Visual acuity
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
OPTICAL LIMITATIONS
• Diffraction pattern – AIRY DISC
• ὼ=2.44λ/p
▫ Where ὼ = diameter in radians
▫ λ=wavelength of light
▫ p=pupil diameter
• ὼ ∝ 1/p
• Rayleigh criterion for resolution
• βmin = 1.22 λ/p (or) 2.3/p
▫ Where βmin in min of arc & p is in mm
• When p=4.6mm, βmin = 0.5min of arc
=2.3mm,
= 1.0min of arc…etc
• Refractive errors/focusing errors
• Small pupils=diffraction
• Large pupils=aberrations
▫ Chromatic
▫ Optical
• P=2.5mm (max VA..,MAR =0.92)
Neural limitations
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Packing density of neural receptors
Neural interactions in retina
Visual pathway
Unstimulated receptor(2μm-4μm)
0.82 min of arc resolution..
 16.67mm assumed nodal point dist from retina
• ≈optical limit
Tests of visual resolution
Measuring the limits of visual performance…to
discern & to detect & to recognize
• MINIMUM DETECTABLE
• MINIMUM SEPERATION
• RECOGNITION RESOLUTION
Minimum detectable
• Threshold size for being recognized against a
background
• Width of image…???!!!???
 Dependence??
 Diffraction..!!
• Very small – contrast
▫ Min. size?!?
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 targets
Recognition resolution
• Most widely used for clinical testing
• Correct identification / recognition
• OPTOTYPES
▫ Test targets used for these tests
▫ Eg.,
 Landolt C rings
 Letters
 Lh symbols..etc
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 target
Letter optotypes
• Grid patterns
▫ 5×5
▫ 5×4
▫ 5×6
• SNELLEN-serifs-British
standard(D,E,F,N,H,P,R,U,V,Z)-5×4 grid
• MODERN-sanserifsSloan(C,D,H,K,N,O,R,S,V,Z)-5×5 grid
• 2003 British bi{(C,H,N,V,Z)(K,R,D)(E,F,P,U)}
• Standardization of variability and legibility of
optotypes universally
• Average legibility at each acuity level
• Clues and combinations
▫ Eg.,
N & H
Tumbling E
• Illiterate E - 5×5
• Different orientation at each acuity level
▫ Four alternative
▫ Eight alternative
Numerical & pictorial targets
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Animals,Catoons,Numbers…etc
Used in case of Infants, Toddlers..
No specific grid pattern and not standardized
Recognition better than usual optotypes
Threshold size of identification is very small
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.
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.
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.
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.
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)
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 size
• This 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 sizes
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.
Designation of VA
• Snellen fraction
• Decimal notation
• MAR
• logMAR
• VAR & fAS
• VE
logMAR
• Logarithm of MAR
• Eg., 20/20
 MAR = 1
 LogMAR = log10(1.0) = 0.00
• 20/40
 MAR = 2
 logMAR = log10(2.0) = 0.30
VAR
• VAR = 100 – 50 logMAR
▫ 20/20 – logMAR = 0.00
 100 – 50(0.00) = 100
▫ 20/200 – logMAR = 1.00
 100-50(1.00) = 50
▫ 20/2000 – logMAR = 2.00
 100-50(2.00) = 0
▫ 20/16 – logMAR = 105
Functional acuity score
• FAS = (VARod+VARos+3VARou)/5
• VAR guides to the evaluation of permanent
impairment
• 5:4 change btw MAR & VAR
Visual Efficiancy
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1925 – Snell & Sterling
Use for legal and compensating purposes
Diffusing filters – Equal decrease in vision
20/20 – VE = 1.0/ 100%
20/200 – VE =0.2/20%
VE = 0.2(mar-1/9)
• Log(VE%) = 2.0777 – 0.0777 (MAR)
• Adopted by AMA
• Extended to quantification of VF & OM
• Overall efficiency – a product of acuity, fields &
motility efficiency scores
• MONOCULAR efficiency of both eyes with
THRICE the weightage to the better eye
▫ Guides to the evaluation of permanent impairement,5th ed.,
Chart Designs
• SNELLEN
• Bailey lovie
▫ TAYLOR – tumbling E chart
▫ Ferris & colleagues – ETDRS – Sloan letters
▫ Strong & Woo – columnar size progression with
masking bars
▫ Others( Johnston, Hyvarinen, Landolt)
SNELLEN CHART
• 7 Different lines
• One letter – largest at the top
• Classical chart size sequence is
200,100,70,50,40,30&20
• Modified Snellen charts
• STANDARD Snellen charts
Bailey – Lovie design principles
1. A logarithmic size progression
2. Same no. of letters at each size level
3. Spacing b/w letters and rows proportional to
letter size
4. Equal and average legibility for letters at each
size level
• logMAR units for lines and letters
Design features
• Logarithmic size progression
▫ Green , Sloan , Westheimer
▫ Peripheral visual acuities @ dif retinal
eccentricities on a logarithmic scale is constant
▫ 0.1 log units progression commonly used
▫ 1.2589:1 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….)
