Abstract: For measuring surface motion or profile, laser beams of two different frequencies .omega..sub.1 and .omega..sub.2 (from one laser or two) are each split into two parts. Frequency shifts of .delta..omega..sub.1 and .delta..omega..sub.2 are introduced into one part of the respective beams. One part of each beam (preferably the parts which are not frequency-shifted) are directed on to, and reflected from, the surface of interest. These reflected parts of the beams, together with the non-reflected parts, are received at either a single or two separate photosensitive detectors so that the parts of similar frequency interfere coherently with one another. From the detector(s) is derived a difference frequency (.delta..omega..sub.1 -.delta..omega..sub.2) whose phase is a measure of the distance to the surface.