the role of contrast sensitivity charts and contrast...

O P T O M E T R Y

The role of contrast sensitivitycharts and contrast letter charts in

clinical practice

Russell L Woods PhD MBCO FAA0 and Practitioners frequently make clinical decisions based on changes in visual acuity. How-Joanne M Wood PhD MBCO FAA0 ever, acuity is recognised to be only one aspect of visual performance. In recent years,School of Optometry, Queensland Univer- a number of clinical chart-based tests which measure different aspects of vision basedsity of Technology on contrast sensitivity have become available. These tests allow a more subtle investiga-

tion of visual problems. In this review of currently available contrast sensitivity chartsand contrast letter charts, we examine their clinical application and some of the problems which may be encountered in the use of these tests in practice. This should per-mit a better understanding of some patients’ visual problems, better clinical decisionmaking and a better understanding of studies which report visual performance meas-

Accepted for publication: 13 December urements. Information on currently available equipment and a list of suppliers in Aus-1994 tralia is included. (Clin Exp Optom 1995; 78: 2: 43-57)

Keywords: contrast sensitivity charts, contrast letter charts, clinical practice

Visual acuity for many years has been themainstay of vision assessment in optomet-ric practice. Visual acuity is a measure ofthe recognition of small (high spatial fre-quency), high-contrast letters. Hence,visual acuity is a measure of the resolu-tion limit of the visual system and there-fore is a sensitive measure of changes inrefractive error. High frequency acuitycharts as described by Medina andHowland’ are even more sensitive to re-fractive blur. Unfortunately, conventionalletter acuity is limited because, despiteproper refractive correction, somepatients will complain of a visual problembut no visual anomalies can be demon-strated with conventional letter charts.This is common with early cataract andcontact lens patients. These subtle visualproblems can be distressing to the patientand confusing to the practitioner.

Because the ‘real world’ is composedof objects of varying sizes (spatial frequen-cies) and contrasts, visual acuity is too sim-plistic an assessment of visual perform-ance for everyday visual tasks. Contrastsensitivity measures, which allow a morecomplete investigation of visual function,may be used to detect visual problems atan earlier stage, to understand thepatient’s problem and to help managethat problem (for example, by advising apatient of increased risks if driving in lowcontrast conditions).

Though the importance of vision inmany daily tasks seems obvious, conven-tional visual acuity tests are rarely relatedto functions such as mobility,2 face recog-nition,s sports4 or driving.5-g The relativesuccess of measures such as contrast sen-sitivity in predicting ‘real world’ visualperformance suggests that more advanced

tests will become more commonly used invision assessment, for example, to predictdriving ability,8,g or to investigate differ-ences between children who are ‘good’readers and those who are ‘disabled’ read-ers.‘O

It is difficult to introduce these testsinto clinical practice when there is onlylimited information about them and of-ten this information is in journals whichare not readily available to practitioners.We begin by briefly reviewing contrast sen-sitivity as there are many good reviews.11-13Then, we introduce contrast sensitivitycharts and contrast letter charts with anemphasis on those which are commer-cially available to practitioners in Aus-tralia, discuss the application of each test,consider clinical studies of the usefulnessof the tests, examine their sensitivity andreliability and consider their l imitations.

Clinical and Experimental Optometry 78.2 March-April 1995

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Contrast charts in practice Woods and Wood

(1)

Contrast sensitivityThe contrast sensitivity function (CSF) isa measure of contrast thresholds for arange of object sizes and is convention-ally measured by finding the thresholdcontrast of sine wave gratings of varyingspatial frequencies (sizes). The spatial fre-quency of a grating is the number of cy-cles (one dark and one light band) perdegree of visual angle. The finer the grat-ing, the more cycles per degree (c.p.d.),the higher the spatial frequency. In Fig-ure 1 three sine waves are shown, A and Bhave the same spatial frequency, while Chas a higher spatial frequency. Contrast isa measure of the difference between theluminance of the object and the lumi-nance of its surround. For grating-basedtests, where there are equal areas of lightand dark, contrast ( CFaf,,) may be definedas:

Lmex and L,,, are the maximum andminimum luminances in the grating re-spectively. In Figure 1, B and C have thesame contrast, while A has a higher con-trast. The contrast of A = (35-5) / (35+5) =0.75, while the contrast of B and C = (22.5-17.5)/(22.5+17.5) = 0.125. Contrast is of-ten expressed as a percentage, hencethese become 75 per cent and 12.5 percent respectively. If A and C representedgratings at threshold, the contrast sensi-tivity would be l/O.75 = 1.3 and l/O.125= 8. At medium spatial frequencies (twoto five c.p.d.) contrast sensitivity may beas high as 500, and hence a grating with acontrast of l/500 = 0.002 is at threshold.

For measurement of contrast sensitiv-ity, typically the contrast of a grating at agiven spatial frequency is varied and thisis repeated over the range of spatial fre-quencies of interest. This is then plotted,as shown in Figure 2, with contrast sensi-tivity versus spatial frequency. For higherspatial frequencies (greater than 10c.p.d.), the CSF is approximately limitedby the optical performance of the eye, theneural aspects of the visual system beingas good as, or better than the optical

4 0

mm .A

mm B

- C

Figure 1. Three sine waves. A and B have the same spatial frequency, while C has ahigher spatial frequency. B and C have the same contrast (Equation l), while A has ahigher contrast.

1 I

1 1 0

spatial frequency (cpd)

Figure 2. A typical contrast sensitivity function. Higher contrast is required to detectsmaller objects (high spatial frequency). Even with 100 per cent contrast (contrastsensitivity = 1) objects finer than about 30 to 60 cycles per degree (c.p.d.) cannot bedistinguished. This resolution limit is related to visual acuity. Peak contrast sensitivityof better than one per cent contrast (contrast sensitivity > 100) is found at mediumspatial frequencies of about 3 to 5 c.p.d.


