A luminometer is provided comprising a flow through waveguide and one or more detectors. The flow through waveguide has at least two openings and the sample is free to enter from one opening and exit from the other. The flow through waveguide can be made of material that guides emission light to a bottom end of the flow through waveguide. One or more detectors may be provided which detect the emission light coming out of the bottom of the flow through waveguide. A fluorometer/photometer is also provided that comprises a flow through waveguide, one or more excitation light sources, and one or more optical detectors. The flow through waveguide has a hollow region to hold the sample. The excitation light is introduced at an angle or perpendicular to one surface of the flow through waveguide. The flow through waveguide is made of material that can guide absorption and/or emission light to the bottom end of the flow through waveguide. There are one or more detectors that detect the emission light coming out of the bottom of the flow through waveguide.