Introduction — EW Academy

Infrared (IR) is the part of the EMS between radio waves and visible light: from 300 GHz to ~394 THz, or wavelengths from 1 mm to 760 nm. Invisible to the eye, but felt as heat — and a goldmine for sensors.

The physical basis was laid in Temperature: every object above absolute zero emits IR radiation, and the hotter the object, the more radiation and the shorter the peak wavelength. A sniper at night, a tank under a camouflage net, a jet fighter at 20 km — they all radiate their own heat, unasked and continuously. An IR sensor only has to look.

Thermal image (FLIR) of a person: warm parts of the body glow brightly, the cooler surroundings stay dark. — A thermal image (FLIR) of a person: the warmer, the brighter. In this way every object above absolute zero betrays its own heat to a passive IR sensor. Image: Emilian Robert Vicol / Wikimedia Commons — CC0.

That makes IR the most important passive sensor family on the battlefield: thermal sights, FLIR cameras in targeting pods, IRST systems on fighters and frigates, and the seekers of countless heat-seeking missiles — from MANPADS to modern air-to-air missiles.

Two physical laws are worth remembering:

Wien's law: λ_peak ≈ 2898 / T λ_peak in µm, T in kelvin — the hotter, the shorter the peak wavelength human (310 K) → peak ~9.3 µm · exhaust plume (900 K) → peak ~3.2 µm

Stefan-Boltzmann law: radiated power ∝ T⁴ 2× as hot (in kelvin) = 16× as much radiation — hot parts completely dominate the IR signature

§6.1 Introduction