Contrast of speckle correlation fringes in presence of a longitudinal displacement between the two exposures

The visibility of speckle correlation fringes is studied in the case of a longitudinal displacement of the photographic plate between the two exposures. If the two speckle patterns are recorded at a large distance from the ground glass, the fringe visibility varies in the Fourier plane. It is high in the center and at the edge, and very low in the intermediate zone. The same experiment is repeated with the difference that the photographic plate is now quite near the ground glass. In this case the diameter of the central zone of high visibility fringes is related to the angular width of the Fourier spectrum of the ground glass.