Disclosed is an illumination system for delivering incoherent radiation to a metrology sensor system. Also disclosed is an associated metrology system and method. The illumination system comprises a spatial filter system for selective spatial filtering of a beam of said incoherent radiation outside of a module housing of the metrology sensor system. At least one optical guide is provided for guiding the spatially filtered beam of incoherent radiation to the metrology sensor system, the at least one optical guide being such that the radiation guided has a substantially similar output angle as input angle.