44

_._ -~.-~. , Ht~-~-.-. ~~

------lst+3rd- - 1 s t + + 1 5 t t-square

I

Figure 3. A square wave can be produced by the addition of suitable sine waves whichinclude the fundamental frequency and all the odd harmonics each at the appropriateamplitude.

performance. i4 For lower spatial frequen-cies the CSF is attenuated principally dueto neural factors.i5 The shape of the CSFvaries with many factors including lumi-nance,“j temporal characteristics,” targetsize’* grating motioni and grating shape(for example, square, sine)15a2” and thuscan vary between different tests and be-tween different studies. If the same con-ditions are always used with the same testthen the shape of the CSF will remain thesame unless there is some visual dysfunc-tion.

Optical degradation can affect contrastsensitivity. Spherical refractive error re-duces contrast sensitivity in proportion tothe spatial frequency, with a minimaleffect on low, a moderate reduction formedium and a greater reduction forhigher spatial frequencies. Astigmatic re-fractive error can produce ‘notch’ defects,where only the medium spatial frequen-cies are reduced.21 Monocular diplopia,such as with a bifocal contact lens, can alsoproduce a notch defect.22,2s

So what does it all mean ?Theoretically, any object can be ‘decom-posed’ into a series of sine waves (Fourieranalysis). In other words, with sine waves

of just the right size and shape, it is possi-ble to ‘recreate’ the object. As an exam-ple, it is possible to produce a square waveusing a series of sine waves of suitableamplitude and frequency as shown in Fig-ure 3. The square wave is formed from acombination of the fundamental fre-quency (the same spatial frequency as thesquare wave) and the odd harmonics(three, five, or seven times the spatial fre-quency of the square wave) of appropri-ate amplitude. These harmonics are im-portant when we consider letter targetswhich are effectively composed of smallsquare wave elements. Modern imageanalysis computer programs are able toperform Fourier analysis and can modifyimages by enhancing or removing differ-ent spatial frequencies.*“ It is possible toproduce a letter chart using these princi-ples where, unlike a Snellen chart, theletters disappear almost as soon as theyare out of focus.’ This mathematical ‘trick-ery’ is a fundamental principal of opticsand implies that the CSF is a relativelybasic aspect of vision. In practical terms,low spatial frequencies (less than 0.5c.p.d.) are related to the detection of largeobjects. Low spatial frequency detectionhelps us to avoid being run down by a bus,


although the bus would be unrecognisablefrom any other large ‘blobs’. Medium spa-tial frequency (2 to 6 c.p.d.) detection al-lows recognition of the ‘blob’ as a busrather than a truck and helps one find thedoor. Fine detail requires high spatial fre-quency (greater than 10 c.p.d.) detection,allowing us to read the number of the bus.Most tasks require medium spatial fre-quencies and, fortunately, medium spatialfrequencies are at the peak of the CSF.While letters comprise many spatial fre-quencies,25 a 6/60 letter is approximately3 c.p.d., a 6/6 letter approximately30 c.p.d. and a 6/3 letter approximately60 c.p.d.

The patient who experiences a reduc-tion in contrast sensitivity at low and me-dium spatial frequencies may have agreater functional visual loss and requireearlier referral than a patient who onlyexperiences a reduction in high spatialfrequency contrast sensitivity (that is,visual acuity).n~26-28 A reduction in con-trast sensitivity for low and medium spa-tial frequencies would reduce the abil-ity to detect large to moderately sizedobjects under reduced contrast condi-tions (for example, a rainy day) therebyseriously compromising a patient’s ori-entation and mobility.*~29 Contrast sen-sitivity reductions and information fromother vision tests should be used to as-sess functional vision. For example, peakcontrast sensitivity and visual field ex-tent are correlated with orientation andmobility2,30 and reduced contrast lettersensitivity is correlated with reduceddriving skills.*s9

Measurement of a full CSF can be verytime-consuming. While the CSF is conven-tionally measured at a range of spatial fre-quencies and many early studies measuredsuch a range, it would appear that oftenthis is unnecessary. It has been suggestedthat the measurement of two or three well-chosen spatial frequencies will adequatelydescribe the CSE In conjunction with con-ventional visual acuity a measure at or nearthe peak of the CSF appears to be a goodclinical compromise.3’~32 The new clinicalcharts allow a measurement of mediumspatial frequency sensitivity to be takenquickly.


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Contrast sensitivity in clinicalpracticeContrast sensitivity measurement in prac-tice has been suggested for:1. screening for ocular disorders,2. diagnosis of ocular disease,3. monitoring visual function,4. assessment of visual function,5. prediction of vision-related ability.

Contrast sensitivity is affected by manyocular diseases and conditions.33 Of mostinterest are reports that contrast sensitiv-ity for low or medium spatial frequenciesmay be reduced when visual acuity is nor-mal in conditions such as amblyopia, as-tigmatic error, cataract, diabetes, glau-coma, keratoconus, multiple sclerosis,ocular hypertension, optic neuritis andpapilloedema. 33 However, the ability ofcontrast sensitivity tests to effectivelyscreen for ocular disease appears to belimited.1i,34 For example, Elliott andWhitaker34 reported an evaluation of arange of contrast charts on 535 selectedpatients in optometric practice. As shownin Table 1, of the 89 patients with ocularor visual dysfunction, 16 had a reduction

Table 1. In a clinical trial of contrastsensitivity charts (Vistech and Cambridge)and a contrast letter chart (Pelli-Robson),Elliott and Whitaker34 reported that 116of the 535 patients who were tested hadan ocular or visual dysfunction. Resultswere categorised for 89 of the 116patients. The contrast sensitivity score wassignificantly reduced in 25 of the 89 listedand this reduction in vision would nothave been noted in 16 patients if visualacuity alone was measured.

