Optical glass

#This invention relates to an optical glass which has optical constants of a refractive index (nd) of 1.88 or over and an Abbe number (νd) within a range from 22 to 28 and is suitable for precision press molding.

#A typical optical glass having a high refractive index is one which contains a large quantity of lead oxide. Since this type of optical glass is stable because of having high resistance to devitrification and has a low glass transition point (Tg), it has been used for precision press molding. For example, Japanese Patent Application Laid-open Publication No. 1-308843 discloses an optical glass suited for precision press molding which comprises a large quantity of lead oxide.

#Since, however, an environment in which precision press molding is performed is kept in a reducing atmosphere for preventing oxidization of the mold, reduced lead precipitates from the surface of a glass if the glass contains lead oxide and this precipitating lead is deposited on the surface of the mold with the result that difficulty arises in maintaining the precision surface of the mold. Further, lead oxide is detrimental to the environment and hence a lead-free optical glass has been desired for.

#Examples of the optical glass of the present invention will be described below. The scope of the present invention however is not limited by these examples.

#The following Tables 1 to 3 show compositions of Examples Nos. 1-13 of the optical glass of the invention as well as refractive index (nd), Abbe number (νd), glass transition point (Tg) and yield point (At) of these examples.

#For manufacturing the optical glasses of Examples Nos. 1-13, conventional raw materials such as oxides, carbonates and nitrates were weighed and mixed at a predetermined ratio to constitute the compositions shown in the tables. These raw materials were put in a platinum crucible, melted at a temperature within a range from 1100° C. to 1300° C. for two to five hours and stirred and homogenized. After lowering the temperature to a proper temperature, the melt was cast into a mold and annealed. As to Examples Nos. 1-13, colorless and transparent glasses were obtained. 3 TABLE 1 Examples 1 2 3 4 5 SiO2 20.0 21.0 21.0 24.0 18.0 B2O3 1.5 2.0 2.0 0.0 5.0 La2O3 0.0 3.0 3.0 0.0 4.0 TiO2 12.0 12.0 12.0 15.0 10.0 ZrO2 6.0 6.0 6.0 8.0 6.0 Nb2O5 40.4 38.9 38.9 34.0 39.9 WO3 0.0 2.0 2.0 00 0.0 CaO 4.0 2.0 2.0 1.0 0.0 BaO 4.0 4.0 4.0 6.0 8.0 Li2O 9.0 9.0 6.0 10.0 9.0 Na2O 0.0 0.0 0.0 0.0 0.0 K2O 0.0 0.0 3.0 0.0 0.0 Sb2O3 0.1 0.1 0.1 0.0 0.1 GeO2 3.0 0.0 0.0 2.0 0.0 Total 100.0 100.0 100.0 100.0 100.0 nd 1.9011 1.9011 1.8973 1.9091 1.9191 νd 25.2 25.2 24.4 25.9 24.0 Tg 539 540 552 542 521 At 589 589 601 572 572