An illumination system (IL) for a lithographic apparatus comprising a polarization adjustment apparatus (15) arranged to receive linearly polarized radiation, the polarization adjustment apparatus comprising regions which are configured to rotate the polarization orientation by different amounts, a directing apparatus (6) operable to direct the radiation through one or more regions of the polarization adjustment apparatus, a controller (CN) configured to control the directing apparatus so as to control which of the one or more regions of the polarization adjustment apparatus radiation is directed through, wherein the controller is configured to limit which of the regions radiation is directed through to one or more regions which rotate the orientation of the linear polarization by substantially the same amount, and a diffuser configured to receive radiation output from the polarization adjustment apparatus and increase a range of angles at which the radiation propagates while substantially conserving the polarization state of the radiation.