How to Configure an Eye Chart
Configuring an eye chart is not as easy as one might think. The chart is officiallly known as a Snellen Chart, because it was invented by ophthalmologist Hermann Snellen in 1862. The Snellen chart contains eleven lines of block letters, traditionally with an E at the top, with letters that decrease in size with each line. The ability to read the decreasing letter sizes indicates how strong a patient's vision is. The top E is made to be read with an acuity, or sharpness of vision, of 20/200 at a distance of 20 feet. This is the smallest-sized letter that can be read by a person who is not legally blind.
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Instructions
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For an average Snellen Chart that will be viewed at a 20-foot distance, use a piece of paper measuring 22 inches long and 11 inches wide. Ample space should be left on the right side to clearly number the rows and the acuity of vision. For example, line one would be labeled 20/200, line eight would be labeled 20/20, and so on.
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Letters on a Snellen Chart are not random. They are called "optotypes," meaning they are standardized to reliably measure vision.
One may only use the "Sloan letters," which are C, D, E, F, L, N, O, P, T and Z. Sloan letters were determined by vision researcher Louise Sloan in 1959 and are standardized to measure visual acuity on the Snellen Chart. These letters are considered the most consistently recognizable at a distance.
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The letters should be written in block format, and they must be presented in a form employing similar geometry. According to specifications, the thickness of the lines should be equivalent to the spaces between lines and also the thickness of the gap in the letter C. Height and width must equal five times the thickness of the line. Follow these steps, or the chart will not be in conformation to an actual Snellen Chart.
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The number eight line is the 20/20 line, which means it is the smallest line a person with normal vision can read at a distance of 20 feet. The number one line is the 20/200 line, which represents the largest letter one can read without being considered legally blind. If a patient cannot read this line without glasses, he or she is legally blind.
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Tips & Warnings
Although you can make your own Snellen Chart, it is advisable to either download one or purchase one from a medical supply store. These charts have many caveats and rules as to their measurements and utilization. Any variance may lead to erroneous measurement of visual acuity.
