http://www.knowyoureye.com/category/ppt-zone/

1. www.knowyoureye.com

2. Distance Visual Acuity

3. ∗ Visual Acuity is the spatial resolving capacity of the
visual system
∗ Expresses the angular size of the detail that can be
resolved by the observer
Definition

4. ∗ Visual function
∗ Refractive status of the eye
∗ Medico legal purposes
∗ Major outcome measure for a treatment
∗ Can be used to explain the patient
∗ Criteria for
∗ person’s fitness to drive
∗ ability to gain entrance into a profession
∗ eligibility to avail Government concessions
Uses of VA

5. 1. Optical Limitations
2. Neural Limitations
Physiological Limitations

6. ∗ Focussing Errors
∗ Myopia
∗ Hypermetropia
∗ Astigmatism
∗ Inappropriate correction of refractive error
∗ Chromatic and Monochromatic Aberrations
∗ Pupil size
∗ Small pupil - Diffraction
∗ Large pupil - Aberrations
Optical Limitations

7. Point Spread Function due to
Diffraction
AiryDisc

8. ∗ Angular size SinΦ= 2.44λ/P
∗ H/F= 2.44 λ/P
∗ H=2.44 λXF/P
∗ H= 1.22 λXF/P
∗ H=1.22 F/P
∗ H= 2.3/P

9. Rayleigh’s Criterion

10. ∗ Photoreceptor spacing
∗ Two point images are when the images fall on two cones
separated by an unstimulated cone
∗ In the fovea separation between the cones is 2 microns
∗ 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. ∗ Increase in photoreceptor spacing - Poor VA
∗ In the Periphery the cones are less densely packed
Neural Limitations

13. ∗ Refractive error
∗ Size of the pupil
∗ Illumination
∗ Time of exposure of the target
∗ Area of retina stimulated
∗ State of adaptation of eye
∗ Eye movement
∗ Cognitive status
Factors Affecting the VA

14. Refractive error
∗ Defocus at the retina
∗ Defocus will blur out
∗ Fine details
∗ Sharp edges
∗ Contrast sensitivity
Factors Affecting the VA

15. Effect of Blur on Line Spread
Function

16. Pupil Size
∗ Large pupil-Aberrations (reduces diffraction)
∗ Small pupil-Diffraction (reduces aberrations)
∗ Mid-size pupil-3-5mm optimal
∗ compromise bet. diffraction & aberrations limits
Factors Affecting the VA

17. Illumination
∗ For recognition tasks VA largely affected by
illumination levels
∗ Higher level of VA in photopic conditions -
Higher density of cones at fovea
Factors Affecting the VA

19. Time of exposure
∗ To detect a bright spot, detection is more dependent
on brightness of the spot
∗ To detect a line, the acuity is proportional to exposure
time
∗ No simple acuity-exposure time relationship to
resolve a target
Factors Affecting the VA

20. Area of retina stimulated
∗ VA greatest at the center of fixation
∗ At 10’ of arc from center of fixation - 25% loss of VA
∗ Negligible loss of VA till 2’of arc eccentricity
Factors Affecting the VA

21. Change in Visual Acuity with
Eccentricity

22. State of adaptation of the eye
∗ Higher level of VA achieved when eye is adapted to
same level as test luminance
(34 cd/m2
to 34,000 cd/m2
)
∗ Test luminance < 34cd/m2
, adapting to lower
luminance achieves better VA
Factors Affecting the VA

23. Eye Movement
∗ During steady fixation the eye, the eye is in constant
motion
∗ Retinal images traverse a distance of 3 min of arc per
second
Factors Affecting the VA

24. ∗ Minimum Distinguishable or Visible Resolution
∗ Minimum Separable Resolution
∗ Minimum Recognizable or Legible
Types of Visual Resolution

25. ∗ Minimum Distinguishable or Visible Resolution
∗ The minimum detectable resolution is the threshold size
of a spot or a line against its background
∗ Contrast discrimination
∗ Visual Fields charting(Eg)
Types of Visual Resolution

26. ∗ Minimum Separable Resolution
∗ The least separation between two adjacent points or lines
that allows them to be seen as separate
∗ e.g. Grating Acuity
∗ Vernier Acuity
∗ 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

