Detection of fires and explosions

A radiation-responsive system is described for measuring the distance to a fire. A radiation detector produces an output I1 dependent on the intensity of radiation received in a narrow wavelength band where there is insignificant atmospheric absorption while another detector produces an output I2 dependent on the intensity received in a narrow wavelength band where there is significant atmospheric absorption. The ratio I1/I2 will thus be dependent on the distance to the fire because each of the terms will be equally affected by the normal square law decrease of intensity with distance, but the term I2 will decrease as the distance increases. A ratio unit measures the ratio I1/I2 which is converted into digital form and passed to a processing unit. The latter is programmed according to the precise form of the relationship between I1/I2 and distance to produce an output proportional to distance.