Patent ID: 8451443

Claim:
A diffraction optical element used for a Rowland type spectrometer, wherein: the Rowland type spectrometer detects wavelengths in a range including a wavelength λ 1 or more and a wavelength λ 2 or less (λ 1 <λ 2 ); the diffraction optical element has a diffractive efficiency which shows a maximum value at a wavelength λ a satisfying, λ 1 ≦ λ a < 7 ⁢ λ 1 + 3 ⁢ λ 2 10 ; the diffraction optical element includes a reference surface having an anamorphic shape; and the following condition is satisfied: R>r, where R indicates a curvature radius of the reference surface in a circumference direction of a Rowland circle, and r indicates a curvature radius thereof in a direction perpendicular to the Rowland circle.