedenominator of the fraction indicates the equivalentdistance at which a normally sighted observer canidentify the letters. With 20/200 visual acuity, forexample, the observer would have to be at 20 feet toidentify the same letter that a 20/20 sighted observercould identify at 200 feet.

Figure 1. Comparison of Snellen acuity and contrast acuity.

Figure 2. Michaelson formula.

Figure 3. Contrast and spatial frequency.

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moderate-to-severe amblyopia,cataracts, and advanced chronicopen-angle glaucoma with wide-spread field defects (Figure 11).

Diabetic retinopathy will often produce broad spatial frequency loss(Figure 12). The incidence of dia-betes mellitus is assuming epidemicproportions in the United States atthis time, with an estimated 17 mil-

lion Americanscurrently affected;of these, roughly5.9 million areundiagnosed. Anaverage of 5 yearsmay elapse beforethe onset of typeII diabetes and itsclinical recogni-tion. Because ofthe strong associ-ation betweendiabetes and ocu-lar disease—par-

ticularly diabetic retinopathy—the eye care practition-er must be aware that visual complaints may some-times point to a diagnosis of unsuspected diabetes andscreen patients accordingly. This is especially impor-tant since diabetic retinopathy produces 12000-14000cases of potentially preventable or treatable blindnesseach year.

In many of the previously mentioned conditions,Snellen acuity will be affected as well, indicating tothe clinician that ocular health has been compro-mised. Occasionally, however, significant ocular dis-ease may not be manifested by changes in Snellen acu-ity, and it is only after contrast sensitivity is found tobe affected that more careful clinical evaluationreveals the presence of such sight-threatening disor-ders as optic neuritis, RP, or glaucoma.

Contrast Sensitivity and Normal Eyes

The real world is not black and white. It is the variousshades of gray encountered in everyday life that makecontrast sensitivity crucial in determining how welleven a normally sighted individual truly sees. If theproperly prescribed modern spectacle lens defines theblack and white for the wearer, spectacle lens treat-ments—such as photochromics and anti-reflectioncoatings—may serve to fill in the gray in between.And, although contrast sensitivity testing may appearto be primarily a laboratory tool, the fact is that meas-uring contrast sensitivity acuity clinically is one way togo beyond the simple quantification of vision and gainimportant information about quality of vision.

Recent research studied the effects of various spec-tacle lens tints at different levels of transmittance oncontrast sensitivity acuity in normal subjects and inthose with incipient senile cataracts. The impetus forthe research was the realization that while spectaclelens tints are important in attenuating excessive lightexposure and promoting visual comfort by decreas-ing illumination when necessary, the very decrease inillumination that is produced might adversely affectcontrast sensitivity. The aims of the study were todetermine: 1) if contrast sensitivity function wasaffected by spectacle lens tints and 2) whether therewere differences in how various tints affected con-trast sensitivity.

Results demonstrated that all spectacle lens tintstested (gray, brown, yellow, green, purple, and blue)produced an increase in contrast thresholds underglare conditions. There were definite differences in theamount of increase with the various tints, however,and these differences varied between the normal and

Contrast Sensitivity and Ocular Disease

Perhaps the most important determinant of contrastsensitivity is the health of the eye. A corollary of thisis that abnormalities in contrast sensitivity may pointto the possibility of underlying ocular disease andalert the clinician to the need for additional testing.

Contrast sensitivity losses can occur at high, low, andbroad spatial frequencies. Various ocular and systemicdiseases can affect contrast sensitivity functions in dif-ferent ways and at different frequencies (Figures 4-6). Itis especially important in patients presenting with nor-mal Snellen acuity but with persistent visual complaintsto consider evaluating contrast sensitivity to rule outpossible contributing ocular—or even systemic—disease that might be affecting the quality of vision.

