Anything warm emits infrared. The hotter something is, the shorter the wavelength at which that radiation is strongest. A hot exhaust (600–1000 K) radiates mainly at 3–5 µm (MWIR), a person or vehicle (around 300 K) at around 10 µm (LWIR).
In practiceThermal weapon sights and the thermal cameras in armoured vehicles work in LWIR (8–12 µm): that is where people and engines light up at night. A MANPADS seeker, by contrast, looks in MWIR, where the hot exhaust plume of an aircraft or helicopter is brightest.
The IR region is subdivided into bands, named after their distance from visible light:
| Band | Name | Wavelength | Typical source / use |
|---|---|---|---|
| NIR | Near IR | 0.76 – 1.4 µm | image intensifiers, IR searchlights, laser (1.06 µm) |
| SWIR | Short-Wave IR | 1.4 – 3 µm | imaging based on reflection contrast, eye-safe lasers (1.55 µm) |
| MWIR | Mid-Wave IR | 3 – 8 µm | hot sources: exhausts, plumes, engines → missile seekers |
| LWIR | Long-Wave IR | 8 – 15 µm | objects at ambient temperature → thermal imaging of people & vehicles |
| FIR | Far IR | 15 µm – 1 mm | scientific; little military use |
Why this division matters operationally: the atmosphere is not equally transparent everywhere. Water vapour and CO₂ absorb whole stretches of the IR spectrum; in between lie atmospheric windows where IR radiation does carry far. The two most important windows are 3–5 µm (in MWIR) and 8–12 µm (in LWIR) — exactly the bands in which almost all military IR sensors operate.
- MWIR (3–5 µm) "sees" hot sources best (Wien: 600–1000 K peaks here). This is the band of heat-seeking missiles that aim at exhausts and plumes, and of many naval IRSTs.
- LWIR (8–12 µm) "sees" everything at ambient temperature (~300 K peaks around 10 µm). This is the band of thermal sights for personnel and vehicles, and it penetrates smoke and dust better.
Why are the windows exactly there? The "dips" explained
The dark stretches between the windows (see the graph) are no coincidence: they are absorption bands of molecules in the air, mainly water vapour (H₂O) and carbon dioxide (CO₂). These molecules vibrate and rotate at fixed frequencies and soak up IR radiation at exactly those wavelengths.
- The dip around 1.5–2 µm: water vapour has strong absorption bands around 1.4 µm and 1.9 µm. Only a narrow transparent strip remains between them, which makes this part of SWIR of limited use. The "eye-safe" laser at 1.55 µm (Division of the infrared spectrum) was deliberately placed in that narrow gap.
- The dip between 5 and 8 µm: here lies a very broad, strong water-vapour absorption band (centred around 6.3 µm). The air is almost opaque to IR there — which is exactly why the MWIR window already stops at ~5 µm and the LWIR window only starts at ~8 µm. CO₂ cuts in too, with a sharp band around 4.3 µm and a broad one around 15 µm.
The usable windows of 3–5 and 8–12 µm are thus literally the "gaps" that H₂O and CO₂ leave open. That is why almost every IR sensor follows these limits, and why the windows shrink in humid air: more water vapour = broader absorption bands = less range (Disadvantages of infrared).