with a contrast sensitivity chart or a con-trast letter chart measure despite normalvisual acuity. These 16 included patientswith diabetes, glaucoma, multiple sclero-sis and those who were at risk from glau-coma. Similarly, Maugdal and colleagues,35using a contrast sensitivity chart, notedthat there was a medium spatial frequencysensitivity reduction with normal highspatial frequency sensitivity in 22 of 211patients who had a range of ocular dis-eases. However, not all patients with thesame disease exhibit the same changes incontrast sensitivity. 33-35 Differences in con-trast sensitivity reduction between patientswith the same ocular disease have beenused to suggest that there are subtypes ofamblyopia,36 glaucomas7 and optic neuri-tis,38,39 although the clinical significanceis not clear. Also, as the contrast sensitiv-ity changes in many ocular diseases aresimilar, there is a limited ability to distin-guish between ocular diseases.1’~40 For ex-ample, contrast sensitivity tests cannot dis-tinguish between patients with diabeticretinopathy and patients with cataract.41Since many diseases cause changes whichare detected routinely with conventionaltests, the usefulness of contrast sensitivityas a screening device has been ques-tioned.“J4 The exceptions are patientswith optic neuropathies who may other-wise be undetected.“s42 Though the inci-dence of ocular disease is low, the newclinical charts allow quick, easy screeningof patients, particularly when performedby appropriately trained staff. Given thereduced reliability of contrast sensitivityand low contrast charts in patients withocular disease43a44 or degraded vision45 theability to monitor changes in vision is lim-ited. Conversely, the usefulness of contrastsensitivity for assessment ofvisual functionand in the prediction of skills such asmobility and driving is becoming moreapparent.*-lo

Psychometric methodIt is very important to separate visualfunction (sensitivity) from the willing-ness of an observer to report a weakvisual sensation (decision criterion). Atest which has a poor measurement tech-nique (psychometric method) may

measure changes or differences in thedecision criterion rather than changesin sensitivity. As an example, there maybe differences in the decision criterionbetween a timid young patient and aconfident older person. Though their‘true’ visual acuity (sensitivity) may bethe same, the timid patient will probablygive up two lines early in case a mistakeis made, while the confident person willprobably attempt the line beyond, eventhough there is little chance of readingit. Thus, even in the measurement ofvisual acuity, it is important to use a goodpsychometric method. In this example,the ‘true’ visual acuity is found by en-couraging the recalcitrant patient tocontinue to try (‘guessing’) until a com-plete line is read incorrectly. Having thepatient read until no correct responses aremade is a forced choice psychometricmethod and is the recommended scoringtechnique for logMAR charts.46

In a recent study, Woods andThomson,47 repeating a previous study,48found an apparent improvement in con-trast sensitivity after exercise. Contrastsensitivity was measured using the methodof limits (similar to the method used withthe Arden gratings). When the contrastsensitivity of the same patients was meas-ured after exercise using a two-alternativeforced-choice presentation, a psychomet-ric method free of decision criterioneffects, there was no change in contrastsensitivity. Hence, the apparent improve-ment in contrast sensitivity after theperiod of exercise was due to changes inthe decision criterion (for example, moodor motivation). Good psychometric tech-niques avoid such problems and improvethe reliabil ity of a test.49 Poor psychomet-ric methods explain why some of the com-mercially available charts do not meet ex-pectations. Unfortunately the betterpsychometric methods usually take longerto perform.4q

CONTRAST SENSITIVITY CHARTS

Traditionally, contrast sensitivity has beenmeasured using electronically generatedtargets which are expensive, difftcult to setup and calibrate, and often use time-


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consuming psychometric methods. There-fore, they are unsuited to clinical practice.Over the last decade, a number of rapidand relatively inexpensive chart-basedtests have been introduced with the aimof making contrast sensitivity a clinicallyviable technique. While some of thesetests have been promoted as screeningtests, their principal use is probably assupplementary tests. If used for screeningthe test should be quick, reliable and ableto detect potential visual problems.

Some of these tests may also have a rolein testing during domiciliary visits. Adomiciliary test should be portable, robustin construction and robust in its lightingand distance requirements. Details oflighting requirements, test distances, spa-tial frequencies and contrast levels testedare listed in Table 2, while details of sup-pliers and the purchase costs are listed inTable 3 .

Arden grating (A0 contrastsensitivity test plates)The Arden grating test,50 the first attemptto develop a simple and inexpensive con-trast sensitivity technique, was marketedby American Optical (AO). Photographicplates of seven spatial frequencies (0.2 to6.4 c.p.d. at 57 cm) are presented to thepatient. Each plate contains a single spa-tial frequency with the sine wave gratingoriented vertically. The contrast of thegrating varies from high at the bottom tolow at the top. One plate at a time is placedin a neutral grey holder and then slowlydrawn upwards, out of the holder, untilthe patient reports that the grating is vis-ible. The score on all seven plates is thensummed. There is a small effect of age onscores.50,51 The lighting requirement isspecified as 100 ft candles (1076 lux) or‘a 60 watt bulb 14 inches (36 cm) abovethe plates’. This simple lighting specifi-

cation and the size of the charts makethem easily portable and suitable fordomiciliary visits.

The Arden grating scores have beenreported to be reduced in diseases suchas ARM, cataract and glaucoma.11~50~5*~53The test has a reasonable ability to screenfor ocular disease.51s54 The Arden gratingtest has been criticised for the use of apoor psychometric method (method oflimits) which is subject to large variationsin the patient’s decision criteria (that iswhere the patient decides to say ‘yes’). Theclinicians must establish ‘normal’ valuesfor their practices as the ‘normal’ valuesprovided with the test may not be correctin different clinical settings.51 Results mayvary between operators, mainly due to dif-ferences in the speed of exposure of theplates50 and due to differences in the in-terpretation of the instructions.55

Despite their initial popularity and a

Table 2. A list of clinical contrast sensitivity charts and contrast letter charts in Australia


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Table 3. A list of suppliers of contrast sensitivity charts, contrast letter charts and related computer-based tests available in Australia


reasonable ability to screen for ocular dis-ease, the Arden gratings are no longerregarded as an efficient or accuratemethod for measuring contrast sensitiv-ity. Though the test has been largely su-perseded it is still in use in some clinics.

