How do rain-sensing wipers work?
Rain-sensing wipers are powered by a circuit board, mounted inside the windshield glass near the rearview mirror. The circuit board houses a small infrared lightbulb that is the key to everything.
The concept is simple: the bulb emits a stream of low-energy infrared light that is invisible to the naked eye, similar to the light projected from your TV remote. The light from the bulb reflects back off the inside of the windshield and is detected by an optical sensor on the circuit board.
When the windshield is dry, most of the infrared light emitted by the bulb is reflected by the glass, and picked up by the optical sensor. When the windshield is wet, the droplets of water mean the infrared light is scattered, not reflected. This means less of the infrared light winds up being reflected back into the optical sensor. It is this change that then triggers the wipers.
In a nutshell, rain-sensing wipers constantly measure the amount of infrared light returning to the optical sensor, to determine how wet the windshield is. That information is used to operate the wipers accordingly, with no driver action required.