Have you ever wondered why you read the chart with all those letters in the optometrist’s office?

Herman Snellen was the Dutch ophthalmologist who devised an eye chart in 1862 to test a person’s visual acuity. Acuity is the sharpness, clearness and distinctness of a person’s vision. This chart is still used today, in several different versions. It consists of rows of letters which decrease in size but increase in number with each descending row. 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. 

To briefly describe the traditional chart and its use: 

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.

A person standing 20 feet from the chart who can clearly read the sixth row down is said to have 20/20 vision.
If a person standing 20 feet from the chart cannot read this line, but can read the line 3 rows higher, he is said to have 20/40 vision. These letters are twice as wide and twice as high as those on the 20/20 row. A person with 20/20 vision could read these letters standing 40 feet away.
If a person standing 20 feet away cannot read even the top row with a single very large letter, even wearing the best possible glasses, she is defined as being legally blind. This person’s vision is 20/200.