Glass with high refractive index - for welding to photochromic glass in mfg. multifocal spectacle lenses

摘要

Glass with good- chemical resistance, -meltability and -weldability with photochromic glass; with refractive index (RI) ca. 1.7; softening temp. (T1) 615-665 degrees C and dispersion coefft. (vd) 35-40 comprises by wt. 21-28% SiO2, 6-12% B2O3, 9-14% Al2O3, 40-46% PbO. The glass pref. also contains one or more of 6-9% La2O3; 0.2-0.5% Li2O; 0.3% As2O3; 2.2% ZrO2; 0.5% SrO; 4% MgO + ZnO; 1% Nb2O5 or WO3. The glass is used for welding to photochromic- or phototropic- glass to make multifocal spectacle lenses. The new glass can be made in continuous furnaces, and provides a value of vc more satisfactory than the vd of 31-32 provided by a conventional glass and producing unwanted irisation of the edges of the lenses.