In testing for visual acuity at a distance, a range of 20 feet (six meters) is used, since rays of light from this distance are, for all practical purposes, parallel. There is therefore no need for the eye to accommodate, since, in the normal
eye, parallel rays come into focus on the retinal surface.
Many subjective tests using pictures, circles, dots, letters, and numerals have been proposed for the determination of visual acuity. The Snellen system uses a series of letters and numerals constructed so that visual acuity can be expressed by a number. In the Snellen test chart, each letter is inscribed within a square that occupies five minutes of arc at a specific stated distance. Each stroke or space of the letter occupies one minute of arc, the minimum visual angle for the normal eye, for two objects separated by less than one minute of arc will no longer be seen separately.
Snellen Test Types are square- shaped letters arranged on a chart, with the size of the letters progressively smaller from the top down. The top line is a single letter that a person with normal vision should be able to read from 200 feet. There follow rows of letters designed to be read by persons with normal vision at 100, 70, 50, 40, 30, 20, 15 and 10 feet. With normal visual acuity, the average individual can read the “20 foot” line, and his visual acuity is expressed as 20/20. The visual acuity of an individual who can make out only the third line from the top is 20/70. Young people can sometimes read the bottom line on the chart; in this case visual acuity is 20/10.
It is important to bear in mind that the visual acuity figures do not represent merely a fraction of normal vision. That is, a visual acuity of 20/100 does not mean that the individual has only one-fifth of normal vision; rather it signifies that at 20 feet he only can see letters that are normally read at 100 feet.
A special Snellen chart for illiterates is constructed with a series of the letter E, in which the openings point in different directions. Visual acuity is determined by the smallest row where the individual can correctly tell the direction in which the figures are open. A special chart using pictures of common objects is available for children who have not yet learned the alphabet.