The spatial uniformity of two nonimaging concentrators is measured, each with a silicon photodetector positioned immediately behind the concentrator. The comparison is made between a compound parabolic concentrator (CPC) and a ?i/?o concentrator. The latter consists of a parabolic section followed by a cone section to restrict the divergence angle of the exit beam. The spatial uniformity of the ?i/?o concentrator measured with an f/4 beam diverging from a circular aperture is 1.2% across the central 4 mm of the entrance aperture of the concentrator, and demonstrates the use of this type of concentrator for effectively maintaining high throughput when using small-area (<2-mmdiam) detectors in applications requiring small fields of view. The uniformity of the gold-coated ?i/?o concentrator measured with a nearly collimated HeNe laser beam is not nearly as good, due in part to the imperfect reflectance of gold. The uniformity of the CPC in the f/4 beam is not nearly as good as that of the ?i/?o concentrator in the f/4 beam.