Means for detecting a wetting of a window

The invention relates to a device for detecting the wetness of a disc which has an optical rain sensor that detects extraneous light and evaluates it with regard to day-night recognition. In one stage of integration (16), the quantity of light falling on a detector (14) is integrated and sent to a comparator stage (20). The total light quantity composed of light from a transmitter (12) and possible extraneous light, and the extraneous light quantity detected when the transmitter (12) is disconnected, are measured in individual time intervals (Tg, Tf, Tf'). After a predetermined number of measuring cycles (M), or after a predetermined length of time, the time interval used to measure the extraneous light quantity can be extended, and/or the threshold (S2) can be lowered.