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Visual AcuityVisual Acuity
Walter Huang, ODWalter Huang, OD
Yuanpei UniversityYuanpei University
Department of OptometryDepartment of Optometry
DefinitionDefinition
The resolving power of the eye used to The resolving power of the eye used to assess and quantify the eye’s ability to assess and quantify the eye’s ability to resolve varying letter sizesresolve varying letter sizes
Visual acuity is dependent on:Visual acuity is dependent on: The refractive error of the eyeThe refractive error of the eye The health and the integrity of the eyeThe health and the integrity of the eye The test targets usedThe test targets used The test conditionsThe test conditions
MeasurementMeasurement
The measurement of VA is done to The measurement of VA is done to monitor change in vision with disease monitor change in vision with disease progression of disease and/or treatment progression of disease and/or treatment planplan
It is a way to measure “blur” visionIt is a way to measure “blur” vision
Visual acuity must be measured on every Visual acuity must be measured on every patient at every visit!patient at every visit!
Standard Test DistancesStandard Test Distances
Distance visual acuity (DVA) Distance visual acuity (DVA) 20ft or 6M is equivalent to optical infinity20ft or 6M is equivalent to optical infinity
Near visual acuity (NVA)Near visual acuity (NVA) 40cm40cm
Weymouth Classification of Visual Weymouth Classification of Visual Acuity MeasurementsAcuity Measurements
Minimum distinguishable Minimum distinguishable (or minimum visible)(or minimum visible) The ability to see something as being distinguishable The ability to see something as being distinguishable
from the backgroundfrom the background
Minimum separableMinimum separable The ability to determine a group of points or lines as The ability to determine a group of points or lines as
separate and distinctseparate and distinct
Minimum cognizable or legibleMinimum cognizable or legible The ability to form sense (e.g., Landolt C) or minimum The ability to form sense (e.g., Landolt C) or minimum
legible (e.g., letters or numbers)legible (e.g., letters or numbers)
HistoryHistoryEarly astronomers found two stars Early astronomers found two stars resolvable as separate if the distance resolvable as separate if the distance between the stars subtend an angle of at between the stars subtend an angle of at least 1 minute of arcleast 1 minute of arc
Snellen TestSnellen Test
In 1862 Snellen and Giraud-Toulon In 1862 Snellen and Giraud-Toulon introduced a system for measuring VAintroduced a system for measuring VAA form of A form of minimum legible minimum legible visual acuity visual acuity tasktaskSnellen acuity is based on a minimum Snellen acuity is based on a minimum angle of resolution of 1 minute of arcangle of resolution of 1 minute of arcVisual acuity target designs: optotypesVisual acuity target designs: optotypes The Snellen optotype has overall dimensions The Snellen optotype has overall dimensions
that are 5 times the detailthat are 5 times the detail
Snellen NotationSnellen Notation
Snellen fractionSnellen fraction Test distance divided by the distance at which optotype Test distance divided by the distance at which optotype
would subtend 5 minutes of arc at the retina would subtend 5 minutes of arc at the retina
The denominator of the Snellen fraction is termed The denominator of the Snellen fraction is termed the the foot-letter sizefoot-letter size The letter’s overall size, both angularly and linearlyThe letter’s overall size, both angularly and linearly
The average emmetrope has a VA of 20/20 or betterThe average emmetrope has a VA of 20/20 or better This is based on a test distance of 20ftThis is based on a test distance of 20ft
The classic Snellen fraction is the reciprocal of the The classic Snellen fraction is the reciprocal of the minimum angle of resolution (MAR)minimum angle of resolution (MAR)
Snellen Acuity and Minimum Angle Snellen Acuity and Minimum Angle of Resolutionof Resolution
Snellen AcuitySnellen Acuity Snellen AcuitySnellen Acuity Minimum Angle Minimum Angle of Resolutionof Resolution
FractionFraction DecimalDecimal Minutes of ArcMinutes of Arc
20/2020/20 1.01.0 11
20/2520/25 0.800.80 1.251.25
20/3020/30 0.670.67 1.51.5
20/4020/40 0.500.50 22
20/5020/50 0.400.40 2.52.5
20/10020/100 0.200.20 55
20/20020/200 0.100.10 1010
Snellen TestSnellen Test
Snellen equivalent Snellen equivalent If the test distance is not 20ft, the measured Snellen If the test distance is not 20ft, the measured Snellen
fraction may be used to predict what the patient’s VA fraction may be used to predict what the patient’s VA would be for a 20ft distancewould be for a 20ft distance
ExampleExample Patient can resolve a 30ft letter at a test distance of Patient can resolve a 30ft letter at a test distance of
15ft. The Snellen acuity is 15/30.15ft. The Snellen acuity is 15/30. 15/30 =20/x15/30 =20/x x = 40x = 40 The Snellen equivalent is 20/40The Snellen equivalent is 20/40
Calculating a Visual Acuity Test Calculating a Visual Acuity Test Object’s Foot-Letter DesignationObject’s Foot-Letter DesignationExample of a letter that is 2cm tallExample of a letter that is 2cm tall
Types of Distance Visual Acuity Types of Distance Visual Acuity ChartsCharts
Wall chartsWall charts Testing VA in literate adultsTesting VA in literate adults Printed on cardboard and mounted on a wallPrinted on cardboard and mounted on a wall Well-suited for vision screenings and doctors’ Well-suited for vision screenings and doctors’
officesoffices Variations: Tumbling E chart, Landolt C chart, Variations: Tumbling E chart, Landolt C chart,
Bailey-Lovie chartBailey-Lovie chart
Types of ChartsTypes of Charts
