Phototropic optical glass

Claim 1

A phototropic glass having the following starting composition in weight percent: SiO 2 12.1 to 13.9 P 2 o 5 30.4 to 33.9 A1 2 o 3 22.5 to 25.7 ZrO 2 1.0 to 2.6 Na 2 O 3.0 to 7.5 K 2 o 5.3 to 10.5 CaO 3.1 to 5.0 BaO 3.1 to 7.0 SrO 0 to 0.5 PbO 0.4 to 2.5 TiO 2 0.0 to 0.6 La 2 O 3 0.00 to 2.0 Ag 2 O 0.10 to 1.0 CuO 0 to 0.1 and at least one halogen selected from the group consisting of bromine and chlorine in an amount sufficient to provide upon analysis 0.2-1.0 wt.% of bromine plus chlorine in the glass, said glass upon analysis containing 0.05-0.5 wt.% Ag calculated as Ag 2 O, the sum of the alkali earth oxides being 8.6-12.5 wt.%, the sum of the alkali oxides being 11.2-16.2 wt.%, wherein: the glass contains as the essential phototropic agent a non-crystalline separation phase rich in said silver and said halogen, said silver-rich and halogen-rich, non-crystalline phase separations having a diameter between 40 and 350 A; compared to said non-crystalline separation phase, less than 10% thereof being present in a silver halide crystalline phase; the glass shows substantially no devitrification in the range between 10 1 and 10 5 poises; the glass has a refractive index n d between 1.5225 and 1.5238, a linear thermal expansion coefficient between 20° and 300° C of from about 99 to about 105 × 10 -7 per degree C, and a sufficient chemical stability, characterized by a titration value in accordance with DIN 12,111 of less than 0.2 ml HC1, and ophthalmic pressing thereof can be made by normal production procedures for such pressings; the glass can be strengthened chemically by ion exchange in a medium containing potassium ions at a temperature below 10 14 .5 poises, whereupon alkali ions smaller than potassium diffuse out of the glass; and the glass has sufficient of said non-crystalline separation phase to possess the following phototropic properties, as determined by monochromatic transmissions at 545 nm, with excitation by an unfiltered xenon light of an intensity of 2 cal cm -2 min -1 , at a temperature of 20° C, on 2 mm thick specimens of the glass; a darkening of at least 50% transmission difference and a regeneration of the transmission after 10 minutes of at least 20% after the end of the illumination.