27. ∗ Cycles /degree
∗ 30 cpd
∗ 1 min of arc
∗ Spurious results
∗ Teller acuity cards
∗ Visual evoked potential
∗ Optokinetic Nystagmus Drum

30. Vernier Acuity

31. ∗ Minimum Recognizable or Legible
∗ Form Sense
∗ Landolt rings where orientation is used to determine the
VA
∗ True Minimum Legible
∗ Complex patterns such as letters or numbers used as test
stimulus
Types of Visual Resolution

33. ∗ Optotypes
∗ On grid pattern
∗ Height 5 units
∗ Width 5,4,6
∗ Serif letters
∗ Sans-serif

34. 1. Detection - presence or absence of a test stimulus
Task Specific Classification

35. 2. Discrimination - separation of points
Task Specific Classification

36. 3. Recognition - orientation of symbol or ability to pick a
particular symbol out of group of symbols
Task Specific Classification

37. 4. Identification - complex symbols
Task Specific Classification

38. Designation of Visual Acuity

39. ∗ Smallest letter
∗ Longest distance
∗ Angular size –
∗ Several types

40. ∗ Snellen Fraction
∗ Decimal Notation
∗ Minimum angle of resolution
∗ Logarithm of the minimum angle of resolution
∗ Visual Acuity Rating
∗ Visual Efficiency

41. 1. Snellen Fraction
Designation of Visual Acuity
VA =
Test distance
Distance at which letter
subtends 5 min of arc

42. ∗ Dutch Ophthalmologist - Herman Snellen
∗ Type of minimum legible VA task
∗ Fraction (eg. 6/30 or 20/100)
∗ Numerator - test distance
∗ Denominator - letter size
∗ Letter size - dist. at which height of overall subtends 1
min.of arc
Snellen Notation

43. ∗ Typical letters on chart - stroke width 1/5th their
height
∗ Def. modified - distance at which the limb of the letter
subtends 1 min of arc
∗ 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

44. E
Snellen Notation
E

45. 5 x 5 grid

46. E 5’
20 ft / 6m
h
Calculation
1 min of arc

47. ∗ Tan 5’ = h/6
∗ Letter size h = tan 5’ 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. ∗ For 6/60, 20/200
Letter size h = tan 5’ x 60
= 0.0145x 60
= 1.458 x 60
= 87.5mm
Calculation

49. ∗ Considered to be optical infinity
∗ Definition of refractive error - when accommodation
relaxed
∗ Shorter distance - role of accommodation
∗ Longer distance - poor legibility
Why 6m or 20ft distance?

50. ∗ Eye resolves letter that subtends 5’ of arc at 20ft or 6m
∗ 20/20 can just resolve a 20/30, 6/9 letter from 30ft/9m
distance - as it subtends 5’ of arc at 30ft/9m
∗ 20/20 Or 6/6 IS NOT PERFECT VISION???
∗ BETTER THAN 20/20 OR 6/6 -POSSIBLE
∗ SUPER ACUITY
What is 20/20 or 6/6 vision?

51. ∗ Does not mean person will not have a functional problem
∗ Contrast sensitivity loss
∗ Color vision loss
∗ Visual Field loss
∗ Does not correlate with REAL WORLD VISION
Is 20/20 or 6/6 perfect vision?

52. ∗ C and G
∗ F and P
∗ N and H
∗ D and O
Commonly confused letters

53. ∗ 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. ∗ A circle with a break in it
∗ Entire diameter of the ring is 5 times the stroke width
of the circle so that the diameter is 3 stroke-widths
∗ Break or gap is 1 stroke width wide
∗ Critical detail is 1/5th of the height of the optotype
Landolt Rings

55. C
Landolt C

57. ∗ Illiterate chart
∗ 3 limbs of equal length
∗ Presented in various orientation
∗ Patient’s task - identify direction to which the limbs of E
point
∗ Four alternative directions - up, down, right and left
∗ School Screening
Tumbling E

58. EE
E
E
Right
Down
Left
Up

60. Decimal notation
∗ Reduces the Snellen fraction to a decimal
quantity
20/20 = 1.0 20/200 = 0.1
∗ Higher number indicates better VA
∗ Most widely used in European continent
∗ It gives a single number to quantify an angle
∗ It does not indicate the test distance
Designation of Visual Acuity

