A supercontinuum radiation source for an alignment mark measurement system comprises: a radiation source; illumination optics; a plurality of waveguides; and collection optics. The radiation source is operable to produce a pulsed radiation beam. The illumination optics is arranged to receive the pulsed pump radiation beam and to form a plurality of pulsed sub-beams, each pulsed sub-beam comprising a portion of the pulsed radiation beam. Each of the plurality of waveguides is arranged to receive at least one of the plurality of pulsed sub-beams beam and to broaden a spectrum of that pulsed sub-beam so as to generate a supercontinuum sub-beam. The collection optics is arranged to receive the supercontinuum sub-beam from each of the plurality of waveguides and to combine them so as to form a supercontinuum radiation beam.