This product named photoelectric smoke detector detects the smoke by a couple of infrared diodes. The principle of detecting is granule in the smoke can reflect infrared light. Infrared diodes are placed in the special chamber. The chamber can shield external light, but doesn't affect the smoke into it. While there is no smoke, the diode can receive very weak infrared light. When the smoke entering the chamber, diode can receive more and more light, and the detector can give out alarm signal when the smoke attains the certain density.