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Snellen fraction

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An index of visual acuity based on the familiar Snellen chart used by optometrists and opticians, on which are printed upper-case letters arranged in size from the largest at the top to the smallest at the bottom. The person being tested normally stands 20 feet or 6 metres from the chart and reads as many letters as possible starting at the top, and a score is assigned in the form of the ratio or fraction d/dn, where d is the viewer's distance from the chart and dn is the distance at which a viewer with normal visual acuity could read the smallest letters that the person being tested can read. On this scale 20/20 vision is normal by definition, and 20/200 vision is a criterion of blindness originally introduced by the American Foundation for the Blind, indicating that the smallest letters read by the person being tested from 20 feet could be read from 200 feet by a viewer with normal (20/20) visual acuity, which means that the letters are approximately ten times as tall as the letters that a viewer with normal visual acuity could read. Normal visual acuity is usually defined as a minimum separable of one minute of arc or 1/60 of a degree of visual angle. [Named after the Dutch ophthalmologist Hermann Snellen (1834–1908) who devised it]

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