Shafee The definition SPATIAL RESOLVING CAPACITY OF THE VISUAL SYSTEM Angular size of the object that can just be resolved Limitations Optical factors Neural factors Combination of the above two ID: 524510
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Slide1
Visual acuity
ShafeeSlide2
The definition
SPATIAL RESOLVING CAPACITY OF THE VISUAL SYSTEMAngular size of the object that can just be resolvedLimitationsOptical factorsNeural factorsCombination of the above twoSlide3Slide4
OPTICAL LIMITATIONS
Diffraction pattern – AIRY DISCὼ
=2.44
λ
/p
Where
ὼ
= diameter in radians
λ
=wavelength of light
p=pupil diameterὼ ∝ 1/pRayleigh criterion for resolutionβmin = 1.22 λ/p (or) 2.3/pWhere βmin in min of arc & p is in mmSlide5
When p=4.6mm,
βmin = 0.5min of arc =2.3mm, = 1.0min of arc…etcRefractive errors/focusing errorsSmall pupils=diffraction
Large pupils=aberrations
Chromatic
Optical
P=2.5mm (max VA..,MAR =0.92)Slide6
Neural limitations
Packing density of neural receptors
Neural interactions in retina
Visual pathway
Unstimulated receptor(2
μ
m-4
μ
m)
0.82 min of arc resolution..
16.67mm assumed nodal point dist from retina≈optical limitSlide7
Tests of visual resolution
Measuring the limits of visual performance…to discern & to detect & to recognizeMINIMUM DETECTABLEMINIMUM SEPERATIONRECOGNITION RESOLUTIONSlide8
Minimum detectableThreshold size for being recognized against a background
Width of image…???!!!???Dependence??Diffraction..!!Very small – contrastMin. size?!?Slide9
Minimum separable
Least separation b/w two adjacent entities two be seen as two…Commonly used to test human visual efficiency
Grating line targets…
DUTY CYCLE(1.0)..three line target
cpd – units
Spurious resolution – checker board targetsSlide10
Recognition resolution
Most widely used for clinical testingCorrect identification / recognition
OPTOTYPES
Test targets used for these tests
Eg.,
Landolt C rings
Letters
Lh symbols..etcSlide11
Landolt rings
Landolt ring target or Landolt C
External diameter is 5 times the STROKE width
Thus the internal diameter is 3 times the stroke width
Gap positions
4 directions
8 directions
Well defined and unambiguous detail as targetSlide12
Letter optotypes
Grid patterns5×55×45×6SNELLEN-serifs-British standard(D,E,F,N,H,P,R,U,V,Z)-5×4 gridMODERN-sanserifs-Sloan(C,D,H,K,N,O,R,S,V,Z)-5×5 gridSlide13
2003 British bi{(C,H,N,V,Z)(K,R,D)(E,F,P,U)}
Standardization of variability and legibility of optotypes universallyAverage legibility at each acuity levelClues and combinationsEg.,N
&
HSlide14
Tumbling EIlliterate E - 5×5
Different orientation at each acuity levelFour alternative Eight alternativeSlide15
Numerical & pictorial targets
Animals,Catoons,Numbers…etcUsed in case of Infants, Toddlers..No specific grid pattern and not standardizedRecognition better than usual optotypesThreshold size of identification is very smallSlide16
Events in the HISTORY
1843 German ophthalmologist Heinrich Kuechler wrote a treatise advocating the need for standardized vision tests and developed a set of three charts.1854 Eduard von Jaeger published a set of reading samples to document functional vision. He published samples in German, French, English and other languages.He used fonts that were available in the State Printing House in Vienna in 1854 and labeled them with the numbers from that printing house catalogue.Slide17
Events in the HISTORY
1861 Franciscus Donders coined the term visual acuity to describe the “sharpness of vision” and defined it as the ratio between a subject's VA and a standard VA.1862 Hermann Snellen published his famous letter chart.His most significant decision was not to use existing typefaces but to design special targets, which he called optotypes.
This was crucial because it was a physical standard measure to reproduce the chart. Snellen defined “standard vision” as the ability to recognize one of his optotypes when it subtended 5 minutes of arc, thus the optotype can only be recognized if the person viewing it can discriminate a spatial pattern separated by a visual angle of 1 minute of arc.Slide18
Events in the HISTORY
1875 Snellen changed from using feet to meters (from 20/20 to 6/6 respectively)Today, the 20-foot distance prevails in the United States and 6 meters prevails in Britain.1875 Monoye proposed to replace the fractional Snellen notation with its decimal equivalent (e.g., 20/40 = 0.5, 6/12 = 0.5,5/10 = 0.5)Decimal notation makes it simple to compare visual acuity values, regardless of the original measurement distance.Slide19
Events in the HISTORY
1888 Edmund Landolt proposed the Landolt C, a symbol that has only one element of detail and varies only in its orientation.The broken ring symbol is made with a "C" like figure in a 5 x 5 grid that, in the 20/20 optotype, subtends 5 minutes of arc and has an opening (oriented in the top, bottom, right or left) measuring 1 minute of arc.This proposal was based in the fact that not all of Snellen's optotypes were equally recognizable.
