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Snellen Eye chart: The familiar eye chart used to measure how well one can see.
Snellen's chart is imprinted with block letters that decrease line-by-line in size, corresponding to the distance at which that line of letters is normally visible. The letters on Snellen's chart are called Snellen's Test Font. Each block letter is scientific in design (so that at the appropriate distance the letter subtends a visual angle of 5 degrees and each component part subtends an angle of 1 minute). The chart and the letters are named after a 19th-century Dutch ophthalmologist: Hermann Snellen (1834-1908) who came up with this chart as a test of visual acuity. Visual acuity refers to the clarity or clearness of a person's vision, a measure of how well that person sees. The word "acuity" comes from the Latin word acuitas which means sharpness. The chart is read while standing at a distance of 20 feet. Acuity is represented as a fraction, with the distance at which a person is standing being the numerator (top part of fraction), and the normal maximum legible viewing distance ("Distance" on the chart above) as the denominator (bottom of fraction). So if, at 20 feet, one can read the letters on the row marked 5 or "20/40", this means that they have visual acuity of 20/40 or better: 1/2 normal. The Snellen Eye Chart is available from most medical supply houses for about $8.00 American. To find one, return to the main page or your favorite search engine and key in "Snellen Eye Chart".
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