Vision Screening with a Snellen Chart

Before modern vision testing equipment was invented it was difficult to determine people's vision prescription needs. When glasses were first invented, a vendor would have several pairs in a both and people would come by and try to find some that worked. It was hit or miss, and people rarely got exactly what they needed. In 1862, Herman Snellen created a chart to test visual acuity.

A Snellen chart has one large bold letter at the top. The letters become smaller and more frequent as the rows progress, and the smallest row the patient can read determines the patient's vision. The typical Snellen chart only contains ten letters (C, D, E, F, L, N, O, P, T, Z). However, more advanced forms of the Snellen charts, used in ophthalmologists' offices, use several more letters, and contain a more in depth test. In other countries or when screening children often a "tumbling E" chart is used. The advantages of this are that the person does not have to be able to read the letters, but simply indicate the direction the letter is pointing. The disadvantage of the tumbling E is that there are only four possible answers instead of 10 or 26, so it's easier to guess.

When vision screening with a Snellen chart a person stands 20 feet from the chart. The patient's vision is then determined by the smallest line he can read accurately.

There are some drawbacks to vision screening with a Snellen chart. One problem is that it only tests for myopia or nearsightedness. Farsighted people can often read the smallest line clearly. Also, a person can have difficulty seeing at distance for multiple reasons, and the Snellen chart is not terribly exact. Simple myopia is a lengthening of the eyeball, but astigmatism, a twisting or warping of the cornea or lens can cause similar problems. A snellen chart cannot determine why a person has difficulty seeing distant objects. A final problem with the Snellen chart is that it assumes perfect vision to be 20/20, while average vision for people with perfect eyes is closer to 20/15 or 20/10.

Although there are several criticisms of the Snellen chart, it is a good way to determine ballpark figures when doing preliminary vision screening.