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A germanium window is an optical window which is opaque to UV and visible light but has a broad transmission range in IR. It has minimal chromatic aberration due to low dispersion, and we recommend using an anti-reflection coating because of its high index of refraction (4.0 from 2 – 14μm). This reduces undesirable reflections and optimizes light transmission.
Germanium (Ge) is a heavy, hard substance, with a Knoop Hardness of 780 — nearly twice that of MgF. It is often a good choice for applications where robust optics are key. It density is 5.33 g/cm3. Its transmission properties do decrease steeply as temperature increases, a property called thermal runaway, and we do not recommend its use for applications where temperature will routinely rise above 100 C.
It is, however, well suited for IR laser applications, especially in situations where rugged optics are necessary. It can also act as a long pass filter, transmitting only wavelengths greater than 2 µm. Germanium is nonreactive to air, water, alkalis, and many acids, and highly scratch resistant.
We manufacture germanium windows in sizes from 2 to 100 mm. 2-14µm or 8-12 µm (BBAR coating or DLC /BBAR) coatings are available on request, and we can work with you to design germanium windows to perfectly fit your application.
Our factory standard windows have a flatness of ½ lambda. This number refers to the deviation from an ideal flat surface, and is measured by reference to a high quality, highly precise reference optical flat. 1 lambda flatness is generally considered to be typical grade, and the lower the number is the better the flatness.
The deviation in parallelism of these windows is less than 3 arc minutes. This number describes how parallel the two optical surfaces of the window are with respect to each other. The lower the degree, the better the parallelism. Precisely tooled windows with low parallelism like ours minimize image distortion and light degradation.
Our factory standard for surface finish is 20-10, with no digs, and the transmission rate is >47.5%@3.7-4.8um. This can be changed to meet custom specifications.
Germanium dust is hazardous, and care should be taken when handling any germanium optics. We recommend using gloves whenever possible, and washing hands after you are finished handling these optics.
Germanium lenses can be cleaned using reagent-grade acetone, ethanol, isopropyl alcohol or methanol and lint-free germanium lens wipes. Nitric acid may also be used for germanium, but care should be taken as it may corrode neighboring optics or mounts.
An alternate way to clean germanium windows is with liquid CO2 dispensed from a specialized nozzle. Although this method requires more equipment it does lead to more precise, controlled cleaning and can remove even otherwise stubborn compounds and thin films. Cleaning with C02 also requires a controlled, moisture-free work environment.
Germanium optics should ideally be stored between 15 and 25 °C, with humidity controlled to below 30 percent. When not mounted, they can be wrapped in lens tissue.
Contact us for manufacturing limit or custom specifications.