2 ms·

Here's a photo of a motion detector with a lens and a light pipe above it: https://d1unzhqf5a606m.cloudfront.net/images/product/honeywell-5898-wireless-dual-tec

• delinka 8y ago
  The “light pipe” is a fat fiber optic channel. Light comes in the end, gets reflected by internal sides and curves, makes its way out the other end and to a detector on the circuit board. The way the end surface interacts with light allows the detection of things touching the surface of the pipe.