A lithographic projection apparatus comprising: an illumination system comprising: beam diverging optics and an array of reflective elements, the beam diverging optics being configured to expand a radiation beam into a plurality of sub-beams, each of which is associated with a reflective element of the array of reflective elements, the array of reflective elements further being configured to direct the sub-beams in order to define an intensity distribution of the radiation beam in accordance with an illumination mode, and a polarization member positioned before the array of reflective elements in a path of the beam, the polarization member configured to provide a polarization to the radiation beam; a support structure configured to support a patterning device, the patterning device configured to pattern the radiation beam according to a desired pattern; a substrate table configured to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate; the apparatus being characterised in that the relative positions of the radiation beam, the reflective elements and the polarization member are such as to cause the radiation beam to have a first part across the cross section of the radiation beam formed by a first set of sub-beams which have a first polarization state, and to cause the radiation beam to have a second part across the cross section of the radiation beam formed by a second set of sub-beams which have a second different polarization state. an illumination system comprising: beam diverging optics and an array of reflective elements, the beam diverging optics being configured to expand a radiation beam into a plurality of sub-beams, each of which is associated with a reflective element of the array of reflective elements, the array of reflective elements further being configured to direct the sub-beams in order to define an intensity distribution of the radiation beam in accordance with an illumination mode, and a polarization member positioned before the array of reflective elements in a path of the beam, the polarization member configured to provide a polarization to the radiation beam; a support structure configured to support a patterning device, the patterning device configured to pattern the radiation beam according to a desired pattern; a substrate table configured to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate; the apparatus being characterised in that the relative positions of the radiation beam, the reflective elements and the polarization member are such as to cause the radiation beam to have a first part across the cross section of the radiation beam formed by a first set of sub-beams which have a first polarization state, and to cause the radiation beam to have a second part across the cross section of the radiation beam formed by a second set of sub-beams which have a second different polarization state. beam diverging optics and an array of reflective elements, the beam diverging optics being configured to expand a radiation beam into a plurality of sub-beams, each of which is associated with a reflective element of the array of reflective elements, the array of reflective elements further being configured to direct the sub-beams in order to define an intensity distribution of the radiation beam in accordance with an illumination mode, and a polarization member positioned before the array of reflective elements in a path of the beam, the polarization member configured to provide a polarization to the radiation beam; a support structure configured to support a patterning device, the patterning device configured to pattern the radiation beam according to a desired pattern; a substrate table configured to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate; the apparatus being characterised in that the relative positions of the radiation beam, the reflective elements and the polarization member are such as to cause the radiation beam to have a first part across the cross section of the radiation beam formed by a first set of sub-beams which have a first polarization state, and to cause the radiation beam to have a second part across the cross section of the radiation beam formed by a second set of sub-beams which have a second different polarization state.