Using infra-red (IR) sensing technology flames can be detected through such elements as dust, steam and smoke; they are also immune to the effects of wind or draughts of air. With the addition of an ultra-violet (UV) sensor a flame detector becomes highly immune to false signals such as sunlight. The optical sensors within the detector receive the IR and/or UV radiation emitted by the flames. The processor within the detector analyses the optical sensor signal waveforms and determines if they represent flames and if so accepts them. If the signals do not match the internal algorithms for flames then they are considered to be false sources and rejected.