Abstract
A point diffraction interferometer (PDI) consists of a partially transmitting film having a pinhole smaller than the central ring of the Airy disk under test. The wavefront under test is focused onto the film in the vicinity of the pinhole, and the interference between the spherical wavefront produced by the pinhole and the test wavefront is observed. The fringe contrast depends on the test beam f/No., the size of the pinhole, the transmittance of the thin film, and the location of the pinhole in the Airy ring pattern. Consequently, high-contrast fringes are only observed in particular configurations. A thin film of aluminum deposited at oblique incidence displays a polarization effect due to its columnar microstructure. Using such a film as the partially transmitting layer in a PDI, it is possible to adjust the ratio of the test and reference beam intensities using an analyzer located after the interferometer. This allows compensation for the reduction in fringe contrast resulting from a change in f/No., etc. Problems associated with the fabrication and durability of this interferometer are discussed.
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