Visual Acuity

1. Different forms of visual acuity
Manoj Aryal
IOM, Maharajgunj Medical Campus

2. Visual acuity
Definition:
• Resolving power of eye.
• Considered a measure of form sense , so refers to the
spatial limit of visual discrimination.
• In terms of visual angle: VA is reciprocal of min. resolvable
visual angle measured in minutes of arc for a standard test
pattern.
• Visual angle: is the angle subtended at nadal point of eye
by the physical dimention of object in visual field.

3. Forms of visual acuity
1. minimum visible
– Detection acuity.
– Is the ability to wheather or not an object is present in
an otherwise empty visual field.This kind of task is
referred as minimum visible or minimum detectable.
– Task is to see something as being distinguishable from
the background.
– Reflects the absolute threshold of vision.

4. • Clinically: detection acuity is tested by:
– Catford- drum test.
– Dot visual acuity test.
– Boek candy bead test.

5. 2. minimum separation
– Discrimination of two spatially separated target.
– Min separation b/w two points, which can be
discriminated as two is referred as min. resolvable.
– Is essentially an assessment of function of fovea
centralis and is termed ordinary VA.
– Normal angular threshold of determination for
resolution called MAR. measures appx. about 30-60
sec of arc .

6. Clinically : resolution acuity is tested by
– OKN test
– Preferential looking test
– Visually evoked response.

7. • Recognition acuity
o Faculty by virtue of which an individual not only
discriminates the spatial chrascteristic of the test pattern
but also identifies the pattern with which one had some
experience.
Separated into 2 forms:
– Form sense
• Simple form is used and some tasks such as orientation
is used to determine visual acuity,
• E.g. Landolts C-chart test.
– The minimum legible
• Complex pattern such as numbers and letters are used
as test stimulus.

8. • Clinically recognition acuity is tested by :
– Snellen”s E-chart
– Landolts C-chart tests
– Snellens letter chart tests

9. • Localization acuity (minimum discriminable or
hyper acuity)
– Refers to spatial discrimination by an observer when the
threshold is much lower than visual acuity.best e.g is
vernier acuity.
– Vernier acuity
• Ability to distinguish b/n two lines placed end to end
with one of them laterally placed.
Threshold value -2-10 sec of arc.
– E.g keratometer, applation tonometer.