Patent ID: 6844973

Claim:
A plane diffraction grating with grooves formed on a surface thereof, the plane diffraction grating being rotated about a rotational axis which is normal to the surface, and being characterized in that a profile of the grooves at a radial area is determined depending on an azimuthal position φ of the area about a rotational center defined as a foot of the rotational axis on the surface of the plane diffraction grating for maximizing a diffraction efficiency of the radial area, wherein the plane diffraction grating is a blazed type, and a blaze angle θ 0 of the grooves in an area along an original line at the azimuthal angle φ=0 which is perpendicular to the grooves is set as: θ 0 = α + β 2 , where α is an angle of an incident ray from a normal to the surface of the plane diffraction grating in the area, and β is an angle of a diffraction ray from the normal, and a blaze angle θ φ of the grooves in an area along a line at the azimuthal position is set as sin ⁢ ⁢ θ ϕ = sin ⁢ ⁢ θ 0 ⁢ 1 + tan 2 ⁢ ϕ 1 + tan 2 ⁢ ϕsin 2 ⁢ θ 0 .