An illumination system for a lithographic or inspection apparatus. A plurality of optical waveguides transmit radiation from the illumination source to an output. A switching system enables selective control of one or more subsets of the optical waveguides. An inspection method uses an illumination system and inspection and lithographic apparatuses comprise an illumination system. In one example, the optical waveguides and switching system are replaced by a plurality of parallel optical bandpass filter elements. The optical bandpass filter elements each only transmit a predetermined wavelength or a band of wavelengths of radiation. At least two of the parallel optical bandpass filter elements each being operable to transmit a different wavelength or band of wavelengths.