A speckle shearing interferometer for studying the aberration of an optical system

Two images of a plane diffuser, illuminated by a beam of coherent light, are recorded successively on the same photographic plate. The angle of incidence of the light beam on the diffuser is changed between the two exposures. For a partial diffuser, the photographic plate, after development, can be considered to be a hologram, which gives a lateral shearing interferogram of the pupil. For an ordinary diffuser giving no direct beam (as ground glass) the same fringes are observed in the Fourier plane, but with a lower visibility. This interference pattern, too, characterizes the aberrations of the optical system. In the case of a perfect instrument, this experiment may be used to study some properties of the diffuser itself.