3. â Visual Acuity is the spatial resolving capacity of the
visual system
â Expresses the angular size of the detail that can be
resolved by the observer
Definition
4. â Visual function
â Refractive status of the eye
â Medico legal purposes
â Major outcome measure for a treatment
â Can be used to explain the patient
â Criteria for
â personâs fitness to drive
â ability to gain entrance into a profession
â eligibility to avail Government concessions
Uses of VA
10. â Photoreceptor spacing
â Two point images are when the images fall on two cones
separated by an unstimulated cone
â In the fovea separation between the cones is 2 microns
â Hence the spacing between two cones should be 4
microns
Neural Limitations
11. a) two lines can be resolved
b) two lines cannot be resolved and
perceived as one thick line
12. â Increase in photoreceptor spacing - Poor VA
â In the Periphery the cones are less densely packed
Neural Limitations
13. â Refractive error
â Size of the pupil
â Illumination
â Time of exposure of the target
â Area of retina stimulated
â State of adaptation of eye
â Eye movement
â Cognitive status
Factors Affecting the VA
14. Refractive error
â Defocus at the retina
â Defocus will blur out
â Fine details
â Sharp edges
â Contrast sensitivity
Factors Affecting the VA
16. Pupil Size
â Large pupil-Aberrations (reduces diffraction)
â Small pupil-Diffraction (reduces aberrations)
â Mid-size pupil-3-5mm optimal
â compromise bet. diffraction & aberrations limits
Factors Affecting the VA
17. Illumination
â For recognition tasks VA largely affected by
illumination levels
â Higher level of VA in photopic conditions -
Higher density of cones at fovea
Factors Affecting the VA
18.
19. Time of exposure
â To detect a bright spot, detection is more dependent
on brightness of the spot
â To detect a line, the acuity is proportional to exposure
time
â No simple acuity-exposure time relationship to
resolve a target
Factors Affecting the VA
20. Area of retina stimulated
â VA greatest at the center of fixation
â At 10â of arc from center of fixation - 25% loss of VA
â Negligible loss of VA till 2âof arc eccentricity
Factors Affecting the VA
22. State of adaptation of the eye
â Higher level of VA achieved when eye is adapted to
same level as test luminance
(34 cd/m2
to 34,000 cd/m2
)
â Test luminance < 34cd/m2
, adapting to lower
luminance achieves better VA
Factors Affecting the VA
23. Eye Movement
â During steady fixation the eye, the eye is in constant
motion
â Retinal images traverse a distance of 3 min of arc per
second
Factors Affecting the VA
24. â Minimum Distinguishable or Visible Resolution
â Minimum Separable Resolution
â Minimum Recognizable or Legible
Types of Visual Resolution
25. â Minimum Distinguishable or Visible Resolution
â The minimum detectable resolution is the threshold size
of a spot or a line against its background
â Contrast discrimination
â Visual Fields charting(Eg)
Types of Visual Resolution
26. â Minimum Separable Resolution
â The least separation between two adjacent points or lines
that allows them to be seen as separate
â e.g. Grating Acuity
â Vernier Acuity
â Ability to distinguish between two lines placed one of them
laterally placed end-to-end with one of them overlapping
and displaced, then stereoacuity can be measured
Types of Visual Resolution
31. â Minimum Recognizable or Legible
â Form Sense
â Landolt rings where orientation is used to determine the
VA
â True Minimum Legible
â Complex patterns such as letters or numbers used as test
stimulus
Types of Visual Resolution
32.
33. â Optotypes
â On grid pattern
â Height 5 units
â Width 5,4,6
â Serif letters
â Sans-serif
34. 1. Detection - presence or absence of a test stimulus
Task Specific Classification
42. â Dutch Ophthalmologist - Herman Snellen
â Type of minimum legible VA task
â Fraction (eg. 6/30 or 20/100)
â Numerator - test distance
â Denominator - letter size
â Letter size - dist. at which height of overall subtends 1
min.of arc
Snellen Notation
43. â Typical letters on chart - stroke width 1/5th their
height
â Def. modified - distance at which the limb of the letter
subtends 1 min of arc
â 20/20, 6/6 = test conducted 20ft/6m and that the
limbs of the smallest letters subtend 1 min of arc at
20ft, 6m
Snellen Notation
47. â Tan 5â = h/6
â Letter size h = tan 5â x 6
= 0.000291X5 X 6
= 8.75 mm
Width of each stroke = 8.75 / 5 = 1.75 mm
(6/6, 20/20 letter)
Calculation
48. â For 6/60, 20/200
Letter size h = tan 5â x 60
= 0.0145x 60
= 1.458 x 60
= 87.5mm
Calculation
49. â Considered to be optical infinity
â Definition of refractive error - when accommodation
relaxed
â Shorter distance - role of accommodation
â Longer distance - poor legibility
Why 6m or 20ft distance?
50. â Eye resolves letter that subtends 5â of arc at 20ft or 6m
â 20/20 can just resolve a 20/30, 6/9 letter from 30ft/9m
distance - as it subtends 5â of arc at 30ft/9m
â 20/20 Or 6/6 IS NOT PERFECT VISION???
