Interferometric measurements of deviations from flatness: some new techniques

The deviations from flatness of a set of three surfaces can be mapped by averaging the results of intercomparisons of these surfaces, taken two at a time, with both the surfaces rotated in their own plane in a series of steps. This procedure makes it possible to separate the surface errors into their rotationally invariant and rotationally dependent components. The errors of the individual surfaces can then be obtained directly from the differences of these averages. Averaging techniques can also be used for direct measurements of small- scale surface irregularities (ripples), with very small amplitudes, on high-quality surfaces. These techniques can be extended to make measurements on highly reflecting surfaces, as well as to obtain, directly, the spatial frequency spectrum of such surface irregularities