61. Minimum Angle of Resolution (MAR)
∗ Expressed in min of arc
∗ Indicates angular size of critical detail with just-
resolvable optotype
∗ Dividing letter size by test distance
20/20 ⇒ 1 min of arc
20/200 ⇒ 200/20 = 10 min of arc
20/40 ⇒ 40/20 = 2 min of arc
Designation of Visual Acuity

62. LogMAR
∗ logarithm of MAR
∗ VA = 20/20, 6/6, the MAR = 1 min of arc
Log MAR = log10 (1.0) = 0.0
20/40, 6/6 ⇒ MAR = 2 min of arc,
LogMAR = 0.30
20/200,6/6 ⇒ MAR = 10 min of arc,
LogMAR = 1.0
Designation of Visual Acuity

63. ∗ When VA score >20/20, 6/6
∗ LogMAR value becomes negative
20/16, 6/4.8 ⇒ MAR = 0.8 min of arc
LogMAR = - 0.10
∗ 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)
∗ VAR = 100 - 50 logMAR
∗ A score of 100 corresponds to 20/20,6/6
∗ VAR = 50 corresponds to 20/200,6/60
∗ VAR = 0 corresponds to 20/2000,6/6000
∗ VAR > 100 when VA > 20/20, 6/6
20/16,6/4.8 ⇒ VAR = 105
Designation of Visual Acuity

65. ∗ For charts with size progression ratio of 0.1 log units
and 5 letters per row, each letter carries a VAR value
of 1
∗ On the VAR scaling difference of 15 points represents
a 2 fold change in MAR
∗ 5-point change - represents a ratio of 5:4 change in
MAR
Designation of Visual Acuity

66. Visual Efficiency
∗ Introduced in 1925 by Snell & Sterling for use in
quantifying visual loss for legal and compensation
purposes
∗ VE = 1.0 or 100% ⇒ 20/20,6/6
∗ 20/200,6/60 ⇒ 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)
∗ Series of light and dark stripes with a sinusoidal
luminance profile
∗ Widely used for testing contrast sensitivity
∗ Testing Infants (Teller Acuity)
∗ Size of the grating and thus the VA is of the patient
are specified in CPD
Designation of Visual Acuity

68. ∗ 30cpd would have 60 alternating light and dark
stripes per degree of visual angle
∗ The ability to see such a grating
∗ 1 min of arc
∗ 20/20, 6/6
Designation of Visual Acuity

69. Snellen Chart
∗ Seven different size levels
∗ One letter at largest size level
∗ The number at each size level increased
progressively
∗ 8 optotypes at the smallest size level
∗ Size sequence in
∗ feet 200,100,70,50,40,30 and 20
∗ metric 60,30,21,15,12,9 and 6
∗ Modified - 60, 36, 24, 18, 12, 9 and 6
Visual Acuity Chart Design

70. Bailey - Lovie Design Principles
∗ Task difficulty same at each size level
∗ Only significant variable - size
∗ 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

71. ETDRS chart

72. Principles
∗ A logarithmic size progression (constant ratio
from one size to the next)
∗ Same number of letters at each size level
∗ Between letter and between row spacing
proportional to the letter size
∗ Each (or similar) average legibility for
optotypes at each size level
Visual Acuity Chart Design

73. Log size progression
∗ 0.1 log unit
∗ Each successive step represents a change in
size by the ratio 1.2589:1 (approx. 5:4)
∗ For a row of 5 letters each letter can be
assigned a value of 0.02 on logMAR scale
∗ A change of 10 increments on this scale
⇒ a change of exactly 10 times
∗ A change of 3 steps ⇒ a change of approx 2
times
Design Features of VA Charts

74. Number of optotypes at each level
∗ Reliability of VA measure increases with the increased
number of letters at near threshold sizes
∗ 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
∗ Spacing between neighboring letters reduce their
legibility
∗ VA better with wider spacing
∗ Eye movement control and fixation tremors may
contribute to reduction in VA when letters tightly spaced
∗ Influence - more in small print
Design Features of VA Charts

76. ∗ Printed Panels
∗ Opaque or Plastic
∗ Directly Illuminated
∗ Translucent material
∗ Indirect Illumination
∗ Usual testing distance - 6m or 20ft
∗ Closer test distance
∗ Small examination room
∗ Low vision patient
Chart Formats

