Laser speckle reduction: equivalence of the moving aperture method and incoherent spatial filtering

The moving aperture and the moving random mask are two methods suggested for speckle reduction in recording images of diffuse objects with coherent light. An analysis is given that shows that these methods are equivalent to a two-step recording process: (1) recording the image with the full pupil in coherent light and (2) reimaging and recording this speckled image in incoherent light with the aperture or random mask in a fixed position. The two-step process makes it easy to understand the filtering effect and can be physically implemented to offer short exposure time in the coherent step and a wider choice of filtering functions in the speckle reduction step. Experimental results to support the analysis are presented.