Fire Safety Science Digital Archive

This note described an experimental investigation of the response to fire of a novel fire detection system. This system consists essentially of a focused laser beam, projected beneath the ceiling of a compartment, and a photocell receiver whose area is just large enough to contain the incident spot of laser light under no-fire conditions. When turbulent hot combustion gases from a fire intercept the path of the beam, the laser spot is randomly deflected on and off the photocell. Frequency analyses have been made of the photocell's output under normal ambient conditions and in response to fires of different powers. It was found that fire can best be discriminated from ambient heat sources by selecting that part of the photocell's output which is in the range 40-80 Hz. It is shown that a fire of less than 140 kW, at any position in a building 12 m high, 15 m x 41 m, can be detected in less than 45 s. This performance compares very favourably with the estimated performance of a point heat-sensitive detector of maximum permitted sensitivity.