Patent ID: 7021765

Claim:
An illumination optical system comprising: a light source which is a discharge gas exciting arc tube of a direct current drive type having a cathode electrode and an anode electrode, and has a light emission area being closer to the cathode electrode than the anode electrode; an optical integrator which uses a lens array to perform splitting of a luminous flux incident from the light source in a first axis direction in a two-dimensional section orthogonal to a traveling direction of the luminous flux; a polarization conversion element which includes a polarization beam splitter array, a plurality of ½ wave plates, and a mask, the polarization beam splitter array having a plurality of polarization beam splitters arranged corresponding to a plurality of predetermined lens areas in the lens array, each of the ½ wave plates rotating a polarization direction of first polarized light substantially 90 degrees out of the first and second polarized light with polarization directions orthogonal to each other split by each of the polarization beam splitters, and the mask covering a plurality of areas out of incident surfaces of the polarization beam splitter array to prevent incident of the second polarized light on each of the ½ wave plates, each area covered by the mask and each of the ½ wave plates are opposed to each other on both sides of the polarization beam splitter array.