Vistech contrast sensitivity chartsThe Vistech charts,56 probably the mostcommercially successful charts, improvedon the At-den plates by using a superiorpsychometric method as suggested byVaegan and Halliday.52 Each chart containsfive rows and nine columns of circularphotographic plates (discs) on a greybackground. Each plate contains a sinewave grating. Each row has a different spa-tial frequency (1.5 to 18 c.p.d. at threemetres) and the contrast within each rowreduces from left to right. The gratingsare presented in three orientations: verti-cal (go”), 15” left (105”), or 15” right(75”). The claim of a forced-choice pro-tocol is not accurate because the choiceof ‘blank’ is allowed. The last disc correctlyidentified at each spatial frequency is plot-ted on a graph provided. A set of ‘normal’contrast sensitivity values is provided withthe charts, but these have been shown tovary between practicess4r5’ and do not in-clude information about changes in con-trast sensitivity with age.56,58,5q Three dis-tance charts (94 x 66 cm) are availablewith different orientations. There is alsoa near chart (17.5 x 14 cm) for use at45 cm. A luminance of between 103 and240 cd/m2 is recommended and a smallphotometer is provided. However, it canbe difftcult to produce even lighting overthe entire chart without producing surfaceglare.

These charts probably provide the mostcommon measure of contrast sensitivity inclinical practice. Some extravagant claimswere made regarding their ability to:1. screen for eye disease,2. monitor visual function (for example,

to determine the efficacy of vision train-ing of amblyopes and the progressionor remission of disease),

3. evaluate occupational vision by dis-criminating small differences in thevisual performance of normal observ-ers,

4. document the performance of lowvision patients60Unfortunately, few if any of these claims

have been substantiated as the reliabilityof the Vistech charts is poor,43,6163 the abil-ity to detect changes in vision is limited43z5qand the ability of the test to screen forocular disease has been disappointing.35The Vistech test will detect reductions invision which would not otherwise benoted. Maugdal and co-workerP reportedthat 22 of 211 randomly selected patientshad a reduction in medium spatial fre-quency sensitivity despite having normalhigh spatial frequency sensitivity. Analy-sis of test results indicates that the meas-urement of five spatial frequencies mayb e redundant and measurement of onespatial frequency (for example 3 c.p.d.)i n conjunction with visual acuity may besufficient to assess visual function.31z63,64

Cambridge contrast chartsThis simple test, an A4size (28 x 22 cm),spiral-bound book with square wavegratings of decreasing contrast, measuresa single median spatial frequency (4 c.p.d.at six metres). 65,66 The gratings are com-posed of fine dots, which while not visibleat six metres, may be visible at shorter dis-tances. Each grating is presented on onepage with a facing page of uniform greya n d the patient must indicate on whichside the grating is seen. This is a trueforced choice procedure. Gratings arepresented until an incorrect response ismade, this is then repeated, and the sumof four incorrect responses is recorded.The normal score (total of four responses)should reduce from about 35 at age 25years to about 29 at age 70 years.66 Simi-larly, an abnormal score (95 per cent con-fidence limit) is reported to reduce froma score of less than 27 at age 25 years toless than 23 at age 70 years.66 Reliabilityof the test is moderate.67 The luminancerequirement of 100 cd/m* is relatively eas-ily met on this small chart which can bedisplayed via a mirror. Despite the lowprice, the very robust methodology andt h e simplicity of this test, it has not beencommonly used in clinical practice. Thesix-metre viewing distance may be a dis-advantage for domiciliary use.

The Cambridge gratings attracted wide-spread interest when Wilkins and col-1eagueP reported their ability to detectdeficits present in patients with normalSnellen letter acuity, who also sufferedfrom diabetes, multiple sclerosis, opticneuritis or glaucoma. However, theseclaims have not been substantiated byother workers. Wood and Lovie-ISitchin68~69found the Cambridge gratings to be poorin the detection of ocular hypertensionand primary open-angle glaucoma.

Melbourne Edge TestThe Melbourne Edge Test (MET)70 is asimple measure of the ability to detect anedge of varying contrast. Edge detectionis related to the peak contrast sensitivity7iand has been correlated with mobility inlow vision patients.29 This small, robust testconsists of 20 test patches (discs) of 25 mmdiameter in which the contrast betweenthe two sides of the edge is reduced inlogarithmic steps. The patient must indi-cate at which of the four possibleorientations (45”, 90”, 135”, lSO”) theedge is seen. The last disc identified cor-rectly is recorded. A repeated measure isobtained by rotating the chart through90 degrees. There is probably no signifi-cant effect due to age,72 as average scoreswere found to be about 20.5 at age 50 yearsand about 19.5 at age 70 years.“’ The small(30 x 25 cm) chart is held at 40 to 57 cmwith a luminance of 18 to 85 cd/m2. Thetest is robust to variations in viewing dis-tance and to poor refractive correction.Edge sensitivity is luminance dependent70so that if the lighting is not even acrossthe chart the contrast of the edge can bealtered. If a single light source is used theangle of incidence should be 45 to 60degrees.

The MET was advocated as a reliableindicator of the peak of the CSF withpotential for screening for abnormal ocu-l a r conditions.70 Verbaken and Johnston70also alluded to the fact that the MET couldbe used for the functional assessment ofpatients with low vision. Despite theseclaims, studies have demonstrated that theMET is a poor screening device for thedetection of primary open angle glau-coma6* and general eye disease.72 Similarly,


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the claims regarding the use of the METfor low vision assessment have yet to besubstantiated, apart from a conference re-portzg which suggested that MET scoreswere correlated with mobility.

CONTRAST LETTER CHARTS

Letter charts have the great advantage ofbeing familiar to the patient and to thepractitioner. Contrast letter charts comein two forms. The first is a reduced con-trast version of a visual acuity chart andmay be called a low contrast acuity chart.The second uses letters of a fixed size andvaries the contrast of the letters and henceis probably best called a variable contrastletter chart. Both of these charts have arole in clinical practice and are provingto be extremely useful adjuncts to conven-tional visual acuity.

