An illumination system for a microlithographic projection exposure step-and-scan apparatus has a light source, a first optical raster element and a second optical raster element. The first optical raster element extends in a first pupil plane of the illumination system and is designed such that the geometrical optical flux of the system is increased perpendicular to a scan direction of the projection exposure apparatus. The second optical raster element extends in a second pupil plane of the illumination system, which is not necessarily different from the first pupil plane, and is designed such that the geometrical optical flux of the system is increased in the scan direction and perpendicular thereto. This makes it possible to improve the irradiance uniformity in a reticle plane.