Optical flats are optical-grade glass structures lapped and polished to be extremely flat on one or both sides, usually within a few millionths of an inch. They are used with a monochromatic light to determine the flatness of other optical surfaces by interference. When a flat surface of another optic is placed on the optical flat, interference fringes are seen due to interference in the tiny gap between the two surfaces. The spacing between the fringes decreases where the gap is larger, indicating a departure from flatness in one of the two surfaces.

Optical flats are sometimes given an optical coating and used as precision mirrors for special purposes, such as in a Fabry-Pérot interferometer or laser cavity. Optical flats have uses in spectrophotometry as well.

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