Measuring Snellen Visual Acuity
When Dr. Herman Snellen developed the famous, ground-breaking Snellen visual acuity test in 1862, he launched a defining movement in the vision testing field that would be followed by eye care practitioners to this day. Snellen’s innovations were critical.
For his Snellen eye test, the Dutch ophthalmologist did not use the prevailing type fonts of the mid-1800s. Those fonts usually featured characters with a mix of rather heavy and rather thin lines. Instead, Snellen designed special targets for visual acuity measurement where all lines are equally thick; he called them optotypes. Snellen also defined the size of his optotypes objectively in minutes of arc, as opposed to using the catalogue numbers of printers’ fonts as Jaeger had done for his reading tests. This crucial step made it possible to reproduce the Snellen eye test at any location, so that it could become a worldwide standard for vision testing.
Visual acuity depends not only on the size of the letter seen, but also on the distance at which it is presented. Snellen’s original formula offers a fraction, in which the numerator indicates the viewing distance, and the numerator is the “distance at which the letter subtends 5 min of arc”. The value of this fraction increases (visual acuity is better) when the same letter size is seen at a longer distance or if smaller letters can be seen at the same distance, as expressed in the following formula. The product is decimal acuity with 1.0 representing 20/20 vision.
The diagram demonstrates that different combinations of letter size and test distance can result in the same visual acuity value. Snellen’s original intention was that the numerator should indicate the actual test distance. Thus, 4/4 would indicate standard acuity measured at 4 meters, 20/20 would indicate the same acuity value measured at 20 feet. Under this system, tests in lanes of different length would yield different notations. This is not practical. So, today a system of “Snellen equivalents” is used. In the U.S. all Snellen values are converted to a fraction with 20 as the numerator, regardless of the actual test distance, be it 20 ft, 18 ft, 10 ft or even reading distance. In continental Europe, use of the decimal equivalent is common; in Britain and related countries (such as India, Australia) 6/… is common, even if the test distance is not 6 meters.
Continue to learn more about Snellen Eye Test Charts.
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