Abstract: A light beam (21) is focused proximate to a (partly) reflecting surface (13, 14). The radiation reflected in the surface is split into two sub-beams and imaged on a radiation-sensitive detection system (50). The distance between the radiation spots formed on this system is determined and yields an output signal (S.sub.f) which is a measure of the distance between the focus (22) and the reflecting surface (13, 14). By moving the focus up and down at a fixed amplitude which corresponds to the distance between focus and surface, at which the output signal is maximum, the amplitude of the output signal is also maximum when the up and down movement is symmetrical with respect to the reflecting surface. The presence of a second partly reflecting surface (13, 14) at a short distance from the first surface has a negligible influence on this amplitude.