The invention relates to a lithography system comprising an optical column, a moveable target carrier for displacing a target such as a wafer, and a differential interferometer module, wherein the interferometer module is adapted for emitting three reference beams towards a first mirror and three measurement beams towards a second mirror for determining a displacement between said first and second mirror. In a preferred embodiment the same module is adapted for measuring a relative rotation around two perpendicular axes as well. The present invention further relates to an interferometer module and method for measuring such a displacement and rotations.