Foucault knife-edge test

A technique for testing the accuracy with which the surface of a mirror or lens is figured. It was introduced by J. B. L. Foucault (1819-68) in 1851. The method utilizes the fact that a knife-edge moved slowly across the focal plane will cause the image of a beam of light to darken uniformly only if the imaging surface is perfectly spherical: any depressed or raised areas on the lens or mirror cause light and dark zones to appear in the image as the test is carried out.