A flame detector (1) including: a fire sensor (2a), capable of detecting a characteristic blackbody-type radiated heat signature emitted by a flaming material; and a guard sensor (2b), for detecting an at least further part of the spectrum emitted by the material and which serves to assist in rejecting false alarms, wherein, in use during detection of a flame, the guard sensor (2b) detects an amount of radiation G and the fire sensor (2a) detects an amount of radiation F, and positive detection of a flaming material depends upon the following criteria: F>0; G>0; and F>G.