All optical rain sensors used in cars and trucks use the same underlying principle. They bounce beams of light through the windshield, and look for disturbances in the beams caused by raindrops. Typically, a rain sensor will have an emitter that emits pulses of light, coupled into the windshield with a lens.
A simplified diagram of the Rain Tracker.
The beams travel through the windshield at about 45 degrees. The beams are infrared and, and thus visible only to dogs. (We're kidding. Dogs can't see infrared, either.) The beams are totally internally reflected by the outside surface of the windshield, so they bounce back into the sensor. A detector picks up the beams and measures them.
If rain drops are present on the outside surface of the windshield, some of the beams escape. This reduces the intensity of the beams. The detector senses this, and circuitry measures the reduction in intensity and figures out that it must be raining. The rest of the control system then runs the wipers.
It's Not Really That Simple
That's the idea in theory. To make a real rain sensor that works really well is complicated. The first problem is ambient light from the sun. The sun is hitting the windshield at about a thousand watts of intensity per square meter. The emitters are putting out a few thousandths of a watt. How does the detector know the difference between the light from the emitter and the light from the sun?
To start with, the Rain Tracker picks out only the color of light that is used by the emitters. That is, it uses optical filtering. That's a start (the easy part, actually), but the energy from the sun is broadband, and will have plenty of energy at whatever detector color you choose, including infrared. And to make matters worse, some very smart people at the windshield companies are working diligently to make their products pass as little infrared energy as possible. So, Rain Tracker uses clever circuit techniques that are able to tell the difference between the pulses from the emitter and sharp shadows cast by telephone poles.
Clever optics and clever circuits are still not enough. Sensing sub-millimeter drops in the harsh automotive environment is a truly formidable engineering problem. The Rain Tracker also uses Digital Signal Processing to be able to sense amazingly tiny drops (a fifth of a millimeter in diameter!) even in the presence of strong ambient light. Truly amazing is that the Rain Tracker can sense these drops even when deployed on high performance solar-absorbing glass-- when roughly 1% of the infrared energy makes its way through the round-trip of the windshield.
But it is not enough to just accurately sense rain. Where the system to simply "run the wipers when hit", the control would be subjectively erratic. If the control were to average out the hits over a long period of time, it would appear subjectively sluggish. The Rain Tracker control module uses a patented technique that is designed to mimic human perception, and actuate the wipers just as a driver would. The result is both smooth and responsive.
The Rain Tracker packs an amazing amount of technology into a tiny package. The sensor snaps into a coupler that bonds permanently to the windshield. Field service is greatly simplified: if the windshield should break, the sensor can be put on a new windshield with a new coupler. The simple, one-piece, single material design discourages the formation of air-bubbles.
The Rain Tracker sensor communicates with the vehicle wiper system using a simple three-wire interface, consisting of ground, digital signal, and power.
