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.).

Flame detectors are available in a number of sensor types.  The most common sensor types include UV detectors, IR detectors, UV/IR detectors, IR/IR detectors, and IR^3 (triple IR) detectors, and triple IR spectral band detectors.

UV only flame detectors (ultraviolet spectral band detection) work with wavelengths shorter than 300 nm (solar blind spectral band). They detect flames at high speed (3-4 milliseconds) due to the UV high-energy radiation emitted by fires and explosions at the instant of their ignition. These devices are quite accurate, although they are subject to interference (false alarms) from random UV sources such as lightning, arc welding, radiation, and solar radiation.

IR only detectors work within the infrared spectral band. The mass of hot gases emits a specific spectral pattern in the infrared spectral region. They are subject to interference (false alarms) by any other “hot” surface in the area.

UV and IR spectral band detectors compare the threshold signal in two spectral ranges and their ratio to each other to confirm the reliability of the fire signal. This style minimizes false alarms.  Dual IR (IR/IR) band spectral band flame detectors function similarly.

IR^3 triple IR spectral band detectors compare three specific wavelength bands within the IR spectral region. Mathematical techniques are used to correlate the three bands to discriminate between fire and false alarm.

Flame detectors are available with a number of features to help them better perform their tasks, and to better survive flame laden or explosive environments.  These features include adjustable time delays and automatic self-tests, explosion-proof enclosures, and integral air conditioning systems.