What Is LWIR? Long Wave Infrared Bands, Detectors and Specifications
LWIR, or long wave infrared, is the band of the spectrum from about 8 to 14 µm. It is where objects at everyday temperatures, such as people, vehicles, terrain and buildings, emit most of their own thermal radiation, and where the atmosphere lets that radiation through. That combination is why almost every uncooled thermal camera, weapon sight and driver’s viewer works in this band.
This reference explains where the band begins and ends, the physics behind it, the detectors that use it and how to read a thermal core datasheet. It then compares the band with MWIR and SWIR and covers the conditions that limit it in the field. It is written for engineers, integrators and buyers who specify thermal sensors for defence vehicles and equipment.
Where the Long Wave Infrared Band Starts and Ends
Infrared runs from just beyond visible red to the edge of the microwave region. Thermal imaging uses two parts of it: mid wave infrared (MWIR) at 3 to 5 µm and long wave infrared at 8 to 14 µm. Between them, from 5 to 8 µm, water vapour blocks almost all radiation from the lower atmosphere, so no imaging band sits there.
The limits are a convention, not a standard. Detector and camera manufacturers mostly quote 8 to 14 µm. Meteorological satellite training material describes the clearest part of the window as 8 to 13 µm, and Betalight’s own thermal identification material is specified for 8 to 12 µm. At the long end, carbon dioxide absorbs strongly around 15 µm; inside the window, ozone adds an absorption band at about 9.7 µm.
When two datasheets state different band limits, compare the spectral response of the complete camera, lens and window included, rather than the headline range of the detector.
Why 8 to 14 µm: Peak Emission and the Atmospheric Window
Wien’s displacement law gives the wavelength at which a blackbody radiates most strongly: about 2,898 µm·K divided by its absolute temperature. At 300 K (27 °C) the peak lies at 9.7 µm, for skin at about 305 K at 9.5 µm, and at 0 °C it moves out to 10.6 µm. People, terrain and a vehicle hull at ambient temperature all peak inside the long wave window.
Planck’s law shows how the energy divides between the bands. For a blackbody at 300 K, about 38% of the total emission falls between 8 and 14 µm, roughly 170 W per square metre, against 1.3% (about 6 W per square metre) between 3 and 5 µm. An ambient scene therefore delivers around 30 times more energy in the long wave band, which is one reason an uncooled detector can form a usable image there without cryogenic cooling.
The balance shifts as targets get hotter. At 300 K the photon flux in the 3 to 5 µm band rises by about 3.6% per kelvin, against 1.5% between 8 and 14 µm, so the mid wave band shows more contrast per degree. For a surface at 500 °C the peak moves to 3.7 µm and the mid wave band carries almost twice the energy of the long wave band. That is why MWIR is favoured for hot targets such as engines and exhaust plumes, and the long wave band for targets near ambient temperature.
Emissivity, Reflection and Apparent Temperature
A long wave imager measures radiance, not temperature. What leaves a surface is its own emission, scaled by its emissivity, plus whatever it reflects from its surroundings. A surface with high emissivity shows close to its true temperature; a surface with low emissivity acts as a mirror and shows the temperature of what it reflects.
| Surface | Typical emissivity | What the imager mostly shows |
|---|---|---|
| Human skin | About 0.98 | Its own temperature |
| Water | About 0.98 | Its own surface temperature |
| Flat-finish paint | Around 0.90 | Mostly its own temperature |
| Roughened or oxidised metal | 0.6 or more, depending on condition | A mix of its own and reflected temperature |
| Polished copper or aluminium | Below 0.10 | Reflected surroundings: sky, sun, people |
On a vehicle this means a polished or bare metal fitting can show the cold sky instead of the warm hull, while a painted panel at the same temperature next to it looks warmer. It also means a thermal image is not a thermometer. Radiometric cores that report temperature need the emissivity and reflected temperature to be set, and one camera manufacturer advises that readings on surfaces with an emissivity below 0.5 are unlikely to be accurate.
