In the recent significant development of smaller-size and lighter-weight optical equipment, aspherical lenses have been used increasingly. The aspherical lens is advantageous in that aberration of light can readily be corrected and that the number of lenses can be decreased so as to allow reduction in size of the equipment.
For fabricating an aspherical lens or the like, a glass preform is softened by heating, which is then formed into a desired shape by precision-mold press molding. There are generally two ways of obtaining the preform: one is to cut a piece of glass out of a glass block or bar and process it into a preform, and the other is to drop a molten glass from a distal end of a nozzle so as to obtain a glass preform in the spherical form.
In order to obtain a molded product of glass by way of precision molding, it is necessary to press-mold the preform under the temperature condition near the deformation point (At). Therefore, when the preform has a higher deformation point (At), the mold coming into contact with the preform will be exposed to a higher temperature, causing the surface of the mold to suffer oxidization and corrosion. This gives rise to the need of maintenance of the mold, hindering mass production at a low cost. Accordingly, it is desired that the optical glass constituting the preform can be molded at a relatively low temperature, or, that it has a low glass transition point (Tg) and/or a low deformation point (At).
As to the glass used for a molded lens, a glass having various optical characteristics suitable for its specific use is demanded. In particular, there is an increasing demand for a glass having a high refractive index, low dispersion, and a low deformation point.
The conventional glasses satisfying the above-described optical characteristics include a barium flint glass. This not only contains PbO (lead oxide) hazardous to humans, but also poses other unfavorable problems. For example, metallic lead would be deposited on a surface of the product upon precision press molding, and a glass surface would be likely to become rough due to fusion with the mold.
In a digital camera, it is necessary to reduce the lens surface reflection as much as possible, and anti-reflection coating is used for that purpose. In order to restrict the reflectance as well as incident angle dependence and also to broaden the wavelength band, however, a considerable number of layers of coating films are required, resulting in complicated and expensive process steps.
As a way of achieving low reflectance without the coating films, it is known to form, on the surface of a lens or the like, a fine structure that is smaller in size than the wavelength of light. This may be done, for example, by nanoimprinting using a resin. A material having a low softening temperature such as a resin is relatively easy to form and shape using a microfabricated mold. However, temperature dependence of refractive index of the resin is about −1×10−4 (K), which is greater than that of the glass by two orders of magnitude. This means that for a part intended for higher image quality, the change in refractive index will affect the image quality more severely. In view of the foregoing, a glass for use in transferring a fine structure has been studied in order to achieve higher functionality of an optical part. A fine structure-transferred glass is an ultra-precision-mold press-molded product having a glass surface onto which the mold's concavo-convex pattern on the order of μm to nm has been transferred. For example, a conventional optical part may be replaced with a lens provided with such a fine structure so as to advantageously achieve a compact device with higher functionality at a reduced cost. A mold made up of Ni and P, which has conventionally been used primarily for molding a resin lens, may be used as well. As the characteristics required for the glass, it is crucially important that the deformation point is 500° C. or lower in order to restrict deterioration of the mold. Furthermore, in order to eliminate the need of a coating film on a lens, climate resistance of the glass itself is required as well.
As a glass free of PbO and having the above-described optical characteristics, a P2O5—R12O—R2O-(rare earth oxide or the like) type glass (where R1: alkali metal oxide, and R2: divalent metal oxide) has been disclosed. This optical glass has a refractive index (nd) of 1.63 to 1.67, an Abbe number (υd) of 47 to 59, and a deformation point (At) of 500° C. or lower (Patent Document 1).
There is also disclosed a P2O5—R12O—BaO—ZnO-(high-valent oxide) type optical glass. This optical glass has a refractive index (nd) of 1.52 to 1.7, and an Abbe number (υd) of 42 to 70 (Patent Document 2).
Also disclosed are a P2O5—R12O—R2O—Nb2O5 type glass, a P2O5—R12O—Nb2O5—WO3 type glass, and a P2O5—R12O—Bi2O3 type glass. These optical glasses each have a refractive index (nd) of 1.57 or greater, and a deformation point (At) of 570° C. or lower (Patent Documents 3 to 7).
There are also disclosed a P2O5—B2O3—R12O—R2O—Gd2O3 type glass and a P2O5—B2O3—R12O—BaO—ZnO type glass. These optical glasses each have a refractive index (nd) of L54 or greater, and an Abbe number (υd) of 57 or greater (Patent Documents 8 and 9).
Further disclosed is a P2O5—R12O—R2O—ZnO—Al2O3 type glass. This optical glass has a refractive index (nd) of 1.55 to 1.65, an Abbe number (υd) of 55 to 65, and a deformation point of 500° C. or lower (Patent Documents 10 to 12).
Patent Document 1: Japanese Patent Application Laid-Open No. 11-139845
Patent Document 2: Japanese Patent Application Laid-Open No. 2004-217513
Patent Document 3: Japanese Patent Application Laid-Open No. 2002-293572
Patent Document 4: Japanese Patent Application Laid-Open No. 2005-247659
Patent Document 5: Japanese Patent Application Laid-Open No. 2003-335549
Patent Document 6: Japanese Patent Application Laid-Open No. 2004-2153
Patent Document 7: Japanese Patent Application Laid-Open No. 2003-238197
Patent Document 8: Japanese Patent Application Laid-Open No. 2006-52119
Patent Document 9: WO 2003/072518
Patent Document 10: Japanese Patent Application Laid-Open No. 2004-168593
Patent Document 11: Japanese Patent Application Laid-Open No. 2004-262703
Patent Document 12: Japanese Patent Application Laid-Open No. 2005-53749