The detector features two parts: a transmitter-receiver and reflector. The transmitter-receiver directs a continuous infra-red beam towards the reflector which utilising a prism then sends back the beam to the transmitter-receiver which in turn registers the strength of the beam.

A fire or fault is signalled when the continuous beam is interfered between the two parts by smoke; reducing the strength of the beam. The transmitter-receiver is preset to indicate a fire or fault when the beam’s strength is below a certain value.