Contrast for letter-based tests (C,,,,)may be defined as:

Lma, - Lm?l (2)

cl& =Lmar

Though this definition is differentfrom that used for grating-based tests(Equation l), it is possible to convert be-tween these formulae as:

2 - cl&r

Bailey-Lovie chartsTo overcome many of the shortcomingsof the traditional Snellen chart, Bailey andLovie proposed a new design for visualacuity charts. This design uses 10 lettersof approximately equal legibility, live to aline, spaced such that the separation be-tween lines and between letters gives simi-lar ‘crowding’ effects at all levels. Thisavoids the major objection to Snellencharts that the task varies at different lev-els (lines on chart or distances from thechart). As the letter size varies on a loga-rithmic scale, visual acuity can be scoredaccording to the logMAR system. By thissystem, each letter correct scores -0.02

logMAR units and each correct line of liveletters scores -0.1 logMAR units. Thepatient must read until no correct re-sponses are made on a line.46 A Snellenfraction of 6/6 scores 0 logMAR, 6/60 (10lines larger) scores 1.0 logMAR and 6/3(three lines smaller then 6/6) scores -0.3logMAR. Average visual acuity reducesfrom about -0.15 logMAR (6/4) at age 30years, through 0 logMAR at age 60 yearsto about 0.3 logMAR (6/12) at age 80years.74

Measurement of high contrast (90 percent) visual acuity, in contrast sensitivityterms, involves a horizontal movementtowards the right at a contrast sensitivityof approximately 1 (just above the x-axisin Figure 2). The spatial frequency is in-creased (letter size is reduced) until theresolution limit is reached.

Practitioners should be using Bailey-Lovie charts as these have become thestandard chart for accurate visual acuitymeasurements.75m78 The charts are morereliable than Snellen charts and are thechart of choice for low vision assess-ment.44s79 Charts are available in a numberof different formats from the NationalVision Research Institute of Australia(Table 3).

Low contrast acuity chartsDespite 19th~century descriptions of lowcontrast acuity charts, it is only in the lastdecade that low contrast acuity has beenapplied clinically. 8o-82 Low contrast acuitycharts conventionally have grey letters ona white background. *“,*I Measurement oflow contrast (10 per cent) visual acuityinvolves a horizontal movement at a con-trast sensitivity of approximately 10 (Fig-ure 2). The typical difference betweenhigh (90 per cent) and low contrast (10per cent) acuities in normal patients isjustover two lines.5ga62 The difference in visualacuity between the high and low contrastacuity charts is an indicator of the slopeof the right hand portion of the CSF andhence an indicator of changes in mediumspatial frequency contrast sensitivity.There may be a small increase in the acu-ity difference with age from approximately0.21 logMAR at age 30 years to about 0.24logMAR worse by age 60 years.59s62

Regan and co-workerss’,s3 pioneeredthe use of low contrast charts in the de-tection of ocular disease and producedcommercially available charts which areunfortunately a Snellen-like design. Thelocally made Australian Vision Charts usea Bailey-Lovie design.73ss4 The luminancerequirement of 85 cd/m*, or an illumi-nance of 280 lux, is easily met in mostpractices. Fluorescent lights at an angleof 30 to 60 degrees are recommended.Spotlights are not recommended as theluminance gradient (hot spot) may alterthe contrast by introducing a veilingglare, 85 although in practice we have foundthat with careful selection of the spotlightgood even illumination is possible. Thecharts come in a range of sizes and for-mats as detailed in Table 3.

Low contrast acuity charts have beenpopularised as a screening tool for anumber of ocular diseases including glau-coma, diabetes and neurological disor-ders.*3 However, recent studies seem toconfirm that the low contrast acuity charts,despite convincing theoretical advantages,offer little extra clinical information overaccurate high contrast visual acuity meas-ures in the examination of patients withage-related maculopathy,*“zR7 glaucoma68and other elderly patientsRR However, lowcontrast acuity has been reported to beuseful in describing visual function in softcontact lens wearers,“g bifocal contact lenswearer? and in patients with cataracts,especially in the presence of glare.5gs90

Recently, a different low contrast acu-ity chart with black letters on a dark greybackground has been described for usewith low vision patients.8’,g1 It is presumedthat this chart (the SKILL chart) will givea measure of low contrast acuity similarto that of a ‘conventional’ low contrastacuity chart used under low contrast con-ditions. The relative benefits of this inter-pretation of low contrast acuity have notbeen determined.

Pelli-Robson contrast thresholdchartThe Pelli-Robson”* variable contrast letterchart consists of 16 groups of threeletters (a ‘triplet’) arranged on eight lines.The contrast of each triplet reduces in


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logarithmic steps. As the letter size (spa-tial frequency) is fixed and the contrast isvaried the test procedure is more like con-ventional contrast sensitivity measure-ment. As the patient reads down the chartthe letters reduce in contrast and in con-trast sensitivity terms they move up Fig-ure 2 until the detection limit is reachednear the peak’of the CSF. In the manufac-turer’s description and in most reports thetest is administered at one metre andwould be expected to measure sensitivityat approximately one cycle per degree(fundamental frequency) but higher har-monics (3, 5 and 7 c.p.d.) will also be de-tected. We suggest that the test may bebetter at three or four metres, where thefundamental spatial frequency tested is ap-proximately three or four cycles perdegree45 and therefore nearer the peak ofthe CSF.

