Thermal imaging settled on the 8–14 µm band because physics gave it no better option: the atmosphere is transparent there, and objects near Earth-ambient temperature emit their peak radiation there. Everything about drone thermal follows from those two facts.
- 8–14 µmThe LWIR atmospheric window
- ~10 µmPeak emission at 300 K
- 3–5 µmMWIR, the cooled alternative
- GermaniumWhat the lenses are made of
Key takeaways
- Water vapour and CO₂ absorb infrared across most of the spectrum. The 8–14 µm corridor passes through kilometres of air with modest loss.
- A 300 K object — a person, a roof, a transformer — radiates most strongly near 10 µm, so LWIR reads targets at their brightest natural wavelength with no illumination.
- MWIR offers finer resolution per aperture, but its high-sensitivity variants need cryogenic cooling, which rules out most UAV payloads.
- Glass is opaque at these wavelengths, which is why thermal cameras cannot image through windows and why their own lenses are germanium.
On this page
The Atmospheric Window
Water vapour and carbon dioxide absorb infrared radiation across most of the spectrum. If you plot atmospheric transmission against wavelength, the result is not a smooth curve but a series of blocked regions separated by a few relatively clear corridors — and the widest useful one for terrestrial imaging sits between roughly 8 and 14 µm.
In that corridor, infrared passes through kilometres of air with modest loss. That is the entire reason standoff thermal imaging is possible: a sensor operating in an absorption band would be looking at the atmosphere rather than through it, and range would collapse to metres regardless of how good the detector was.
The window is not perfectly clear, and how much it degrades matters operationally. Humidity attenuates even inside the band, which is why published DRI figures are laboratory numbers that field planning discounts, and why fog, rain and humidity have measurable effects on detection range.
Wien’s Law Puts Targets in the Band
The second fact is about the targets rather than the air. Every object radiates across a spread of wavelengths, and the wavelength at which it radiates most strongly depends only on its temperature — that relationship is Wien’s displacement law.
For an object at 300 K, which is roughly room temperature and close to a person, a roof or a transformer, peak emission lands near 10 µm. Squarely inside the atmospheric window. This is a coincidence in the sense that nothing arranged it, and a profound convenience in the sense that thermal imaging of the everyday world would be far harder if it were not true.
The practical consequence is that LWIR cameras read targets at their brightest natural wavelength, requiring no illumination whatsoever. A thermal camera in total darkness sees exactly what it sees at noon, because the target is supplying the signal. That is a categorically different proposition from a starlight sensor, which amplifies whatever light happens to be present, or a NIR illuminator, which supplies its own — the three approaches compared covers when each wins.
Why Not MWIR on Drones?
Mid-wave infrared, 3–5 µm, is also an atmospheric window, and it has a genuine optical advantage: shorter wavelengths diffract less, so MWIR offers finer resolution for a given aperture. For long-range systems where aperture is the binding constraint, that matters.
The problem is what MWIR sensitivity costs. The high-sensitivity variants require cryogenic cooling — a Stirling cooler holding the focal plane at very low temperature — and that brings mass, power draw, cost and a cooldown wait before the camera can image at all. Those four together rule out most UAV payloads.
Uncooled LWIR microbolometers won the drone era for exactly this reason. They are not better cameras in the abstract; they are the cameras that fit. Uncooled vs cooled thermal cores works through the four-way trade in detail, and how uncooled VOx microbolometers work covers the detector physics.
| Property | LWIR 8–14 µm | MWIR 3–5 µm |
|---|---|---|
| Atmospheric transmission | Good over kilometres | Good, with different humidity behaviour |
| Peak match to 300 K targets | Near-optimal (~10 µm) | Off-peak for ambient-temperature objects |
| Resolution per aperture | Lower — longer wavelength diffracts more | Higher |
| Cooling requirement | None | Cryogenic for high-sensitivity variants |
| Practical on small UAV | Yes | Rarely |
| Time to first image | Immediate | Minutes of cooldown |
What the Band Means for Optics
A consequence people meet before they meet the physics: ordinary glass is opaque at 8–14 µm. It transmits visible light and blocks long-wave infrared almost completely, which is why a thermal camera cannot image through a window, and why the camera’s own lenses cannot be made of glass.
