Sapphire is transparent from UV to IR (150 nm – 4.5 m). It is often used in infrared laser systems. The ordinary refractive index at 1.064 m is 1.754 and the unusual refractive index is 1.747. The high-quality artificial sapphire crystal materials have good transmittance in the 0.20-5.50 micron band. The infrared transmittance hardly varies with the temperature, and the transmittance to ultraviolet and visible light is also very high. It is widely used in infrared and far infrared military equipment. Because the hardness of sapphire is second only to that of diamond, and its structural strength is very high, the thickness of sapphire window made by enhancing the transmittance can be compared with that of general media. The quality of Windows is thinner, which is very suitable for demanding applications.