The method uses light or infrared radiation with rays reflected in a shifting or distorted condition, either from an object with a diffusely reflecting surface, from a transparent object or from the surface of a mirror. The object (10) is irradiated by a coherent beam (11) of known frequency. The reflected rays (12) pass through an imager (13) to a sensor (15). The sensor is composed of many regularly arranged pixels (16). A superimposed reference ray (14) of equal frequency and defined phase produces an speckle interference pattern (17). The intensity values of the pattern are processed to give the calculated phase difference.
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