WHAT IS VISUAL ACUITY?

Visual acuity is the ability to see fine detail, for example, reading signs on the road, seeing separate stars in the night sky, and seeing a small white spot on a dark surface.

Visual acuity is expressed as a number representing the smallest object or feature that the person can see.

For example, when the eye doctor tests an adult's visual acuity, lines of letters of different size are shown at a fixed distance, usually 20 feet. If the person can read letters made of strokes of 1 minute arc (60 min arc = one degree) of visual angle, their acuity is 20/20. One minute of arc, or 20/20, is considered "normal" acuity.

If the person can read letters only of 2 min arc stroke width, their acuity is 20/40 (2 times wider stroke). 20/200 acuity means the person can read letters of only 10 min arc strokes (10 times wider than normal). So, the poorer the acuity, the larger the bottom number of the fraction. This is called the "Snellen fraction."

FOUR KINDS OF VISUAL ACUITY

There are four distinct types of visual acuity: Detection, Resolution, Recognition, and Localization.

Detection Acuity means the smallest line width or dot size you can detect.

Resolution Acuity is the smallest separation between dots or between bars in a grating you can resolve.

Recognition Acuity is the smallest letter size you recognize and discriminate.