Infrared sees differences in heat; it does not see through everything. Fog, rain and clouds absorb the radiation, and when everything is equally warm, the contrast disappears.
In practiceAround sunrise and sunset the ground and the objects on it briefly have about the same temperature (thermal crossover). A thermal image then shows little contrast — a moment observers know about and an opponent can exploit. Heat-reducing camouflage nets work on the same principle.
IR is powerful but no miracle cure. Its limitations determine how you employ IR sensors — and how you defend against them:
- No range information. A passive IR sensor measures direction only (azimuth/elevation). Distance requires a laser rangefinder, triangulation or kinematic ranging. For a fire control solution IR alone is therefore not enough (Target acquisition and tracking by IR).
- Weather dependence. Rain, fog, cloud, snow and high humidity absorb and scatter IR strongly. The range can drop from tens of kilometres (dry, cold air) to hundreds of metres (thick fog). Radar suffers much less from this.
- Atmospheric attenuation and thermal contrast. Even without precipitation, dust, smoke and heat shimmer (scintillation) degrade the image. And a target at the same temperature as its background (thermal crossover, especially around sunrise/sunset) all but disappears.
- Countermeasures are relatively simple. Flares (hot decoys), IR jammers (DIRCM: a modulated laser that blinds or deceives the seeker), exhaust cooling and shielding, IR-suppressing paint and camouflage nets, and smoke screens with IR-active particles.
- Solar and background reflections. The sun, sunlight reflected off clouds or water, and hot backgrounds can distract older seekers and saturate sensors.
- Cooling needed for top performance. Sensitive MWIR detectors have to be cryogenically cooled (~77 K); that costs time (cool-down), energy and maintenance. Uncooled LWIR sensors do exist, but are less sensitive.
The logic of the duel: because IR is passive, the target cannot hear that it is being watched — but conversely, the IR user does not know whether his sensor has "locked" the target without an active step of his own (laser ranging). The contest between IR seeker and flare/DIRCM is a classic EW cycle: better seekers (two-colour, imaging) against better countermeasures.
Video reportedly shows a Russian Ka-52 attack helicopter missing a Ukrainian FP-1 — a slow long-range drone with a low thermal signature — with a missile from the Igla family. Defense Express attributes the miss to insufficient thermal contrast for lock-on (see above) and names the Vikhr air-to-air variant with semi-active laser guidance as a possible Russian alternative against this type of target. It illustrates the principle of this section with a concrete (but unverified, Ukrainian-sourced) case — source: Defense Express, RU Mil Tracker e-20260823-s03.