In the original description of the test,the last triplet where at least two lettersare read correctly is scored.32 To improvereliability, Elliott and colleaguesg2 sug-gested scoring correct a call of ‘C’ for ‘0’or a call of ‘0’ for ‘C’. To further improvereliability, theyg3 then suggested scoringeach correct letter as 0.05 units. At onemetre, the average Pelli-Robson scoreshould reduce from about 1.90-l 85 at age20 to 30 years to about 1.80-l .75 at age 70years.5g~94~95 If a different distance such asthree metres is used, ‘normal’ values willneed to be established. Lighting require-ments of 85 cd/m2 (acceptable range 60to 120 cd/m*) can require some manipu-lation to get this large (64 x 85 cm) chartevenly lit.

Pelli-Robson charts have proven to bequick and reliable in clinical practice andto have significant uses in the descriptionof visual performance. Studies by anumber of workers have demonstratedthat the Pelli-Robson chart can detect areduction in visual function in patientswith cataracts,5gzg6 probably due to narrowangle light scatter from the ‘white’ areasof the chartz6 When the test is combinedwith a peripheral glare source it has a highcorrelation with stray light and excellentdiscrimination between patients.sg Simi-larly, studies have shown that the Pelli-Robson chart is useful in reflecting the

loss in contrast sensitivity experienced byintraocular lens wearers” and bifocal con-tact lens wearers.‘Lp

Recent studies by the vision and driv-ing group at the Queensland Universityof Technology found that Pelli-Robsonscores correlated highly with driving per-formance of patients with simulated visualimpairment and these results were sup-ported by studies of patients with truevisual impairment9 Whitaker and Lovie-Kitching8a99 have described the use of thePelli-Robson chart to determine the con-trast reserve which is used to predict theability of low vision patients to read text.

Low luminance testingIt has been suggested that testing underlow luminance conditions is more sensi-tive to changes in vision.xg,‘“O Greater dif-ferences between single visions9 and bifo-cal”” contact lens designs have been notedwith low luminance-low contrast acuitytests than with conventional high lumi-nance-high contrast tests. Differences inthe effects of changes in luminance onhigh contrast acuity have been reportedfor a range of ocular diseases.‘02J03 In op-tic nerve lesions (optic atrophy and opticneuritis), suppression amblyopia, retinallesions (disease of Bruch’s membrane andmacular toxoplasmosis), although visualacuity is reduced under normal lumi-nances, under reduced luminance visualacuity does not decrease as much as fornormal patients. “* Patients with age-related macular degeneration have a re-duction in acuity with reduction in lumi-nance similar to that experienced by nor-mal patients. lo2 Those with retinitispigmentosa have a greater reduction inacuity with reduced luminance than nor-mal subjects. lo3 Whether this test can beused for early detection of retinitispigmentosa has not been evaluated.

Rather than reducing the lighting onthe chart, it is usually simpler and moreaccurate to reduce the amount of lightreaching the eye, typically by using neu-tral density filters. Low luminance testingcan be performed with neutral density(ND) filters in a pair of goggles (weldinggoggles with the lenses replaced workwell). Typically a ND of two (reduction in

luminance of 1O-2) is used. As pupil size isnot controlled, much of the reported dif-ferences may have been due to variationsin ocular aberrations which occur with theincrease in pupil size resulting fromchanges in luminance, but this may stillbe related to functional vision. Depend-ing on the conditions, with youngpatients, the reduction with a two ND fil-ter is about 0.1-0.35 logMAR with highcontrast visual acuity, about 0.3-0.7logMAR with low contrast (10 per cent)acuity,8g and O-O.4 log units with the Pelli-Robson charts.‘04,‘05 The differences aregreater for older patients.‘06 As these val-ues vary between studies, due to differ-ences in the conditions, it is necessary toestablish ‘in practice’ normal values. Thevalue of low luminance testing for detec-tion of disease and the assessment of ‘realworld’ visual function has yet to be dem-onstrated.

COMPUTER-BASED TEST UNITS

Medmont AT-20The Medmont AT-20 visual acuity tester isa commercially-available computer-basedsystem for the presentation of variablecontrast Bailey-Lovie visual acuity chartsand gratings. Eight contrast levels can beused on a range of visual acuity charts anda grating display. A randomised display ofdifferent characters can be used to avoidlearning effects. In addition to standardvisual acuity chart displays, a staircase pro-cedure can be used to determine averageacuity. Other facilities include duo-chrome, cross cylinder targets, binocularvision tests (Worth four dot and fixationdisparity), astigmatic fan and animatedfixation targets for children. The AT-20(board and controller) is fitted to a PCcomputer. The system comes with a hand-held control unit and has many morefacilities than projector charts. ‘Normal’values are not yet available.

Mentor B-VAT IIThe Mentor B-VAT II video acuity systemis a commercially-available computer-based system for the measurementof variable contrast visual acuity and grat-ing acuity. Optotypes include letters,


5 1


numbers, Landolt rings, tumbling E,HOTV and children’s symbols. Arandomised display of different characterscan be used to avoid familiarity. Unfortu-nately the visual acuity charts are not of aBailey-Lovie design. Nine contrast levelsare available. It also has the ability topresent duochrome, astigmatic fan, bin-ocular vision tests, stereopsis and ani-mated targets for children. The more ex-pensive Mentor B-VATII-SG system canalso present sine wave gratings for themeasurement of contrast sensitivity at upto 16 spatial frequencies. The systemcomes with a hand-held control unit andhas many more facilities than projectorcharts, ‘Normal’ values for contrast sensi-tivity are available.io7

LSV Acuity programA shareware (pay if you use it) computerprogram, developed at the Visual Func-tions and Optical Sciences Laboratoriesof the University of Melbourne, allows as-sessment of high and low contrast visualacuity Visual acuity scores might be ex-pected to be slightly different from thoseof a Bailey-Lovie chart as only single let-

ters are presented. The low contrast let-ters are approximately 30 per cent con-trast. The program runs under Windows3.1 on a PC computer.

PU’lTING THE TESTS INTOOPTOMETRIC PRACTICE

Before contrast sensitivity charts or con-trast letter charts are introduced intoclinical practice the chart must be prop-erly lit. Age and ophthalmic disease willaffect scores and the reliability of meas-urement. To enable correct detection ofreduced visual performance the vari-ance of the expected scores must be con-sidered.

