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Abstract
The autocorrelation function of the spatially integrated speckle intensity fluctuation is investigated theoretically and experimentally by using both the circular hard aperture and the circular gaussian soft aperture for detection. This investigation shows that the use of the circular gaussian soft aperture simplifies an evaluation of the autocorrelation function of the spatially integrated speckle intensity fluctuation and that the resultant autocorrelation functions obtained using the circular gaussian soft aperture sufficiently approximate to the ones obtained using the circular hard aperture which is usually employed in real situations. The autocorrelation function of the speckle intensity fluctuation integrated spatially by an elliptic detecting aperture is also studied theoretically and experimentally as an extension of the circular hard and gaussian soft detecting apertures.