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Germanium (Ge) is a crystalline material with the highest refractive index (4.0026 at 11µm) of the commonly used infrared materials. It is also comparatively hard (HK780) with a high density which is advantageous in systems where a robust infrared material is needed.

Uncoated germanium acts as a natural 50% beamsplitter between 2-14µm because of its high refractive index. Only around 45% of light is transmitted through the germanium at these wavelengths. To decrease the amount of surface reflection, and therefore increase the transmission, single sided or double sided antireflection (AR) coatings can be applied. These can bring the transmission average to above 90%. Knight Optical offers stock germanium windows with ultra-broadband AR coatings designed for use between 7-14µm and 3-12µm, covering both MWIR and LWIR wavebands. We can also provide custom coating to meet individual requirements, including diamond-like carbon (DLC) coatings to increase the durability of the germanium optic.

Germanium makes an ideal substrate for a variety of optical components:

  • Spherical germanium lenses have low optical dispersion, due to its high refractive index, and therefore display minimal chromatic aberration. Meniscus lenses can offer superior performance, over planoconvex/concave and biconvex/concave lenses, as they can be designed to produce very little spherical aberration, and be more cost effective than using multiple elements or an aspheric lens.
  • Infrared germanium aspheric lenses offer diffraction-limited performance. Their design eliminates spherical aberration, and can therefore greatly simplify an optical system by removing the need for multiple elements.
  • Germanium is often used as the substrate for infrared longpass filters as it already blocks the UV and visible regions. It is a semiconducting material and therefore has an abrupt cut-on (intrinsic edge) at 1.7µm. It can also be the substrate for IR bandpass filters.

Considerations when using Germanium optics in your system:
• It has a high density and therefore consideration needs to be taken if the optical system is weight sensitive.
• It is subject to thermal runaway – as the temperature increases, germaniums absorption increases and therefore its transmission decreases. 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.

Germanium is used in a wide range of infrared applications including:

  • In thermal imaging and forward-looking infrared (FLIR) applications due to its transmission in MWIR (3-5µm) and LWIR (8-12µm) wavebands. An AR coating needs to be applied to increase the transmission to above 90% as to not degrade the performance of the camera.
  • As a substrate for optical components used in low to medium powered CO2 laser systems due to its transmission at 10.6µm. Zinc selenide is more commonly used for high power CO2 lasers due to its low absorption at 10.6µm.
  • In Fourier Transform Infrared (FTIR) spectroscopy as the substrate for ATR crystals.
  • A protective window in pyrometers (remote thermometers).

Knight Optical has a range of stock germanium windows and ATR prisms ready to order. Our infrared germanium windows, with sizes from 10 to 100mm, have a precision flatness of lambda/10 at 10.6µm, high surface quality and different coating options to suit your needs, including an AR/AR coating working at 3-12µm and an AR/DLC coating working at 7-14µm, we also have uncoated germanium windows.  Our ATR (attenuated total reflection) prisms are designed for spectroscopic applications, germanium is an ideal substrate for ATR crystals when the sample to be measured also has a high refractive index.

Large germanium optical components are becoming more and more desirable in industries such as Oil & Gas and Aerospace, as they offer many benefits to your system. Knight Optical can offer high quality large germanium windows to meet your requirements, our stock windows are up to 100mm diameter but we can provide custom germanium with dimensions exceeding 200mm with bespoke coatings to ensure usability in your application.

Knight Optical supplies high quality germanium components made to your specific custom specification.  Every component is individually tested by our highly skilled technicians in our state of art metrology lab to ensure all components meet our high quality standards. We also hold a large range of stock not listed on our website, so please enquire.

Contact our multilingual technical sales team and discover how Knight Optical’s high quality IR Optical Filters and service can improve your instrumentation and supply chain experience.

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