Hand-held visual acuity cardsHand-held visual acuity cards Primarily for children in vision screeningsPrimarily for children in vision screenings Simplest form is the letter E in different sizes Simplest form is the letter E in different sizes
printed onto hand-held cardsprinted onto hand-held cards Free space testingFree space testing
Types of ChartsTypes of Charts
Chart projectorsChart projectors Projects test objects onto a screenProjects test objects onto a screen Standard chart projector can be calibrated for Standard chart projector can be calibrated for
varying testing distances between 10ft and varying testing distances between 10ft and 20ft20ft
Optically “fold” the testing distance using Optically “fold” the testing distance using mirrors to create appropriate testing distance mirrors to create appropriate testing distance (20ft is the standard)(20ft is the standard)
Calibration of Chart ProjectorsCalibration of Chart Projectors
Measure the testing distanceMeasure the testing distance
Calculate how large an optotype would Calculate how large an optotype would have to be on the screen to represent have to be on the screen to represent 20/200 for that testing distance20/200 for that testing distance
Adjust the lenses in the chart projector Adjust the lenses in the chart projector until the letter size from the previous step until the letter size from the previous step is achievedis achieved
Calibration of Chart ProjectorCalibration of Chart Projector
A 20 foot-letter is 8.87mm. May use direct A 20 foot-letter is 8.87mm. May use direct proportions to calculate the letter size that proportions to calculate the letter size that would be needed to represent 20/20 for would be needed to represent 20/20 for any possible testing distance.any possible testing distance.
ExampleExample 8.87mm/20ft = x/17ft8.87mm/20ft = x/17ft x = 7.54mmx = 7.54mm
RecordingRecording
Record the VA line for which more than 2 Record the VA line for which more than 2 letters (or less than half the number) are letters (or less than half the number) are read incorrectlyread incorrectly
RecordingRecording
ExamplesExamples Two out of five letters on the 20/20 line are read Two out of five letters on the 20/20 line are read
incorrectlyincorrectly20/20 -220/20 -2
All letters on the 20/20 line are read correctly, and All letters on the 20/20 line are read correctly, and one letter on the next line down is read correctlyone letter on the next line down is read correctly
20/20 +120/20 +1 One of the letters on the 20/20 line is read incorrectly, One of the letters on the 20/20 line is read incorrectly,
and two letters on the 20/15 line are read correctlyand two letters on the 20/15 line are read correctly20/20 -1/+220/20 -1/+2
RecordingRecording
Points to considerPoints to consider Ask the patient to read a line over again or Ask the patient to read a line over again or
show the letters to the patient one line at a show the letters to the patient one line at a timetime
Isolate lines on the chart to avoid distracting Isolate lines on the chart to avoid distracting influencesinfluences
May consider showing the patient a line of the May consider showing the patient a line of the same VA on a different slide, if the projector same VA on a different slide, if the projector has more than one slidehas more than one slide
Expected FindingsExpected Findings
A visual acuity of 20/20 or better is A visual acuity of 20/20 or better is considered normalconsidered normal
The difference between the two eyes The difference between the two eyes should be no greater than one lineshould be no greater than one line
Pinhole Visual AcuityPinhole Visual Acuity
PurposePurpose To determine if a decrease in vision is correctable by To determine if a decrease in vision is correctable by
lenseslenses
Pinhole visual acuities are taken when the Pinhole visual acuities are taken when the patient’s entering VA is worse than 20/30 at patient’s entering VA is worse than 20/30 at distance distance andand near through the habitual near through the habitual correctioncorrection
Expected FindingsExpected Findings
If the cause of the patient’s decreased If the cause of the patient’s decreased visual acuity is due to an uncorrected visual acuity is due to an uncorrected refractive error, the VA is expected to refractive error, the VA is expected to improve through the pinholeimprove through the pinhole
If the cause of decreased visual acuity is If the cause of decreased visual acuity is not optically based, no improvement, and not optically based, no improvement, and possibly a decrease, will occur through the possibly a decrease, will occur through the pinholepinhole
Near Visual AcuityNear Visual Acuity
Testing the VA at close range (usually Testing the VA at close range (usually 40cm)40cm)Near visual acuities are taken through the Near visual acuities are taken through the habitual correctionhabitual correctionThe purpose is to detect people with near The purpose is to detect people with near vision difficulties (e.g., uncorrected high vision difficulties (e.g., uncorrected high hyperopia, accommodative dysfunction)hyperopia, accommodative dysfunction)In patients over 40 years old, the reduced In patients over 40 years old, the reduced near visual acuity is one of the symptoms of near visual acuity is one of the symptoms of presbyopiapresbyopia
Near Vision ChartsNear Vision Charts
Types of notationTypes of notation Reduced Snellen Acuity cardReduced Snellen Acuity card
Test distance at 16in (or 40cm)Test distance at 16in (or 40cm) Jaeger Acuity CardJaeger Acuity Card
20 letter sizes classified J1 to J2020 letter sizes classified J1 to J20Test distance at 14inTest distance at 14in
Point systemPoint systemEach point is 0.35mmEach point is 0.35mm
M notationM notationBased on meter unitBased on meter unit