The SCD 573 (Smoke Criteria Detector) provides early detection of smouldering, glowing and open fires with smoke development. Light transmitters and receivers are arranged at right angles in the sampling chamber to prevent the light beam of the transmitter from striking the receiver directly. First, the smoke-scattered light (Tyndall effect) reaches the receiver and then is converted to an electronic signal. If it reaches the alarm value, an alarm is sent automatically to the control unit. An advance warning stage constantly monitors the detector for contamination and communicates its condition to the control unit. This self-monitoring feature guarantees that the detector functions optimally.