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 Herman Snellen who developed the chart in 1862
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, N, or A. Subsequent rows have increasing numbers of letters that decrease in size. A patient taking the test covers one eye, and reads aloud the letters of each row, beginning at the top. The smallest row that can be read accurately indicates the patient’s 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 letters C, D, E, F, L, N, O, P, T, Z are used in the traditional Snellen chart.
Visual acuity = Distance at which test is made / distance at which the smallest optotype identified subtends an angle of 5 arcminutes.
Snellen defined “standard vision” as the ability to recognize one of his optotypes when it subtended 5 minutes of arc. Thus the optotype can only be recognized if the person viewing it can discriminate a spatial pattern separated by a visual angle of 1 minute of arc.
The fact that the number of letters increases while the size decreases introduces two variables, rather than just one. Some people may simply (or unconsciously) memorize the Snellen chart before being tested by it, or between tests of one eye and the other, to give the impression that their vision is good.
Several studies indicate that the crowding together of letters makes them inherently more difficult to read. Another issue is that there are fairly large and uneven jumps in acuity level between the rows.