Method for measuring a relative displacement between a first mirror and a second mirror in a lithography system, wherein said second mirror is connected to an exposure tool of said system and said first mirror is connected to a target to be exposed by said system, wherein said first mirror is moveable relative to said second mirror, wherein said method is performed using a differential interferometer module and comprises the steps of: a) generating three coherent beams (b1, b2, b3), b) splitting said three coherent beams into three measurement beam and an associated reference beam pairs (mb1, rb1; mb2, rb2; mb3, rb3), wherein said three coherent beams are split using a unitary beam splitter, wherein said generating is performed before said splitting, the method further comprising the steps of: c) after said splitting, directing said three measurement beams to be incident on said first mirror to be reflected thereby, wherein said three measurement beams are non-coplanar, directing said three reference beams to be incident on said second mirror to be reflected thereby, wherein said three reference beams are non-coplanar, wherein said three reference beams and said three measurement beams incident on said second and first mirror respectively are all substantially parallel to each other, d) combining said three reflected measurement beams with their three associated reflected reference beams to provide three combined beams (cb1, cb2, cb3), each combined beam being a combination of one reflected measurement beam and one reflected reference beam, e) projecting each one of said combined beams onto a corresponding beam receiver (51, 52, 53), each receiver adapted for converting a combined beam into a signal representative of a change in position and/or orientation of the first mirror relative to the second mirror, wherein said differential interferometer module comprises: a beam source (31, 32, 33, 34, 35) adapted for generating said three coherent beams (b1, b2, b3); said unitary beam splitter adapted for splitting said three coherent beams into said respective pairs of measurement beams (mb1, mb2, mb3) and associated reference beams (rb1, rb2, rb3); at least one beam combiner for combining said three reflected measurement beams with their associated three reflected reference beams to said three corresponding combined beams (cb1, cb2, cb3); and three beam receivers (51, 52, 53) each for receiving said one of said three combined beams. a) generating three coherent beams (b1, b2, b3), b) splitting said three coherent beams into three measurement beam and an associated reference beam pairs (mb1, rb1; mb2, rb2; mb3, rb3), wherein said three coherent beams are split using a unitary beam splitter, wherein said generating is performed before said splitting, the method further comprising the steps of: c) after said splitting, directing said three measurement beams to be incident on said first mirror to be reflected thereby, wherein said three measurement beams are non-coplanar, directing said three reference beams to be incident on said second mirror to be reflected thereby, wherein said three reference beams are non-coplanar, wherein said three reference beams and said three measurement beams incident on said second and first mirror respectively are all substantially parallel to each other, directing said three reference beams to be incident on said second mirror to be reflected thereby, wherein said three reference beams are non-coplanar, wherein said three reference beams and said three measurement beams incident on said second and first mirror respectively are all substantially parallel to each other, d) combining said three reflected measurement beams with their three associated reflected reference beams to provide three combined beams (cb1, cb2, cb3), each combined beam being a combination of one reflected measurement beam and one reflected reference beam, e) projecting each one of said combined beams onto a corresponding beam receiver (51, 52, 53), each receiver adapted for converting a combined beam into a signal representative of a change in position and/or orientation of the first mirror relative to the second mirror, a beam source (31, 32, 33, 34, 35) adapted for generating said three coherent beams (b1, b2, b3); said unitary beam splitter adapted for splitting said three coherent beams into said respective pairs of measurement beams (mb1, mb2, mb3) and associated reference beams (rb1, rb2, rb3); at least one beam combiner for combining said three reflected measurement beams with their associated three reflected reference beams to said three corresponding combined beams (cb1, cb2, cb3); and three beam receivers (51, 52, 53) each for receiving said one of said three combined beams.