Snellen chart

A Snellen chart is an eye chart used by eye care professionals and others to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Hermann Snellen who developed the chart in 1862. Vision scientists now use a variation of this chart, designed by Ian Bailey and Jan Lovie (see "visual acuity").

         The traditional Snellen chart is printed with eleven lines of block letters. The first line consists of one very large letter, which may be one of several letters, for example E, H, or N. Subsequent rows have increasing numbers of letters that decrease in size. A person taking the test covers one eye from 20 ft. away, and reads aloud the letters of each row, beginning at the top. The smallest row that can be read accurately indicates the visual acuity in that eye.

        The symbols on an acuity chart are formally known as "optotypes". In the case of the traditional Snellen chart, the optotypes have the appearance of block letters, and are intended to be seen and read as letters. They are not, however, letters from any ordinary typographer's font. They have a particular, simple geometry in which:

• the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter "C"
• the height and width of the optotype (letter) is five times the thickness of the line.

Only the ten Sloan letters C, D, E, F, L, N, O, P, T, Z are used in the traditional Snellen chart. The perception of five out of six letters (or similar ratio) is judged to be the Snellen fraction.

http://en.wikipedia.org/wiki/Snellen_chart