The rain sensor is based on an optical functional principle. A light diode emits light so that nearly the entire amount of light is reflected on a light sensor if the windshield is dry. If the windshield is wet or covered with snow, the reflection behaviour changes; the more water or snow there is on the surface, the less light is reflected. With the new rain sensor, infrared light is brought into action instead of the visible light usually used until now. Because of this, the sensor can be fastened in the blackened area of the windshield and is not visible from the outside.

The sensor controls the speed of the windshield wipers depending on the measured amount of snow or rain. Together with electronically regulated wiper drives, the regulation of the wiper speed can take place continuously in intervallic operation. In a snowstorm or torrents of water – as when overtaking a truck – the system immediately switches to the highest speed level.

The rain sensor offers still more possibilities today, however. For example, it can be used for closing windows and sliding roof automatically. Equipped with an additional light sensor, it can even control the driving light: it is turned on when it becomes dark outside or when entering a tunnel without the driver having to do anything.

It is even conceivable that one could use the signals of the rain sensor for modern traffic telematic systems in the future, in order to register the present weather situation on a particular route.