An illumination system of a microlithographic projection exposure apparatus includes a pupil forming unit directing light on a spatial light modulator that transmits or reflects impinging light in a spatially resolved manner. An objective images a light exit surface of the spatial light modulator on light entrance facets of an optical integrator so that an image of an object area on the light exit surface completely coincides with one of the light entrance facets. The pupil forming unit and the spatial light modulator are controlled so that the object area is completely illuminated by the pupil forming unit and projection light associated with a point in the object area is at least partially and variably prevented from impinging on the one of the light entrance facets.