The buv instrument design as originally conceived in the late 1960s provided a direct measure of atmospheric reflectance by observing incident solar flux as well as backscattered Earth radiance. The key element of the design was, and remains, the solar diffuser. This rather elegant design proved to be anything but simple, as instrument builders and analysts spent the next 20 years learning to cope with optical degradation of the diffusers. Scene-based “soft calibration,” where measured radiances are compared to those modeled in radiative transfer calculations, was born of necessity when solar calibrations failed to provide sufficient accuracy for ozone retrievals. Flight calibration systems have improved in more recent instruments, but scene-based methods continue to be important for refining pre-launch calibration and addressing instrument problems that standard calibrations cannot. Newer data products also rely more heavily on spectroscopy, making them less sensitive to the types of calibration errors the buv approach was intended for. There has been a design trend toward eliminating the expense of solar measurements in favor of soft calibration and calibration insensitive retrievals. But the buv technique, when properly applied, still provides unique calibration capabilities over climate time scales that are difficult to duplicate by other means.