Apparatus and method for surface contour measurement

Apparatus and method of measuring position information, typically the depth coordinate, of a point on the surface of an object. The apparatus includes two sources of radiation (P 1 ,P 2 ) positioned to illuminate the point (P 0 ) on the surface of the object (10) with radiation from each of the sources. The radiation from each of the sources is coherent with respect to the radiation form the other source. A control system (32) changes the phase of the radiation from at least one of the sources relative to the phase of the radiation from the other source as measured at the point on the surface of the object. A detector (22) is positioned at the point on the surface and a processor (28), in communication with the detector, calculates position information in response to the change in phase of the radiation from the sources and the radiation detected at the point on the surface of the object.