Principle of visual acuity charts class

Visual acuity introduction, principle, notation, near and distance visual acuity charts, log MAR charts, Jeager's chart, snellen's chart

1. Principle of Visual Acuity Charts,
techniques and Notations
Raju Kaiti, M Optom.
Consultant Optometrist
Nepal Eye Hospital (NEH)

2. Objective
•Students should able to
– tell the fundamental basis of Visual Acuity
–Principle of VA charts
–Visual acuity notations
–Able to Calculate the letter size for different testing
distances.

3. Visual Acuity
•Definition:
–Resolving power of the eye
–Ability to see two separate objects as separate
–Minimum angular size of detail that can just be
resolved

4. Visual Acuity
•Vision:
–complex integration of visual perceptions-
•light sense
•form sense
•sense of contrast
•colour sense
•derived from ‘acuitas’ (latin); meaning- sharpness
•defined as the eye's ability to see fine details
•the resolving power of the eye
•considered as a measure of form sense

5. Principles
–Receptor theory
–Resolution theory

6. Receptor theory
•At fovea,
–A cone’s diameter = 1.5 µm
–Space between two cones = 0.5 µm
•Effective separation between the center of two cone is 2
microns
–E.g. if two adjacent objects fall on the 2 adjacent cones then
they would be seen single
–But if there is a single cone unstimulated resolution would
occur
•For resolution: an unstimulated cone in between
•The two illuminated cones would be thus 50 (49”) seconds
of arc apart

7. Types of Visual Acuity
•Minimum detectable(Detection)
–Detect and find a target
•Minimum separable (Resolution)
–locate and resolve a difference between two targets
–Eg: Grating Acuity (Teller Acuity Cards)
•Recognition Acuity (minimum cognizable or minimum legible)
–recognize and identify a series of targets
–Eg: Snellen Optotypes
•Localization
–Vernier Acuity – ability to distinguish when two lines placed end-to-end are
displaced laterally

8. Types of VA Assessment:
•Objective
–10
∆
Fixation Test
–Preferential Looking Test
–Optokinetic Nystagmus (OKN) Test
–Catford Drum
–Galvanic Skin Response
–Visually Evoked Potential (VEP)

9. •Subjective
–Snellen’s Test Types
–Landolt’s Test Types
–Tumbling E Chart
–Allen Picture Chart
–Sheridan Gardiner Test
–HOTV Chart
–Ivory Balls
–Hiding Heidi
–Ffooks’ Test
–Stycar Test
–Light House cards
–Lea Symbols
Types of VA Assessment:…

10. Optotypes
•Landolt Rings (Landolt ‘C’)
–External diameter of ring is 5x5
–The gap is one stroke-width wide
•The critical detail is 1/5 the height of the optotype
•Numbers
•Figures
•Tumbling E
•Letters (snellen)
–Based on a 5x5 grid (sometimes a 5x4)

11. Snellen Acuity
•Visual acuity expresses the angular size of the SMALLEST
target that can just be resolved by the patient
•Snellen Fraction is the most common notation of acuity
•Snellen Fraction is an expression of angular size of an optotype
at the eye
•Assume 5x5 grid for letters with detail separation of 1/5 of letter
size.

12. 5’
5’1’

13. Distance Visual Acuity Testing
•VA - Visual acuity
•OD - ocular dexter
•OS - ocular sinister
•OU - oculus uterque
•20/20 or 6/6
•Herman Snellen 1862
•Layman understanding.....
Distance between the patient and the eye chart
VA= _____________________________________________
Distance at which the letter can be read by a person with normal acuity

15. •Place patient at 20 ft or 6 m from Snellen chart (distance of
6 mts is taken as at this distance it is assumed that the
rays are almost parallel and patient exert minimum
accommodation)
•The chart should be properly illuminated.
•Instruct the patient to cover eye not to press the eyes
•OD then OS
•Pinhole if < 6/12

