Refractive index-corrected copper-cadmium halide photochromic glasses

Claim 1

A refractive index-corrected copper-cadmium halide photochromic glass having a composition, in weight percent, of about 45-56% SiO 2 , 14.5-21% B 2 O 3 , 9.0-15% Al 2 O 3 , 1.4-2.4% Li 2 O, 2-12% Na 2 O, 0-6.0% MgO, 0-3.5% BaO, 0-2.5% TiO 2 , 0-1.4% ZrO 2 , 0-1.5% La 2 O 3 , at least 8% total of refractive index-correcting oxides MgO+BaO+TiO 2 +ZrO 2 +La 2 O 3 , 0.1-0.5% Cuo, 0.3-1.5% CdO, 0.3-0.9% Cl, 0-0.6% SnO, and 0-2% F, said glass having: (a) a refractive index (n D ) in the range of about 1.52-1.54; (b) chemical durability characterized by freedom from visible surface attack following a 10-minute exposure to 10% (wt.) HCl at 25° C.; (c) a viscosity of at least 3000 poises at the liquidus temperature thereof; (d) photochromic properties which provide, in 2 mm. thickness, a transmittance not exceeding about 55% in the fully darkened state and fading of at least 12 percentage points of transmittance in a 5-minute fading interval from the fully darkened state; said glass being chemically strengthenable by sodium-for-lithium ion-exchange to provide a modulus of rupture strength of at least about 35,000 psi with an ion-exchange layer depth of at least about 3 mils, and (l) being essentially free of Ag, PbO and ZnO.