The value of contrast sensitivity testing in assessingvisual impairment in cataract patients has already beendiscussed. However, the usefulness of contrast sensitiv-ity, along with glare testing, in determining the need forcataract surgery with mild-to-moderate anatomicalcataracts cannot be overemphasized. With most types of cataracts, a broad spatial frequency loss is encountered.

High spatial frequency losses can be produced byoptical or non-optical abnormalities (Figure 4).Conditions affecting the optical quality of the eye thatmay lead to high spatial frequency losses includerefractive errors, mild cortical or nuclear cataracts,and various corneal disorders (eg, edema, irregulari-ties, or opacities) (Figures 7 and 8). Among the non-optical abnormalities leading to high spatial frequencylosses are mild amblyopia, macular disease, chronicopen-angle glaucoma with moderate visual field loss,and retinitis pigmentosa (RP) in its early stages(Figures 9 and 10).

Selective loss at low spatialfrequency (Figure 5) is most com-monly seen with optic nerve dis-ease, especially in cases of opticneuritis.

Broad spatial frequency loss(Figure 6) is characteristic of

Figure 4. High spatial frequency loss and eye disease.

Figure 5. Low spatial frequency loss and eye disease.

Figure 6. Broad spatial frequency loss and eye disease.

Figure 12. Diabetic retinopathy. (Slidecourtesy of Dr. Carol Lee).

Figure 11. Extensive visual field loss in an advanced case ofchronic open-angle glaucoma.

Figure 7. Moderate cataract.

Figure 9. Macular degeneration.

Figure 10.Retinitis pigmen-tosa. (Slide cour-tesy of Dr. IrwinSiegel).

Figure 8. Irregularcorneal surface andopacification in a case of granularcorneal dystrophy.

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How Does Glare Affect the Normal Eye?

The normal cornea, lens, and vitreous scatter 10%-20% of incident light. Glare is caused by light scatterand is influenced by the dynamics of light-to-dark-to-light adaptation and by retinal photoreceptor satura-tion. In the normal eye, increasing levels of glare willincrease baseline incident light scatter and adverselyaffect contrast sensitivity. The result is visual discom-fort and fatigue.

A frequent complaint in normally sighted individualsrelates to difficulties encountered with glare duringnight driving. Studies have demonstrated that in sub-jects with healthy eyes, glare sensitivity correlates wellwith simulated nighttime driving performance.

How Does Glare Affect the Abnormal Eye?Since glare depends on light scatter in the ocularmedia, any abnormality or inhomogeneity in the ocu-lar media that further increases intraocular light scat-ter will increase glare sensitivity. It is generally accept-ed that glare tests are more specific for anterior seg-ment disorders than simple contrast sensitivity tests.For example, it has been shown that corneal edemaproduces only minimal effects on contrast sensitivity,but can lead to a threefold increase in glare sensitivity.

Among the ocular diseases which affect glare sensi-tivity are corneal edema, irregularity, and opacifica-tion; cataract and aftercataract; vitreous syneresis; and

macular edema (Figures 13 and 14). An area of greatinterest relating to glare sensitivity at this time is in thepost-operative refractive surgery patient. Both witholder techniques (radial keratotomy and photorefrac-tive keratoplasty or PRK) and newer laser techniques(LASIK and LASEK), a variety of qualitative visionissues may arise, even in the face of an excellent quan-titative (20/20) result (Figure 15). These include prob-lems with night vision, distortion, ghost images,monocular diplopia, and glare. Many of these areattributable to corneal haze and surface irregularitiesafter surgery, producing increased incident light scatterwith resultant Discomforting or even Disabling glare.

PhotophobiaWhat Is Photophobia?

Photophobia is a symptom, not a disease. Photophobia,as its name implies, is “a fear of light.” It is not the sameas glare, although individuals who are photophobictend to be more bothered by the effects of glare thanthose who are not. With glare, it is the amount of lightor how it is presented that produces the problem. Withphotophobia, it is not necessarily the amount or thepresentation that is the problem, it is simply the lightitself.

