The COLENBRANDER DIOPTER RULER
The ruler shows inches and centimeters. Most important, however, is the center scale, which shows DIOPTERS. Recording the reading distance in diopters (the reciprocal of the distance in meters) greatly simplifies calculations and comparisons. It replaces the use of a visual acuity fraction
(V = m / M = distance in meters / letter size) with a multiplication (1/V = M.x.D = letter size x distance in diopters).
Measuring the reading distance in diopters provides a simple check on the power of the reading add.
The diamonds on the ruler represent steps that are equivalent to the print sample steps. For each diamond that the print is brought closer, the patient should be able to read one print size smaller, provided that the reading add is adjusted accordingly.
The short end of the ruler provides an M-unit scale to measure the size of unknown print samples.