Microbolometers and Cooled Long Wave Detectors
Almost all long wave cameras outside long-range sights use an uncooled microbolometer. It is a focal plane array of thin membranes, each held on narrow legs above a readout circuit and sealed in a vacuum package. Absorbed radiation warms each membrane slightly and changes its electrical resistance. The two dominant membrane materials, vanadium oxide (VOx) and amorphous silicon (a-Si), both have a high temperature coefficient of resistance. Both work at ambient temperature, so the core needs no cryocooler and no cool-down time.
Pixel pitch keeps shrinking. 17 µm and 12 µm arrays are both in current production, and one manufacturer’s 2026 defence range includes a 1024 × 768 array at 8.5 µm. A smaller pitch lets a shorter, lighter germanium lens deliver the same angular resolution: a 12 µm pixel behind a 50 mm lens covers the same angle as a 17 µm pixel behind a lens of about 71 mm.
Cooled long wave detectors serve the most demanding long-range sights. They are photon detectors, made of mercury cadmium telluride (HgCdTe) or of type-II superlattice and related barrier structures, and they run close to liquid nitrogen temperature. One published 640 × 512, 15 µm type-II superlattice array is background limited at 77 K with a cut-off of about 9.5 µm; NASA JPL reports a long wave barrier array in HDTV format with an 11 µm cut-off, operating at 80 K. High operating temperature mid wave detectors can run at up to 150 K, so a cooled long wave detector needs a colder and more demanding cooler, and it often covers only part of the 8 to 14 µm band.
How cooled and uncooled cores compare for helmet-mounted use, and how pixel count and lens set detection, recognition and identification ranges, is covered in our article on thermal goggles.
Reading a Thermal Core Datasheet
Integrators compare LWIR cores on a short list of figures, and almost every one of them depends on a condition stated in small print. The ranges below are typical of current uncooled cores from several manufacturers.
LWIR vs MWIR, and Where Short Wave Fits
The three infrared imaging bands used in defence do different jobs. MWIR and the long wave band image emitted heat. Short wave infrared images reflected light, much like a visible camera, and is covered in our article on SWIR.
| Attribute | Short wave | Mid wave | Long wave |
|---|---|---|---|
| Band | About 1 to 1.7 µm; extended designs to 2.2 or 2.5 µm | 3 to 5 µm | 8 to 14 µm |
| Signal | Reflected light: moonlight, nightglow or an illuminator | Emitted heat, strongest from hot objects | Emitted heat from objects near ambient temperature |
| Typical detector | InGaAs | Cooled photon detector, near 77 K, or up to 150 K in high operating temperature designs | Uncooled VOx or a-Si microbolometer; cooled HgCdTe or superlattice for long range |
| Optics | Glass lenses and windows work | Infrared optical materials | Germanium or chalcogenide; ordinary glass is opaque |
| Favoured for | Surface detail and markings, imaging through glass | Hot targets, humid and maritime air, long-range cooled sights | People and terrain, smoke and dust, compact low-power equipment |
| Limited by | Needs some light; shows no thermal contrast | Cooler cost, power and service life | Water vapour in humid air; a larger diffraction spot for a given lens |
Diffraction scales with wavelength. The smallest spot a lens can form is about 2.44 times the wavelength times the f-number, so at 4 µm a lens forms a spot about two and a half times smaller than the same lens at 10 µm. A mid wave sight with the same aperture can therefore resolve finer detail at long range. Against that, the long wave band offers far more signal from ambient scenes and works with uncooled detectors.
Atmosphere, Humidity, Fog and Glass
Humidity. Water vapour absorbs inside the long wave window too, most strongly towards its long end around 12.5 µm. Humid air attenuates the signal, and summer atmospheres usually attenuate more than winter ones. In hot, humid and maritime conditions, MWIR generally performs better.
