Integrated lens arrays or aspherical lenses are used in various projectors as important optical components to condense light from a high-brightness light source or to diffuse such light uniformly. Nowadays, such lenses for optical components are required to have high precision. In particular, microlens arrays utilized for image processing are required to have high optical precision to provide high-definition images.
Precision press-molding, wherein polishing is not necessary, has been widely adopted as a method for producing such optical lenses. In order to produce high-precision lenses by this method, it is necessary to decrease the thermal deformation of molds and to enhance transferability. Therefore, an important technical problem is to lower the deformation temperature of the glass material.
Low-melting point borosilicate crown glasses are known as glass materials having low deformation temperatures. However, since the chemical durability of low-melting point borosilicate glasses tends to decrease with lowering deformation temperature, they are not suitable for practical use in lens arrays or aspherical lenses.
In contrast, conventional glass materials such as Pyrex (registered trademark) and white plate glasses have good chemical durability. However, such glass materials have high deformation temperatures; therefore, when they are used for press molding, mold thermal deformation tends to occur, which makes it difficult to produce high-precision lenses.
Japanese Unexamined Patent Publication No. 1993-19379 discloses, as a glass for press molding, an optical glass for precision press molding comprising SiO2, B2O3, and Al2O3 as principal components, and further comprising 3% to 12% by weight of Li2O. Although this glass is described as having a deformation temperature of 560° C. or lower and allowing precision press molding at a temperature of 600° C. or lower, an improvement is desired in terms of press moldability at low temperature. This glass is also unsatisfactory in terms of chemical durability.
Japanese Unexamined Patent Publication No. 2001-89183 discloses an optical glass for press molding comprising, as an essential component, an alkali metal oxide comprising Li2O, Na2O, and K2O. Although this glass has improved press moldability at low temperature, a further improvement is desired in terms of chemical durability.