Patent ID: 7916292

Claim:
A concave diffraction grating device, comprising: a concave and aspherical reflection plane including a diffraction grating formed on a surface thereof, wherein the diffraction grating is plane symmetrical with respect to an imaginary reference plane passing a center point of the reflection plane and aligned in a dispersing direction of the diffraction grating; and has such a configuration that a projected image of the diffraction grating on an imaginary tangential plane in a direction normal to the tangential plane in contact with an imaginary reference spherical plane at the center point has a linear shape in a direction orthogonal to the reference plane, and the reflection plane has a cross section in a direction of the reference plane configured in such a manner that a curvature of one cross-sectional portion of the reflection plane with respect to the center point decreases relative to a curvature of the reference spherical plane, as a distance from the center point is increased; and that a curvature of the other cross-sectional portion of the reflection plane increases relative to the curvature of the reference spherical plane, as the distance from the center point is increased.