1. Field of the Invention
The present invention relates to optical glass, and more particularly, to optical glass having low melting point and which is suitable for a press lens.
2. Description of the Related Art
There has been, as shown in Japanese Patent Application Laid-open Specification Nos. 56-59641, 56-149343 and the like, a study for a press lens which can be produced by press molding a glass heated above a softening point in a metal mold without having to subsequently grind and polish glass.
When producing such kind of press lens, there has been problem regarding the melting point of optical glass employed. When the melting point is high, the metal mold used for molding can readily cause roughness of the surface by oxidation and the like and the life of the metal mold is thus shortened. Accordingly, it may be said that such optical glass having a low melting point is advantageous.
On the other hand, such press lenses are generally applied to minute parts so that the specific gravities of these presses must be as small as possible. It is preferable to decrease the thermal expansion coefficient since the contraction amount of the press lens must be small. In addition, the press lens naturally needs to have excellent weather-resistent characteristics.
As to optical glass, lenses having an intermediate index of refraction of 1.65 to 1.75 are widely available commercially. There has been a need for producing a lens having an intermediate index of refraction by press molding. In view of foregoing problems, when the conventional glass compositions are examined, SK glass and the like are difficult for press lenses since they have a higher melting point than that of SF series optical glass.
In conventional SF series glass, there have been known glass having components of SiO.sub.2, Al.sub.2 O.sub.3, Na.sub.2 O, K.sub.2 O, TiO.sub.2, PbO and As.sub.2 O.sub.3. This glass has an intermediate index of refraction of 1.75 to 1.9, specific gravity of not more than 5 g/cm.sup.3, and excellent weather-proofness. In such conventional glass, the life of the metal model can not be lengthened since many of these glasses have high melting points of not less than 440.degree. C.
In addition, in the conventional SF series, there has been no glass having an index of refraction approximately in the range of 1.65 to 1.75.