The core of an optical (photoelectric) smoke detector is the optical chamber, which contains an infrared light-emitting diode and a light sensor (photodiode). At regular intervals, a beam of light is emitted by the light-emitting diode, which does not hit the light sensor due to the arrangement of both components in relation to each other. Only when smoke particles have penetrated the chamber to scatter the light does light hit the sensor and an alarm is triggered.
Source: bauwion