16. Ask the patient to read with one eye from the top letter while the
contralateral eye is closed gently with the patient palm or with
occluder in the trial frame.
•Now patient is asked to reads the Snellen’s chart and depending
upon the smallest line which the patient can read from distance of
6mts his vision is recorded as 6/6, 6/9 ,6/12,6/18, 6/24, 6/36,
6/60.
•But if patient is not able to see the top line from 6mts he is asked
to come towards Snellen’s charts step by step and vision
recorded at
5,4, 3, 2, 1 meter and noted as 5/60,4/60,3/60,2/60,1/60 resp.
•If < 1/60
–CF (include distance), CF3FT, CF 2FT, CF1FT OR CF
close to face .
–HM+
–PL and PR
–NPL

17. •When patient cannot distinguish hand movements the
examiner notes whether the patient can perceive light
(PL) or not. If he perceive light it is noted as PL +ve
otherwise as PL-ve.
•Also examiner then throw the light from four directions
(nasal, superior, temporal, inferior) and record
accordingly. if present patient perceive light from all
directions it is marked as PR (Projection of rays )
present or else mark as absent or defective. The test is
repeated for the other eye in similar fashion
•English, Hindi and regional language charts
•Landolt ring chart
•E-chart – used in illiterate patients
•Simple picture charts for children.

18. •6/6
•6/9
•6/12
•6/18
•6/24
•6/36
•6/60
Measurement of Snellen Acuity:
5/60
4/60
3/60
2/60
1/60
CF 3F
CF 2F
CF 1F
CF CF
HM+
PL+
PR Accurate
PR Inaccurate
NPL
Þ Þ

20. •After noting vision unaided patient is asked to
read Snellen’s chart while holding a pin hole
(hole size is 1mm) exactly in center of pupil in
front of eye.
•Now patient’s vision is noted and similar pin
hole vision is recorded for other eye also
•If patient vision is improved with pin hole it
means the poor acuity is due to refractive
error.
•If static acuity means may be due to structural
or organic cause.
•If reduced the poor visual acuity may be due to
corneal opacity or lenticular opacity occupying
papillary area or macular pathology.
Pin hole test

21. Snellen’s Fraction
VA =
Testing Distance
Distance at which letter
subtends 5 min of arc

22. Snellen Acuity
h
θ
h
d
tan θ = h / d

23. Snellen Acuity
How big is the 20/20 letter at 20 feet?
Solution: tan 5/60 = h/20
0.0014544 = h/6, h = 8.73mm
Each detail is 1.75mm
N
6 metres
8
.
7
5
1.75
8.7
5
5'
1'

24. Types of Notation
•Decimal
•Percentage
•Minimum Angle of Resolution
•Log MAR
•Visual Acuity Rating (VAR)

25. Decimal Notation
•Snellen Fraction notated as a decimal
•20/20 = 1.00
•20/40 = 0.50
•20/100 = 0.20

26. Percentage Acuity
•Represent in %
•20/20 = 100%
•20/200 = 10%
•20/2000 = 1%
•Confusing to patient
•Used for research purpose

27. Minimum Angle of Resolution
(MAR)
•The MAR is expressed in minutes of arc
•The angular size of the just-resolvable optotype.
•Normally 1/5 of the letter height
•Reciprocal of the Snellen Fraction or decimal acuity
–20/20: MAR = 1.0 min of arc
–20/40: MAR = 2.0 min of arc
–20/200: MAR = 10 min of arc

28. Bailey-Lovie Chart

29. Log MAR Snellen Equivalents
•20/10 = -0.30
•20/15 = -0.13
•20/20 = 0.00
•20/25 = 0.10
•20/32 = 0.20
•20/40 = 0.30
•20/50 = 0.40
•20/63 = 0.50
•20/80 = 0.60
•20/100 = 0.70
•20/125 = 0.80
•20/160 = 0.90
•20/200 = 1.00
•20/317 = 1.20
•20/502 = 1.40