â BETTER THAN 20/20 OR 6/6 -POSSIBLE
â SUPER ACUITY
What is 20/20 or 6/6 vision?
51. â Does not mean person will not have a functional problem
â Contrast sensitivity loss
â Color vision loss
â Visual Field loss
â Does not correlate with REAL WORLD VISION
Is 20/20 or 6/6 perfect vision?
52. â C and G
â F and P
â N and H
â D and O
Commonly confused letters
53. â If test chart designed for 6m/20ft is kept at 3m/10ft
E.g. 3/60 = 1/20 x 6/6 = 6/120
= 1/20 x 20/20 = 20/400
Conversions
54. â A circle with a break in it
â Entire diameter of the ring is 5 times the stroke width
of the circle so that the diameter is 3 stroke-widths
â Break or gap is 1 stroke width wide
â Critical detail is 1/5th of the height of the optotype
Landolt Rings
57. â Illiterate chart
â 3 limbs of equal length
â Presented in various orientation
â Patientâs task - identify direction to which the limbs of E
point
â Four alternative directions - up, down, right and left
â School Screening
Tumbling E
60. Decimal notation
â Reduces the Snellen fraction to a decimal
quantity
20/20 = 1.0 20/200 = 0.1
â Higher number indicates better VA
â Most widely used in European continent
â It gives a single number to quantify an angle
â It does not indicate the test distance
Designation of Visual Acuity
61. Minimum Angle of Resolution (MAR)
â Expressed in min of arc
â Indicates angular size of critical detail with just-
resolvable optotype
â Dividing letter size by test distance
20/20 â 1 min of arc
20/200 â 200/20 = 10 min of arc
20/40 â 40/20 = 2 min of arc
Designation of Visual Acuity
62. LogMAR
â logarithm of MAR
â VA = 20/20, 6/6, the MAR = 1 min of arc
Log MAR = log10 (1.0) = 0.0
20/40, 6/6 â MAR = 2 min of arc,
LogMAR = 0.30
20/200,6/6 â MAR = 10 min of arc,
LogMAR = 1.0
Designation of Visual Acuity
63. â When VA score >20/20, 6/6
â LogMAR value becomes negative
20/16, 6/4.8 â MAR = 0.8 min of arc
LogMAR = - 0.10
â For charts with size progression ratio of 0.1
log units and 5 letters per row, each letter
can be assigned a value of 0.02 on logMAR
scale
Designation of Visual Acuity
64. Visual Acuity Rating (VAR)
â VAR = 100 - 50 logMAR
â A score of 100 corresponds to 20/20,6/6
â VAR = 50 corresponds to 20/200,6/60
â VAR = 0 corresponds to 20/2000,6/6000
â VAR > 100 when VA > 20/20, 6/6
20/16,6/4.8 â VAR = 105
Designation of Visual Acuity
65. â For charts with size progression ratio of 0.1 log units
and 5 letters per row, each letter carries a VAR value
of 1
â On the VAR scaling difference of 15 points represents
a 2 fold change in MAR
â 5-point change - represents a ratio of 5:4 change in
MAR
Designation of Visual Acuity
66. Visual Efficiency
â Introduced in 1925 by Snell & Sterling for use in
quantifying visual loss for legal and compensation
purposes
â VE = 1.0 or 100% â 20/20,6/6
â 20/200,6/60 â 0.2 or 20%
VE = 0.2(MAR-1)/9
Log (VE%) = 2.0777 - 0.777 (MAR)
Designation of Visual Acuity
67. Gratings and Cycles per Degree (CPD)
â Series of light and dark stripes with a sinusoidal
luminance profile
â Widely used for testing contrast sensitivity
â Testing Infants (Teller Acuity)
â Size of the grating and thus the VA is of the patient
are specified in CPD
Designation of Visual Acuity
68. â 30cpd would have 60 alternating light and dark
stripes per degree of visual angle
â The ability to see such a grating
â 1 min of arc
â 20/20, 6/6
Designation of Visual Acuity
69. Snellen Chart
â Seven different size levels
â One letter at largest size level
â The number at each size level increased
progressively
â 8 optotypes at the smallest size level
â Size sequence in
â feet 200,100,70,50,40,30 and 20
â metric 60,30,21,15,12,9 and 6
â Modified - 60, 36, 24, 18, 12, 9 and 6
Visual Acuity Chart Design
70. Bailey - Lovie Design Principles
â Task difficulty same at each size level
â Only significant variable - size
â Clinical scoring of VA - LogMAR units as well as a method
for giving equal additional credit for each additional
letter read correctly
E.g. ETDRS
Visual Acuity Chart Design
72. Principles
â A logarithmic size progression (constant ratio
from one size to the next)
â Same number of letters at each size level
â Between letter and between row spacing
proportional to the letter size
â Each (or similar) average legibility for
optotypes at each size level
Visual Acuity Chart Design
73. Log size progression
â 0.1 log unit
â Each successive step represents a change in
size by the ratio 1.2589:1 (approx. 5:4)
â For a row of 5 letters each letter can be
assigned a value of 0.02 on logMAR scale
â A change of 10 increments on this scale
â a change of exactly 10 times
â A change of 3 steps â a change of approx 2
times
Design Features of VA Charts
74. Number of optotypes at each level
â Reliability of VA measure increases with the increased
number of letters at near threshold sizes
â For 5 letters / rows and 0.1 log unit size progression,
SD of letter chart acuity is about 0.028
Design Features of VA Charts
75. Spacing between letters and between rows
â Spacing between neighboring letters reduce their
legibility
â VA better with wider spacing
â Eye movement control and fixation tremors may
contribute to reduction in VA when letters tightly spaced
â Influence - more in small print
Design Features of VA Charts
76. â Printed Panels
â Opaque or Plastic
â Directly Illuminated
â Translucent material
â Indirect Illumination
â Usual testing distance - 6m or 20ft
â Closer test distance
â Small examination room
â Low vision patient
Chart Formats
77. â The dist from the pt. to the chart and size of the letter
- to determine VA
â VA chart designed for 4m
â 6/60 (5â of arc at 4m)
â Top row at 4m - 4/40 â 6/60
â at 1m - 1/60 â 6/360
â 1/40 â 6/240
Chart Formats
78. â Projector Charts
â Angular size of the Optotype
â If the projection dist. = observation dist.
