We discuss the uncertainty limit in distance sensing by laser triangulation. The uncertainty in distance measurement of laser triangulation sensors and other coherent sensors is limited by speckle noise. Speckle arises because of the coherent illumination in combination with rough surfaces. A minimum limit on the distance uncertainty is derived through speckle statistics. This uncertainty is a function of wavelength, observation aperture, and speckle contrast in the spot image. Surprisingly, it is the same distance uncertainty that we obtained from a single-photon experiment and from Heisenberg's uncertainty principle. Experiments confirm the theory. An uncertainty principle connecting lateral resolution and distance uncertainty is introduced. Design criteria for a sensor with minimum distanc uncertainty are determined: small temporal coherence, small spatial coherence, a large observation aperture.
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.
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