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An effect of the extended detecting aperture on the contrast of speckle patterns produced with monochromatic and white light is investigated at the focused and defocused image planes. An expression for the degree of contrast reduction of integrated speckle patterns due to the extended detecting aperture is derived by using the method of Goodman (1975). The dependence of the speckle contrast of the size of the detecting aperture is experimentally investigated together with an autocorrelation function of the speckle intensity fluctuation.
Nakagawa et al. (Sat,) studied this question.