The device for the detection of a wetting of a pane of glass, has an optical rain sensor which detects extraneous light and evaluates it for purposes of day-night detection. The amount of light striking the receiver (14) is integrated in an integration stage (16) and supplied to a comparator stage (20). The total light quantity formed by the light of a transmitter (12) and extraneous light and the extraneous light quantity detected when the transmitter (12) is switched off are measured in separate time intervals (Tg, Tf, Tf′). The time interval (Tf, Tf′) utilized for measurement of the extraneous light quantity is to be extended after a predeterminable number of measurement cycles (M) or after a predeterminable time period and/or the threshold (S2) is to be lowered for day-night detection.