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snellen fraction


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  • A ratio (such as 20/20) that measures the acuity of a person's eyesight for objects at a distance. The denominator represents the distance at which the detail of a letter on a standard test chart would subtend an angle of one arcminute, and the numerator represents the testing distance (the distance at which the person being tested correctly identifies the letter). By custom, the ratio is often stated with a standard testing distance such as 20 feet or 6 meters, depending on which country you're in. So, for example, 20/100 in the United States (or 6/30 in countries using the metric system) means that the person located 20 feet (or 6 meters) from the chart can identify a letter subtending an angle of one arcminute at a distance of 100 feet (or 30 meters). Named The for the Dutch ophthalmologist Herman Snellen, who developed the first standard methods for measuring visual acuity.

    November 6, 2007