Abstract: An apparatus for measuring light in samples using a high intensity light source, is presented. The system utilizes bifurcated fiber bundles to transmit light at the excitation and emission wavelength bands. It also uses a band-pass filter for eliminating extraneous light, including that which corresponds to the excitation wavelength range, while permitting the emitted light to pass to a detector for quantitation. The system employs a shutter to shield the detector while the laser light source is activated, and a controller to intermittently activate the laser light and close the shutter. The apparatus preferably includes lenses for better illumination and read out conditions. The apparatus is employed in Luminescence Oxygen Channeling Immunoassays. The method has high sensitivity, accuracy and precision, and the apparatus is highly compact. Accordingly, the analyzer can perform assays in nanoliter to microliter sample volumes in standard microplates having at least 96, 384 or 1536 wells.