Quote:
Originally posted by Pete Hanlin:
Actually, "centrad," "diopter," and "degree" are not synonymous terms... A centrad will also deviate light by one centimeter over the span of one meter- but the meter is measured as an arc with a radius of one meter (meaning that a centrad is a slightly more precise, if less used, measure of prism than the diopter)... One degree is actually equivalent to about 1.75 diopters (and one diopter is equivalent to about 0.57 degrees)...
As an added bit of optical trivia for this answer (or a trivial bit of optics?)... I'll add that a "prism diopter" is not a true unit of measure, while the centrad and degree are. The effective value of a prism diopter changes as the angle of deviation increases, so it is not a uniform unit of measurement... Meaning that 8 prism diopters do not produce the same amount of deviation as 2 x 4 prism diopters. For low prism powers, like we typically deal with in ophthalmics, the units are all almost equivalent.