1. A contrast enhancement system for use in a lithographic projection apparatus arranged to project a pattern from a patterning device onto a substrate using a projection system, said system comprising: a sensor arranged to measure contrast in an aerial image projected by said projection system; and a controller responsive to contrast values measured by said sensor and connected to manipulators for adjusting one or more elements of the projection system, said controller being arranged to adjust elements of the projection system to enhance contrast in the aerial image.