For LWIR (8 to 12 µm) applications we favour germanium or chalcogenide glasses; for MWIR (3 to 5 µm) work, ZnSe, ZnS or chalcogenide substrates perform best; for SWIR and NIR, silicon, sapphire and selected oxide glasses do the job. Whatever the band, we always specify the internal-absorption coefficient or equivalent transmittance metric at operating wavelengths, alongside the maximum operating temperature, before any material gets shortlisted.
TL;DR:
- Choose germanium or chalcogenide for LWIR, ZnSe or ZnS for MWIR, and silicon, sapphire, or oxide glass for SWIR and NIR.
- Set bulk window absorption below 1 cm⁻¹, about 99% internal transmittance through 500 µm; thin film coatings can qualify below 10 cm⁻¹ at 10 µm.
- Germanium transmission falls at elevated temperatures, so hot LWIR systems need cooling or another substrate; mounts must allow differential expansion to prevent cracking.
- List wavelength, absorption, maximum operating temperature, load, mounting, and incidence angle in the RFQ; request transmission spectra, coating curves, wavefront maps, and batch traceability.
- Use single layer antireflection coatings for narrow band lasers and multilayer stacks for broader coverage; specify allowed transmission loss and require tested damage thresholds.
Table of Contents
- Comparing infrared window materials by band and performance
- Which materials actually transmit in each infrared band?
- Operating temperature, mounting and mechanical constraints
- Coatings and surface treatments for infrared windows
- Manufacturing routes, tolerances and the test data to request
- Building a spec-first RFQ for your infrared window
- How a precision manufacturer reduces risk on infrared window projects
- An engineer’s view on margin and early collaboration
- Request a quote or an RFQ checklist for your next infrared window
- FAQ
- Sources
Comparing infrared window materials by band and performance
Choosing a substrate starts with eliminating options that cannot meet the transmission or mechanical requirement, not with picking a favourite.
- Germanium offers a high refractive index (around 4.0) and strong LWIR transmission, but its absorption rises sharply with temperature, and machinability demands diamond tooling.
- Silicon suits SWIR and MWIR windows, with a phonon-related cutoff limiting deep-infrared use, and wafer-grade material is widely available at low cost.
- ZnSe and ZnS give broad MWIR transparency and are common in laser-window applications, though ZnSe is typically more expensive than ZnS for similar aperture sizes.
- Chalcogenide glasses split into moulded and crystalline forms, with moulded variants covering the atmospheric 3 to 5 µm and 8 to 14 µm windows and scaling well for mass production.
- Sapphire and oxide glasses handle SWIR and NIR duty with superior mechanical durability, making them a sound choice where impact resistance matters as much as transmission.
- Fluoride, halide and specialised ceramics such as ALON or spinel fill niche roles where chemical resistance or extreme hardness outweighs the cost penalty.
For deeper spec data on one of the most common substrates, our germanium transmission reference sets out curve data engineers can quote directly in an RFQ.
Which materials actually transmit in each infrared band?
Optics engineers generally work with four infrared bands: near-infrared (NIR, roughly 0.75 to 1.4 µm), short-wave infrared (SWIR, 1.4 to 3 µm), mid-wave infrared (MWIR, 3 to 5 µm) and long-wave infrared (LWIR, 8 to 12 µm). Band names alone do not tell you whether a material qualifies for a window; the figure that matters is the internal-absorption coefficient.
- For bulk optical windows, we apply the threshold α < 1 cm⁻¹, equivalent to roughly 99% internal transmittance at a 500 µm path length, as the qualifying criterion for window-grade material.
- For thin-film coatings, the requirement relaxes to α < 10 cm⁻¹, around 95% internal transmittance at 10 µm thickness, which opens up coating-material choices that would fail a bulk-window test.
- This distinction matters because a material rejected for a thick substrate may still work perfectly as a thin coating layer on a different host window.
Operating temperature, mounting and mechanical constraints
Thermal behaviour deserves its own line item in every spec sheet. Germanium transmission degrades significantly at elevated temperatures and designs should avoid operation at high temperatures without cooling, so LWIR designs running hot need a different substrate or active cooling. CTE mismatch between window and housing is the other common failure mode: a stiff bond across a temperature swing cracks glass long before it fails optically, so mounts should allow for differential expansion rather than clamping rigidly. Hardness and scratch resistance vary enough between substrates (sapphire sits far above chalcogenide glass, for instance) that exposed windows in harsh environments often need a sacrificial cover or a hard protective coating rather than relying on the optical substrate itself to survive abrasion.
Pro Tip: Request measured transmission data at your actual operating temperature, not just room-temperature curves, especially for germanium-based designs.
Coatings and surface treatments for infrared windows
Anti-reflective coatings should be tuned to the design wavelength and bandwidth, and the choice between a single-layer and multi-layer stack depends on how wide that bandwidth needs to be.
- Single-layer AR coatings suit narrow-band laser windows; multi-layer stacks cover broader bands but add cost and complexity.
- Environmental coatings such as hydrophobic or anti-fog layers protect against field conditions but can trade off a fraction of peak transmittance, so specify the acceptable loss explicitly.
