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Custom Germanium

Germanium (Ge) is a crystalline material with the highest refractive index (4.0026 at 11µm) of the commonly used infrared materials. This high refractive index means that uncoated germanium can act as a natural 50% beamsplitter between 2 and 14µm. It also has a comparatively high hardness and density, making it an ideal substrate in optical systems requiring a more robust infrared optical component.
Below is a brief summary of Germanium’s optical properties, a full data sheet is available here.

Transmission Range

1.8 to 23µm

Refractive Index

4.002 @ 11µm


5.33 g/cm3

Melting Point



Knoop 780

Operating Temperature

< 100°C


Knight Optical supplies high quality germanium components including, but not limited to, windows, lenses, aspherics and prisms made to your custom specification. We can offer uncoated and coated optics, including antireflective (AR) and diamond-like carbon (DLC) coatings. Our typical manufacturing specifications are listed below however we are always expanding our capabilities so please do not hesitate to contact our technical sales team who will guide you through your enquiry.



5mm to 300mm +

Form error (@633nm)

< 0.5 waves


< 1 arc minute


< 60/40

Typical coatings

AR/AR — T(avg) > 85% @ 3-12µm

AR/DLC — T(avg) > 85% @ 7-14µm


Uncoated, 1mm thick

AR/AR @ 3-12µm

AR/DLC @ 7-14µm

With our in-house state-of-the-art metrology laboratory you can be confident that your optical components will meet your requirements. You can find out more about our testing facilities here.

Additional Information

  • Spherical germanium lenses have a low optical dispersion, due to its high refractive index, and therefore display minimal chromatic aberration.
  • Aspheric germanium lenses offer diffraction-limited performance, their design eliminating spherical aberration and can therefore greatly simplify an optical system by removing the need for multiple elements.
  • Germanium is often used as a substrate for infrared longpass filters as it naturally blocks the UV and visible wavelengths. It is a semiconducting material and therefore has an abrupt cut-on (intrinsic edge) at 1.7µm.
  • Due to germanium’s naturally high reflectivity, AR coatings are usually applied to increase transmission, reaching over 85%. A DLC coating can be applied to an exposed surface, increasing durability and transmission.


  • Germanium has a high density which needs to be taken into account if the optical system is weight sensitive, silicon has almost half the density of germanium so could present a better option if weight is an issue.
  • It is also subject to thermal runaway – as the temperature increases, germanium’s absorption also increases meaning its transmission is reduced, at 100°C it is almost opaque and by 200°C it does not transmit at all. Therefore it should be used in systems working below 100°C to reduce the risk of optical failure
  • Germanium dust is hazardous if inhaled, so care should be taken when handling

Typical Applications

FLIR and Thermal Imaging
  • Germanium transmits in both the MWIR and LWIR wavebands making it ideal for thermal imaging systems both as protective windows and/or lenses within the systems.
Low powered CO2 Lasers
  • Germanium transmits at 10.6µm and therefore works in a CO2 laser systems, however it cannot withstand the higher powered lasers, zinc selenide is more commonly used here.
Fourier Transform Infrared (FTIR) Spectroscopy
  • Germanium is often the material of choice for attenuated total reflection (ATR) prisms used in FTIR spectroscopy. It is especially used when the sample being measured has a high refractive index.
  • Due to germaniums comparative robustness, especially when a DLC coating has been applied to the exposed surface, it is commonly used as a protective window in pyrometers.

Our 2022/2023 Catalogue

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