30. Log MAR
•Every letter read counts as 0.02 of each line (every line
has 5 letters)
•Letter to Letter scoring system
• Each line = 0.1 log unit
–25% larger, than the preceding line
–Every third line is double/ half sized = 0.3 Log unit diff.
•Variable testing distance
•The log MAR chart is available in Landolt C,
Tumbling E as well as in numeric and alphabet
optotypes

31. Visual Acuity Rating (VAR)
•A relative scale of vision
–If the patients visual acuity is 6/6 then VAR
is 100
–If he/she reads all except 1 letter in 6/6 line
then VAR is 99
–Mathematically VAR = 100 – 50 x log
MAR

32. Technique

33. Visual Acuity Assessment
in
Children

34. Normal visual response
•Newborn: Light perception
•4-7 weeks: Eye contact
•4-12 weeks: Fixates and follows
•3 months: Facial expressions

35. Normal visual response
•3-4 months: Reaches objects
•6-9 months: Crawls and walks
avoiding objects*
* Pediatric Ophthalmology and Strabismus, Kenneth W. Wright,
Mosby 1995

36. Gross VA Estimation
TechniqueTechniqueBirth Birth 2 mos2 mos4mos4mos6 mos 6 mos 1 yr 1 yr Age for Age for
20/2020/20
OKNOKN 20/40020/40020/40020/40020/20020/20020/10020/10020/6020/6020-30 mos20-30 mos
FPLFPL 20/40020/40020/40020/40020/20020/20020/15020/15020/5020/5018-24 mos18-24 mos
VEPVEP 20/80020/80020/15020/15020/6020/6020/4020/4020/2020/206-12 mos6-12 mos

37. Visual Scene in children

38. Assessment of VA in Children
Challenging Task
–Immature Visual system
–Poor Cognitive ability
–Communication barrier
–Rapid development of visual system

39. How to overcome?
–Several techniques of VA assessment
–Selection of technique (age appropriate)
–Understanding the development of visual
system in children

40. Visual Acuity Assessment
in
Non-verbal Children

41. 10∆ Fixation Test

42. Hundreds and thousands sweet test
•D = 33 cm
•Age: 15-18 mos

43. Cat ford Drum
•D = 60 cm

44. OKN Drum
•D = 33 cm

45. Preferential looking chart
•Resolution VA
•D = 38,55,84 cm
•Age: below 1 yr

46. Lea Grating Paddles

47. Hieding Hidie

48. Visual Acuity Assessment
in
Pre- verbal Children

49. Lea Symbols
• Recognition Acuity
•Age: 2-3 yrs

50. Allen Picture card
•Recognition Acuity
•Age: 2-3 yrs

51. Sheridan-Gardiner Chart
•D = 6 m
•Age: 3 yrs

52. Visual Acuity Assessment
in
Verbal Children

53. Tumbling E

54. HOTV CARD

55. Snellen’s / Bailey Lovie Chart

56. Discussion
•Child friendly environment
•Mother’s lap is the best couch
•Better to avoid VA when the child is in
stress (cry, hunger)
•Don’t take VA when the child is fatigue
•Interest creating surrounding

57. Discussion
•Child may peep through
•may press with palm
•Always use Occluder

58. Discussion
•Gross VA assessment techniques
- CSM
–Hundreds and thousands sweet test
–10Δ fixation preference test
–Catford drum
–OKN Drum

59. Discussion
•Reliable VA assessment techniques:
–Preferential looking Chart
–Sheridan-Gardiner Chat
–HOTV Chart
–Snellen’s Chart/Bailey Lovie Chart

60. Recommendations
•VA assessment – challenging task
•It should be given a great importance
•VA measurement should be repeated in
each visit
• The best , realiable , reproducible & age
appropriate techniques should be choosen.

