An illumination system is disclosed having a polarization member that includes first and second polarization modifiers movable into at least partial intersection with a radiation beam such that the respective polarization modifier applies a modified polarization to at least part of the beam. The illumination system further includes an array of individually controllable reflective elements positioned to receive the radiation beam after it has passed the polarization member, and a controller configured to control movement of the first and second polarization modifiers such that the first and second polarization modifiers intersect with different portions of the radiation beam.