Glass is opaque to long-wave infrared — a thermal camera behind a normal lens would see nothing at all. Thermal optics are built from germanium and a small family of similar exotic materials, and three design consequences of that choice explain both the performance and the price of every thermal payload: fixed fast apertures, no iris, and the need for athermalization.
Key takeaways
- Germanium transmits the 8–14 µm band and machines to precision surfaces. It is also expensive, soft-coated and easy to ruin — a scratched thermal lens is a replacement, not a repair.
- Thermal lenses are fixed-aperture and almost universally f/1.0, because signal scales with collected radiation and f/1.0 gathers roughly four times the energy of f/2.0.
- There is no iris to stop down and no optical zoom on a thermal channel at this scale — field of view is set by lens choice, and any “zoom” is digital scaling.
- Refractive index shifts with temperature, so an unathermalized design drifts out of focus between a winter dawn and a summer roof.
On this page
Germanium and friends
Germanium transmits the 8–14 µm window superbly and machines to the precision surfaces an imaging lens requires, which is why it is the standard material for LWIR optics. Anti-reflection coatings push transmission higher still — uncoated germanium reflects a substantial fraction of what hits it, so the coating is not a refinement, it is load-bearing.
| Property | Germanium | Consequence for a payload |
|---|---|---|
| Visible transmission | Opaque | Lens looks like a dark mirror; you cannot judge cleanliness by eye the usual way |
| LWIR transmission | Excellent across 8–14 µm | Matches the band the detector senses |
| Cost | High, and it tracks raw material price | A significant fraction of payload cost is the front element |
| Coating hardness | Soft AR coatings | Abrasive cleaning removes the coating permanently |
| Thermal behaviour | Refractive index shifts strongly with temperature | Athermalization required for outdoor use |
Cleaning discipline is not optional. The soft AR coating is what makes the lens work, and it is thinner and softer than anything on a visible-light camera. Dry wiping grit across it removes coating in a single pass. Follow the procedure in the gimbal maintenance guide rather than improvising with a shirt.
Why f/1.0 is everywhere
Microbolometers measure very small heat differences, and the signal available to them scales with the radiation the optics collect. Aperture area goes as the square of the f-number, so a fast f/1.0 lens delivers roughly four times the energy of f/2.0 — which translates directly into effective sensitivity. This is why f/1.0 is standard across UAVThermal thermal channels rather than a premium option.
It also explains a spec-sheet trap. An NETD of <50 mK measured at f/1.0 and the same figure measured behind a slower lens describe different systems. When a datasheet states NETD without stating the aperture, the number is not comparable — a point covered in how to read a thermal spec sheet and in detector performance modelling.
Thermal lenses are fixed-aperture: there is no iris, nothing to stop down, and no exposure control in the photographic sense. Brightness in the displayed image comes from gain and mapping, not from the optics. Field of view is set by focal length at build time — see the focal length guide for the coverage-versus-range trade.
Athermalization
The refractive index of germanium changes appreciably with temperature, and so does the length of the aluminium barrel holding it. Left unmanaged, a lens focused correctly on a −5 °C winter dawn is measurably soft after soaking above a 60 °C summer roof, and nothing in the image processing recovers the lost resolution.
| Approach | How it compensates | Trade |
|---|---|---|
| Passive optical | Element materials chosen so their index shifts cancel | No moving parts; constrains the optical design |
| Passive mechanical | A barrel material whose expansion moves the element correctly | Simple and reliable; tuned for one design |
| Active refocus | A motor refocuses against a temperature sensor | Flexible; adds a mechanism, power and a failure mode |
This matters more on aircraft than on handheld instruments. A payload climbs through a temperature gradient, soaks at altitude, and then descends — all within one sortie, and often while the sensor body is still warming from its own electronics. Pixel pitch and optics resolve together, so an optic that drifts soft wastes detector resolution you paid for; see pixel pitch explained.
Windows, filters and what not to put in front
Ordinary glass and acrylic are opaque in the LWIR band. A clear protector that looks harmless on a visible camera blinds a thermal one completely — this is the most common self-inflicted failure on a new integration, and it presents as a uniform grey image that operators initially read as a dead sensor.
If a protective window is genuinely required, it must be an IR-transmissive material specified for 8–14 µm, and it will still cost transmission and add a reflective surface inside the optical path. On a gimbal that already seals against the environment, the honest answer is usually to leave the front element exposed and manage cleaning instead.
The dark mirror is normal. Germanium is opaque to visible light and its AR coatings reflect what little interacts with them, so a healthy thermal lens looks like a dark, slightly gold mirror. It is transparent only in the band it serves — see why thermal cameras live at 8–14 µm.
What this changes when you specify
- Require NETD stated with its f-number and sensor temperature, or the figure cannot be compared.
- Choose focal length from the ground sample distance the mission needs, and accept the field of view that comes with it.
- Ask whether the optic is athermalized and over what temperature range, if the aircraft operates across seasons.
- Do not plan on a protective window unless it is an IR-grade one, and budget the transmission loss if you use it.
- Write lens cleaning into the maintenance procedure, with the approved method named.
Related reading
- Why Thermal Cameras Live at 8–14 µm: The LWIR Window
- Thermal Lens Focal Length on Drones: Wide vs Narrow, Range vs Coverage
- Thermal Pixel Pitch: Why 12 µm Replaced 17 µm on Drone Payloads
- Gimbal Camera Maintenance: Keeping a Stabilized Payload Healthy
- How to Read a Thermal Payload Spec Sheet (and Where the Padding Hides)
- How Uncooled VOx Microbolometers Work
- From NETD to DRI: How Thermal Detector Performance Is Actually Calculated
- Continuous Zoom Optical Blocks: What 10x to 40x Costs in Size, Weight and Light
- MV-2P — 640×512 thermal micro payload
FAQ
Why do thermal lenses look mirror-like and dark?
Germanium is opaque to visible light, and its anti-reflection coatings reflect what does interact with them. The lens is transparent only in the infrared band it serves, so a dark, slightly gold mirror finish is what a healthy thermal optic looks like.
Can I add filters or protectors over a thermal lens?
Only IR-transmissive windows designed for the 8–14 µm band. Ordinary glass or acrylic protectors are opaque in the LWIR and blind the camera completely — the symptom is a uniform grey image that looks like a dead sensor.
Why is there no aperture control on a thermal camera?
Thermal lenses are fixed-aperture, almost always f/1.0, because the detector needs all the energy the optics can collect. There is no iris to stop down; image brightness comes from gain and display mapping rather than from the optics.
Questions about the technology? Talk to our engineers — we reply within 2 business days.