The rain sensor detects precipitation on the windshield using an optoelectronic method of measurement. The sensor element comprises one or more light-emitting diodes (transmitters), a prism and a photodiode (receiver). A light beam produced by the light-emitting diode passes via a prism to the windshield, is reflected several times by the outer windshield surface and forwarded to the photodiode.
The combination of sensor position and dry windshield surface enables maximum reflection of the light beam.
Raindrops on the windshield change the reflection properties to the extent that all light beams no longer reach their target and are deflected by the drops of water.
The greater the intensity of the rain, the less light reaches the photodiode. The evaluation electronics use the irradiance to calculate the amount of rain currently on the windshield and transmit the information needed from the windshield electronics in order to control the wiper speed. The sensor continually takes measurements so that wiping can be individually adapted to the amount of precipitation. If heavy rain or spray from a vehicle driving in front is detected, the system automatically switches from the intermittent setting to the highest wiper setting.
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