A ioptre, or diopter, is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (that is, 1/metres). For example, a 3 dioptre lens brings parallel rays of light to focus at 1/3 metre. The same unit is also sometimes used for other reciprocals of distance, particularly radii of curvature and the vergence of optical beams. The term was proposed by French ophthalmologist Felix Monoyer in 1872.