This chart is actually the preferred visual acuity measurement symbol for laboratory experiments but gained only limited acceptance in clinical use.Slide20
Events in the HISTORY
1923 Soviet ophthalmologists Sergei Golovin and D. A. Sivtsev developed the table for testing visual acuity. Later this table became known as Golovin-Sivtsev Table.1959 Louise Sloan designed a new optotype set of 10 letters, all to be shown in each and every line tested, in order to avoid the problem that not all letters are equally recognizable.
The larger letter sizes thus required more than one physical line.
Louise Sloan also proposed a new letter size notation using the SI system stating that standard acuity (1.0, 20/20) represents the ability to recognize a standard letter size (1 M-b unit) at a standard distance (1 meter)Slide21
Events in the HISTORY
1976 Ian Bailey and Jan Lovie published a new chart featuring a new layout with five letters on each row and spacing between letters and rows equal to the letter sizeThis layout was created to standardize the crowding effect and the number of errors that could be made on each line, so letter size became the only variable between the acuity levels measured.These charts have the shape of an inverted triangle and are much wider at the top than traditional charts.
Like Sloan's chart, they followed a geometric progression of letter sizesSlide22
Events in the HISTORY
Lea Hyvärinen created a chart, the Lea chart, using outlines of figures (an apple, a house, a circle and a square) to measure visual acuity in preschool children.Hugh Taylor used these design principles for a "Tumbling E Chart" for illiterates, later used to study the visual acuity of Australian Aborigines.Slide23
Designation of VA
Snellen fractionDecimal notationMARlogMARVAR & fASVESlide24
logMARLogarithm of MAR
Eg., 20/20MAR = 1LogMAR = log10(1.0) = 0.0020/40MAR = 2logMAR = log10(2.0) = 0.30Slide25
VARVAR = 100 – 50 logMAR
20/20 – logMAR = 0.00100 – 50(0.00) = 10020/200 – logMAR = 1.00100-50(1.00) = 50
20/2000 – logMAR = 2.00
100-50(2.00) = 0
20/16 – logMAR = 105Slide26
Functional acuity scoreFAS = (VARod+VARos+3VARou)/5
VAR guides to the evaluation of permanent impairment5:4 change btw MAR & VARSlide27
Visual Efficiancy
1925 – Snell & SterlingUse for legal and compensating purposesDiffusing filters – Equal decrease in vision20/20 – VE = 1.0/ 100%20/200 – VE =0.2/20%VE = 0.2
(mar-1/9)
Log(VE%) = 2.0777 – 0.0777 (MAR)Slide28
Adopted by AMA
Extended to quantification of VF & OMOverall efficiency – a product of acuity, fields & motility efficiency scoresMONOCULAR efficiency of both eyes with THRICE the weightage to the better eyeGuides to the evaluation of permanent impairement,5th
ed.,Slide29Slide30
Chart DesignsSNELLEN
Bailey lovieTAYLOR – tumbling E chartFerris & colleagues – ETDRS – Sloan lettersStrong & Woo – columnar size progression with masking barsOthers( Johnston, Hyvarinen, Landolt) Slide31
SNELLEN CHART7 Different lines
One letter – largest at the topClassical chart size sequence is 200,100,70,50,40,30&20Modified Snellen chartsSTANDARD Snellen chartsSlide32
Bailey – Lovie design principles
A logarithmic size progression
Same no. of letters at each size level
Spacing b/w letters and rows proportional to letter size
Equal and average legibility for letters at each size level
logMAR units for lines and lettersSlide33
Design features
Logarithmic size progressionGreen , Sloan , WestheimerPeripheral visual acuities @ dif retinal eccentricities on a logarithmic scale is constant0.1 log units progression commonly used1.2589:1 ratio(5:4 approx.)
1.0,1.25,1.6,2.0,2.5,3.2,4.0,5.0,6.3,8.0,10,12.5,16…
(6.0,7.5,9.5,12,15,19,24,30,38,48,60,75,95….)Slide34
Number of optotypes at each size level
More letters – more reliabilityFiner size progression – more reliabilitySD = k√p/n SD= standard DeviationK= constant
p= size progression in log units
N= no. of letters at each lineSlide35
Letter Legibility
Concilium Ophthalmological Universale – CETDRS – golden standardLETTERS preferred to C&EGuessing ratioRelativity b/w gesture & vocal decisions
ETDRS – SLOAN letters
Bailey-lovie – 1968 British standard Slide36
Spacing b/w letters and words
Flom & colleagues – Contour InteractionCrowding effectBAILEY & RAASCH – expt.(0.5 – 3.0)Two fold spacing improved VA score by 0.03,0.04&0.07 respectively for
british
,
sloan
&
landolt
rings
Wider Spacing – better VASlide37
Chart formatsPrinted Panel Charts
Projector ChartsSelf illuminatedBack illuminatedCharts On Display Screens150cd/m
2
300cd/m
2Slide38
Chart luminance
Moderate photopic luminance+subdued room lightningRecommendations –85-300cd/m2(general)
Sheedy
& colleagues – twice luminance(5%MAR)
160m2(
stnd
)
120m2 min(British
stnd
)
80-320cd/m2(clinical tolerance)&low contrast targetsSlide39
Refractive correction
Unaided VAUncorrected VAHabitual VACorrected VA/BCVAOptimal VAPinhole VA& PAMVA under Special IlluminationSlide40
Testing distanceVisual infinity…??!!??
