Germanium or Ge has high transmission in the range of 2 to 16 μm and is best used in mid-IR and long wave IR applications. Due to its low dispersion, it is a common choice for use in low power CO₂ laser applications. Germanium also is opaque throughout the entire visible spectrum, making it ideal for applications where only IR transmission is desired.

• Germanium is inert to air, water, alkalis and many acids, which makes it a robust, versatile material
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Germanium windows are optical windows that are completely impermeable to UV and VIS light giving them a dark, metallic appearance to the naked eye. However, when it comes to the IR range this is where germanium truly shines with an excellent, broad transmission range from 2 to 16 μm making it an ideal candidate for Mid-Wave-IR (MWIR) and Long-Wave IR (LWIR) applications. The sharp transmission cut-off before 2 µm also enables germanium to be used as a long pass filter, only transmitting wavelengths in excess of 2 µm and fully blocking everything before it.