An illumination and detection apparatus for a metrology tool, and associated method. The apparatus includes an illumination arrangement operable to produce measurement illumination having a plurality of discrete wavelength bands and having a spectrum having no more than a single peak within each wavelength band. The detection arrangement includes a detection beamsplitter to split scattered radiation into a plurality of channels, each channel corresponding to a different one of the wavelength bands; and at least one detector for separate detection of each channel.