Patent ID: 7393409

Claim:
A method of making CaF 2 single crystals with low scattering, small refractive index differences and few small angle grain boundaries, which are temperable at elevated temperatures, said method comprising the steps of: a) washing CaF 2 waste with an aqueous surfactant solution and then drying the CaF 2 waste to obtain a CaF 2 material, said aqueous surfactant solution comprising deionized water and a surface-active compound selected from the group consisting of non-ionic surfactants, amphoteric surfactants and cationic surfactants; b) heat treating said CaF 2 material for at least five hours at heat treatment temperatures between 1000° C. and 1250° C.; c) after the heat treating of step b), subliming said CaF 2 material at a sublimation temperature of at least 1100° C. while drawing a vacuum of at most 5*10 −4 mbar to form a vapor; d) condensing the vapor at a condensation temperature of at least 500° C. to form a condensate, said condensation temperature being at least 20° C. below said sublimation temperature; e) making a CaF 2 melt from said condensate; f) cooling the CaF 2 melt so as to grow a CaF 2 single crystal; and g) subsequently tempering the CaF 2 single crystal.