The effect of speckle in the imaging of diffusely illuminated gratings and continuous-tone objects was studied. It was found that when imaging diffuse gratings, the aperture of a coherently illuminated system must be 2.6 times as large as that of an incoherent system to obtain comparable resolution. This factor must be increased to five when imaging continuous-tone objects and to a factor of seven if the coherent image is subsequently low-pass filtered. A coherent system can achieve resolution which is comparable to an incoherent system of equal aperture if the coherent image is smoothed so that the mean to standard deviation ratio is ten or more.
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