1. Field of the Invention
The present invention relates to an optical glass and an optical element comprising this optical glass, more specifically an optical glass suitable for press molding, and an optical element comprising this optical glass.
2. Description of the Related Art
In recent years, what is called press molding, wherein glass heated at not lower than the yield-temperature (At) thereof is pressed with a mold, which is made of a pair of upper and lower mold parts, thereby molding the glass, has widely been used as a process for producing an optical element such as a glass lens since the press molding has a smaller number of producing steps than any conventional process of polishing glass so that the press molding makes it possible to produce the element inexpensively in a short time.
This press molding can be roughly classified into a reheating process and a direct press process. The reheating process is a process of forming a gob pre-form or polished pre-form having a shape very similar to a shape of a final product, heating the pre-form at not lower than the softening temperature thereof again, and then press molding the pre-form with a heated mold, which is made of a pair of upper and lower mold parts, thereby making the pre-form into a final product shape. On the other hand, the direct press process is a process of dropping melted glass droplets from a glass melting furnace directly onto a heated mold, and then press molding the melted glass, thereby making the glass into a final product shape. In the press molding according to any one of these processes, it is necessary that when glass is formed, a press mold is heated at a temperature near the glass transition temperature, which may be designated as the “Tg” hereinafter, or a higher temperature. For this reason, when the Tg of the glass is higher, surface oxidization of the press mold or a change in the metal composition of the mold is more easily generated so that the lifespan of the mold becomes shorter. Thus, costs for the production increase. When the molding is performed in the atmosphere of an inert gas such as nitrogen, deterioration in the mold can be restrained. However, a machine for the molding becomes complicated in order to control the atmosphere, and further running costs of the inert gas is also required. Thus, the production costs increase. Accordingly, the glass used in press molding is desirably a glass the Tg of which is as low as possible. In order to improve the devitrification resistance of the glass, the liquid phase temperature thereof, which may be designated as the “TL” hereinafter, is desirably low in the same manner as the Tg.
In recent years, however, people have begun to fear that lead compounds which have been conventionally used to make the Tg low produce a bad effect onto the human bodies. Therefore, it has been intensely desired in the market not to use any lead compound. Thus, various techniques have been investigated and suggested for making the Tg and the TL of glass low without using any lead compound (see, for example, EP 552932A, Japanese Published Unexamined Patent Application No. 6-107425, and Japanese Published Unexamined Patent Application No. 2003-176151.
However, the optical glass in the EP 552932A has a larger linear thermal expansion coefficient; thus, the glass is easily cracked in the step of cooling the glass when the glass is formed. Furthermore, the viscosity is too low so that striae are easily generated in the glass. About the optical glasses of the Japanese Published Unexamined Patent Application Nos. 6-107425 and 2003-176151, the Tg thereof is not sufficiently low, and the TL is not low yet, so as to result in a problem in devitrification resistance.