The core of this chapter:
- Everything above 0 K radiates IR; Wien determines the peak wavelength (λ ≈ 2898/T), Stefan-Boltzmann the amount (∝ T⁴).
- Military IR works in the atmospheric windows: MWIR 3–5 µm (hot sources, missile seekers) and LWIR 8–12 µm (ambient temperature, thermal imaging).
- Passive IR is the norm: silent, invisible, excellent angle measurement. Active IR gives itself away and has largely been abandoned.
- The price: no range, strong weather dependence and relatively accessible countermeasures (flares, DIRCM, signature reduction).
IR provides the eyes, but lacks range measurement. The instrument that fills exactly that gap is the laser — the subject of the next chapter.