Photochromic glass

Abstract

The instant invention is concerned with transparent photochromic glass compositions which not only exhibit desirably rapid darkening and fading characteristics, but which also demonstrate the other physical and optical properties required for use in ophthalmic applications. Such glasses utilize the presence of silver chloride and/or silver bromide crystals to impart photochromic behavior in a narrowly-defined composition range within the alkali metal aluminoborosilicate base system. Copper ions are also included in the composition to perform as a sensitizing agent. The glasses have the capability of being either thermally tempered or chemically strengthened to comply with the United States Food and Drug Administration regulations for lenses used in ophthalmic applications. Improved independence of temperature effects upon the photochromic properties can be achieved where the copper level, expressed as CuO, is maintained between about 0.004-0.02% by weight, the chloride value, expressed as Cl, is held between about 0.1-0.25% by weight, the silver content, expressed as Ag, is included between about 0.15-0.3% by weight, and the quantity of bromide, expressed as Br, is varied between about 0.1-0.2% by weight. The fading rate of the glasses is enhanced by maintaining the molar ratio of alkali metal oxides to B.sub.2 O.sub.3 between about 0.55-0.85, the molar ratio of alkali metal oxides minus Al.sub.2 O.sub.3 to B.sub.2 O.sub.3 is about 0.25-0.4, and the weight ratio Ag:(Cl+Br) is about 0.65-0.95.

Dependencies are inferred from the claim text.