Optical flats or test plates are polished surfaces that are used as references against the flatness of unknown surfaces for comparison. They use the property of interference to measure the flatness of a test surface. Placing optical flats on a test surface forms an air wedge where the surfaces do not touch. The change in the thickness of the air wedge shows the direction and shape of the interference bands. When viewed under monochromatic light, light and dark bands will be visible. These bands are interference fringes whose shape gives off a visual representation of the flatness of the test surface.
Optical flats are measured for their accuracy through the fractions of a reference wavelength. Surface accuracy is derived from the amount of curve and spacing between the interference fringes. If the interference fringes are straight and parallel, then the tested surface has a flatness more than or equal to the reference surface. With optical flats, an uneven interference pattern indicates that the flatness of the test surface is less than that of the reference surface.
Optical flats are often used in to test gauge blocks for precision and wear. Other applications for optical flats include the testing of optical components in filters, windows, mirrors, prisms, and more. Optical flats can also be used as flat optical windows in interferometry requirements. Because they may lose some of their precision with repeated use, optical flats can be refinished or resurfaced to return them to their original level of functionality. Although manufacturers of optical flats typically perform this procedure, third-party services are also available.
Materials for optical flats include fused silica or Zerodur® (Schott Slaswerke). Optical flats made from fused silica have a low thermal expansion. Fused silica is durable and maintains good resistance to abrasion. These types of optical flats are ideal for high wear applications. Additional mechanical, thermal, electrical, and chemical specifications are available for both fused silica optical flats and Zerodur optical flats. Zerodur optical flats consist of a clear glass ceramic. Zerodur® has an extremely low level of thermal expansion. Zerodur optical flats are ideal for applications in which temperature fluctuation is a concern.