a method of making below 250 - nm uv light transmitting optical fluoride lithography crystals includes applying heat along a shortest path of conduction of a selected optical fluoride crystal , heating the optical fluoride crystal to an annealing temperature , holding the temperature of the optical fluoride crystal at the annealing temperature , and gradually cooling the optical fluoride crystal to provide a low - birefringence optical fluoride crystal for transmitting below 250 - nm uv light .