An illumination system (IL) for a lithographic apparatus comprises an array of lenses (2a-h) configured to receive a beam of radiation (B) and focus the beam of radiation into a plurality of sub-beams (4a-h), an array of reflective elements (6a-h) configured to receive the sub-beams and reflect the sub-beams so as to form an illumination beam (8), a beam splitting device (10) configured to split the illumination beam into a first portion (12) and a second portion (14) wherein the first portion is directed to be incident on a lithographic patterning device (MA), a focusing unit (22) configured to focus the second portion of the illumination beam onto a detection plane such that an image is formed at the detection plane and wherein the image is an image of the sub-beams in which the sub-beams do not overlap with each other and an array of detector elements (24) configured to measure the intensity of radiation which is incident on the detection plane.