Abstract: A photo-electric transducer, comprising a semiconductor wafer with, near its free surface, a region of the opposite conductivity type which is transparent for radiation, adapted for generating photo-currents when being irradiated, said currents flowing through said region towards electrodes connected to means for measuring the ratio between the currents flowing towards the respective electrodes so as to derive therefrom the distance between the point of incidence of the radiation and said electrodes, said region being defined by substantially concentric circles in conformity with the trajectory of a radiation spot; in particular switching means are provided which either disable an electrode when being crossed by said spot, or, in the case of multiple spots incident on different portions of said region, disable a spot when crossing an electrode.