Photophobia, or light sensitivity, is one of the mostcommon complaints made to the eye doctor. It shouldbe separated into two categories: pathological photo-phobia, where there is demonstrable ocular disease to account for the light sensitivity; and non-patholog-ical photophobia, where there is no obvious ocular abnormality to explain the symptom.

Pathological Photophobia

A variety of eye diseases may lead to pathologicalphotophobia. This type of photophobia can be trulydisabling, even incapacitating at times, serving tocompound the basic visual deficit caused by theunderlying disease.

the cataractous eyes. In the latter, brown or yellowtints caused the least change in thresholds, while inthe former, purple and gray tints were preferable.These differences are probably related to changes inclarity and transmission characteristics of the normalversus the cataractous lens.

Glare What Is Glare?

Glare is the loss in visual performance or visibility, orthe annoyance or discomfort, produced by a lumi-nance in the visual field greater than the illuminance towhich the eyes are adapted. Luminance is defined interms of the lumen: a unit of measurement of theamount of light incident on a surface. The higher theluminance, the brighter the surface.

Optimal lighting is in the range of 1000-1400lumens. Examples of typical environmental lumi-nances include:

Indoor, artificial light 400 lumensSunny day, shady side of street 1000-1400 lumensSunny day, sunny side of street 3500 lumensConcrete highway 6000-8000 lumensBeach or ski slopes 10000-12000 lumens

Glare may come directly from a light source (eg,facing toward the sun) or be reflected. There are fourtypes of glare: Distracting glare, Discomforting glare,Disabling glare, and Blinding Glare.

Distracting Glare

Distracting glare results from light being reflectedfrom the surface of an optical medium. Wherever theincident light moves from one optical medium toanother (eg, from air to glass) some of the incidentlight is reflected. This results in reflections from thelens surface or in the presence of halos around brightlights at night. Distracting glare can represent anannoyance to the viewer and lead to eye fatigue.

Discomforting Glare

Discomforting glare may result from direct or reflectedglare. It ranges from 3000 lumens up to about 10000lumens, at which point the glare becomes disabling. Even mild degrees of discomforting glare produce ocular discomfort, often manifested bysymptoms of asthenopia or fatigue. The unprotectedeye will respond to discomforting glare by squinting

and constriction of the pupil. Often the affected indi-vidual will try to avoid the glare by shielding the eyesor turning the head in another direction.

Disabling Glare

Disabling or veiling glare is when the level of lightincreases to 10000 lumens or more and it produces aglare that can actually interfere with or block vision.This type of glare causes objects to appear to havelower contrast than they would were there no glare. Itoccurs because the eye is not a perfect optical systemdue to inhomogeneities in the optical media that leadto light scattering which, in turn, reduces visual acuityand raises the differential light threshold. Disablingglare tends to become more problematic in the elderly,as the decreasing transparency of the crystalline lensthat comes with age leads to incipient cataract forma-tion.

Blinding Glare

Blinding glare results from incident light reflectingfrom smooth shiny surfaces such as water and snow,and becoming plane polarized. It can block vision to the extent that the wearer becomes visually compromised.

How Does Glare Differ From Contrast Sensitivity?

There exists considerable confusion about the difference between contrast sensitivity and glare. Thisis because glare is used in testing contrast sensitivity.Simply put, contrast sensitivity is about differentiat-ing the various shades of gray. Glare, on the otherhand, relates to how it becomes more difficult to differentiate those various shades of gray when illumi-nance is excessive. Contrast sensitivity tests measurethe amount of contrast necessary to recognize a target. Glare sensitivity tests measure the change invisual function that results from a glare source inanother part of the field of vision.

How Is Glare Tested?

Glare sensitivity may be tested using contrast detec-tion tasks or by acuity-based measures. The majorityof glare tests in current use assess the effects of glareon contrast sensitivity by measuring contrast thresh-olds in the presence or absence of glare, since veilingluminance reduces image contrast. Acuity-based testsrely on the fact that acuity is affected by changing thecontrast of the acuity targets. Targets used in glaretesting may be point sources or extended-glaresources. Subjects generally are more comfortable withthe latter.

