Abstract: A method and apparatus for determining the fluorescence, luminescence, or absorption of a sample is provided. The sample may either be contained within a cuvette or within one or more sample wells within a multi-assay plate. A combination of a broadband source, monochromator, and a series of optical filters are used to tune the excitation wavelength to a predetermined value within a relatively wide wavelength band. A similar configuration is used to tune the detection wavelength. In one aspect, multiple optical filters are coupled to the excitation source subassembly, thus allowing the system to be quickly converted from one optical configuration to another. In another aspect, the excitation light and the detected sample emissions pass to and from an optical head assembly via a pair of optical fibers. In another aspect, an optical scanning head assembly is used that includes mirrored optics for coupling the excitation source to the sample and the emitted light to the detector.