Dutch ophthalmologist Hermann Snellen (1834-1908) first came up with the idea of a standardized test to measure how well a person can see and also allow comparisons of different people's visual capabilities. The Snellen Chart, developed in 1862, consists of eleven rows of block letters. The first row consists of very large letters; subsequent rows decrease in size. A person 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 patient then reads the letters with the other eye, and then again with both eyes. A traditional Snellen Chart features only the letters C, D, E, F, L, N, 0, P, T, and Z.
The relationship between the size of letters and the distance at which they are seen has become the standard method of recording visual acuity: Snellen determined that a letter that was approximately,..0.3 inches (8.75 mm) high (size 20) could be identified by most people with "normal" eyesight at a distance of 20 feet (6 meters). Therefore, the Snellen fraction of 20/20 became the model for "normal" vision. The top number of the fraction is always 20 because it represents the testing distance, and the bottom number identifies the smallest letter the person taking the test can see at 20 feet. If the smallest letter that a person can identify is 0.6 inches (17.5 mm)—or twice the size of a size 20 letter—then the Snellen fraction for that person would be 20/40. The biggest letter on the chart represents an acuity of 20/200, the measurement that is considered "legally blind."
For children who cannot read, charts were made using pictures of common objects or broken circles with missing segments. The Snellen Chart remains the most popular chart design among eye doctors.
