A thermal camera forms an image from emitted infrared radiation, so it sees a fire in darkness and, importantly, through thin smoke that blinds a visible camera. Two bands are used. Longwave infrared, roughly 8 to 14 micrometres, is where objects near ambient temperature radiate most, and it is the band of uncooled microbolometer cameras: cheap, small, no cryocooler, instant on, and the reason the FLIR-class handheld exists at all. Midwave infrared, roughly 3 to 5 micrometres, is where much hotter sources peak, and it penetrates some atmospheric paths better; midwave imagers are usually cooled photon detectors, giving far better sensitivity and shorter integration times at much higher cost, size and warm-up delay. For fire work the practical rules are that a longwave uncooled camera is what a crew or a drone carries, that hot targets can saturate a camera set for ambient scenes, and that thermal imaging measures surface radiance, so smoke, glass, water and a wet canopy all lie to it. After this Concept you can pick a band and a detector type for a fire imaging task and say what the image cannot tell you.
Thermal imaging sensor selection is governed by which atmospheric transmission window a detector operates in: the mid-wave infrared (MWIR, 3-5 micron) and long-wave infrared (LWIR, 8-12 micron) bands…