Differential interferometer module adapted for measuring a displacement between a reference mirror and a measurement mirror movable with respect to said reference mirror, said differential interferometer module adapted for emitting measurement beams (Mb) and associated reference beams (Rb) towards, respectively, the measurement mirror provided on a target carrier of an exposure tool and reference mirror provided on an optical column of said exposure tool, characterised by said differential interferometer module comprising: a beam source adapted for providing three coherent beams (b1, b2, b3), a beam splitter unit adapted for splitting said three beams (b1, b2, b3) into respective pairs of measurement beams (mb1; mb2; mb3) and associated reference beams (rb1; rb2; rb3), wherein said three measurement beams are arranged to be incident on said measurement mirror and said three reference beams are arranged to be incident on said reference mirror; a beam combiner for combining a reflection of each of said measurement beams (Mb) and its associated reference beam (Rb) into a combined beam (Cb), hence creating three substantially parallel combined beams, a non-polarizing beam splitter for splitting each of said combined beams (cb1; cb2; cb3) into a first split beam and a second split beam, each split beam comprising a component of said associated reference beam and said measurement beam, a first polarizing beam splitter, for splitting each of said first split beams into a first polarized beam having a first polarization and a second polarized beam having a second polarization, a second polarizing beam splitter, for splitting each of said second split beams into a third polarized beam having a third polarization and a fourth polarized beam having a fourth polarization, three first, second, third and fourth detectors (108, 109, 110, 111) for detecting a beam energy of the said first, second, third and fourth polarized beams respectively, the detectors providing interference signals; wherein said polarizations of said first, second, third and fourth polarized beams are different polarizations. a beam source adapted for providing three coherent beams (b1, b2, b3), a beam splitter unit adapted for splitting said three beams (b1, b2, b3) into respective pairs of measurement beams (mb1; mb2; mb3) and associated reference beams (rb1; rb2; rb3), wherein said three measurement beams are arranged to be incident on said measurement mirror and said three reference beams are arranged to be incident on said reference mirror; a beam combiner for combining a reflection of each of said measurement beams (Mb) and its associated reference beam (Rb) into a combined beam (Cb), hence creating three substantially parallel combined beams, a non-polarizing beam splitter for splitting each of said combined beams (cb1; cb2; cb3) into a first split beam and a second split beam, each split beam comprising a component of said associated reference beam and said measurement beam, a first polarizing beam splitter, for splitting each of said first split beams into a first polarized beam having a first polarization and a second polarized beam having a second polarization, a second polarizing beam splitter, for splitting each of said second split beams into a third polarized beam having a third polarization and a fourth polarized beam having a fourth polarization, three first, second, third and fourth detectors (108, 109, 110, 111) for detecting a beam energy of the said first, second, third and fourth polarized beams respectively, the detectors providing interference signals; wherein said polarizations of said first, second, third and fourth polarized beams are different polarizations.