Snellen Eye Chart
All of us would be familiar with the Snellen eye chart. It is the chart used to test the clarity of your vision when you visit an eye specialist. The chart gets its name from Herman Snellen, a Dutch ophthalmologist, who was instrumental in preparing the chart way back in 1862.
The chart has eleven lines in all and the first line has just one alphabet which is very large in size. In the lines below the number of alphabets increases and at the same time the size of the alphabets decreases. The test is taken in a way where one eye is covered and the person is told to read out the alphabets. The visual acuity is determined depending upon the smallest alphabet that the person is able to read. The other eye is tested in the same fashion. The alphabets shown on the Snellen Eye Chart are known as optotypes. The letters are not printed in an ordinary manner but the length, width and height do have some geometrical implications.
The customary Snellen chart has only the following 10 alphabets C, D, E, F, L, N, O, P, T, Z. The chart that is wall mounted is inexpensive and is generally used for a preliminary examination of the eye. The Visual acuity is determined by a formula which is:
Distance from where the test is conducted/ distance where the smallest optotype recognized subtends at an angle of 5 arcminutes. The chart is placed at a standard distance of 20 feet in most countries for the test. The chart also has certain specifications, it should be made on non-reflective material, it should have a matte finish, should be made of durable plastic and also the standard size is 22”*11”.
The Snellen eye chart also has its variations i.e. for illiterate persons and for young children who do not know how to read. For the illiterate, the chart has the alphabet E written in various directions and the ones for young children have symbols on it. The Snellen eye chart has also been a subject for a lot of criticism. Many are of the opinion that patients might memorize the chart and this would end up in a wrong result. Also after testing one eye they may be able to memorize it because of which the other eye may seem perfect. But even with all the criticism the chart has been in use all these years. Even with so much progress made in the field of medicine, the Snellen Eye Chart is the primary test one has to go through which speaks volumes about it.