We have all made trips to the eye doctor, hopefully more often then not. We sit on the chair get some machines put in our face and asked to read a bunch of letters on a chart. But, did you ever think about the history and background of the famous eye chart we see in every eye doctor’s office?
An eye chart, also known as a Snellen chart was named after its developer, Dutch ophthalmologist Hermann Snellen 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, or N. Subsequent rows have increasing numbers of letters that 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 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.
For over 150 years this low-tech, affordable and easy-to-use chart has been used by eye professionals on patients during an eye exam.
Sometimes “if it isn’t broke, don’t fix it”, so next time you are visiting your Ophthalmologist to see if you need a new pair of reading glasses, mention Snellen chart to them. They will be very impressed!