- Laser-damage threshold data should be requested as a test output, not assumed from a datasheet, particularly for high-power MWIR or LWIR sources.
Our coatings and RFQ specification guidance sets out what to demand in terms of spectral test curves before accepting a coated window.
Manufacturing routes, tolerances and the test data to request
Manufacturing method shapes both achievable precision and cost. Crystal growth and float-zone processes produce the high-purity single-crystal blanks needed for germanium and silicon, while hot-pressing and precision moulding suit chalcogenide glasses at volume. Each route constrains available sizes and shapes differently, and that affects lead time as much as price.
- Thickness and tolerance: state both the nominal thickness and the acceptable deviation, since thin windows flex and thick ones absorb more.
- Surface flatness: specify wavefront error as peak-to-valley or RMS at a stated wavelength.
- Parallelism: tight parallelism matters for any window in an imaging path, less so for a simple protective cover.
- Surface quality: give a scratch-dig rating appropriate to the application, not a generic “optical grade” label.
- Edge finish: specify chamfer or bevel requirements, particularly for windows that will be bonded or clamped.
Ask every supplier for a transmission spectrum, an interferogram or transmitted-wavefront map, coating spectral curves and batch traceability records before sign-off. Our tolerance and interferogram reference shows the kind of RFQ-ready data that should accompany a quote.
Building a spec-first RFQ for your infrared window
A correct RFQ the first time saves weeks of back-and-forth. Before writing one, confirm the target wavelength, the required transmittance or α value, the maximum operating temperature, the mechanical load and mounting style, the incidence angle or field of view, and any laser-power constraint the window must survive.
- List material, thickness, flatness and coating requirements in that order so a supplier can price accurately on first pass.
- Name the acceptance tests you expect (transmission spectrum, interferogram, coating curve) rather than leaving them implicit.
- State delivery conditions, including packaging and any batch-traceability documentation required for regulated industries.
The most common omission we see is a missing operating-temperature figure, which forces a second quotation round once thermal performance turns out to be inadequate. A close second is specifying a coating by name only, with no spectral bandwidth or damage-threshold requirement attached. Our thermal-camera window RFQ guide walks through a complete parameter list for one common application, imaging windows from 1 to 20 µm.
How a precision manufacturer reduces risk on infrared window projects
We approach every custom infrared window as a specification problem first and a manufacturing problem second. Our capabilities span optical windows, coatings and tight-tolerance machining under quality processes, with batch-level traceability carried through from raw material to finished component. When an engineer’s RFQ is incomplete, our role is to clarify the missing parameter (usually operating temperature or coating bandwidth) before cutting any glass, and to support prototype iteration where mounting or coating trade-offs only become visible once a part is in hand.
Traceable production and documented export compliance matter most when a window ends up in a regulated defence, aerospace or medical programme.
An engineer’s view on margin and early collaboration
Build thermal margin into material selection rather than trusting a datasheet curve at room temperature, and always ask for measured transmission at your real operating point. Bring your manufacturer in during prototyping, before tolerances are frozen, to catch mounting and coating conflicts early.
— Alexandra
Request a quote or an RFQ checklist for your next infrared window
We supply custom infrared optical windows with the tight tolerances, coating options and batch traceability regulated programmes demand, built from prototype through to volume production. For mounting hardware and bespoke frames alongside your optical window, JIENCO LTD offers precision metal fabrication under ISO 9001:2015 across the UK.
| What you need | Where to start |
|---|---|
| Custom optical windows, coatings, machining | Our services |
| Lenses, filters and finished optical components | Optical components catalogue |
| RFQ-ready tolerance and test data | Optical window tolerances |
Request a quote through our services page, or use the tolerance reference above to prepare a complete RFQ before you contact us.
FAQ
What is an infrared window?
An infrared window is an optical component made from a material transparent to infrared wavelengths, used to let IR radiation pass into a sensor, camera or laser system while sealing or protecting the internals. The material and coating are chosen to match the specific band, such as MWIR or LWIR, and the operating environment.
Does infrared work through windows?
Ordinary glass blocks most mid- and long-wave infrared radiation, so standard window glass will not work for thermal imaging. Purpose-built IR windows made from materials such as germanium, ZnSe or chalcogenide glass, meeting the bulk-window transparency criterion of α < 1 cm⁻¹, are required instead.
What are IR windows made of?
Common materials include germanium and chalcogenide glasses for LWIR, zinc selenide and zinc sulfide for MWIR, and silicon, sapphire or oxide glasses for SWIR and NIR. The right choice depends on the target wavelength band, required transmittance and operating temperature.
Can infrared go through glass?
Near-infrared radiation passes through conventional silica glass to a limited extent, but mid- and long-wave infrared is largely absorbed by standard glass compositions. Specialised infrared-transparent materials, selected against a quantitative absorption threshold rather than by appearance alone, are needed for any genuine IR optical application.
Sources
- Material selection for mid-infrared thin-film coatings and windows
- Recent progress in chalcogenide glasses applicable to infrared optical elements manufactured by molding technology
- A critical review of infrared transparent oxide glasses
