Abstract: A surface to be tested is illuminated with two beams polarized perpendicularly to each other, extending symmetrically to an optical axis and focussed on the surface by a lens. The light scattered at the surface is separated from the directly reflected light by a reflecting diaphragm, and by means of electro-optical compensation (with an electro-optical modulator and photodetector) the scattered light is tested for phase difference which is a function of the distance of a scattering element of the surface from the optical axis. The phase difference within the illuminated spot is a unique function of the distance of the scattering element from the optical axis, if a diffraction-limited optical system is used.