Any object above absolute zero emits thermal (blackbody) radiation across a spread of wavelengths. A perfect radiator is a blackbody; real surfaces emit some fraction of that ideal, set by their emissivity.
Total emitted power climbs with the fourth power of absolute temperature (Stefan-Boltzmann, \(M_{\text{total}} \propto T^4\)). Doubling the temperature radiates roughly 16× as much power.
By Wien's displacement law, \(\lambda_{\text{peak}}\,[\mu\text{m}] \approx 2898 / T\,[\text{K}]\). As temperature rises, the peak wavelength shifts shorter (bluer).
About 0.5 µm, in the visible band (\(2898/5800 \approx 0.5\) µm).
About 8.9 µm, in the LWIR (8–12 µm) band (\(2898/325 \approx 8.9\) µm).
MWIR 3–5 µm (hot targets: plumes, engines) and LWIR 8–12 µm (cool targets: skin, terrain, FLIR).
It is largely opaque, blocked mainly by atmospheric water vapor, with CO₂ also cutting a notch near 4.3 µm. Water vapor and CO₂ close broad stretches of the IR, and humidity/weather shrink IR range — the IR advantage is real but not all-weather.
Reflective band (~0.4–3 µm) sees reflected light (sun, moon, laser) and goes dark without a source. Emissive/thermal band (~3–14 µm) sees the target's own radiated heat and works in total darkness.
It senses the heat objects radiate on their own (emissive band), so no external illumination is needed.
It lives on contrast — the temperature difference between target and background. Thermal crossover is when that difference vanishes (e.g., cool skin against sun-warmed desert) and the target can disappear.
Photon (quantum) detectors free a charge carrier per photon: fast, sensitive, band-selective (InSb MWIR, HgCdTe/MCT LWIR), but must be cooled. Thermal detectors (microbolometer) sense a temperature rise: broadband and uncooled, cheaper and smaller, but slower and less sensitive.
Silicon (Si) for VIS/NIR, indium antimonide (InSb) for MWIR, and mercury cadmium telluride (HgCdTe, MCT) for LWIR.
Warm, a photon detector drowns in its own thermally generated noise. Cooling (liquid nitrogen or a Stirling cooler) suppresses that noise and raises SNR so faint targets are detectable, letting the limit become the background itself (BLIP).
Radar sees the energy you reflect (active, can warn/jam); IR sees the energy you emit (passive, no warning, hard to jam, but line-of-sight and weather-limited).
Wien's law: hotter parts peak at shorter wavelengths (MWIR ~3–5 µm for plumes and tailpipes), while cool skin and background peak longer (LWIR ~8–12 µm).