Contrast sensitivity charts or contrastletter charts may also be used to demon-strate the effects of cataract, refractive bluror bifocal contact lenses on vision (Table 4).

Lighting requirementsWhile the precise lighting requirementsfor each of these tests are different, all areperformed under photopic conditionssimilar to those found in most practicesand most perform equally well with small

changes in overall illuminance. Most ofthese tests can be used in clinical practicewith few changes to lighting arrange-ments. The larger charts are more prob-lematic as even lighting may be difficultto achieve. Glare sources on the chartfrom poorly placed lights must be avoided.Smithlo* has described how a photo-graphic light meter (separate or built intoa camera) can be a useful substitute for aphotometer for measurement of the lu-minance of the charts. Luminance (L) canbe estimated by the equations:io8

13.1 . (F)2 (4)L= cd/m2

t .A

13.1 . 2” (5)L= cd/m*

A

where F is the F-number, t is the exposureduration, A is the ASA rating, and E is theexposure value. The illuminance require-ment is specified for some charts. In sim-ple terms, illuminance is the light falling

Table 4. Some practical demonstrations of the use of contrast sensitivity charts or contrast letter tests. These examples may be usedto demonstrate the value of these tests to the practitioner, to patients or to a patient’s relative or carer.


52


on a surface, while luminance is the lightcoming from the surface. The illuminance(I) can be approximated by the equation:

n:.L (6)I = -

r

The reflectance (r) of the white areas ofmost charts is about 0.8 to 0.9. The reflect-ance of grey charts will be different (for ex-ample, MET reflectance = 0.3).s5 A smallphotometer is supplied with Vistech charts.

Population normsPopulation norms (the average scoresexpected within the ‘normal’ populationwhere there is no ocular or visual distur-bance) allow the practitioner to comparethe visual performance of a new patientwith the age-matched norms to determinewhether visual function is reduced. Published population norms have been in-cluded where available. These norms areapplicable only under certain conditionsas detailed in the relevant publications.The conditions include the luminanceand test protocol used during the study.Most contrast measures are sensitive tochanges in luminance. As an example ofthe effects of changes in protocol, Reevesand co-workers55 demonstrated that differ-ences between the mode of application ofthe Arden grating test accounted for muchof the difference in measured contrastsensitivity. If the same conditions are usedthe published norms are very useful. Al-ternatively, the scores of the first few hun-dred ‘normal’ patients may be used to es-tablish population norms in a particularsetting. The population norm is the meanof the responses of normal patients asshown in Figure 4. Limits to the normalrange are typically established by multiply-ing the standard deviation by two or 2.6 togive the 95 per cent or 99 per cent confi-dence limits respectively. Only five per centor one per cent, respectively, of normalindividuals will fall outside these ranges.

ReliabilityReliability is a measure of the ability toget the same result on repeated measure-ment. Two methods for assessing reliabil-

Figure 4. A normal distribution shown as the frequency of visual performance scores.This normal frequency distribution has been converted into standard deviation units,where 68 per cent of the patients are between f 1 standard deviations and 95 per centof the patients between f 2 standard deviations of the mean. The shaded area indicatesthe five per cent of the patients who are outside the + 2 standard deviations (95%confidence limits).

ity of visual performance have been sug-gested. The first approach uses the coeffr-cient of repeatability as proposed by Blandand Altman.1og The second approach usesthe variability of the individual patient’svisual performance as proposed by Brownand Lovie-Kitchin.“’ Both approacheshave a place in optometric practice.

Coefficient of repeatabilityThe variance (spread) of the distributionof differences between two (repeated)measures made on separate occasions(that is, test-retest) can be used to assessrepeatability. This is achieved by measur-ing vision twice on the same person un-der the same conditions and subtractingone from the other to get the differencescore. As in the determination of popula-tion norms, the distribution of the differ-ence scores for the group of patients isused to determine the coefficient ofrepeatability (twice the standard deviationof the difference scores). The coefficientof repeatability represents the 95 per cent

confidence limits (Figure 4). In practicalterms, this means that a difference be-tween vision scores greater than the coef-ficient of repeatability will only occur bychance five per cent of the time, that is,there is a 95 per cent probability that thedifference is a real change in vision.

Individual variabilityThe coefficient of repeatability measurestypical changes in a population. Somepatients are more reliable in their re-sponses than others and hence the vari-ability within the group measured by thecoefficient of repeatability may over-estimate the reliability of many patients.“OMeasurement of the visual performanceon a number of occasions (for example,during initial consultation, after frameselection, at collection, subsequent con-sultation, et cetera) allows determinationof the standard deviation of the responsefor that individual. Again, typically the 95per cent confidence limit (twice the stand-ard deviation) is used to determine what


53


constitutes a real change in vision for thatindividual (Figure 4). Brown and Lovie-Ritchinlio demonstrated that individualvariability can be a much more sensitivemeasure of change for an individual. Un-fortunately, it is not possible to determinean individual reliability for all patients andthus the coefficient of repeatability andpopulation norms are still important.

The effects of ageVision as assessed with most visual per-formance measures shows a reduction af-ter about the fourth decade.‘4z1i’ Much ofthe loss is due to optical changes in theolder eye, but reductions in neural integ-rity at all levels of the visual system arelikely to play a role. Visual acuity peaks inthe third decade and then reduces withincreasing age. The effect on low contrastletters is slightly greater.5g,62 The reductionin visual acuity is more pronounced un-der low luminance and most pronouncedfor low contrast charts under low lumi-nance.ro6 Contrast sensitivity reduces withage for median and high spatial frequen-cies and the peak contrast sensitivitymoves to a lower spatial frequency.in Thisis noted over a wide range of luminances.‘4

Age-related changes must be consideredwhen tests of visual function are applied.Increases in the range of responses for olderpatients, whereby some older patients haveexcellent vision and some have poor visionwithout any apparent visual disease, makesthe detection of visual dysfunction in theelderly more difficult.

