Flame detectors use optical sensors working at specific spectral ranges (usually narrow band) to record the incoming radiation at the selected wavelengths. 30 - 40% of the energy radiated from a fire is electromagnetic radiation that can be read at various spectral ranges (such as UV, VIS, IR). The signals are then analyzed using a predetermined technique (flickering frequency, threshold energy signal comparison, mathematical correlation between several signals, correlation to memorized spectral analysis, etc.).