The rain sensor activates the windshield wipers automatically when rain or other moisture appears on the windshield, and adapts the wiper interval to match the amount of rain. How it works: light-emitting diodes inside the rain sensor emit an invisible infrared light which is reflected by the outside surface of the windscreen and measured by photodiodes. If the full amount of light returns to the photodiode, the windshield is dry. A film of water or drops on the windscreen break up the light emitted. As the measuring process is permanent, the electronics can control the wiper intervals perfectly.