61. Snellen Acuity Theory
•Fundamental Principle of Visual acuity chart
•Developed by Snellen in 1862
•Visual acuity :
–Reciprocal of Minimum angle of resolution
•Snellen Fraction is an expression of angular size of an
optotype at the eye
•The angular size of the SMALLEST target that can just
be resolved by the patient

62. Types of Optotypes
•Sloan (5 x 5)
•SanSharief (5x 4) with ornamentals

63. Snellen Acuity: Theory
•The relationship between Snellen letter
angles is linear
•Comparing to 20/20 (6/6) letters
–A 20/40 (6/12) letter ; 2x larger
subtends 10 min of arc
17.46mm tall at 20 feet
–A 20/200 = 6/60 letter ; 10x larger
subtends 50 min of arc
87.3mm tall at 20 feet

64. Problems
(A). A 20/200 letter is what size at 20 feet?
(B). If our testing distance is changed to 8 ft, what will
the size of the 20/200 letter need to be?
Solution:
20/20 letter is 8.73mm at 20 feet (6 meters)
Therefore, the 20/200 letter is 87.3mm (8.73 cm) at 20
feet.
Since Snellen fractions are linear:
8.73 cm / 20 ft = x cm / 8 ft x = 3.5 cm

65. Problem
What is the Snellen equivalent at 20 feet if the
acuity provided is 15/45?
Solution:
15 ft is the test distance. 45 ft is the distance at which the
letter subtends 5 min of arc.
15 / 45 = 20 / x x = 60
Therefore the acuity of 15/45 is equivalent to 20/60

66. VA Scoring in Snellen
•Line by line scoring system
•Count a line as “seen” if over 50% of the
Optotypes of line are correctly identified
–Using (-) and (+) signs in VA recording
–T Z V E C L (20/20) – read as “T Z V E C K”
•20/20
-1
–read as “T W W E C M” but all of the 20/25 line
was correctly identified
•20/25
+3

•Some may record as 20/20
-3
but this is not technically
correct

67. Concept behind chart
construction (LOG MAR)

68. Snellen’s Chart
•Line Scoring system - ? 6/9
+ 2
•Unequal letters per line ( 1 to 7)
–Unequal visual task performance
•Calibrated at standard 6m (20ft) only
•
–Other testing distance will introduce error
–??? 5/60 / ??? 3/60
•Unequal spacing between lines and letters
–Without any scientific basis

69. Minimum Angle of Resolution
(MAR)
•The MAR is expressed in minutes of arc
•The angular size of the just-resolvable optotype.
•Normally 1/5 of the letter height
•Reciprocal of the Snellen Fraction or decimal
acuity
–20/20: MAR = 1.0 min of arc
–20/40: MAR = 2.0 min of arc
–20/200: MAR = 10 min of arc

70. Log MAR
•Log of the MAR used to notate the acuity
•Used in research and low vision
•Bailey-Lovie Chart
–Their chart and its derivatives follow the
principles:
•Geometric progression of size & spacing by 0.1
Log unit or 4/5
–Found that the relative spacing between letters
and the number of letters per row can cause
substantial variation in acuity scores

71. Log MAR
•Based on the assumption that perceived
changes in visual acuity are proportional to
the log of the MAR
•Use equally legible symbols with the same
number of symbols in each row (5) and
spacing between rows and symbols
proportional to the size of the symbol
•Acuity becomes worse as the log MAR
value increases

72. Log MAR Snellen Equivalents
•20/10 = -0.30
•20/15 = -0.13
•20/20 = 0.00
•20/25 = 0.10
•20/32 = 0.20
•20/40 = 0.30
•20/50 = 0.40
•20/63 = 0.50
•20/80 = 0.60
•20/100 = 0.70
•20/125 = 0.80
•20/160 = 0.90
•20/200 = 1.00
•20/317 = 1.20
•20/502 = 1.40

73. Log MAR
•Every letter read counts as 0.02 of each line (every line
has 5 letters)
•Letter to Letter scoring system
•Each line = 0.1 log unit &
– 25% larger, than the preceding line
–Every third line is double/ half sized = 0.3 Log unit diff.
• Usual testing distances : 4 or 3 meters (depending on charts)
•Can be done at 2 or 1.5 m,
–VA value = Log MAR score + 0.3
•The log MAR chart Optotypes
– Landolt C, Tumbling E ,Numeric and Alphabet optotypes

74. Log MAR : exmp.
–Letter to letter Acuity assessment
–Every letter that is correctly read deducts 0.02
from that line
–Example:
•If the patient reads all the letters of the logMAR
0.40 line (20/50) and two letters from the 0.30 line
(20/40), it is scored as 0.36
•Snellen Equivalent to this would be 20/50
+2 ?

