Fast fading photochromic glass

Claim 1

A transparent, comfort photochromic glass which, in bodies of 2 mm cross section, will exhibit the following optical properties: (a) a clear (undarkened) luminous transmittance higher than 88%; (b) a darkened luminous transmittance after 15 minutes' exposure to actinic radiation over the temperature range of 0°-25° C. higher than 35%, but less than 60%; (c) a darkened luminous transmittance after 15 minutes' exposure to actinic radiation at a temperature of 40° C. lower than 63%; and (d) a faded luminous transmittance after five minutes' removal from actinic radiation of at least 75%; said glass having a base composition consisting essentially, expressed in terms of weight percent on the oxide basis, of about ______________________________________ SiO.sub.2 46-60 MgO 0-3.5 B.sub.2 O.sub.3 16-28 CaO 0-6 Al.sub.2 O.sub.3 4-11 SrO 0-6 ZrO.sub.2 2-6 BaO 0-6 Al.sub.2 O.sub.3 + ZrO.sub.2 6-13 MgO + CaO + SrO + BaO 1-7 Li.sub.2 O 2-5 P.sub.2 O.sub.5 0-5 Na.sub.2 O 0-4 TiO.sub.2 0-3 K.sub.2 O 2.5-9 ______________________________________ and containing photochromic elements, as analyzed in weight percent, of about ______________________________________ Ag 0.13-0.18 Br 0.08-0.14 Cl 0.16-0.34 CuO 0.001-0.004 ______________________________________ said photochromic elements satisfying the ratios Br:(Cl+Br)≧0.24 and 0.25≦Ag:(Cl+Br)≦0.60 and wherein 6≦Al 2 O 3 +ZrO 2 ≦15, when the composition contains P 2 O 5 , and wherein the cationic ratio Li 2 O:Li 2 O+Na 2 O+K 2 O=0.4-0.8.