Patent ID: 7283449

Claim:
A light beam splitting element that splits a beam incident on a beam receiving surface into a plurality of beams, the beam receiving surface having: at least one predetermined area that directs a portion of the incident beam to proceed in a predetermined direction, the portion of the beam incident on the one predetermined area being insusceptible to spherical aberration; and at least another predetermined area that directs another portion of the incident beam to proceed in another predetermined direction, the another portion of the beam incident on the another predetermined area being susceptible to spherical aberration, wherein: the at least one predetermined area and the at least another predetermined area are defined in accordance with a third-order spherical aberration contained in the incident light beam, the third-order spherical aberration being expressed by a Zernike polynomial: SA =1−6ρ 2 +6ρ 4 , where, ρ denotes a normalized ray height with respect to the optical axis of the light beam splitting element, the portion of the beam susceptible to the spherical aberration corresponds to a non-extreme value of the polynomial, and the portion of the beam insusceptible to the spherical aberration corresponds to a vicinity of an extreme value of the polynomial.