75. Summary of Log MAR chart
Log MAR Chart Design:
•5 letters per row
•Each letter has same legibility
•Size progression ratio of 0.1 log unit per row
•Each letter read correctly can be assigned a value
of 0.02 LogMAR
•Every third line is double/ half sized = 0.3 Log
unit diff
•Usual testing distance : 4 / 3 m. (depending on charts)
– Can test in all test distances.
–Suggested testing distance 2, 1, 0.5 m or 1.5, 0.75 mI

76. Characteristics of optotypes
•Not all letters are used
•Equal eligibility –
–more or less Equal vertical/ Horizontal and
Oblique strokes
–10 letters = Z,N,H,R,V,K,D,C,O & S
•How can be calculated eligibility?

77. Calculation of Letter size for new
testing distance
h
θ
h
d
θ = 5’of arc
Letter size/ 6 = New Letter size / New Testing distance

78. Calculation of Letter size for new
testing distance
•For Ex, Letter size = 8.75 mm for VA =6 /6
•For 3 Meter Testing Distance
•Letter size
–8.75/ 6 = x / 3
–X = 4.275 mm

79. Snellen Notation
Metric Imperial
MAR logMAR Decimal
6/60 20/200 10 1.0 0.10
6/48 20/160 8.0 0.9 0.13
6/38 20/125 6.3 0.8 0.16
6/30 20/100 5.0 0.7 0.20
6/24 20/80 4.0 0.6 0.25
6/19 20/60 3.2 0.5 0.32
6/15 20/50 2.5 0.4 0.40
6/12 20/40 2.0 0.3 0.50
6/9.5 20/30 1.6 0.2 0.63
6/7.5 20/25 1.25 0.1 0.80
6/6 20/20 1.00 0.0 1.00
6/4.8 20/16 0.80 -0.1 1.25
6/3.8 20/12.5 0.63 -0.2 1.58
6/3.0 20/10 0.50 -0.3 2.00
Letter size
(mm)
3/30 42.75
3/27 ?
3/24 ?
3/21 ?
3/15 ?
3/12 ?
3/9 ?
3/6 ?
¾.5?
3/3 4.275 mm

81. Near VA Test

82. How to Score in a Log MAR Chart

83. Log MAR Chart Scoring ; Example
VA = 0.9
Testing Distance = 4m
VA = 0.92
VA = 0.98

84. Log MAR Chart Scoring ; Example
Testing Distance = 4m
VA = 0.8
Testing Distance =
2m
VA = 0.8+0.3 = 1.1
Testing Distance = 1m
VA = 0.8+0.6 = 1.4

85. Log MAR Chart Scoring ; Example
Testing Distance = 2m
VA = 0.8
Predicted VA at 1m
= 0.8 – 0.3 = 0.5
Predicted VA at 4m
= 0.8 + 0.3 = 1.1

86. RELATIONSHIP BETWEEN VISUAL ACUITY NOTATIONS
SNELLEN DECIMAL MAR* logMAR+ VAR# Cycles per degrees
20/10 2 0.5 -0.3 115 60
20/12 1.67 0.6 -0.2 110 50
20/16 1.25 0.8 -0.1 105 38
20/20 1 1 0 100 30
20/25 0.8 1.25 0.1 95 24
20/30 0.67 1.5 0.2 90 20
20/40 0.5 2 0.3 85 15
20/50 0.4 2.5 0.4 80 12
20/60 0.33 3 0.5 75 10
20/80 0.25 4 0.6 70 7.5
20/100 0.2 5 0.7 65 6
20/200 0.1 10 1.0 50 3
2/400 0.05 20 1.3 45 1.5

87. Near Visual Acuity
•Measured at distances within arm’s length
•Usually testing distance of 40cm considered standard.
•Under standard testing conditions near visual acuity
score is equal to score of distance VA provided eye is
accommodated or optically corrected.
•Do not use letter chart formats comparable with those
for distance visual acuity.
•Several different methods like M units, points, N
notations and Jaegar Notations.

