Glass for press molding, and lens

#The present invention relates to a glass for press molding, which contains substantially no lead as a component and which has a low deformation point and a high refractive index to a light having a wavelength of 0.8 μm, and a lens prepared by press molding such a glass.

#The material of optical glass to be used for an instrument utilizing light, such as a lens, is desired to have characteristics such that it has a refractive index as high as possible and a deformation point as low as possible. If the material has a high refractive index, the optical design can advantageously be made, and if the deformation point is low, a lens can be produced at a low cost by press molding. Here, the deformation point is the temperature at which expansion of glass along with the temperature rise stops and shrinkage starts, and it substantially corresponds to the temperature at which glass becomes moldable. In recent years, in addition to these characteristics, the material is desired to be composed of components containing substantially no lead with a view to facilitating e.g. disposal of waste materials.

#However, many of conventional glasses for press molding have been developed on the premise that they will be used mainly in a visible light range, and no glass has been found which satisfies three conditions i.e. a no lead content, a low deformation point and a high refractive index to a light having a long wavelength, particularly a wavelength at a level of from 0.8 to 1.8 μm. Among conventional glass materials for press-molded lenses, there have been some which have reached to a level of a deformation point of 543° C. and a refractive index of about 1.88 to a wavelength of about 0.8 pm, with no lead content. However, there has been no glass material which satisfies both a refractive index higher than this and a deformation point lower than this on the premise that it contains no lead.

#The glass for press molding of the present invention contains substantially no lead. Here, “contains substantially no” means the content is not higher than an impurity level. Specifically, 0.1 mol % (hereinafter referred to simply as %) or less, may be regarded as “contains substantially no”.

#The glass for press molding of the present invention contains from 25 to 70 mol % of Bi2O3. If the content is less than 25%, the desired high refractive index may not be obtained. It is preferably at least 30%, more preferably at least 38%. On the other hand, if it exceeds 70%, vitrification tends to be difficult, or devitrification may take place during the molding. It is preferably at most 60%, more preferably at most 55%, particularly preferably at most 48%.

#B2O3 and SiO2 are network formers, and at least either one of them is contained. If their total amount is less than 5%, vitrification tends to be difficult, or devitrification may take place during the molding. Their total amount is preferably at least 20%, more preferably at least 25%, particularly preferably at least 30%, most preferably at least 40%. If their total amount exceeds 75%, devitrification tends to take place during the molding. It is preferably at most 70%, more preferably at most 65%, particularly preferably at most 60%.