77. ∗ The dist from the pt. to the chart and size of the letter
- to determine VA
∗ VA chart designed for 4m
∗ 6/60 (5’ of arc at 4m)
∗ Top row at 4m - 4/40 ⇒ 6/60
∗ at 1m - 1/60 ⇒ 6/360
∗ 1/40 ⇒ 6/240
Chart Formats

78. ∗ Projector Charts
∗ Angular size of the Optotype
∗ If the projection dist. = observation dist.
∗ Angular size - independent of obs. dist
∗ Proj. dist. ≠ obs. dist.
∗ Letter size proj. should be changed for that dist.
Chart Formats

79. ∗ Not widely used in clinical practice
∗ For research purposes
∗ Provide means
∗ diff. optotypes
∗ change letter order
∗ change stimulus parameters
∗ contrast
∗ spacing arrangements
∗ presentation time
Video Display Charts

80. ∗ Moderate photopic luminance
∗ Recommended range
∗ 85 - 300 cd/m2
∗ Widely used as std - 160 cd/m2
Chart Luminance

81. ∗ Alphabets
∗ Numbers
∗ Symbols
∗ Preferential looking
Type of Target

82. ∗ Std. dist. - 20 ft / 6 m
∗ Mirrors
∗ Low Visual acuity
∗ 3m, 1m, 40cm
∗ Close dist. - add plus lenses (not practiced)
∗ Legibility of the letters not affected
∗ large angular size
Test Distance

83. ∗ Place occluder in front of one eye
∗ Not hand
∗ Usual practice - right eye first
∗ Begin from larger letter or from the size level little
larger than expected
∗ E.g. 20/20 - 20/40 level
Procedure

84. ∗ Ignore occ. errors if all letters in the next level are read
correctly
∗ Encourage to guess - close to threshold level
∗ Testing stopped - missed more than a total of 5 letters
∗ Pin hole acuity
∗ Other eye
∗ B/o acuity
Procedure

85. ∗ Best Corrected VA
∗ With best ref cx
∗ Provides a benchmark for detecting any future
changes of vis. sys
Procedure

86. ∗ Most widely used
∗ Too coarse to reliably detect small changes in VA
∗ Poor Sensitivity
∗ Partial credit
∗ Adding + or - signs
∗ 20/25+2
(6/7.5+2
)
Row-by-Row Scoring

87. ∗ Giving credit for every letter read
∗ More sensitive to changes in VA
∗ Can use + or - signs
∗ Bailey-Lovie Design - the qualifiers (+2,+1,-1,-2) carry
same value at all levels
∗ E.g. 20/25+1
, 20/25-1,+2
Letter-by-Letter Scoring

88. ∗ If VA in LogMAR
∗ Each letter is assigned a value of 0.02 logMAR
∗ For each additional letter read - 00.02 is deducted from the
LogMAR score
∗ If VA recorded in VAR
∗ Each letter is assigned a value of 1 point
∗ 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 –1,+3
6/6 +2

90. ∗ Hearing Impaired
∗ Any Optotypes depending on the pt’s age and ability as
any other normal child
∗ Speech Impaired
∗ Ask the pt to write
∗ Matching cards
∗ Sign language with the help of the attendent
VA in Other Impairments

91. ∗ Hearing Impaired & Speech Impaired
∗ Ask he pt to write
∗ Matching cards
∗ Sign language
VA in Other Impairments

92. ∗ Intellectually Handicap
∗ If the pt can identify letters or numbers
∗ Symbols
∗ If not possible
∗ Rely on observation skills
∗ Lower the dev level & severe the impairement - greater the
dependence on observation
VA in Other Impairments

93. ∗ Begin examination with non-threatening things
(playing with blocks, toys, etc)
∗ For pt. - Game
∗ For Optometrist - Observe pt’s visual behavior
∗ Select procedures appropriate for the dev. age
regardless of the Chronological age
VA in Other Impairments

94. ∗ Forced choice preferantial looking
∗ Teller Acuity
∗ Preferential Looking
∗ OKN
∗ Rewarding
VA in Other Impairments

95. Reduced Visual Acuity
Refractive Errors Non- Refractive Error
Media
Opacities
Retinal
Pathology
Malingering
Amblyopia
Optic
nerve
disorders

96. Thank You