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An optical flat by definition is a polished flat surface that can be used as a reference against which the flatness of an unknown surface may be compared by measurement. An optical flat utilizes the property of interference to exhibit the flatness on a desired surface. Optical flats are versatile optical components used in many applications, such as inspection of gauge blocks for wear and accuracy, as well as the testing of various components including windows, prisms, filters, mirrors, etc. 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.