| 摘要 |
PURPOSE: Disclosed is a glass substrate material for an optical filter having preferable thermal expansion property and mechanical characteristics as well as durability and ray transmittance of the substrate material itself by using a glass for an optical filter having a specified coefficient of thermal expansion. CONSTITUTION: A glass for an optical filter having 90 x10-7 to 120x10-7/ deg.C coefficient of thermal expansion at -20 deg.C to +70 deg.C is used. As for the thermal expansion characteristics, the temperature stability of the center wavelength of the band is extremely important, and a glass having a larger coefficient of thermal expansion than that of the substance constituting a film is required. The proper value is mentioned above, and a range from 95x10-7 to 115x10-7/ deg.C is preferable, and a range from 105±5x10-7/ deg.C is more preferable. The glass satisfying the requirement is fused, formed and slowly cooled (if necessary, annealed) and then lapped by a normal method and polished to obtain a product having the surface roughness ranging from more than 1.0 angstrom and <5.0 angstrom.
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