599. How do motion detectors work? — MK, Port St. Joe, FL
According to Gabriel Lombardi of Torrance, CA, most home motion detectors use infrared light to sense motion. Moving objects change the amount of infrared light striking a detector at the focus of an array of fresnel lenses. He points out that you can see this array on the front of many motion sensors. Such devices are known as passive infrared or PIR detectors. The motion detectors used in automatic door openers, such as those at the supermarket, usually use radio frequency electromagnetic waves to detect motion.