â Angular size - independent of obs. dist
â Proj. dist. â obs. dist.
â Letter size proj. should be changed for that dist.
Chart Formats
79. â Not widely used in clinical practice
â For research purposes
â Provide means
â diff. optotypes
â change letter order
â change stimulus parameters
â contrast
â spacing arrangements
â presentation time
Video Display Charts
80. â Moderate photopic luminance
â Recommended range
â 85 - 300 cd/m2
â Widely used as std - 160 cd/m2
Chart Luminance
82. â Std. dist. - 20 ft / 6 m
â Mirrors
â Low Visual acuity
â 3m, 1m, 40cm
â Close dist. - add plus lenses (not practiced)
â Legibility of the letters not affected
â large angular size
Test Distance
83. â Place occluder in front of one eye
â Not hand
â Usual practice - right eye first
â Begin from larger letter or from the size level little
larger than expected
â E.g. 20/20 - 20/40 level
Procedure
84. â Ignore occ. errors if all letters in the next level are read
correctly
â Encourage to guess - close to threshold level
â Testing stopped - missed more than a total of 5 letters
â Pin hole acuity
â Other eye
â B/o acuity
Procedure
85. â Best Corrected VA
â With best ref cx
â Provides a benchmark for detecting any future
changes of vis. sys
Procedure
86. â Most widely used
â Too coarse to reliably detect small changes in VA
â Poor Sensitivity
â Partial credit
â Adding + or - signs
â 20/25+2
(6/7.5+2
)
Row-by-Row Scoring
87. â Giving credit for every letter read
â More sensitive to changes in VA
â Can use + or - signs
â Bailey-Lovie Design - the qualifiers (+2,+1,-1,-2) carry
same value at all levels
â E.g. 20/25+1
, 20/25-1,+2
Letter-by-Letter Scoring
88. â If VA in LogMAR
â Each letter is assigned a value of 0.02 logMAR
â For each additional letter read - 00.02 is deducted from the
LogMAR score
â If VA recorded in VAR
â Each letter is assigned a value of 1 point
â Each extra letter adds one extra point to the score
Letter-by-Letter Scoring
89. LogMAR VAR Correct LogMAR VAR Correct LogMAR VAR
6/15 0.40 80 5 of 5 0.40 80 5 of 5 0.40 80
6/12 0.30 85 5 of 5 0.30 85 5 of 5 0.30 85
6/9.5 0.20 90 5 of 5 0.20 90 5 of 5 0.20 90
6/7.5 0.10 95 1 of 5 0.18 91 5 of 5 0.10 95
6/6 0.0 100 0 0.18 91 4 of 5 0.02 99
6/4.8 -0.10 105 0 0.18 91 3 of 5 -0.04 102
6/9.5+1
6/6 â1,+3
6/6 +2
90. â Hearing Impaired
â Any Optotypes depending on the ptâs age and ability as
any other normal child
â Speech Impaired
â Ask the pt to write
â Matching cards
â Sign language with the help of the attendent
VA in Other Impairments
91. â Hearing Impaired & Speech Impaired
â Ask he pt to write
â Matching cards
â Sign language
VA in Other Impairments
92. â Intellectually Handicap
â If the pt can identify letters or numbers
â Symbols
â If not possible
â Rely on observation skills
â Lower the dev level & severe the impairement - greater the
dependence on observation
VA in Other Impairments
93. â Begin examination with non-threatening things
(playing with blocks, toys, etc)
â For pt. - Game
â For Optometrist - Observe ptâs visual behavior
â Select procedures appropriate for the dev. age
regardless of the Chronological age
VA in Other Impairments
94. â Forced choice preferantial looking
â Teller Acuity
â Preferential Looking
â OKN
â Rewarding
VA in Other Impairments