Range sensing by shearing interferometry: influence of speckle

We discuss the limits of optical range sensing by shearing interferometry for diffusely reflecting objects. The basic principle is the following: the radius of a wave that is scattered at the object under test is measured by shearing interferometry. This radius is the desired distance. We show that the limits of the method are mainly determined by speckle. With coherent light depth resolution cannot be increased considerably beyond the Rayleigh depth of focus. With partially coherent light a rms depth resolution of 68 microm at a distance of 380 mm (1:5500) was achieved. This resolution is 25 times better than the Rayleigh limit. The working aperture is very small (0.013); shading problems are minimized.