Patent ID: 7265905

Claim:
An optical filter provided with a substrate and a thin film that is formed on the substrate, wherein the thin film has an outermost layer portion that is in contact with an optical medium whose refractive index is lower than the refractive index of the substrate, and has laminated portions having predetermined refractive profiles, and wherein the refractive index profiles are formed by laminating low refractive index layers alternatingly with high refractive index layers approaching the substrate side, and wherein the refractive indices of the low refractive index layers is higher than the refractive index of the optical medium, and the refractive indices of the high refractive index layers is relatively higher than the refractive index of the low refractive index layers, and wherein the outermost layer portion is provided with an outermost low refractive index layer, a first outermost high refractive index layer that is laminated on the outermost low refractive index layer, and a second outermost high refractive index layer that is laminated on the first outermost high refractive index layer, and wherein the refractive index of the outermost low refractive index layer is higher than the refractive index of the optical medium, the refractive index of the first outermost high refractive index layer is higher than the refractive index of the outermost low refractive index layer, and the refractive index of the second outermost high refractive index layer is higher than the refractive index of the first outermost high refractive index layer, and wherein the refractive index profile is represented by a first laminated portion that is laminated on the outermost layer portion side, a second laminated portion that is laminated on the substrate side of the first laminated portion, and a third laminated portion that is laminated on the substrate side of the second laminated portion, and wherein in the first laminated portion, the refractive indices of the high refractive index layers are higher than the second outermost high refractive index layer, and become gradually higher approaching the substrate side, in the second laminated portion, the refractive indices of the high refractive index layers are substantially equal to the highest refractive index of the high refractive index layers constituting the first laminated portion, and in the third laminated portion, the refractive indices of the high refractive index layers become gradually lower approaching the substrate side.