Patent ID: 8717671

Claim:
A projection optical system for projecting an image on a surface to be projected, the image being an enlarged image of a formed image which is formed on an image forming element, the projection optical system comprising: a coaxial optical system having an optical axis; and a non-coaxial optical system including a rotationally asymmetric curved-surface mirror, the non-coaxial optical system not sharing the optical axis with the coaxial optical system, wherein the coaxial optical system and the non-coaxial optical system are arranged in this order on a light path from the image forming element to the surface, wherein the coaxial optical system includes, at least, a first lens having a positive refractive power and being an aspheric plastic lens; and a second lens having a negative refractive power and being an aspheric plastic lens, wherein the first lens has a first refractive index distribution along a first direction from the center of the first lens to a peripheral portion of the first lens, and the second lens has a second refractive index distribution along a second direction from the center of the second lens to a peripheral portion of the second lens, and wherein one of the first lens and the second lens is arranged at a position closest to the curved-surface mirror among the optical elements included in the coaxial optical system.