Snellen fraction

   a ratio (such as 20/20 or 20/100) measuring the acuity (sharpness) of a person's eyesight for objects at a distance. The denominator (number after the slash) is the distance at which the detail of a letter on a standard test chart would subtend an angle of one arcminute, and the numerator (number before the slash) is the testing distance, the distance at which the person correctly identifies the letter. By custom, the ratio is often stated with a standard testing distance such as 20 feet or 6 meters; thus 20/100 (or 6/30) means that the person located 20 feet (or 6 meters, respectively) from the chart can identify a letter subtending an angle of one arcminute at a distance of 100 feet (30 meters). The fraction is also stated as a percentage, for example "20% vision" rather than 20/100. The unit is named for the Dutch ophthalmologist Herman Snellen (1834-1908), who developed the first standard methods for measuring visual acuity.

Dictionary of units of measurement. 2015.

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