Figure 15. Corneal hazeafter LASIK (Slidecourtesy of Dr. WilsonKo).

Figure 13. Severecorneal edema in acase of congenitalglaucoma.

Figure 16 a & b. Ocular albinism: (a) Iris tran-sillumination; (b) Albinoid fundus (Slides cour-tesy of Dr. Irwin Siegel).

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Figure 14.Pseudophakic cystoidmacular edema (Slide courtesy of Dr. Carol Lee).

Probably the most commonlyrecognized ocular conditionassociated with photophobia isalbinism (Figures 16a and b).The characteristic white-blondhair, brows, and lashes; palecomplexion; light blue (almosttransparent) irides; hypopig-

mented fundi; nystagmus; and poor vision usually pointto the diagnosis in individuals with this genetic disease.

Certain rare metabolic disorders where crystallinedeposits accumulate in the cornea may also producepathological photophobia (eg, cystinosis). Secondarycrystalline or lipid deposition can occur in the cornea asa result of chronic inflammato-ry disease (Figure 17). Transientirregularities or defects in thecorneal surface can induce tem-porary disease-based photopho-bia. These clinical manifesta-tions are most commonlyencountered in traumaticcorneal abrasions. In individu-als with severe keratoconjunctivitis sicca and associatedcorneal epithelial damage, photophobia is a frequentcomplaint (Figure 18).

Since the iris serves to control the amount of light enter-ing the eye, any abnormality in the structure or integrity ofthe iris may lead to pathological photophobia. This can

develop primarily (eg, in con-genital aniridia or mesodermaldysgenesis [Figure 19]) or sec-ondarily, after damage to the irisas a result of surgery, trauma, orinflammation. Pharmacologicaldilation of the pupil will alsocause temporary photophobia.

Non-Pathological Photophobia

Most individuals who complain of photophobia, how-ever, fall into the category of non-pathological photo-phobia. These are typically the same people who com-plain of having “sensitive eyes.” Although this has tra-ditionally been associated with fair-skinned, light-eyedindividuals, there does not appear to be any provenracial or ethnic bias in this regard, with many people ofAfrican-American and Hispanic descent having sympto-matic non-pathological photophobia. In these individu-als, visual acuity is typically normal and there is noobvious ocular pathology to account for the light sensi-tivity described.

Optical Solutions

All of the subjects discussed so far share one commonelement: a problem with light—too much, not enough,the wrong kind, and difficulties with presentation.Actually, they share a second element: the possibility ofemploying available spectacle lens treatments to corrector alleviate the problem.

A variety of spectacle lens treatments can be usedalone or in combination to moderate or modulate theamount and quality of light presented to the eye. Theyare helpful in alleviating the ocular discomfort resultingfrom excessive illumination, bothersome reflections,and glare, and, in so doing, serve to enhance both visu-al comfort and performance. But to work well, thesetreatments must be used selectively and efficiently.

Contrast sensitivity depends on light levels and isgenerally optimal under photophobic (ie, daylight) con-ditions, with contrast thresholds increasing progressive-ly as light dims (mesopic and scotopic conditions)(Figure 20). Filters or tints artificially produce a shiftfrom the photopic towards the scotopic state, with theamount of this shift directly proportional to the density

of the tint. The darker the tint, the more the light isfiltered, and the greater the shift towards the scotopicside, with a resulting increase in contrast sensitivitythreshold and a corresponding decrease in CSF. Tints,then, should be expected to adversely affect vision.But the real-life, real-vision situation is significantlymore complicated. Glare is intimately related to con-trast sensitivity function (Figure 21). Excessive glaredecreases CSF. Spectacle lens treatments that decreaseglare should improve contrast sensitivity. So dothese t r ea tment s help or hinder t h e v i s u a lexperience? The answer lies in the balance—the balance between the amount of light and the amount of glare. Depending on the specific

