Vertometer was the trade name of an instrument made by Bausch & Lomb (B&L) and later by Reichert. Lensometer was the trade name of the nearly similar instrument made by American Optical. Neither the Vertometer or the Lensometer is now being made. Lensmeter is a generic name that can be applied to any lens measuring instrument. 'A vertometer' is an ophthalmic bench instrument used to measure dioptric power of a lens. It differs from a lensmeter or lensometer which is a general dispensary tool for opticians to verify spectacle prescriptions at the laboratory. A vertometer, on the other hand, is a more precision instrument used to locate, mark, and index a lens optical center for edging, mounting, and construction of a spectacle prescription.
The theory of geometrical optics uses ray tracing techniques. In many of his models, Galileo used a mathematical formula that measured a vertex distance. This nomenclature prevailed throughout hundreds of years, during a time that studies of optics may have been misunderstood. Derived from the word, the measure of the focal length of a lens is represented by the reciprocal of the vertex distance in meters, or diopters. Vertometers measure sphere ± cylinder x axis, and can be used to calculate prismatic power by indexing the optical center offset from the visual axis. An ink coated stylus is used to mark optic centers on the lens in the optical laboratory. A millimeter rule would be nearby for spectacle eyesize fittings.