Onboard calibration sources for the mid-infrared instrument (MIRI) on the James Webb space telescope

We have developed a novel light source for flat fielding and transmission monitoring of the Mid-Infrared Instrument on the JWST. The source uses a hot tungsten filament, mounted in a hemispherical, non-imaging flux concentrator. The design is compact, with the hemisphere having a diameter of 20 mm, and dissipates only 10 milliWatts of electrical power when operating. We describe the most important features of the design, and present the first measurements of its photometric performance.