Patent ID: 8345515

Claim:
A deflection optical element that deflects an incident light coming along a first optical axis at a deflection surface and emits an emission light propagating along a second optical axis, so as to couple the emission light to either a diffraction grating or a refracting optical system, wherein the first optical axis and the second optical axis are in unparallel with each other, and either a normal line of the diffraction grating or a third optical axis of the refracting optical system intersects the first optical axis at an angle θn, serving as an acute angle formed at a first intersection thereof, the deflection optical element comprising: at least one diffracting surface; and at least one reflecting surface; wherein the angle θn, an angle θt derived as a total diffraction angle caused by the at least one diffracting surface, and an angle θa, which is formed at a second intersection of the first optical axis and the second optical axis, fulfill a conditional relationship of: 0 ≦θn≦θa<θt.