A clam-shell luminometer that, when closed, completely encloses an assay reaction mixture-containing reaction vessel and some portion of a reaction carousel or ring. The luminometer includes first and second portions that are coupled to each other, a photomultiplier tube, and plural fiber optic bundles that are optically coupled to the photomultiplier tube. First ends of the fiber optic bundles are disposed adjacent to the reaction vessel in the second portion so that the fiber optic bundles completely surround the perimeter or periphery of the reaction vessel.