Patent ID: 7878659

Claim:
An illumination optical system configured to guide a light flux from a light source unit onto an illuminated surface, the illumination optical system comprising: a compression system configured to compress the light flux emitted from the light source unit at a different compression ratio in a first cross section plane and a second cross section plane which are orthogonal to each other, and which both include an optical axis of the illumination optical system; and a polarized light splitting unit including a polarization splitting surface, wherein the second cross section plane is a plane including both the normal to the polarization splitting surface, and wherein the following conditions are satisfied: −0.18*( Fno ) 2 +1.245*( Fno )−1.260<α/β<1 1.4 ≦Fno ≦3.6, where α and β are compression ratios for compressing a light flux in the first cross section plane and in the second cross section plane respectively, and Fno is the smaller of the two F numbers calculated according to a maximum angle of a light flux incident on an illuminated surface in the first and the second cross section planes.