Patent ID: 8179520

Claim:
An optical element which is used for light having a wavelength not more than 250 nm, and receives a light beam at a maximum incident angle not less than 55°, wherein the optical element comprises an optical thin film in an effective aperture thereof, and a film thickness distribution of the optical thin film in the effective aperture includes a distribution in which a thickness of the optical thin in a region from an optical axis to a midpoint between the optical axis and an outermost periphery of the effective aperture is in a neighborhood of a thickness of the optical thin film on the optical axis, and a distribution in which a thickness of the optical thin film in the outermost periphery of the effective aperture is 1.10 times (inclusive) to 1.25 times (inclusive) a thickness of the optical thin film at the midpoint between the optical axis and the outermost periphery of the effective aperture.