Fog and rain. Droplets scatter both thermal bands, and the denser the fog, the shorter the range. In one manufacturer’s analysis of fog categories, the long wave band penetrated fog better than MWIR in every case studied and gave about four times the range of MWIR in Category II fog, but dense fog blocked visible, MWIR and LWIR alike. In rain the two thermal bands perform similarly; rain cuts target contrast, and the same analysis shows range performance dropping sharply between 100 and 500 metres.
Smoke and dust. Longer wavelengths scatter less from fine particles, so the long wave band often sees through ordinary smoke and dust better than shorter bands, depending on the size and density of the particles. Military screening smokes are a different matter: phosphorus and brass-flake obscurants, sometimes called visual and infrared screening smoke, are designed to be opaque to thermal imagers.
Glass and optics. Ordinary window glass is opaque above 5 µm. Long wave lenses are made of germanium or chalcogenide glass. Chalcogenide can be moulded into finished aspheric and diffractive lenses in volume, and one grade changes its refractive index with temperature about eight times less than germanium, which eases athermal lens design. A vehicle-mounted long wave sensor must look out through open air or an infrared window, never through the windscreen.
Military Uses of the Long Wave Band
Driver vision enhancement. The US Army’s AN/VAS-5B(V) Driver’s Vision Enhancer is a lightweight, uncooled, passive thermal imaging camera that lets drivers of tactical wheeled and combat vehicles operate in darkness, fog, smoke or dust. Because it emits nothing, it fits driving under light discipline, where military blackout lights limit what the vehicle shows to the eye and to night vision.
Weapon sights and clip-ons. Low power, low weight and no cool-down suit uncooled LWIR cores to weapon sights and clip-ons. Betalight’s TI-GEAR-S sight uses shutterless uncooled arrays at 12 µm or 17 µm and 9 Hz or 50 Hz, and the TICON clip-on a 640 × 512, 12 µm VOx array. The full range is on our thermal imaging equipment page.
Surveillance and force protection. Mast, perimeter and observation post cameras watch routes and approaches day and night. Larger arrays such as 1024 × 768 cover a wider field at the same angular resolution, or the same field in finer detail.
Vehicle signature. A vehicle radiates in the long wave band whether its lights are on or off. Engines, exhausts and sun-heated surfaces stand out against the background, which is why multispectral camouflage systems are designed to cover the thermal infrared band as well as the visual, near infrared and radar bands. Light discipline and thermal signature management are separate tasks.
Thermal identification. IR strobes and markers emit in the near infrared, at far shorter wavelengths than a long wave sight detects. Passive thermal identification equipment fills that gap: its reflective material mirrors the cold open sky and appears as a distinct cold area on thermal devices working at 3 to 5 µm or 8 to 12 µm.
Frequently Asked Questions
No. Water and ice clouds have an emissivity close to 1 in the long wave window, so an imager sees the cloud’s own surface and not what lies behind it. Weather satellites use exactly this: the 10.7 µm long wave window channel of US geostationary satellites reads cloud top temperature. Thin cirrus is the exception, being partly transparent. Like dense fog, thick cloud blocks visible, mid wave and long wave sensors alike.
A core is the detector with its readout, processing and video electronics, supplied for integration into a sight, mast or vehicle system, usually without housing or display and often without a lens. A camera adds the lens, housing, controls, power supply and a display or network output. Datasheet NETD and power are normally given for the core; the finished camera also depends on its lens f-number, any protective window and the interface chosen.
Not by itself. A smaller pitch gives more pixels on the same die, or the same angular resolution with a shorter, lighter lens. But at 10 µm the diffraction spot of an F/1 lens is about 24 µm across, wider than a 12 µm or 8.5 µm pixel, and each smaller pixel collects less energy. Fine-pitch arrays therefore need fast, high-quality optics. Compare complete cameras: pitch, focal length, f-number and NETD at stated conditions.