Patent ID: 7935920

Claim:
An illumination optical system which introduces a luminous flux emitted from a light source to an image-forming element, said illumination optical system comprising: a optical surface having a light splitting function, wherein said illumination optical system further comprises, in order from a light source side, a first optical lens having positive optical powers in a first cross-section and a second cross-section perpendicular to each other, a second optical element having a first negative optical power in the second cross-section, and a third optical element having a second negative optical power that is different from the first negative optical power in the first cross-sections; wherein the first cross-section is parallel to a normal to the optical surface and a normal to an entrance surface of the image-forming element, and the second cross-section is perpendicular to the first cross-section and is parallel to the normal to the entrance surface of the image-forming element, and wherein a diameter of the luminous flux emerging from the third optical element in the first cross-section is smaller than a diameter of the luminous flux emerging from the second optical element in the second cross-section.