Blackbody Radiation.

Infrared sensors are also effected by Infrared Radiation that is not coming from a fire. The fire can be masked by this Blackbody Radiation. Every object that has a temperature higher than the absolute minimum (0 Kelvin or -273 °C) radiates energy and at room temperature the energy is already detectable for the most sensitive Infrared sensors. Sometimes a moving hand close to the sensor is enough to generate an alarm. At 700 Kelvin a hot object already starts to send out visible energy (glowing).

Dual or Multi Infrared detectors suppress the effects of Blackbody Radiation by sensing energy just besides the CO₂ radiation peak e.g. on 4.1µ. The principle works on the fact that a real Hydrocarbon fire causes a difference between the sensors. See S1 and S2 in the picture above. A drawback is that there must be a larger difference in sensor output than the background radiation present. In other words, the detector gets insensitive when Blackbody Radiation is present. Every multi IR Flame Detector deals with this problem, no matter what the price of the device is.

The spectrum | Fire Radiation | Solar radiation impact | Blackbody Radiation | Cone of Vision
Flame detection Range | Square Law and Sensitivity