lensometry

Lensometry is the practice of determining the optical properties and power of lenses, especially ophthalmic spectacles, by means of specialized instruments. It is used to verify prescriptions, check quality, and assist in dispensing eyewear, as well as in optical laboratories and manufacturing.

Most common instrument is the lensometer (also called a focimeter). The lens is mounted in a holder, and an illuminated target is viewed through the lens. The device calculates the refractive power in diopters from how the lens alters the focal point of the light, reporting sphere, cylinder, and axis for spectacle lenses. For cylindrical lenses, axis orientation is determined by rotating the lens or by measuring meridional powers. Some models can also indicate prism effects, and may estimate edge thickness or diameter for compatibility with frames.

Measurements rely on the thin-lens approximation and proper alignment; accuracy depends on instrument calibration and lens type (e.g., single-vision, progressive, or high-index lenses). Lensometry does not replace full subjective refraction and is used mainly to verify and reproduce prescribed powers.

In research and industry, focimeters or lensometers are used to characterize lenses, test optical materials, and calibrate production processes. The term can also refer to measurement techniques in broader optics contexts, not only ophthalmic lenses.