Snellen chart

Snel·len chart 'snel-ən- n the chart used in the Snellen test with black letters of various sizes against a white background

Snellen Hermann (1834-1908)

Dutch ophthalmologist. Snellen was director of an eye clinic at Utrecht. In 1862 he introduced a test for measuring the acuteness of vision which consisted of printed letters or words in type of various sizes. He also introduced operations for entropion and ectropion and for ptosis, the latter operation being performed by shortening the aponeurosis of the levator palpebrae superioris of the upper eyelid. Snellen is additionally famous for constructing an artificial eye consisting of two concavo-convex plates with intervening empty space.

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the commonest chart used for testing sharpness of distant vision (see visual acuity). It consists of rows of capital letters, called test types, the letters of each row becoming smaller down the chart. The large letter at the top is of such a size that it can be read by a person with normal sight from a distance of 60 metres. A normally sighted person can read successive lines of letters from 36, 24, 18, 12, 9, 6, and 5 metres respectively. There is sometimes a line for 4 metres. The subject sits 6 metres from the chart and one eye is tested at a time. If he can only read down as far as the 12-metre line the visual acuity is expressed as 6/12. Normally sighted people can read the 6-metre line, i.e. normal acuity is 6/6, and many people read the 5-metre line with ease. A smaller chart on the same principle is available for testing near vision. In North America the test is done at a distance of 20 feet: 20/20 vision is the same as 6/6.

H. Snellen (1834-1908), Dutch ophthalmologist

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a standardized chart imprinted with block letters (Snellen test type) in gradually decreasing sizes, identified according to distances at which they are ordinarily visible; used in testing visual acuity. See also Snellen test type, under test type.