Patent ID: 7130125

Claim:
An optical filter comprising a substrate and thin films formed on the substrate, wherein: the thin films comprise an outermost layer portion and a refractive index varying portion; the outermost layer portion contacts an optical medium whose refractive index is lower than the refractive index of the substrate, and comprises an outermost low refractive index layer, a first outermost high refractive index layer, and a second outermost high refractive index layer; the outermost low refractive index layer is provided at a position in contact with the optical medium and has a refractive index that is higher than the refractive index of the optical medium; the first outermost high refractive index layer is provided at a position in contact with the outermost low refractive index layer and has a refractive index that is higher than the refractive index of the outermost low refractive index layer; the second outermost high refractive index layer is provided at a position in contact with the first outermost high refractive index layer and has a refractive index that is higher than the refractive index of the first outermost high refractive index layer; the refractive index varying portion comprises low refractive index layers whose refractive index are higher than the refractive index of the optical medium, and high refractive index layers whose refractive index are higher than the refractive index of the low refractive index layer, the low refractive index layer and the high refractive index layer are alternately stacked towards the substrate side; the refractive index varying portion is divided into a first stacked portion, a second stacked portion, and a third stacked portion; the low refractive index layer which forms the first stacked portion is provided at a position in contact with the outermost layer portion, the refractive index of the high refractive index layer which is adjacent to the outermost layer portion by interposing between the outermost layer portion and itself among the high refractive index layers which form the first stacked portion is higher than the refractive index of the second outermost high refractive index layer, the refractive indices of other the high refractive index layers which form the first stacked portion, is as high as the high refractive index layer near the substrate; the second stacked portion is provided at a position in contact with the substrate side of the first stacked portion, and the refractive indices of the high refractive index layers which form the second stacked portion, is as low as the high refractive index layer near the substrate; and the third stacked portion is provided between the first stacked portion and the second stacked portion, and the refractive index of the high refractive index layer is substantially identical to that of the high refractive index layer which has the highest refractive index among the high refractive index layers which form the first stacked portion.