Speckle Contrast in the Diffraction Field of a Weak Random-phase Screen When the Illuminated Region Contains a Few Correlation Areas

An analysis of the speckle contrast in the diffraction field of a weak random phase screen when the illuminated region contains a few correlation areas is carried out. Curved illuminating wavefronts, and the Fresnel and off-axis terms are considered. The limitations of the method are pointed out. Comparison with a gaussian approximation is made. Some previous studies are examined to obtain a more comprehensive view of the problem. Some of the reasons proposed for observed departures of the theory from experimental results are discussed.