Prior Scientific Inc. has released LumenLED, an illumination system for fluorescence microscopy. It offers two modes of operation. In "constant light" mode, a photodiode provides a closed-loop feedback mechanism to ensure short- and long-term stability of illumination intensity, essential for quantitative experiments. For more general imaging applications, "constant current" mode assures maximum illumination intensity. The modular system is based on a two- or four-position LED combiner directly coupled to the fluorescence port of the microscope to maximize light efficiency and to avoid the light loss associated with fiber guides. The system can be mounted onto most major manufacturers' inverted or upright microscopes using the appropriate adapters. Up to 10 LED modules are available, mounted in the combiner and providing up to four LED wavelengths.