Athermalized LWIR Lenses: Holding Focus from -40°C to +80°C
A practical guide for integrators specifying infrared optics for outdoor thermal cameras and OEM cores. What thermal defocus is, how athermalization removes it without a motor, why germanium-free glass changes your supply risk, and how to pick a focal length from a DRI requirement instead of a habit.
What Thermal Defocus Actually Is
Every infrared lens is mounted in a housing, and the housing expands and contracts with temperature. The infrared glass itself also changes refractive index with temperature. An uncorrected LWIR assembly therefore shifts its focal plane as the day warms up or the season changes, and the detector ends up slightly outside the plane where the image is sharp.
The visible consequence is a soft image that looks like a focus problem: edges lose contrast, small targets blur, and the automatic image processing cannot recover detail that the optics never delivered. Because the drift is slow, it is often misdiagnosed as a detector fault or a bad NUC.
An athermalized design compensates this mechanically and optically, so the focal plane stays on the detector across the specified range. ZanVision athermalized LWIR lenses are specified for -40°C to +80°C operation, which covers heated indoor housings, desert rooftops and winter border deployments without a refocus mechanism.
Why not just use a motorized focus?
Motorized focus works, but it adds a moving part to a system that is otherwise solid-state, needs a control loop and a position reference, adds cost and power draw, and introduces a failure mode that a passive design does not have. For fixed installations that are never physically accessed after commissioning, passive athermalization is the simpler long-life answer. Motorized focus remains the right choice when the scene distance changes deliberately, for example a continuous-zoom system that refocuses as it zooms.
Germanium-Free Optics: A Supply Decision, Not Just an Optical One
Traditional LWIR lenses use germanium, which transmits well in the 8-14μm band but carries two procurement risks: price volatility driven by a concentrated supply chain, and export-control exposure that can complicate cross-border programs. Both risks land on the buyer years after the optical design is frozen.
Chalcogenide glass avoids that exposure while delivering usable transmission across the LWIR band. ZanVision athermalized lenses are germanium-free for that reason: the optical result is a stable specification, and the commercial result is a bill of materials that is easier to quote and easier to ship.
- Cost stability — pricing not tied to a single constrained raw material market.
- Compliance simplicity — fewer germanium-related export considerations on international shipments.
- Thermal stability — passive athermalization maintains focus from -40°C to +80°C.
- Measured sharpness — ZanVision athermalized LWIR lenses achieve MTF ≥0.5 at 30 lp/mm.
The Current Athermalized Lens Family
All six models share one detector format and one mechanical interface, so focal length can be changed without re-qualifying the detector side of the design. Every model below is built for 640×512 12μm uncooled LWIR detectors at F1.0 with an M34 mount.
| Focal length | Model / product page | Typical deployment |
|---|---|---|
| 9.1mm F1.0 | 9.1mm F1.0 germanium-free athermalized lens | Wide-area coverage, short-range perimeter and building monitoring |
| 13mm F1.0 | 13mm F1.0 germanium-free athermalized lens | Entry surveillance, parking and yard coverage |
| 19mm F1.0 | 19mm F1.0 germanium-free athermalized lens | Perimeter security, wide FOV day/night patrol |
| 25mm F1.0 | 25mm F1.0 germanium-free athermalized lens | General security and industrial monitoring |
| 35mm F1.0 | 35mm F1.0 germanium-free athermalized lens | Medium-range detection, road and border watch |
| 50mm F1.0 | 50mm F1.0 germanium-free athermalized lens | Long-range detection with 640×512 12μm cores |
Scope limit, stated plainly: this family covers 640×512 12μm detectors only. ZanVision does not currently offer a standalone lens for 1280×1024 detectors, and does not offer MWIR optics. If your program is HD thermal, the procurement path is an integrated module rather than a separate lens.
Choosing a Focal Length from the Requirement
Selecting focal length by habit is the most common specification error in thermal projects. The reliable method is to start from the task, not the glass.
Step 1 — State the task as a DRI range
Decide whether you need to detect that something is present, recognise it as a person or vehicle, or identify a specific individual. Under EN IEC 62676-4 these map to 3, 6 and 12 pixels on the critical dimension of the target. Naming the task first converts a vague "long range" requirement into a number.
Step 2 — Model it before you buy
Use the DRI calculator to model 9.1mm through 50mm against a 640×512 12μm detector. The calculator applies the standard's pixel criteria, so you can see exactly which focal length clears your range requirement and by how much margin.
Step 3 — Take the shortest focal length that clears the margin
A longer focal length than the task requires narrows the field of view, increases cost and size, and makes installation alignment more critical. The shortest focal length that meets the DRI requirement with margin keeps coverage wide and the bill of materials lean.
Step 4 — Confirm the environment
Match the operating range to the deployment: passive athermalization covers -40°C to +80°C, so confirm the housing, ingress rating and mounting rigidity are specified to the same standard as the optics.
Specifying Athermalized Optics?
Tell us the detector format, DRI range and operating temperature, and we will confirm which focal length clears your requirement.