Lithographic apparatus and device manufacturing method, and device manufactured thereby for calibrating an imaging system with a sensor

A lithographic apparatus comprises an illumination system, an array of individually controllable elements, a projection system, a substrate table, and a sensor system. The illumination system supplies a beam of radiation. The array of individually controllable elements patterns the beam. The projection system projects the patterned beam onto a target plane, the patterned beam comprising an array of radiation spots. The substrate table supports a substrate, such that a target surface of the substrate is substantially coincident with the target plane. The sensor system comprises an array of detector elements arranged to receive at least one of the spots. The sensor system measures an energy of the or each received spot and provides an output signal indicative of the energy of the or each received spot.