Magnesium Fluoride (MgF2 crystals) is used for optical elements where extreme ruggedness and durability is required. Its useful transmission range is 0.12 μm - 8.5 μm. Irradiation does not lead to color centers. It is a rugged, hard material which is resistant to thermal and mechanical shock and can be worked to the highest standards. MgF2 crystal is slightly birefringent and usually supplied with the optic axis cut perpendicular to the window faces.