Differential interferometer system and lithographic step-and-scan apparatus provided with such a system

An interferometer system for measuring a mutual position and movement of a first and a second object in at least one direction, said system comprising, for at least one of all possible mutual directions of movement:

• a first interferometer unit, associated with the first object, provided with a first beam splitter, a first measuring reflector and a plurality of first reflectors, and
• a second interferometer unit, associated with the second object, provided with a second beam splitter, a second measuring reflector and a plurality of second reflectors, characterized in that, in operation, a measuring beam passes through both the first and the second interferometer unit and is reflected at least once by both the first measuring reflector and the second measuring reflector, in that the first and second interferometer units have the same radiation-sensitive detector, and in that a reference beam associated with the measuring beam traverses the same path as the measuring beam between the first and the second interferometer unit.