The eye chart most commonly associated with eye tests – with a large Eat the top followed by progressively smaller rows of letters, is called the Snellen Eye Chart. Named after Dutch ophthalmologist Herman Snellen, who developed the chart in 1862, it is used to this day by optometrists to measure visual acuity.
The letters on the Snellen Eye Chart, as well as the letters and symbols on all versions of eye charts, are called optotypes. By definition, optotypes are standardised symbols used for testing visual acuity. As optotypes, the letters used differ from regular font. More specifically, the thickness of the letters is equivalent to the white space between lines, and the height and width of the optotype is five times the thickness of the line. In addition, only the 10 Sloan letters – C, D, H, K, N, O, R, S, V and Z – are used in the chart, as they are all equally legible and recognisable.
One prominent alternative to the Snellen Eye Chart is the Landolt C, also known as the Landolt broken ring. Itself an optotype, the Landolt C was developed by Swiss ophthalmologist Edmund Landolt. During the visual acuity test, the gap in the “C” shape is placed in various positions – up, down, right, left, or in the 45 degree positions in between. The subject must judge where the gap is, as the lines of symbols get progressively smaller.
The Lea Test is another alternative eye test, particularly useful for children who are not yet literate. Developed by Finish ophthalmologist Lea Hyvärinen in 1976, the test uses optotypes including outlines of apples, houses, squares, and circles. There are various versions to test distance vision, near vision and contrast sensitivity.
Computer-based semi-automatic alternatives have also been developed, though they are not nearly as prevalent as standard eye tests.
Highlighting the significance – and one of the drawbacks – of such eye tests, April 2009 saw dustbin lorry driver William Seago run over and kill colleague and close friend Keith Warman, after cheating on his eye test to obtain a large goods vehicle (LGV) license. Seago is blind in his right eye. This disability would have prevented him from getting a LGV license, but he failed to disclose this fact and memorised the bottom two lines of the optician’s eyesight test to pass the test.
In November, Bristol courts sentenced Seago to a six-month suspended prison sentence, and 150 hours community service. This case illustrates one of the potential problems with tests whose content remains static, enabling vision-impaired people such as Mr. Seago to take advantage of the test. Fortunately, these cases do not arise frequently.