Patent ID: 8089620

Claim:
A method for measuring optical characteristics of a diffraction optical element, comprising: a filtering step of filtering light emitted from a white light source to obtain a beam in a specified wavelength range; a light entering step of allowing the beam in the specified wavelength range to enter an imaging lens composed of the diffraction optical element; a magnifying step of magnifying a spot beam as a result of convergence of the beam by the imaging lens; a projecting step of projecting the magnified spot beam on a photodetector having a plurality of pixels; a distance changing step of changing a distance between the photodetector and the imaging lens in a direction of an optical axis of the spot beam; a measuring step of measuring in-plane intensity distribution of the spot beam in a plane perpendicular to the optical axis by the photodetector; a computing step of computing a sum of light amounts of the spot beam on the basis of the in-plane intensity distribution; and a magnification changing step of changing optical magnification in the magnifying step such that the smaller the computed sum of the light amounts of the spot beam becomes, the lower the optical magnification becomes.