Photochromic glass suitable for ophthalmic applications

Abstract

The present invention is concerned with the production of photochromic glass compositions consisting essentially, expressed in terms of weight percent on the oxide basis, as calculated from the batch, of ______________________________________ SiO.sub.2 52-59 B.sub.2 O.sub.3 18-23 Al.sub.2 O.sub.3 6-8 Li.sub.2 O 1-2.5 Na.sub.2 O 1-3 K.sub.2 O 8-13 ZrO.sub.2 2-6 TiO.sub.2 0-3 ______________________________________ and containing, as analyzed in weight percent, of ______________________________________ Ag 0.17-0.22 CuO 0.012-0.019 Cl 0.29-0.35 Br 0.06-0.12 PbO 0.1-0.15 ______________________________________ exhibiting, in 0.4 mm cross section, a clear luminous transmittance in excess of 90%, a darkened luminous transmittance below about 45% at a temperature of 25.degree. C., a darkened luminous transmittance below about 57% at a temperature of 40.degree. C., and a half fading time at both temperatures of about five minutes.

Dependencies are inferred from the claim text.