For the deliberate attenuation of beam components which originate from multiple reflections, an attenuation filter (18; 19) which produces an attenuation of the intensity by at least a factor of 0.7, preferably at least 0.5 or 0.3, is arranged in each case before a receiver (2) and after a transmitter (1). Alternatively or additionally, deflection means, such as a mirror (10) or a cubic prism (14), can be formed so as to attenuate a beam. Since multiply reflected beam components pass through the attenuation means at least twice as frequently as the regular beam components the former are accordingly more greatly attenuated. Furthermore, an entry surface (5) of the receiver (2) may be surrounded by a conical deflection surface which substantially absorbs light incident on it and reflects the remainder in directions other than the direction of incidence, so that this cannot give rise to multiple reflections.