Optical Flats are most commonly used as test plates to evaluate the accuracy of unknown optical surfaces. Esco manufactures optical flats out of G1 grade fused quartz; the low thermal expansion, abrasion resistance and general stability of this material allows continuous use with minimal degradation over time.

Esco offers optical flats with either one or two precision surfaces. Customers can test flat optical surfaces by holding the precision of an optical flat against the test surface. When placed under a monochromatic light, a fringe pattern can be seen through the back surface of the flat. This pattern describes the contour of the test surface, and is not affected by the accuracy of the rear surface.