Avantier Germanium Windows 

In applications demanding robust optics, Germanium Windows prove to be invaluable. Functioning as optical windows with UV and visible light opacity and an extensive transmission range in the infrared (IR) spectrum, these windows exhibit minimal chromatic aberration thanks to low dispersion. To enhance performance, we recommend the application of an anti-reflection coating due to the high index of refraction (4.0 from 2-14μm).

Germanium Windows
Germanium Windows

Germanium (Ge) possesses notable physical properties, characterized by its heavy and hard composition, boasting a Knoop Hardness of 780—nearly double that of MgF. With a density of 5.33 g/cm3, it becomes a favored choice for applications prioritizing durability. However, caution is advised for applications with routine temperatures exceeding 100°C, as Germanium’s transmission properties experience a sharp decline with increasing temperature, a phenomenon known as thermal runaway.

Despite its sensitivity to temperature, Germanium excels in IR laser applications, particularly in scenarios necessitating resilient optics. Additionally, it serves as an effective long pass filter, transmitting only wavelengths greater than 2 µm. Germanium is nonreactive to air, water, alkalis, and many acids, displaying high scratch resistance.

Avantier Germanium Windows are manufactured in sizes ranging from 2 to 100 mm, offering coatings in the 2-14µm or 8-12 µm range (BBAR coating or DLC/BBAR) upon request. Custom designs tailored to specific applications are also available in collaboration with our clients.

Our standard windows adhere to a flatness of ½ lambda, indicating minimal deviation from an ideal flat surface. Parallelism is maintained with a deviation of less than 3 arc minutes between optical surfaces, reducing image distortion and light degradation. The surface finish is set at 20-10, with no digs, and a transmission rate exceeding 47.5% at 3.7-4.8um, though adjustments can be made to meet custom specifications.

When it comes to cleaning Germanium Windows, precaution is essential due to the hazardous nature of Germanium dust. Handling with gloves is recommended, and hands should be washed after contact. Cleaning can be performed using reagent-grade acetone, ethanol, isopropyl alcohol, or methanol along with lint-free Germanium lens wipes. Nitric acid is an alternative option, but caution is required to prevent corrosion of neighboring optics or mounts.

Germanium windows
Germanium windows

Germanium windows

For a more precise and controlled cleaning process, an alternate method involves liquid CO2 dispensed from a specialized nozzle. While this approach demands additional equipment, it effectively removes stubborn compounds and thin films. Cleaning with CO2 necessitates a controlled, moisture-free work environment.

Ideal storage conditions for Germanium optics involve maintaining temperatures between 15 and 25 °C, with humidity controlled below 30 percent. Unmounted optics should be wrapped in lens tissue when not in use.

Factory StandardGermanium  Window  with different sizes (2 mm – 100 mm)


+0/-0.05 mm


+0.01/-0.03 mm


1/2 Lambda


< 3 arc min

Polished surface finish

20-10 (no dig)


protective bevel

Transmission rate

>47.5%@3.7-4.8um or according to customer’s specifications


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