Infrared (IR)

Infrared, IR, is electromagnetic radiation just beyond the red end of visible light, invisible to the eye but emitted by everything warm as heat. That simple physical fact underlies an enormous amount of military technology, because if you can build a sensor that sees infrared, you can see heat, and seeing heat means seeing in the dark, seeing through smoke and light foliage, and detecting anything warmer than its surroundings, from a running engine to a human body. The military exploitation of infrared transformed warfare in two big ways: it gave forces the ability to see at night and in poor visibility through thermal imaging, and it gave weapons the ability to home on the heat of their targets through infrared guidance.

Seeing heat and homing on it

Thermal imaging is the sensing side of infrared, and it changed the battlefield by ending the cover that darkness once provided. A thermal imager detects the infrared radiation, the heat, that objects emit, rendering warm objects bright against cooler backgrounds, so it sees in complete darkness, when the eye and ordinary cameras see nothing, and it penetrates smoke, haze, and light concealment that block visible light. This gave forces equipped with thermal sensors a decisive advantage in night and poor-visibility fighting, since they could see and engage an enemy who could not see them, and thermal imaging spread from a specialized capability to a common one, on vehicles, aircraft, weapons, and individual soldiers' equipment, so that seeing by heat became a routine part of how modern forces operate, day and night. This is also what made the heat signature such a critical vulnerability, since anything warm, an engine, a body, a heated shelter, glows in the infrared to a thermal sensor, and hiding from view no longer means hiding, driving the whole effort of infrared signature management to reduce and conceal the heat that thermal sensors detect.

Infrared guidance is the weapons side, and it made heat a thing that could be homed on. An infrared-guided missile seeks the heat of its target, an aircraft's hot engines, a vehicle's warm spots, homing on the infrared radiation the target emits, which made these missiles effective and widespread, from the man-portable anti-aircraft missiles that threaten aircraft to the air-to-air missiles carried by fighters. The heat-seeking missile's reliance on infrared also created its vulnerability, since it can be decoyed by flares that burn hotter than the target and by the infrared countermeasures that confuse its seeker, the measure-countermeasure contest that runs through infrared as through everything else in warfare. The proliferation of infrared-guided missiles, especially the portable anti-aircraft ones, made low-flying aircraft and helicopters vulnerable and forced the development of the countermeasures, flares and infrared jammers, that aircraft carry to defeat them, an ongoing duel between infrared guidance and infrared countermeasures.

Infrared across the spectrum of war

Infrared technology pervades modern military systems because seeing and exploiting heat is useful in so many ways, and the capability keeps advancing as sensors improve. Beyond thermal imaging for night vision and infrared guidance for missiles, infrared serves in search and track systems that detect targets by their heat, in the infrared signatures that identify and distinguish objects, and in the sensors that give aircraft, ships, and vehicles the ability to see and target by heat. The improvement of infrared sensors, becoming more sensitive, higher-resolution, and more widely available, has continually extended what forces can see by heat, and the modern battlefield is saturated with infrared sensors, especially on the drones that hunt for heat signatures day and night, making the management of one's own infrared signature a constant survival concern. The infrared spectrum has thus become one of the most important sensing domains in warfare, a realm in which forces see and are seen by heat, in which weapons home on heat, and in which the contest between exploiting heat, to see and to target, and concealing heat, to avoid being seen and targeted, plays out continuously, a contest that the advance of infrared technology keeps intensifying as thermal sensors grow more capable and more ubiquitous, and as the heat that everything warm emits becomes ever harder to hide from the sensors that see it.

Why can't you hide from infrared the way you hide from visible light?

Because you emit infrared continuously as heat, whereas hiding from visible light only requires blocking the light reflecting off you, which concealment can do. Visible-light concealment works by breaking your outline and hiding you behind cover, since visible light is reflected, not emitted, so blocking the line of sight hides you, but infrared is emitted by anything warm, radiating out from your body, your engine, your equipment, so a thermal sensor detects the heat you give off regardless of ordinary visual concealment, seeing through the darkness and light cover that would hide you from the eye. This is why the camouflage and concealment that defeat visual observation do little against thermal sensors, and why hiding from infrared requires the much harder work of reducing and masking the heat you emit, insulating hot components, using special multispectral materials that block infrared, and managing heat-producing activities, rather than simply breaking your outline and staying behind cover. Even these measures rarely hide a heat signature completely, since anything that must generate heat to function keeps emitting infrared that a sensitive sensor can detect, so the realistic goal is to reduce the heat signature and make detection harder and shorter-ranged rather than to achieve the near-invisibility that visual concealment can provide against the eye. The fundamental difference is that visible-light concealment blocks reflected light, which is achievable, while infrared concealment must suppress emitted heat, which is far harder, so you cannot hide from infrared the way you hide from visible light, and the advent of widespread thermal sensing consequently stripped away much of the concealment that darkness and cover once provided, forcing forces to confront the difficult problem of managing the heat they cannot help emitting.