Patent ID: 8531770

Claim:
A diffractive optical element, comprising: a first optical member including a first diffraction grating having a sawtooth cross section; and a second optical member which includes a second diffraction grating having an inverted shape of the first diffraction grating and which has a refractive index different from that of the first optical member, wherein the first and second optical members are stacked in a state in which the first and second diffraction gratings closely contact each other, and a diffraction surface of the first diffraction grating satisfies the following expression (1): 4 × Ra_s ≤ Ra_a ≤ Ra_s ·  Δ ⁢ ⁢ n d ⁢ _s   Δ ⁢ ⁢ n d  ( 1 ) where “Ra_a” represents arithmetical mean roughness (μm) of the diffraction surface, “Δn d ” represents a refractive index difference between the first and second optical members, “Ra_s” represents arithmetical mean roughness (μm) of a surface causing the largest scattering among light entering surfaces other than the diffraction surface in the diffractive optical element, and “Δn d — s” represents a refractive index difference between mediums sandwiching the surface at which the scattering is largest.