| 摘要 |
PROBLEM TO BE SOLVED: To obtain a glass preform material which has the index of refraction, Abbe's number, and glass transition temperature all of which are shown below, also has excellent devitrification resistance and light permeability, and is used for precision press forming and, to provide an optical glass suitable for the precision press forming. SOLUTION: The optical glass has the index of refraction (nd) of 1.70 to 1.75, an Abbe's number (νd) of 45.0 to 54.0, and a glass transition temperature (Tg) of 500 to 580 deg.C, and contains each component of, by mass, >5 to 15% SiO2 and 20 to <30% B2 O3 so as to satisfy >25 to 40% SiO2 +B2 O3 , >21 to <30% La2 O3 , >5 to 15% Y2 O3 , and 1 to 8% ZrO2 , 0.1 to 5% Nb2 O5 and >5 to 12% Ta2 O5 also so as to satisfy 7 to 20% ZrO2 +Nb2 O5 +Ta2 O5 , and 0 to 10% ZnO, 0 to 10% CaO, 0 to 5% SrO and 0 to 10% BaO also so as to satisfy 5 to 15% ZnO+CaO+SrO+BaO, and is not devitrified when its temperature is held at 920 deg.C for 2 hr.
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