• Number of optotypes at each size level
▫ More letters – more reliability
▫ Finer size progression – more reliability
▫ SD = k√p/n
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SD= standard Deviation
K= constant
p= size progression in log units
N= no. of letters at each line
• Letter Legibility
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Concilium Ophthalmological Universale – C
ETDRS – golden standard
LETTERS preferred to C&E
Guessing ratio
Relativity b/w gesture & vocal decisions
ETDRS – SLOAN letters
Bailey-lovie – 1968 British standard
• Spacing b/w letters and words
▫ Flom & colleagues – Contour Interaction
▫ Crowding effect
▫ BAILEY & 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 VA
Chart formats
• Printed Panel Charts
• Projector Charts
▫ Self illuminated
▫ Back illuminated
• Charts On Display Screens
▫ 150cd/m2
▫ 300cd/m2
Chart luminance
• Moderate photopic luminance+subdued room
lightning
• Recommendations –
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85-300cd/m2(general)
Sheedy & colleagues – twice luminance(5%MAR)
160m2(stnd)
120m2 min(British stnd)
80-320cd/m2(clinical tolerance)&low contrast targets
Refractive correction
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Unaided VA
Uncorrected VA
Habitual VA
Corrected VA/BCVA
Optimal VA
Pinhole VA& PAM
VA under Special Illumination
Testing distance
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Visual infinity…??!!??
Spatial constraints of examination room
10-30 foot range
Distance appropriate testing charts
Low vision
Pre-presbyopes – proximal accomodation
Testing procedures
• Monocular VA
▫ Occluder
▫ Hand-palm
▫ Worst eye first if known
• Binocular VA
▫ Both eyes open
▫ Better than/equal to VA of better eye usually
• Can be started from a little better than expected
VA
• Guess work?!?
▫ Snellen
 >50%
▫ Landolt
 20%
 All above & None below
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MISSING letters
JUMPING letters
SHUFFLING letters
ECCENTRIC viewing
FLIP CHARTS
Rosser – abbreviated charts
Camparini and colleagues – first letter in each
row
Scores
• Row by Row scoring
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Widely practiced
Too coarse
Results not reliable
Partial credit - ± recording
• Letter by Letter scoring
▫ ± recording
▫ Equal letters – equal points for qualification
In Research
• More sampling
• Standardized procedures and instructions
• Memorizing
▫ Change in distance
▫ Paired charts
▫ Auto generated charts
• S – charts
Pediatric tests
• Limited ability to respond to stnd. Test stimuli
• Capability of patient to respond
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Grating acuity tests
VEP tests
Preferential looking tests
Optokinetic nystagmus
Flash card tests – pictures, letters
Picture or Symbol charts
Grating Acuity Tests
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Striped/Checkered grating targets
Finest pattern that can elicit the response
Response can be elicited subj/obj
Cpd – MAR
MAR = 30/cpd
30 cpd grating equal to 20/20
3 cpd grating equal to 20/200
Visual Evoked Potential
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For least responsive patients
Measuring electrical potentials due to vision
Flickering striped / checkered pattern
EP magnitude declines with finer detail
Smallest spatial frequency(in CPD) with a
measurable response is taken as VA
Preferential looking tests
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Dobson & Teller – Mc Donald & co-workers
Teller acuity cards – finest grating
Cardiff cards
Flanking lines on either side
Optokinetic nystagmus
• Rotating drum/video with spatial pattern
• “Railroad nystagmus” eye movements
• Finest grating showing nystagmus movements
Flash card tests
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A little ability to respond to instructions
Pointing, naming, matching….
Bailey hall cereal test
LH symbols(Lea symbols)
Lighthouse flash cards(umbrella, apple, house)
Broken wheel test
Allen picture cards – simple lined drawings
Letter flash cards
• Naming & matching
• H, O, T, V – Mirror reversible
• E&C
NEAR VISUAL ACUITY
• Arm’s length
• 40 cm stnd. Distance
• Should be comparable with DVA if the
illumination is same
• But accommodation should be at rest
• Pupillary constriction (in PSCC)
• Typeset materials – sentences, paragraphs,
unrelated words
Designation of NVA
• M units
▫ Sloan and Habel
▫ Lower case letters
▫ 5 mins of arc
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1.0 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)
Points
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Printing industry
1 point = 1/72th of an inch
Small letters(a, c, e, m, n, o, r, s, u, v, w, x, z)- 4 p
Descenders (g, j, p, q, y) – 8 p
Ascenders( b, d, f, I, j, k, l, t) – 8 p
4/72 – 1.41mm – 8p = 1.0M
Caps – 8 p = 1.5 M
N Notation
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UK standard
Times New Roman font
Size in points
Recorded by smallest print read by the subject
Distance is specified
▫ N8 @ 40 cm
• Print size divided by 8 gives M units
Equivalent Snellen notation
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Reduced snellen
40 cm usually assumed
1.0 M @ 40 cm = 0.4/1.0 = 20/50
Varies with distance
▫ Inappropriate to use angular distance for height
▫ Suggests 20 foot is suggested but irrelavent
Jaeger Notation
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J followed by number
Used widely by ophthalmologists
No standardization
Smaller no., larger size
Should not be used
IMP things to remember
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Reading & letter chart acuity
Near visual acuity verses near vision adequacy
Resolution limit
Reading efficiency
Logarithmic scaling
Purposes
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Refraction and prescribing decisions
Monitoring ocular health
Insurances
Treatment
Contrast
Disability glare
PAM
Thank you...!!!