Patent ID: 8107163

Claim:
A projection optical system for magnifying an image on a primary imaging plane on a reduction side onto a secondary imaging plane on a magnification side, comprising: a first optical system configured to form an intermediate image of the primary imaging plane, the first optical system including a first lens group having a negative refractive power, a second lens group having a positive refractive power, an aperture, and a third lens group having a positive refractive power in this order in a direction from the intermediate image towards the aperture; and a second optical system including a concave reflecting surface disposed between the intermediate image and the secondary imaging plane; wherein each of surfaces of the first optical system and the second optical system has rotational symmetry with respect to a light axis that is common to all of the surfaces, a light ray traveling from the center of the primary imaging plane to the center of the secondary imaging plane intersects the light axis, is reflected off the concave reflecting surface, intersects the light axis again, and reaches the secondary imaging plane, and wherein the following conditional expressions are satisfied: 0.5<φ1/φ2<3 (1) 1 <AST/ASS< 5 (2) | AST|/L 12<1 (3) where φ 1 =refractive power of the first optical system, φ 2 =refractive power of the second optical system, AST=position at which the intermediate image is formed in the tangential plane by the first optical system, ASS=position at which the intermediate image is formed in the sagittal plane by the first optical system, and L 12 =distance between the first optical system and the second optical system on the light axis, and wherein all lenses included in the first optical system and having a positive refractive power satisfy the following conditional expression: −3 <K — rel (4) where K_rel=temperature coefficient of the index of refraction.