A double exposure technique for speckle pattern interferometry

When a photographic plate is doubly exposed to a laser speckle pattern and is given a small displacement, in its own plane, between exposures then it will exhibit linear fringes in its optical Fourier transform. This effect may be used to display correlation between two different intensity distributions which are obtained by interference of two coherent speckle patterns before and after a relative phase change; surface displacement may be displayed by interference of the speckle pattern image of a surface with another, coherent, speckle pattern.