A measure of the focal power of a lens equal to 40 times the focal length, or 40 divided by the refractive power indiopters. For example, a 2.00 diopter lens in a pair of reading glasses is also described as 20 power
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}