An optical two path switch (2) is used for providing the alternate measuring beam and the reference beam. The light emitter (1) is pref. a laser diode or a LED. The measuring beam is aligned obliquely upwards and vertical to the movement direction across the vehicle. The reference beam is directly coupled again in a glass fibre wave conductor (6), after leaving the transmitter. The light from a light sensor (7) located scattered across the vehicle, the reception direction of which is arranged vertically to the direction of the measuring beam, is thus aligned obliquely upwards. The light is received and an output signal is produced as a measurement for the visual range at a signal processor (9).
