The Lumen 200 series fluorescence illumination systems from Prior Scientific Inc. were designed to replace standard microscope HBO-type mercury lamphouses. Using a 200-W metal arc lamp with a 2000-h lifetime, they illuminate fluorescently labeled specimens. Coupled to the microscope with a 2- or 3-m liquid lightguide, they provide even illumination and eliminate the need for bulb alignment. With the lamphouse physically removed from the microscope, heat transfer is eliminated, enabling thermal stability for experiments over a long period. The series comprises the standard Lumen 200 model and the Lumen 200Pro, which features a six-position, 25-mm-diameter high-speed filter wheel as well as a high-speed, variable-aperture shutter.