The Photoelectric Smoke Detector consists of a transmitter LED and a receiver photo-diode. These two devices are installed at a specific angle to one another and separator by a screen, so that light from the LED cannot impinge directly on the receiver diode.
Photoelectric (Optical) Smoke Detector
The transmitter LED emits infrared light into the detection chamber. In the event of a fire, visible combustion products enter into the chamber and some of the light emitted by the LED is scattered by the particles so that it impinges on the receiver diode.
This naturally increases the signal level generated by the receiver, which is registered by the detector electronics, which in turn trigger the alarm signal.