Lithium fluoride (LiF) crystal shows the smallest refractive index among commonly used infrared optical materials. The lower refractive index allows the crystal to be used directly without an anti-reflective coating. LiF crystal exhibits high transparency in a broad wavelength range from 120nm to 7000nm. It is usually used as a prism and window for thermal imaging systems and aerospace optical systems.
High water solubility and large thermal expansion coefficient are the two disadvantages of lithium fluoride materials. Therefore, special protective measures must be taken when used in humid environments.