An illumination system of a microlithographic projection exposure apparatus includes a beam deflection array including a number beam deflection elements, for example mirrors. Each beam deflection element is adapted to deflect an impinging light beam by a deflection angle that is variable in response to control signals. The light beams reflected from the beam deflection elements produce spots in a system pupil surface. The number of spots illuminated in the system pupil surface during an exposure process, during which a mask is imaged on a light sensitive surface, is greater than the number of beam deflection elements. This may be accomplished with the help of a beam multiplier unit that multiplies the light beams reflected from the beam deflection elements. In another embodiment the beam deflecting elements are controlled such that the irradiance distribution produced in the system pupil surface changes between two consecutive light pulses of an exposure process.