Ultraviolet radiation is utilized in many areas of health, industry, and science, such as in semiconductor lithography, non-destructive testing, UV curing, catalytic processes, environmental monitoring, atmospheric research, water purification, space-based astrophysical observations, medical diagnostics and therapy. The importance of ultraviolet radiation is rapidly increasing and correspondingly ultraviolet calibration services need to be as reliable as possible.
At present, two different types of radiometric facilities are available at NIST for absolute UV irradiance calibrations: The first derive source-based scales from high-temperature blackbodies and the second employ synchrotron radiation. After two major upgrades over more than four decades, SURF III at NIST has been converted into a dedicated primary irradiance source in the spectral region from x-rays to far-infrared with high beam stability, tunable electron energies and wide range of beam currents (approximately 11 orders of magnitude). Especially in the UV and vacuum UV regions, synchrotron radiation (SR) is the only standard source available, since these wavelength regions are beyond the range of the blackbody standard sources.
We recently constructed the Facility for Irradiance Calibration Using Synchrotrons (FICUS), where transfer light sources, such as deuterium lamps, can be calibrated against the new synchrotron-radiation-based NIST irradiance standard in the UV region from 200 nm to 400 nm. FICUS was built on beamline 3, which is a white-light beamline at SURF III. The measurement uncertainty is one of the vital indicators of the facility performance. The use of the facility has improved the current NIST UV irradiance scale to a lower relative measurement uncertainty of 1.2 % (k=2).
The long-term stability of the SR-based UV irradiance scale at FICUS is important and manifests itself in the reproducibility of the irradiance calibrations. To check our SR-based calibrations, we periodically calibrated three deuterium lamps as transfer standards simultaneously with customer's lamps. If the lamps remain temporally stable, than the repeat calibrations serve as an independent reference of the SR-based scale realization. These deuterium lamps are well suited as transfer standards in the air-UV spectral range because of their spectral continuity, high ignition reproducibility and good stability from 200 nm to 400 nm. The results of these irradiance calibrations are used in comparison with previous calibrations to ensure the accuracy, stability, and reproducibility of FICUS.