This document discusses various aspects of visual acuity and its measurement. It defines visual angle and visual acuity, and describes the receptor theory and Rayleigh criterion for visual resolution. It outlines the components involved in measuring vision, including visual acuity, field of vision, color vision and more. Methods for testing visual acuity are provided for both distance and near vision for adults and infants. Factors that can influence visual acuity measurements are also summarized.

1. Visual angle
 The angle subtended at the nodel point of eye by
the physical dimension of an object in visual field.
 Measure in minutes of arc.
 Depends on
 Size
 distance

2. Visual acuity
 Reciprocal of minimum resolvable visual angle.
 Measure of form sense.
 Spatial limit of visual discrimination.

3. Theories of visual acuity
Receptor theory-
If the image fall on 2 cones separated by an
unilluminated cone, then the points of light
would be perceived as 2 distinct source.
 two adjacent point can be recognized as separate
when they subtend an angle of 1 minute of arc.

4. Rayleigh criterion for resolution
2 point (unresolved) sources are resolved from each
other when separated by at least the radius of airy disc.
The central disc is surrounded by less instance
concentric rings. (airy disc)

6. Components of measurement of
vision
 Visual acuity
 Field of vision
 Color vision
 BSV

7. Components of visual acuity
Minimum detectable (visible)
Minimum separable(resolution)
Minimum cognizable(recognition)
Minimum discriminable(hyperacuity)

12. Testing visual acuity
 Distance 6 mtr. & Near 33 cm.
 Properly illuminated (not less than 20 foot candle)
 Recorded as a fraction
-Numerator: dist. Of pt. from chart.
-Denominator: smallest latter read accurately.
eg. 6/6, 6/9 , 6/12 ,6/18, 6/24, 6/36, 6/60, 5/60,4/60,
3/60……

14.  Finger counting CF-1 , CF-2 , CF-3
 HM+/-
 Perception of light
 Projection of rays
 Each eye should be check separate for both dist. & near
, with or without glasses.

16. Variables in visual acuity
external internal
 Lighting level is not constant
 Variability in contrast.
 Chart is not kept clean
 Distance .
 Incidental glare.
 Wearing dirty eye glasses.
 Tear film abnormalities.
 Corneal surface
abnormalities.

17. Neurological impairment
 Motility defects viz. nystagmus.
 Visual field defects.
 Optic nerve defects.
 Pupil abnormality.
 impairment of drugs, legal or illicit.

18. Distance acuity chart

20. Near acuity chart

22. VA Assesment in infants
 Blink response
 Pupillary reflex-after 29th week of gestation.
 Fixation reflex-2 week to 2month.
 Follow movements-horizontal tracking
vertical tracking
 central , steady & maintained (CSM) method of rating.

23.  Optokinetic nystagmus test
 Preferential looking test
 Visual evoked response
 Catford drum test
 Cardiff acuity test

25. b/w 1-2
 Boeck candy test
 Worth’s ivory ball test
 Sheridan’s ball test

26. b/w 2-3
 Coin test
 Dot VA test
 Miniature toy test
b/w 3-5
 Illiterate e-cutout test
 Tumbling e-pad test
 Isolated hand figure test
 Sheridan gardiner test
 Pictorial vision chart
 Broken wheel test
 Light home test
 Lippman’s HOTV test

28. Logmar chart
 Academic and research purpose
 This is a modification of snellens chart, where each
subsequent line differs by 0.1 log unit in the minimum
angle of resolution (MAR) required for that line.
 Dist. 4 meter.
 Equal number of latter in each line.

30. Potential acuity test
These test are used to assess the visual acuity of eye in
which it is not possible to see macula bcz of cat.
 pinhole test
 bluefield entopic phenomena-abiliy to see white dots
when blue light diffusely illuminates retina.
 interferometers

31. Hyperacuity
 Human eye is capable of seeing more than the ability
of the retinal cones to resolve.
 It is due to the involvement of higher cortical centers
in the parietal cortex , for Ex.
 Vernier acuity
 Stereo acuity

32. Vernier acuity
 Ability to discern the vernier separation b/w 2 lines
not in perfect alignment.
 Range-10-20 sec of an arc.
steroacuity
 Ability to perceive sepration in 3D

33. Miscellaneous
 The Directional sensitivity of retina is referred to
Stiles- crawford effect.