Ophthalmic diseaseOcular and visual dysfunction reduce thereliability of visual performance measure-ment. For example, Reeves and co-workers43,44,55J13 have demonstrated withpatients suffering from stable ocular dis-eases that the coefficient of repeatabilityis much larger for Arden, high and lowcontrast acuity, Pelli-Robson and Vistechchart tests than reported in other studieswith ‘normal’ patients. Similarly, when op-tically degraded by wearing bifocal con-tact lenses, the coefficient of repeatabilityis much larger for the scores with high andlow contrast acuity and Pelli-Robsoncharts.45 Hence, as the responses of

patients with a visual dysfunction are lessreliable, detection or monitoring of dis-ease is more difficult.

The clinical significance ofchangeBefore a clinical management decision forreferral or alteration of treatment (forexample, contact lens wear), it is essen-tial to determine whether a measuredchange is significant. The latest findingmust be compared with some ‘standard’or reference. That standard may be thefinding at the previous consultation, thefinding for the other eye or it may be theestablished ‘normal’ value. A knowledgeof the variance of the measurement tech-nique will allow the practitioner to be con-fident that the measured change is signifi-cant. This indicates the importance ofdetermining reliability and norms forclinical tests. While 95 per cent confi-dence limits are often quoted as levels forthe acceptance of a significant (that is,real) change in visual function, Reevesand co-workers43,44 noted that the level ofacceptance of change must be altered toaccount for differences in the possibleoutcome and cost of an inappropriate ac-tion. For example, if the likelihood of aserious result (such as loss of vision) fromfailure to take appropriate action was highand the cost of an inappropriate clinicalaction when no real change had occurredwas low, then acceptance of a lower levelof change may be more appropriate (suchas 1.6 standard deviation change: 90 percent confidence limit). If the probableoutcome of a failure to take clinical ac-tion is negligible and the cost of inappro-priate action is high then a higher confi-dence limit may be more appropriate(such as three standard deviations ofchange: 99 per cent confidence limit).Reducing the confidence limit will in-crease the number of patients without thecondition for whom action is taken (falsepositive rate), while increasing the confi-dence limit will increase the number ofpatients with the condition for whom noaction is taken (false negative rate) .‘13

The decision to take clinical actionbased on the belief that a patient suffersfrom a particular condition must be tem-

pered by a knowledge of the incidence ofthat condition within a population. Forexample, as the incidence of congenitalcolour vision defects differs between malesand females, a ‘failing’ score on a colourvision screening test is more likely to in-dicate a true defect in a male patient thanin a female patient. Where the incidenceof a disease is low, to avoid inappropriatereferral, confirmatory evidence from anumber of tests should be sought. Thelikelihood that a patient has a particularcondition can be formally evaluated givenknowledge of the incidence in the popu-lation and the ability of the test to detectthe condition using Bayesian theory.n”“7As may be appreciated from considerationof reliability, a single response may be subject to considerable variance. Repeatedmeasurement (for example, after a breakor the next day) will increase the confi-dence of a reliable result. A knowledge ofthe expected variance on the particulartest and confirmatory evidence from arange of tests will assist in clinical deci-sion making.

RECOMMENDATIONS

Contrast sensitivity charts and contrastletter tests have a limited ability to screenfor ocular disease. Screening will detectpatients with an otherwise undetectedvisual reduction, but the incidence of suchconditions is low. As most of the chart-based tests can be quickly and easily ap-plied, screening can be performed bytrained ancillary staff. Some diseases pro-duce changes in vision which are not de-tected with visual acuity (for examplesome optic neuropathies) but most dis-eases which reduce contrast sensitivity alsoreduce visual acuity or are detected byother routine ophthalmic tests.“a33.34Where visual acuity is reduced, a measure-ment of contrast sensitivity at low to me-dium spatial frequencies (Cambridge grat-ing or Pelli-Robson test) or low contrastvisual acuity will give further informationabout visual function. If this aspect ofvision is reduced, the patient maysuffer from difficulties in mobility andorientation, especially in poor lightingconditions and where contrast is reduced


54


(such as on a rainy day). The practitioneris then able to assist with advice on howthe patient may reduce these difficulties.

As the results with these tests (Cambridgegrating, Pelli-Robson test and low contrastvisual acuity) are highly correlated onlyone test is necessary.

Similarly, the vision of a patient withnormal visual acuity who complains ofreduced vision should be investigated withone of these tests. A reduction in visionmay warrant further investigation and thepractitioner can offer advice on means ofameliorating the visual diff icult ies . Ofcourse, there may be other reasons for the

complaint of reduced vision such as theeffects of glare. Glare can be assessed eas-ily in practice. For patients whose occu-pation (such as professional drivers) orinterests (such as certain sports) makes de-tection of relatively large objects often oflow contrast very important, the routineuse of a measure of low to medium spa-tial frequencies is recommended. Hence,contrast sensitivity tests and contrast let-ter charts are an essential supplementarytest which optometrists may use to furtherevaluate vision.

C O N C L U S I O N S

Evaluation of the commercially availablecontrast sensitivity charts and contrast let-ter tests indicates that their most impor-tant role in clinical practice is in the de-scription of a patient’s vision and visualproblems. As such, they provide an impor-tant component of a clinician’s testingbattery as they reflect the visual experi-ences of patients in the ‘real world’ bet-ter than visual acuity.

ACKNOWLEDGMENTMuch of this information was presentedas a Practitioner Workshop in the Schoolof Optometry, Queensland University ofTechnology. Our thanks to Associate Pro-fessor Jan Lovie-Kitchin and the anony-mous reviewers for their helpful com-ments.

DECLARATION

The authors have no proprietary interestsin any of the vision tests mentioned here.

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Author’s address:Dr RL WoodsCentre for Eye ResearchSchool of OptometryQueensland University of TechnologyLocked Bag 2, Red HillQueensland 4059AUSTRALIA

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