The use of weak phase-retarding diffusers in coherent imaging systems is analyzed. It is shown that ringing caused by diffraction from dust or blemishes can be significantly reduced by a diffuser that scatters light slightly beyond the first interference minimum of the diffraction pattern, provided that the specular transmission of the diffuser is small. Weak phase diffusers, whose phase standard deviations are a small part of a wavelength, accomplish this is an optimal manner. The characteristics of the speckle patterns that consititute their spatially filtered images are found to depend not only on the size of the aperture of the imaging system, but on the Wiener spectrum of the phase and the statistical law that the phase obeys. It is shown that random diffusers with normally distributed phase can easily be constructed that produce image speckle patterns whose Wiener spectra consist primarily of high spatial frequencies, if the size of the aperture of the imaging system is greater than a specified minimum.Speckle patterns are also found to exist near the image of a phase diffuser even when the aperture of the imaging system is large, and their Wiener spectra are functions of their distances from the image of the diffuser.
M. J. Lahart and A. S. Marathay, "Image speckle patterns of weak diffusers," J. Opt. Soc. Am. 65, 769-778 (1975)