Optical window interferograms: a simple method for their evaluation

The selection of optical windows for photographic aerial reconnaissance is considered, taking into account an interferogram whose fringe pattern represents phase variations across the aperture or pupil of an optical component or system. It is pointed out that fringe departure from parallelism and equispacing implies image degradation. An approach for obtaining a simple measure of this degradation is discussed. In the considered case, a window was judged acceptable if the average magnitude of its wavefront deviation was equal or less than a certain critical value. The establishment of this criterion makes it possible to base future tests on similar components only on interferometric considerations.