Abstract: A sensor device is proposed for detecting the degree of wetting of a transparent pane by precipitation. The sensor device is provided with a beam guide element, which is affixed to the pane with the aid of an optical adhesive, and is acted upon by heat by a heating apparatus. A beam transmitter and a beam receiver are associated each via a lens. Such a sensor device solves the technical problem of guaranteeing optimum conditions for the transfer of heat to the beam guide element. The beam guide element has, in the central region of its rear surface remote from the pane, a recess lying outside of the beam paths. The recess contains a heating apparatus associated via a coupling medium with the beam guide element.