Different forms of visual acuity

Manoj Aryal

IOM, Maharajgunj Medical Campus

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.

Forms of visual acuity1. 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.

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

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 .

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

• 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.

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

• 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.