The Solar Backscatter Ultraviolet Radiometer satellite-borne ozone monitoring instrument and its radiometric calibration are described. BaSO₄ diffusers used as part of the calibration transfer optics were found to be unstable, losing up to 6.5% total reflectance at 230 nm over five years. The resulting calibration errors were corrected after the instrument was in orbit by using data gathered when a more stable aluminum diffuser was used. A small spectral irradiance disagreement between FEL and D2 lamps is described.