a chart for testing *visual acuity in which the rows of letters (optotypes) vary in size in a logarithmic progression. The space between the letters in a row and the space between each row is identical to the size of the letters in that row. This neutralizes the effect of crowding on vision. There are five letters in each row and each letter has a score of 0.02 log unit (the name derives from logarithm of the minimum angle of resolution). This makes it more accurate than a *Snellen chart and it is becoming increasingly used in testing vision. Most tests are done at a distance of 4 metres, allowing it to be used in smaller rooms. The acuity is scored as a number, with 1 being the same as 6/60 on the Snellen chart and 0 being equivalent to 6/6 (normal visual acuity). Vision better than 6/6 is scored as a negative, e.g. –0.18.... logmar chart
the commonest chart used for testing sharpness of distant vision (see visual acuity). It consists of rows of capital letters, called test types (or optotypes), 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. See also LogMAR chart. [H. Snellen (1834–1908), Dutch ophthalmologist]... snellen chart
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