Visual angle

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.