Germanium single crystals are ideal for infrared spectrum applications for its broad transmission range and opacity in the visible portion of the spectrum. Germanium is commonly used in infrared (IR) thermal imaging systems operating in the 2 µm to 14 µm spectral range and covers both the LWIR and MWIR spectrum. Ge windows are relatively more rugged than other infrared (IR) materials, it is often used as protective windows for thermal cameras, or long-pass filters for wavelengths above 2000nm. Germanium also has the virtues of high chemical inertness and low chromatic aberration. But germanium would begin to become opaque in the IR spectrum when the temperature rises above 90℃, one should take caution when it is used in high-temperature applications.