A lens formed of a high-refractivity low-dispersion glass enables the downsizing of an optical system while correcting a chromatic aberration when combined with a lens formed of a high-refractivity high-dispersion glass. It hence occupies an important place as an optical element for constituting an image-sensing system or a projection optical system such as a projector.
JP 2007-269584A discloses such a high-refractivity low-dispersion glass. The glass disclosed in JP 2007-269584A has a refractive index nd of 1.75 to 2.00 and has a Ta2O5 content in the range of 0 to 25 mass %, and all of the glasses that have a refractive index nd of at least 1.85 contain a large amount of Ta2O5. That is because the introduction of a large amount of Ta2O5 is indispensable for securing glass stability in the region of high refractivity such as a refractive index nd of 1.75 or more. For such a high-refractivity low-dispersion glass, Ta2O5 is a main and essential component.
Meanwhile, tantalum (Ta) is an element having a high rarity value and is in itself a very expensive substance. Moreover, rare metal prices are recently soaring worldwide, and the supply of tantalum is deficient. In the field of glass production, tantalum as a raw material is deficient, and if such a situation continues, it may be no longer possible to maintain the stable supply of high-refractivity low-dispersion glasses that are essential and indispensable in the industry of optical apparatuses.