How does an infrared sensor faucet work?

How does an infrared sensor faucet work? — DD, Sacramento, CA

The sensor has two lenses: one that emits a beam of infrared light and the other that looks for a reflection of that light. As long as there is nothing beneath the faucet, there is very little infrared light reflected back toward the sensor and the sensor prevents any water from flowing out of the faucet. But when you hold your hands under the faucet, the infrared light reflects from your hands and some of it returns to the sensor. The sensor detects this light and opens an electronic valve to permit water to flow out of the faucet. The lenses are aimed so that only objects under the faucet itself will reflect the infrared light back toward the lens. A more distance object may reflect some of the infrared light, but the light won’t pass through the sensor at the proper angle and won’t be detected.

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