Abstract: Apparatus for detecting the presence of a fire includes a pair of detectors for radiation characteristic of a fire, e.g. at 4.3 microns, and a further detector for radiation characteristic of a black body, e.g. at 3.8 microns. Filters select the detector output components at flame flicker frequencies. The filtered outputs of the detector pair are cross-correlated to produce a relatively noise-free signal. This signal is divided by a factor obtained by cross-correlating the further detector filtered output with the average of the detector pair filtered outputs. The result of the division is applied to a threshold-responsive circuit. Thus the relatively noise-free signal is effectively subjected to a threshold which is proportional to the degree of similarity between the variations with time of the output signal of the further detector and the output signals of the detector pair.