A measure of the power of a lens or a prism. The diopter (also called dioptrie) is usually abbreviated D. Its dimension is a reciprocal length, and its unit is the reciprocal of 1 m (39.4 in.). Thus a thin lens of κ diopters has a focal length of 1000/κ mm or 39.4/κ in. The lens is collecting for positive κ, diverging for negative κ, and afocal for κ = 0.