As with radar, we distinguish active and passive systems — but with IR, passive is the norm:
Passive IR systems
A passive IR system emits nothing and detects only the target's own thermal radiation (or the temperature contrast between target and background). Examples:
- Thermal imaging camera / FLIR (Forward Looking InfraRed): forms an image of temperature differences; for driving, flying and aiming at night.
- IRST (InfraRed Search and Track): searches for and tracks air targets passively — a "radar without transmission" that measures direction only (no range, 300 GHz – 394 THz: Infrared).
- IR seekers of missiles: home on the target's heat (Passive homing).
- Missile warning sensors: detect the hot plume of an incoming missile (often in UV as well as IR).
The big advantage has already been mentioned: complete silence. No emission means no warning for the target and nothing to home on or to jam in the classic sense.
Active IR systems
An active IR system illuminates the scene itself with an IR source and looks at the reflection. Historically: IR searchlights on tanks with night sights (first-generation night vision). Today: laser illuminators and gated-viewing cameras, and the IR laser of a rangefinder or designator (Laser and military applications) is in effect active IR as well.
The drawback is the same as with radar, but worse: anyone with a (cheap) IR viewer sees your searchlight as a blazing torch, at a much greater distance than you can see the target. Active IR illumination has therefore been almost abandoned on the modern battlefield in favour of passive thermal imaging and image intensification.
Passive = looking at the heat the target itself radiates (standard). Active = emitting IR yourself and looking at the reflection (gives itself away; only used in niche applications now).