Patent ID: 8203787

Claim:
An optical device comprising: a first optical member having a prism shape and a second optical member having a prism shape formed of a medium having a refractive index greater than 1; and a multiple layer film section which is provided between an optical surface of the first optical member and an optical surface of the second optical member, wherein the multiple layer film section has a structure in which, at least two types of thin film layers namely a first thin film layer, and a second thin film layer which has a refractive index higher than a refractive index of the first thin film layer, are stacked alternately, and where a refractive index of the first optical member and the second optical member is let to be n S , the refractive index of the first thin film layer is let to be n L , the refractive index of the second thin film layer is let to be n H , and an angle between a normal of a plane of the multiple layer film section and a light beam which is incident is let to be θ respectively, θ being equal to 45 degrees, then the following conditional expressions (1) and (2) are satisfied 1 ≠ n L 2 × n H 2 ( n L 2 + n H 2 ) ⁢ sin 2 ⁢ θ n S ( 1 ) 1.28 ≤ n L 2 × n H 2 ( n L 2 + n H 2 ) ⁢ sin 2 ⁢ θ n S ( 2 ) and at the multiple layer film section, out of the light incident, a light of a first wavelength band is allowed to be reflected, and a light of a second wavelength band which is shorter than the first wavelength band, and a light of a third wavelength band which is longer than the first wavelength band are allowed to pass; wherein in the first wavelength band, a ratio p/s of an integral value s of an amount of reflected light of S polarization and an integral value p of an amount of reflected light of P polarization is not less than 0.4.