The term Optical Flat can mean different things to different people. A single-sided optical flat, so called because only one side is ground and polished to specification, is most often used as a reference surface to determine the flatness quality of another flat optical surface, such as the face of a prism, optical mirror, optical window, or another optical flat. When the optical flat is placed in contact with the test surface and illuminated with a monochromatic light source, an interference pattern is visible, the shape and character of which indicates the flatness of the test surface. A single-sided optical flat may also serve as the substrate for a high-quality optical mirror.