Spatial constraints of examination room10-30 foot rangeDistance appropriate testing chartsLow visionPre-presbyopes
– proximal
accomodationSlide41
Testing proceduresMonocular VA
OccluderHand-palmWorst eye first if knownBinocular VABoth eyes open
Better than/equal to VA of better eye usuallySlide42
Can be started from a little better than expected VA
Guess work?!?Snellen>50%Landolt20%All above & None belowSlide43
MISSING letters
JUMPING lettersSHUFFLING lettersECCENTRIC viewingFLIP CHARTSRosser – abbreviated chartsCamparini and colleagues – first letter in each rowSlide44
Scores
Row by Row scoringWidely practicedToo coarseResults not reliablePartial credit - ± recordingLetter by Letter scoring
± recording
Equal letters – equal points for qualificationSlide45
In Research
More samplingStandardized procedures and instructionsMemorizingChange in distancePaired chartsAuto generated charts
S – chartsSlide46
Pediatric testsLimited ability to respond to
stnd. Test stimuliCapability of patient to respondGrating acuity testsVEP testsPreferential looking tests
Optokinetic nystagmus
Flash card tests – pictures, letters
Picture or Symbol chartsSlide47
Grating Acuity TestsStriped/Checkered grating targets
Finest pattern that can elicit the responseResponse can be elicited subj/obj
Cpd
– MAR
MAR = 30/
cpd
30
cpd
grating equal to 20/20
3
cpd grating equal to 20/200Slide48
Visual Evoked PotentialFor least responsive patients
Measuring electrical potentials due to visionFlickering striped / checkered patternEP magnitude declines with finer detailSmallest spatial frequency(in CPD) with a measurable response is taken as VASlide49
Preferential looking testsDobson & Teller – Mc Donald & co-workers
Teller acuity cards – finest gratingCardiff cardsFlanking lines on either sideSlide50Slide51
Optokinetic nystagmusRotating drum/video with spatial pattern
“Railroad nystagmus” eye movementsFinest grating showing nystagmus movementsSlide52
Flash card testsA little ability to respond to instructions
Pointing, naming, matching….Bailey hall cereal testLH symbols(Lea symbols)Lighthouse flash cards(umbrella, apple, house)Broken wheel test
Allen picture cards – simple lined drawingsSlide53
Letter flash cardsNaming & matching
H, O, T, V – Mirror reversibleE & CSlide54
NEAR VISUAL ACUITY
Arm’s length40 cm stnd. DistanceShould be comparable with DVA if the illumination is sameBut accommodation should be at rest
Pupillary constriction (in PSCC)
Typeset materials – sentences, paragraphs, unrelated wordsSlide55
Designation of NVAM units
Sloan and HabelLower case letters5 mins of arc1.0 M units subtends 5 min of arc at 1 meter distance
1.45 mm height
Snellen fraction – 0.40/1.0M
Jose & Atcherson – M = 0.7*smallest letter size(mm)Slide56
PointsPrinting industry
1 point = 1/72th of an inchSmall letters(a, c, e, m, n, o, r, s, u, v, w, x, z)- 4 pDescenders (g, j, p, q, y) – 8 pAscenders( b, d, f, I, j, k, l, t) – 8 p 4/72 – 1.41mm – 8p = 1.0M
Caps – 8 p = 1.5 MSlide57
N NotationUK standard
Times New Roman fontSize in pointsRecorded by smallest print read by the subjectDistance is specifiedN8 @ 40 cmPrint size divided by 8 gives M unitsSlide58
Equivalent Snellen notationReduced
snellen40 cm usually assumed1.0 M @ 40 cm = 0.4/1.0 = 20/50Varies with distanceInappropriate to use angular distance for height
Suggests 20 foot is suggested but
irrelaventSlide59
Jaeger NotationJ followed by number
Used widely by ophthalmologistsNo standardizationSmaller no., larger sizeShould not be usedSlide60
IMP things to rememberReading & letter chart acuity
Near visual acuity verses near vision adequacyResolution limitReading efficiencyLogarithmic scalingSlide61
PurposesRefraction and prescribing decisions
Monitoring ocular healthInsurancesTreatmentContrastDisability glarePAMSlide62
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