Fast fading, high refractive index photochromic glass

Claim 1

A photochromic glass having a refractive index between 1.585-1.610, an Abbe number higher than 41, a density less than 2.80 g/cm 3 , being capable of receiving an antireflection coating, and which, in a thickness of 2 mm, exhibits (a) a light transmittance in the clear state (T o ) of greater than or equal to 85%, (b) a light transmittance in the darkened state after exposure for 15 minutes to actinic radiation (TD 15 ) over the temperature range from 0°-25° C. of less than 48%, (c) a light transmittance in the darkened state after exposure for 15 minutes to actinic radiation (TD 15 ) at 25° C. of more than 20%, (d) a fading rate at ambient temperature (20°-25° C.) such that the glass demonstrates a light transmittance (TF 5 ) of at least 60% within five minutes after withdrawing from actinic radiation, (e) a difference in light transmittance in the darkened state over the temperature range from 25°-40° C. of less than 23 points of the degree of transmittance, and (f) a difference in absolute value of light transmittance in the darkened state before and after exposure at 280° C. for one hour, that exposure simulating the application of an anti-reflection coating, of less than five points of transmittance; said glass consisting essentially, in weight percent on the oxide basis, of: ______________________________________ SiO.sub.2 43-52 MgO 0-5 B.sub.2 O.sub.3 12.5-18 CaO 0-5 Al.sub.2 O.sub.3 0-3 SrO 0.8-9 ZrO.sub.2 3-8 BaO 1-9 Li.sub.2 O 1.5-3.5 K.sub.2 O 2-9 Na.sub.2 O 0.3-3 TiO.sub.2 2-8 Nb.sub.2 O.sub.5 4-9 ______________________________________ with the provisos that: < X 2 O <12 where X 2 O=Li 2 O+K 2 O 2<XO <12 where XO =MgO +CaO +SrO +BaO 12 <X 2 O+XO <20 15 <ZrO 2 +TiO 2 +Nb 2 O 5 <22 ZrO 2 +Al 2 O 3 ≦10 Nb 2 O 5 +Al 2 O 3 ≦9 0.15 <Li 2 O/X 2 O <0.40 and of photochromic elements in the following proportions expressed in percent by weight: ______________________________________ Ag 0.100-0.175 Br 0.093-0.180 Cl 0.140-0.350 CuO 0.0080-0.0300 ______________________________________ where the Ag, Cl and Br contents are as analyzed in the glass and the CuO content is as calculated from the glass batch, with the provisos that: Ag +Br >0.21 Br +Cl >0.24.