Patent ID: 7799714

Claim:
Optical glass comprising the following components, denoted as weight percentages: P 2 O 5 in a range of 18 to 70 weight percent, where 70 weight percent is excluded; B 2 O 3 in a range of 0 to 34 weight percent, where 0 weight percent is excluded; Al 2 O 3 in a range of 0 to 6 weight percent; Li 3 O in a range of 0 to 20 weight percent, where 0 weight percent is excluded; Na 2 O in a range of 0 to 18 weight percent; K 2 O in a range of 0 to 15 weight percent; MgO in a range of 0 to 25 weight percent; CaO in a range of 0 to 18 weight percent, where MgO+CaO>4 weight percent; SrO in a range of 0 to 20 weight percent; BaO in a range of 0 to 40 weight percent, where SrO+BaO>1 weight percent and BaO/B 2 O 3 , as a weight percentage ratio, is 0 to 15; ZnO in a range of 0 to 14 weight percent, where the ratio of ΣR″O/ΣR′O, as a weight percentage ratio, is equal to or less than 25, where ΣR″O=MgO+CaO+SrO+BaO+ZnO and ΣR′O=Li 2 O+Na 2 O+K 2 O; Gd 2 O 3 in a range of 0 to 18 weight percent; and Sb 2 O 3 in a range of 0 to 1 weight percent, wherein the content of the component contained in the largest quantity other than P 2 O 5 , B 2 O 3 , Al 2 O 3 , Li 2 O, Na 2 O, K 2 O, MgO, CaO, SrO, BaO, ZnO, Gd 2 O 3 and Sb 2 O3 is lower than the contents of both B 2 O 3 and Li 2 O, wherein said glass has an Abbé number of equal to or greater than 58 and a glass transition temperature of equal to or less than 570 degree Celsius, and has an alkalinity resistance defined as a rate of weight reduction of equal to or less than 17 micrograms/(cm 2 ·hour) when the surface of said optical glass which has been optically polished is immersed in 0.01 mole/liter of NaOH aqueous solution at 50 degree Celsius, and wherein said glass has a thermal stability such that a glass piece of said optical glass, which has been heated at a rate of 30 degree Celsius/minute to 610 degree Celsius, maintained at 610 degree Celsius for 10 minutes, and then stood to cool, comprises equal to or fewer than 10 inside crystal particles 100 nm or larger in size per cm 3 .