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Photonics Dictionary

lensometer

An ophthalmic instrument used to measure the magnitude and direction of the maximum and minimum powers of a spectacle lens, and mark the optical center of the lens by an ink dot. When the lens to be measured is placed in a beam of parallel light between a collimator and projection lens, the X-shaped reticle at the focus of the collimator rotates and moves along the axis until a sharp focus is seen on a ground-glass screen or in an eyepiece. Abbe pointed out that if the rear vertex of the test lens is at the anterior focus of the collimator lens, then the longitudinal movement of the reticle will be strictly proportional to the vertex power of the spectacle, and that the size of the reticle image will be independent of the power of the test lens.
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