Eliminating the middleman: ultraviolet scale realization using a laser-driven plasma light source.

After we replaced the argon mini-arc with a laser-driven light source in the Ultraviolet Spectral Comparator Facility at the National Institute of Standards and Technology (NIST), we realized that the optical power should be sufficient to use the comparator system for absolute-cryogenic radiometry. Calibrating working standard detectors directly against an absolute-cryogenic radiometer in the system used for calibrations would eliminate all uncertainties resulting from the use of transfer standards, which were calibrated in a separate system using a different light source and optics. The transfer standards are the middlemen we refer to in the title. Any uncertainty caused by differences in bandpass, out-off-band radiation, spectral purity, collimation, or data interpolation would be removed. In the end, we successfully set up a twin system resembling the Ultraviolet Spectral Comparator Facility and used this system to perform a primary calibration of several photodiodes, based on an absolute-cryogenic radiometer. Using this system, we were able to reduce relative standard uncertainties at wavelengths below 220 nm from above 1 % (=1) to below 0.5%. We refer to this system as the Ultraviolet Scale Realization Facility or UV-SRF.