An optical flat/protection window is a precisely polished flat surface, used as a reference, against which flatness of an unknown surface may be compared.
When an optical flat is placed on a polished flat surface, with a monochromatic light incident on it, dark and light fringe pattern, called interference pattern is visible. The shape of the fringes gives the visual representation of the flatness of the surface under test.
Both, single surface and dual surface flats are available. Thermal stability is of utmost importance, so it is preferable to select material with low coefficient of expansion like Zerodur or Fused Silica. However for commercial, low cost applications Pyrex or BK 7 may also be used. Dimensional stability is maintained by keeping diameter to thickness ratio at least 8. The surface accuracy offered is l/2, l/4 and l/8.
Optical flats are widely used for optical shop testing of flatness of prisms, windows, lenses etc. They may also be used in mechanical workshop to inspect gauge block for wear and accuracy.