Superposition fringes as a measuring tool in optical testing.

Superposition fringes are suitable for making highly accurate measurements of the thicknesses of air layers. A combination of a multiple beam interferometer (Fabry-Perot) and a two-beam interferometer is discussed. In this case the contrast degradation is small. Two possible applications are discussed to some extent: the first deals with the adjustment of air gaps between lenses or other optical elements, and the second is a special spherical Fizeau interferometer. This interferometer makes possible surface testing of spheres with the help of interference colors or rings as is usual with the proof glass method, with radius differences of several centimeters. The latter application seems especially promising.