Patent ID: 8284378

Claim:
An illumination optical system configured to illuminate a surface to be illuminated, the illumination optical system comprising: a first reflection integrator configured to form a plurality of light sources by using light from a light source; a second reflection integrator having a plurality of cylindrical reflection surfaces that possess parallel meridional line directions, the second reflection integrator being configured to form a plurality of linear light sources by using light from the first reflection integrator; a pair of flat plane mirrors that are arranged parallel to the meridional line direction on the second reflection integrator and opposite to each other so as to sandwich the plurality of linear light sources in between; a unit for changing an aperture shape of an aperture stop arranged at an exit side of the second reflection integrator in a direction perpendicular to the meridional line direction so that the aperture stop has an optical Fourier transformation relationship with the surface to be illuminated; and an adjustment unit configured to adjust an interval between the pair of flat plane mirrors as the aperture shape of the aperture stop is changed.