Most people are familiar with reading the lines of progressively smaller letters projected onto a screen in the eye doctor’s office. The Snellen chart is by far the most likely to be employed in places using the Roman alphabet. It actually only uses a few of the letters available, not all of them. These are specially designed and standardized, called “optotypes.” The 20/20 (6/6 metric) line is not the smallest -it is common for younger people to be able to read even smaller print (e.g. 20/10).
But there are many “flaws” in the 150 year old Snellen eye chart that diminish its reliability in certain situations. Still, it is widely used in routine screening for vision problems. One problem with the Snellen chart is that a projected image rarely provides a full 100% contrast between the black letters and the white background. Contrast sensitivity is actually an extremely important aspect of measuring vision that too often gets ignored. This can be better measured with back -illuminated eye charts and a choice of optotypes charts with low contrast optotypes (letters).
A second problem is that the change in size of each line on the Snellen eye is not equal. This makes evaluating changes in vision difficult, especially in clinical trials (such as in LASIK) that require statistical analysis. The EDTRS eye chart was developed as an improved version of the Snellen eyechart to remedy these problems. Back illumination on the light box is standardized and does not fade with the life of a single bulb as often occurs in projector systems. Furthermore, each line is one exactly log unit (10x) larger than the one under it. The EDTRS chart otherwise looks almost the same as the traditional Snellen chart but can provide some odd-sounding line sizes, such as 20/63 instead of 20/60 or 20/16 instead of 20/15.