1. Visual acuity
Farhana Adnin
B.optom,4th batch
ICO,CU

2. VISION…….?

3. VISUAL ACUITY…?
Resolving power of the eye or
the ability to see two separate
objects as separate.

4. Visual Angle…
 The angle subtended at the nodal point
of the eye by the physical dimensions of
an object in the visual field.

5. Theories of visual acuity…
 Receptor theory:
If the images fell on two cones
separated by an unilluminated cone, then
the points of light would be perceived as
two distinct sources.
Means that two distinct points only can
be recognized as separate when they
subtend an angle of 1’ of arc.

6.  3 types of Cone:
1. Green- Chlorolabe
2. Red- Erythrolabe
3. Blue- Cyanolabe
 Diameter of each cone : 0.002μm

7. Rayleigh Criterion for resolution
Two “point” (unresolved) sources are resolved
from each other when separated by at least
the radius of the airy disk.
The central disk is surrounded by less intense
concentric rings. (airy disk)

9. Types of Visual Acuity
 Minimum Detectable ( visible)
 Minimum Separable (resolution)
 Minimum cognizable (recognition)
 Minimum Discriminable ( hyperacuity)

10. ACUITY TASKS
MINIMUM DETECTABLE:
Ability to determine whether or not an object is
present in a visual field.
The limit for this kind of acuity is ~ 1 arc second.

11. Minimum Separable (resolution)
 Discrimination of 2 spatially separated targets.
 Normal angular threshold of discrimination for
resolution 30-60 seconds of an arc. (minimum
angle of resolution).

12.  MINIMUM COGNIZIABLE: not only discrimination
of the spatial characteristics of the test pattern but
also the pattern with which he has previous
experience
(Identification of faces, letters, symbols, pictures etc.)

13. Minimum Discriminable
 Ability to determine whether or not two
parallel and straight lines are aligned in the
frontal plane.

14. CLINICAL MEASUREMENT OF
VISUAL ACUITY
 Criteria:
 Normal testing distance (distance VA): 6m
 For near : 33cm/ 40cm.
 Illuminated visual acuity chart.
 Proper room illumination.
 Steps:
 Unaided VA
 Aided VA
 Pinhole VA

15. 6m distance

16. Factors affecting visual acuity
Overall
 Illumination of test object.
 Contrast of letters.
 Pupil size
 Cognitive ability
 Ocular health
 Refractive Status

17. Physical factor
 Those which influence the light characteristics
of the distribution & hence influence the
nature of retinal image.

18. Physiological factor
Stimulus related
 Luminance of test object
 Geometrical configuration
 Contrast difference of stimulus from background
 Influence of wavelength
 Exposure duration of stimulus
 Interaction effects of 2 targets

19. Observer related
 Retinal locus of stimulation
 Pupil size
 Accommodation
 Effect of eye movement
 Optical elements of eye
 Development of aspects

20. Distance Acuity chart
 Snellen’s distance acuity chart
 Bailey-Lovie charts
 Landolt ring (or C) chart
 Feinbloom Distance Test Chart
 Chronister Pocket Acuity Chart

21. 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

22. Assessment in Children
 Forced choice preferential looking
 Optokinetic nystagmus test
 Stycar ball test
 Static test
 Catford drum test
 Marble game test
 Tumbling E test
 Sheridan gardiner test
 Pictorial test
 Cardiff acuity test (0-1year)
 Dot acuity test
 Coin test (2-3year)
 Miniature toy test

23. Optotypes
 Landolt Rings (Landolt ‘C’)
 Numbers
 Figures
 Tumbling E
 Letters (snellen)

24. LETTER STYLE VARIATION
 SERIF: ornamental cross stroke at the end of
the limb
 SANS SERIF;
 bold type faces and appear less cluttered
Easier to read than serif letters

25. Distance test charts & Acuity

26. Basics of snellen acuity chart…
Snellen Fraction is the most common notation of acuity.
 The distant acuity is usually tested by snellen chart.
 Consist of a series of black capital letters on a white
board,arranged in lines,each progressively diminishing
in size.
 Each letters fits in a square.
 Assume 5x5 grid for letters with detail separation of
1/5 of letter size.

27. ..

29. Formula
Testing distance
VA=
distance at which letter subtends 5min of
arc

30. …
 The number used to indicate the “size” of the
letter is the distance at which the letter height
subtends 5 min of arc.
 A “20 foot letter” is one that subtends 5 min
of arc at 20 feet.
 A “40 foot letter” is one that subtends 5 min
of arc at 40 feet.
 A “100 foot letter’ is one that subtends 5
min of arc at 100 feet.

31. Snellen chart…6/60, 6/36, 6/24, 6/18, 6/12, 6/9,
6/6
θ
tan θ = h / d

33. Testing & Notation
 One eye at a time, OD then OS
 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

35. TYPES OF NOTATION
 Decimal Notation
 Snellen Fraction notated as a decimal
 20/20 = 1.00
 20/40 = 0.50
 20/100 = 0.20

36. Percentage Acuity
Represent in %
20/20 = 100%
20/200 = 10%
20/2000 = 1%

37. 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

38. Log MAR(LOGarithm of the minimal
angle of resoluation. )
 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

39.  Acuity becomes worse as the log MAR value
increases
…
 Decrease brightness line by line

40. Characteristics of LogMAR
 It is flat and typical “V” shape.
 Testing distance 4m.
 The letter is sanserif and sollan letters. ex:
s,o,c,v,z
 Every line has 5 letters .
 Point one (0.1) logarithmic progression. (5:4)

41.  Between two letter space is the width of
a single letter.
 Between two line space is the height of
the lower line letter.

42. ..
..
 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 The log MAR chart is available
in Landolt C, Tumbling E as well as in numeric and
alphabet optotypes

44. Log MAR Snellen Equivalents
 20/10 = -0.30
 20/15 = -0.13
 20/20 = 0.0020/80 = 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
 20/25 = 0.10
 20/32 = 0.20
 20/40 = 0.30
 20/50 = 0.40
 20/63 = 0.50

45. Exam…
 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

46. Position/LIMITATION OF V/A
 Standard visual acuity depends on two limitation .
1.Anatomical
 two cones stimulate but one cone
separated by one minute of arc angle
 two separate object as separate
2.Optical
 Airy disc form by diffraction
 Diffraction form in smaller in size of pupil
 Aberration

47. References…..
 THEORY AND PRACTICE OF OPTICS &
REFRACTION…A.K.KHURANA
 CLINICAL PROCEDURE OF OPTOMETRY
 INTERNET