Ge windows are available off-the-shelf with three anti-reflection coating options: 3-5μm for mid-infrared applications, 3-12μm for broadband multispectral applications, or 8-12μm for thermal imaging applications.
Due to its high index of refraction (around 4.0 from 2 – 14μm), an anti-reflection coating is recommended on these ge windows for sufficient transmission in the region of interest. Germanium is subject to thermal runaway, meaning that the transmission decreases as temperature increases.
As such, these ge windows should be used at temperatures below 100°C. Germanium’s high density (5.33 g/cm3) should be considered when designing for weight-sensitive systems. The Knoop Hardness of germanium (780) is approximately twice that of magnesium fluoride, making it ideal for infrared applications requiring rugged optics.
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