The law in the United Kingdom requires that a car driver must be able to read, in good daylight with the aid of corrective lenses if necessary, a vehicle number plate containing letters and figures 79.4 mm high at a distance of 20.5 meters. This is a test of binocular static visual acuity and corresponds to a geometric visual angle of 6/15 Snellen acuity. (In the United States this translates into the equivalent of the 20/20 notation, in which the measurement is expressed at a test distance of 20 feet rather than 6 meters as in the Snellen notation. In other parts of Europe people use both the Snellen notation and a system of expressing the visual angle as a decimal fraction—for example 6/6 = 1 6/12 = 0.5 6/60 = 0.1. The rest of the world uses the Snellen notation.) Because of differences in letter types the driving visual test is clinically similar to a Snellen acuity of approximately 6/10. These tests should be performed with both eyes open because the acuity of the better eye when tested separately is often different from the binocular visual acuity. This is the result of interactions in the visual cortex between the input from each eye. (Author/publisher)
Abstract