i l l u m i n a t i o n conditions, a fil-ter or tint caneither facilitate or c o m p r o m i s evision. This iswhy clear lensesand fixed-tintsunglasses are

inadequate and incomplete solutions to thelight/vision equation. What should be ideal is not aconstant or fixed tint, but an as-needed, on-demandtype of light modulation that decreases incident lightwhen levels are too high or glare conditions arise yetallows sufficient light into the eye when levels arelower and glare is not an issue. This is the primaryadvantage that photochromic (ie, “colored by light”)lenses offer to the wearer. These variable tint lenses rely on ultraviolet radiation-induced chemical reactions to produce a darkening of the lensupon exposure to light, with a return to the clear statewhen the light stimulus is removed (Figure 22).Photochromic lenses are indeed “colored by light”and, with newer designs that are essentially clearindoors or under low light conditions but darken tothe level of the standard fixed-tint sunglass outdoorsor under conditions of intense incident light, these

lenses allow light to work best for the eye and permitthe eye to function at its peak under various levels ofillumination (Figure 23).

Another useful spectacle lens treatment is the anti-reflection (AR) coating to reduce Distracting glare.Antireflection coatings function by reflecting light.The reflected light from the lens coating interfereswith the light being reflected from the lens substrateor underlying layer. These coatings do not effectivelyfilter out or attenuate non-glare light stimuli andtherefore do not shift the photopic-scotopic curves.By minimizing unwanted reflections while still maxi-mizing transmitted light, the coatings enhance thequality of vision while decreasing ocular discomfortunder conditions of low-to-moderate light with glare.

The combination of a photochromic lens with an ARcoating would appear to be the ideal in achieving theproper balance between illumination and glare to maxi-mize contrast sensitivity function, and offer visual com-fort and convenience under varying light conditions.

One other important aspect of light and vision whenconsidering spectacle lens treatments is polarized light.Ambient sunlight is unpolarized. With unpolarizedlight, the direction of vibration is random (ie, in alldirections). When light is reflected from a surface, it ispartially or completely plane polarized, with the planeof polarization of the reflected light perpendicular to theplane of incidence of the light. Light incident on smoothsurfaces such as glass, concrete, or water producesbothersome polarized light (Blinding glare). This can beeliminated through the use of a polarizing lens oriented

Figure 19. Multiple irisdefects in a case of meso-dermal dysgenesis.

Figure 20. Contrast sensitivity and light conditions.

Figure 22. Photochromic chemical reaction.

Figure 23. Photochromic lenses: UVR-mediated reaction.

Figure 24. Polarized lenses.

Figure 18.Keratoconjunctivitissicca: Rose Bengalstaining.

Figure 21. Effect of glare on contrast sensitivity.

Figure 17. Corneal crys-talline deposition in acase of rosacea keratitis.

