Germanium is popular for its high index of refraction (around 4.0 from 2-14µm). Due to its high index, an antireflection coating is required for sufficient transmission in the region of interest. 3-12µm for mid IR or broadband multi-spectral applications, or 8-12µm for thermal imaging applications. Germanium is subject to thermal runaway, meaning that the transmission decreases as temperature increases. As such, they 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 8-12µm coated Germanium windows are typically used in thermal imaging and FLIR applications. The Knoop Hardness of Germanium (780) is approximately twice that of Magnesium Fluoride, making it ideal for IR applications requiring rugged optics.