Patent ID: 8182101

Claim:
An illumination optical system comprising: an integrator configured to divide light from a light source into plural light fluxes; and a collection optical system configured to collect the plural light fluxes from the integrator toward an illumination surface, the collection optical system comprising a first optical unit having a positive optical power and a second optical unit disposed closer to the illumination surface than the first optical unit and having a positive optical power, the distance between the first optical unit and the second optical unit is the longest air interspace distance in the collection optical system, wherein a rear principal point of the collection optical system is located closer to the light source than a front principal point of the collection optical system, and wherein the following condition is satisfied: 0.8 <Fc/F 2<0.98 where Fc represents a focal length of the collection optical system, and F 2 represents a focal length of the second optical unit.