lensometer

A lensometer, also known as a focimeter or lensmeter, is an optical instrument used by optometrists and opticians to measure the refractive power of a corrective lens. This measurement, expressed in diopters, indicates how strongly a lens converges or diverges light. The lensometer determines the sphere power, cylinder power, and axis of astigmatism for eyeglasses and contact lenses.

The instrument works by projecting a pattern of illuminated lines or targets through the lens being measured. A movable prism or lens system within the lensometer is then adjusted until the projected pattern appears perfectly sharp and in focus when viewed through the lens. The position at which this sharp focus is achieved corresponds to a specific diopter reading on a calibrated scale. For lenses with astigmatism, the lensometer can also determine the direction of the optical correction required.

By accurately measuring existing spectacle or contact lenses, a lensometer allows eye care professionals to verify the prescription, duplicate lenses, or ensure the proper fit and power of new eyewear. It is an essential tool in fitting contact lenses and adjusting eyeglasses to correct vision effectively.