Patent ID: 7815340

Claim:
An optical element, comprising a mirror surface configured to transmit a part of an incident light beam therethrough and to reflect the other part of the incident light beam, wherein a target cutoff wavelength, which indicates a boundary wavelength between a wavelength band of a desired transmission light beam and a wavelength band of a desired reflection light beam, is defined, the mirror surface includes at least a first region and a second region, each of the first region and the second region includes a reference position at which a cutoff wavelength reference value, which defines whether a light beam having a reference incidence angle should be transmitted or reflected, indicates the target cutoff wavelength, and in each of the first region and the second region, the cutoff wavelength reference value at a certain position is set to be a shorter wavelength or a longer wavelength than the target cutoff wavelength, based on a distance from the reference position to the certain position.