A variety of materials are used. In the near infrared between 1 - 10 microns, lenses made from lithium fluoride ( LiF), aluminum oxide ( Al2O3) and strontium fluoride ( SrF2) have been used. In the mid and far infrared from 10 - 100 microns) germanium, magnesium fluoride ( Mgf) and other materials are used due to their transparency at these wavelengths, even though they appear opaque optically.