To provide an apparatus and a method used to obtain information indicative of the uniformity of a projection system of a lithographic apparatus.

An electromagnetic radiation beam is directed toward a projection system such that the radiation beam passes from a first end of the projection system to a second end of the projection system. The electromagnetic radiation beam is subsequently directed back toward the projection system such that the electromagnetic radiation beam passes from the second end of the projection system to the first end of the projection system. At least a part of the electromagnetic radiation beam is detected after the electromagnetic radiation beam has passed back through the projection system to obtain information indicative of the uniformity of the projection system.