| 摘要 |
PURPOSE:To obtain a glass composition suitable for use in the manufacture of a lens having a refractive index grade which decrases continuously from the central axis to the peripheral surface, by using a prescribed mol% each of SiO2, Tl2O, ZnO, ZrO2, Al2O3 and SnO or SnO2 as principal components and by providing specified relations among the components. CONSTITUTION:This glass composition contains, by mole, 35-80% SiO2, 4-26% Tl2O, 2-26% ZnO, 0-2% ZrO2, 0-8% Al2O3 and 0-5% SnO or SnO2 as principal components and satisfies four conditions represented by the equations. The glass has resistance to fused salt in an ion exchange stage, no vitrifying substance is formed on the surface of the glass, and the glass is not crystallized. The glass is suitable for use in the manufacture of a lens having a refractive index grade and >=40 deg. angular aperture. The lens does not cause deterioration in the degree of resolution due to coloring, devitrification or heterogeneity. |
