An interferometric procedure, such as electronic speckle pattern interferometry, involves generating two signals representing the point-by- point variations in intensity of respective patterns of electromagnetic radiation resulting from the interference of first and second beams of such radiation derived from a coherent source, with at least the first beam from each pattern being scattered, before interference with its respective second beam, from a common object surface, and with a corresponding pair of the beams, one for each pattern, having a predetermined relative phase difference of other than a multiple of &pgr;; and determining from the two signals values for a datum phase of the radiation at the object surface. Preferably, as a preliminary to this last determination. DC components are removed from the two signals. Conveniently, to simplify the determination, the phase difference is an odd multiple of &pgr;/4 or &pgr;/2.
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