A method for mapping and determining the surface defects function of pairs of coated optical flats

A new photographic technique for mapping pairs of coated optical flats, applicable to surfaces matched to lambda/10 or better, is described. By slowly changing the separation between the flats during exposure, interferograms are produced in which surface error is represented almost linearly by photographic transmission, and the surface defects distribution is determined from a large number of samples of the transmission of an interferogram. An illustrative example is discussed in which the surface defects distribution of a pair of Fabry-Perot plates is found to be asymmetric.