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1. Stenson SM, ed. Light, Sight, and Photochromics.Pinellas Park, FL. Transitions Optical; 2002.2. American Academy of Ophthalmology. Contrast sen-sitivity and glare testing in the evaluation of anterior seg-ment disease. Ophthalmology. 1990;97:1233-1237.3. Ross JE, Bron AJ, Clarke DD. Contrast sensitivity andvisual disability in chronic simple glaucoma. Br JOphthalmol. 1984;68:821-827.4. Atkin A, Bodis-Wollner I, Wolkstein M, Moss A,Podos SM. Abnormalities of central contrast sensitivityin glaucoma. Am J Ophthalmol. 1979;88:205-211.5. Wolkstein M, Atkin a, Bodis-Wollner. Contrast sensitivi-ty in retinal disease. Ophthalmology. 1980;87:1140-1149.6. Marron JA, Bailey IL. Visual factors and orientation-mobility performance. Am J Optom Physiol Opt.1982;59:413-426.7. Lempert P, Hopcroft M, Lempert Y. Evaluation ofposterior subcapsular cataracts: with spatial contrastacuity. Ophthalmology. 1987;(pt 2):14-18.8. Sjostrand J, Frisen L. Contrast sensitivity in maculardisease: a preliminary report. Acta Ophthalmol(Copenh). 1977;55:507-514.9. Hyvarinen L, Laurinen P, Rovamo J. Contrast sensitiv-ity in evaluation of visual impairment due to diabetes.Acta Ophthalmol (Copenh). 1983;61:94-101.10. Guyton DL. Preoperative visual acuity evaluation.Int Ophthalmol Clin. 1987;27:140-148.11. van den Berg TJ. Importance of pathological intraoc-ular light scatter for visual disability. Doc Ophthalmol.1986;61:327-333.12. Hess RF, Carney LG. Vision through an abnormalcornea: a pilot study of the relationship between visualloss from corneal distortion, corneal edema, kerato-conus, and some allied corneal pathology. InvestOphthalmol Vis Sci. 1979;18:476-483.13. Carney LG, Jacobs RJ. Mechanisms of visual loss incorneal edema. Arch Ophthalmol. 1984;102:1068-1071.

14. Knighton RW, Stomovic AR, Parrish RK 2nd. Glare measurements before and after neodymium-YAG laserposterior capsulotomy. Am J Ophthalmol.1985;100:708-713.15. Applegate RA, Trick LR, Meade DL, Hartstein J.Radial keratotomy increases the effects of disabilityglare: initial results. Ann Ophthalmol. 1987;19:293-297.16. Neumann AC, McCarty GR, Steedle TO, SandersDR, Raanan MG. The relationship between cataract typeand glare disability as measured by the Miller-NadlerGlare Tester. J Cataract Refract Surg. 1988;14:40-45.17. Abrahammson M, Sjostrand J. Impairment of con-trast sensitivity function (CSF) as a measure of disabilityglare. Invest Ophthalmol Vis Sci. 1986;27:1131-1136.18. Wolf E. Glare and age. Arch Ophthalmol.1960;64:502-514.19. Regan D. Low-contrast letter charts and sinewavegrating tests in ophthalmological and neurological disor-ders. Clin Vision Sci. 1988;2:235-250. 20. Pelli DG, Robson JG, Wilkins AJ. The design of anew letter chart for measuring contrast sensitivity. ClinVision Sci. 1988;2:187-199.21. Naidu S, Lee JE, Holopigian K, Seiple WH,Greenstein VC, Stenson SM. The effect of variably tintedspectacle lenses on visual performance in cataract sub-jects. Eye Contact Lens. 2003;29:17-20.22. Lee JE, Stein JJ, Prevor MB, Seiple WH, HolopigianK, Greenstein VC, Stenson SM. Effect of variable tintedspectacle lenses on visual performance in control sub-jects. CLAO J. 2002;28:80-82.23. Fry GA, King VM. The pupillary response and dis-comfort glare. J Illum Eng Soc. 1975;5:307-324. 24. Seiple WH. The clinical utility of spatial contrastsensitivity testing. In: Tasman W, Jaeger EA, eds. Duane’sFoundations of Clinical Ophthalmology. Philadelphia,Pa: Lippincott; 1991.

with its vibration plane perpendicular to the reflected light(Figure 24). Polarized lenses eliminate reflected glare,improving the quality of vision and relieving visual discom-fort. This is especially important for individuals who workoutdoors or enjoy water sports or skiing.

The relationship between light and sight remains acomplicated one, but fortunately spectacle lens technolo-

gy has evolved to the point where there is a spectacle lenstreatment or combination of treatments to meet the visu-al requirements of most people under most circum-stances. It is the responsibility of the eye care practition-er to assist the vision care consumer in choosing thesetreatments wisely.

References:

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