Thermal optics are germanium — a semiconductor that is opaque to visible light and transparent in the LWIR band, the inverse of glass. Germanium is dense, expensive and requires anti-reflection coating to work well, which is a large part of why thermal payload optics cost what they do. Germanium lenses, f/1.0 apertures and athermalisation covers the design consequences.
It is also the fact that settles most privacy arguments about thermal drones. LWIR does not see through walls, and it does not see through windows either. It reads exterior surface temperatures only — the privacy and regulation primer covers why saying this early defuses most objections.
The Band in Practice Across the Line
Published spectral bands across the UAVThermal thermal payloads sit inside this window, though not all at exactly the same limits.
| Payload | Published spectral band | Thermal resolution |
|---|---|---|
| MV-2M | 8–14 µm | 256×192 |
| MV-2P | 8–14 µm | 640×512 |
| OP-90P | 8–14 µm | 640×512 |
| OP-125A | 8–14 µm | 640×512 |
| LX-9B | 8–14 µm | 640×512 |
| OP-90A | 8–12 µm | 640×512 |
Note the OP-90A. Its published band is 8–12 µm rather than 8–14 µm. A narrower band collects somewhat less energy, though both sit inside the atmospheric window and around the 300 K emission peak. Worth knowing if you are comparing datasheets closely rather than assuming the whole line is identical.
For most operational purposes the difference is not something you will notice against the much larger effects of lens speed, resolution and atmospheric conditions. It is the kind of detail that matters when writing a specification and not when flying one.
What Blocks LWIR, and What Does Not
The list of what interferes is shorter and more specific than most people assume, which makes it worth knowing precisely.
Blocks completely: glass, water, and any solid material. There is no thermal imaging through a window, a wall, a vehicle body or a water surface. What you see is the surface temperature of the nearest opaque thing.
Scatters and attenuates: heavy rain and dense fog. Water droplets in the air of comparable size to the wavelength scatter the radiation, cutting range substantially — though thermal in poor conditions still typically outperforms visual search.
Barely affects it: light smoke and complete darkness. Smoke particles are generally smaller than the LWIR wavelength and much of it passes through, which is why firefighting thermal works at all. And darkness is a non-event, because the target supplies the signal.
That last pairing — sees through smoke, unaffected by darkness, stopped dead by a pane of glass — is counter-intuitive until you hold both facts about the band at once: transparency depends on the material and the wavelength, not on how solid something looks to a human eye.
Related reading
- How Uncooled VOx Microbolometers Work
- Thermal Infrared Optics: Germanium Lenses, f/1.0 Apertures and Athermalization
- From NETD to DRI: How Thermal Detector Performance Is Calculated
- Uncooled vs Cooled Thermal Cores
- Starlight vs NIR vs Thermal: Drone Night Vision Compared
- Thermal Drones in Fog, Rain and Humidity
- Thermal Drones, Privacy and Regulation
- Thermal Pixel Pitch: Why 12 µm Replaced 17 µm
- MV-2P — 640×512 thermal at 130 g
- OP-90A — multi-sensor pod
- OP-125A — 30x zoom multi-sensor pod
- Drone Thermal Cameras for Firefighting
FAQ
Does LWIR see through glass?
No. Glass is opaque at 8–14 µm, which is why thermal cameras cannot image through windows and why their own lenses are made of germanium rather than glass. Germanium is the inverse of glass optically — opaque to visible light, transparent in the long-wave infrared. This single fact settles most privacy questions about thermal drones and also explains a large part of why thermal optics are expensive.
What blocks LWIR?
Heavy rain and dense fog scatter it, cutting range substantially though rarely to zero. Glass, water and any solid material block it entirely — you always see the surface of the nearest opaque object. Light smoke and full darkness barely affect it: smoke particles are typically smaller than the wavelength so much of it passes through, and darkness is irrelevant because the target itself supplies the radiation being imaged.
Are all thermal payloads 8–14 µm?
Nearly, but check rather than assume. Across the UAVThermal line most payloads publish 8–14 µm while the OP-90A publishes 8–12 µm. Both sit inside the atmospheric window and around the peak emission wavelength of ambient-temperature objects, so the operational difference is small compared with lens speed, resolution and weather. It matters when you are writing a specification or comparing datasheets closely.
Questions about the technology? Talk to our engineers — we reply within 2 business days.