Patent ID: 7262890

Claim:
An observation optical system having an entrance surface, a reflecting surface, and an exit surface, at least one of the entrance surface, the reflecting surface, and the exit surface being configured as a free-formed surface, a volume holographic optical element being provided to the free-formed surface, wherein the following conditions are satisfied: −0.20<φ x/Px <0.30 −0.20<φ y/Py <0.20 where φx is a power, in a direction of an X axis, of the volume holographic optical element at a point of intersection of an axial principal ray with a surface of the volume holographic optical element, φy is a power, in a direction of a Y axis, of the volume holographic optical element at the point of intersection of the axial principal ray with the surface of the volume holographic optical element, Px is a total power, in the direction of the X axis, of the observation optical system, and Py is a total power, in the direction of the Y axis, of the observation optical system, the X axis being defined to be perpendicular to the Y axis and a Z axis, the Y axis being defined to be perpendicular to the Z axis and to lie in a plane of decentration of the observation optical system, and the Z axis being defined as the axial principal ray as entering the entrance surface of the observation optical system.