How do automatic windshield wipers work? — Norman, Tampa, Fla.
Marshall Brain Answers:
As mentioned in the video, the sensor is found behind the rear view mirror. You can see what the sensor looks like in this article:
An infrared beam is reflected off the outside surface of the windshield to the infrared sensor array. When moisture strikes the windshield, the system experiences an interruption to its infrared beam. Advanced analog and digital signal processing determines the intensity of rain or snow. The sensor communicates to the wiper control module to switch on the wiper motor and controls the wipers automatically according to the moisture intensity detected.
In other words, the system shoots a beam of infrared light at the windshield at an angle. The light reflects off the glass-air boundary on the outside surface of the glass and ends up hitting a light sensor. When water gets on the glass, the glass-air boundary changes, and the amount of light that gets reflected also changes. A computer can measure the change and turn on the windshield wipers.