The subject invention relates to a broadband optical metrology system that segregates the broadband radiation into multiple sub-bands to improve overall performance. Each sub-band includes only a fraction of the original bandwidth. The optical path--the light path that connects the illuminator, the sample and the detector--of each sub-band includes a unique sub-band optical system designed to optimize the performance over the spectral range spanned by the sub-band radiation. All of the sub-band optical systems are arranged to provide small-spot illumination at the same measurement position. Optional purging of the individual sub-band optical paths further improves performance.