Abstract: The invention is especially suitable for profile measurements on non-specular surfaces. The surface of interest is intersected by projected interference fringes generated by two intersecting coherent light beams. The intersected fringes are imaged at a photo-electric detector having a sensitive cross-sectional area relative to the fringe pitch, suitably half a fringe pitch, such as to produce an output variation if the fringes move relative to the detector. The phase of at least one of the beams is modulated to cause the fringes to shift laterally and cyclically across the surface, thereby causing the detector output to include a sinusoidal component whose phase depends on the distance to the imaged surface. The change in phase of this component, if and as the distance to the surface changes, is used to indicate the change in the distance.To measure distance changes greater than the depth of focus of the detector, it is aranged that, after such a change, the detector moves axially to a refocussed position.