88. NEAR ACUITY CHARTS
•Single letter chart
–Reduced Snellen’s chart:
–lighthouse near visual acuity test
–Designs for Vision Number chart
–Reduced Ferris-Bailey ETDRS Chart
•Word & Continuous Text charts
–Jaeger’s chart
–Roman test types
–Lighthouse game card
–Lighthouse continuous text card for adults
–MNREAD card

89. M Units for Near Visual Acuity
•Most widely recommended system for near VA.
•Are a measure of print size introduced by Sloan &
Habel.
•Used to specify size of print by indicating the
distance in meters at which the height of smaller
letters of the printed materials subtends 5’of arc.

90. M acuity
•(usually tested at 40 cm)
–1M = Letter size that subtends 5 minutes of arc
at one meter (20/20 @ 1meters)
–2M = Letter size that subtends 5 minutes of arc
at two meters (20/20 @ 2meters)
–5M = Letter size that subtends 5 minutes of arc
at five meters (20/20 @ 5meters)
–etc…

91. M acuity
•3M letter is 3x larger than 1M letter
•6M letter is 2x larger than the 3M letter
•6M letter is one-third the size of 18M letter,
etc…
•Rel
n
with N notation (metric)
–N
8
= 1 M
•Rel
n
with Snellen fraction
– For x M letter @ Y meter
•Acuity Value = Y/x = Snellen fraction

92. Conversion into M notation
• If no standard chart available:
Unknown Print size : ? M notation
–smallest Letter read = x mm @ Y meter
–? M notation = x/1.45 = z M
–Acuity = Y/z
–Eg: 8.7 mm letter @ 1 meter
M value = 8.7/1.45 = 6 M
Acuity = 1/6 = 1/6 x 6/6 = 6/36

93. •Print that is 1.0M units subtends 5’ of arc at 1m so it
is 1.45 mm high.
•VA may be easily recorded as a Snellen’s Fraction in
which the testing distance is recorded in meters in
numerator, the denominator indicates the M unit size
of the smallest print that can be read at that distance.
•E.g. A patient who can just read 1.0M print at 40cm
would have visual acuity recorded as 0.40/1.0M.

94. •Jose & Atcherson in 1977 pointed out that the M unit
rating of a sample of print can easily be estimated by
measuring the height of the smallest letters in mm &
multiplying this number by 0.7.

95. Equivalents of M units
1.Equivalent Snellen’s Notation
•Expresses the distance visual acuity that is
mathematically equivalent to near visual acuity.
•e.g. A specimen of print of size 1.0M presented at
40 cm might be labeled as 20/50 equivalent because
Snellen fraction 20/50 equals to 0.40/1.00M
•1M is 2.5 times greater than Snellen 6/6

96. 2Points
•Used to specify size of typeset print, used in
printing industry.
•1 point =1/72 inch
•4/72 inches =1.45mm so 8 point print can be given
an M unit rating of about 1.0M.
•For capital letters, 8 point print = 1.5M
•1.0 M units= 1.45 mm = 8 points (lower case)=
1.5M points (upper case).

97. •One issue remaining unsolved is whether near acuity
test should be based on unrelated words or
meaningful text.
•Argument for unrelated words is that contextual
information promotes guessing and may lead to
overestimate of Near acuity.
•Argument for meaningful text is that purpose of
measuring Near acuity is to gain information about
reading performance. Since context is normally
available to the reader, a reading test that includes
meaningful text will be a better indicator of everyday
performance.

98. Thank youThank you