Long-term monitoring of the ultraviolet irradiance scale at the facility for irradiance calibration using synchrotrons

In 2004, the National Institute of Standards and Technology established the ultraviolet spectral irradiance scale from 200 nm to 400 nm using the calculable irradiance of the Synchrotron Ultraviolet Radiation Facility (SURF). Since the establishment of the scale, spectral irradiance calibrations of many customer lamps have been performed in direct comparison with synchrotron radiation. However, to ensure long-term stability of the scale, three check-standard deuterium lamps were calibrated using SURF III at the same time as customer lamps were being calibrated. Here, we present the results of the long-term monitoring of the scale using these check-standard lamps to ensure customer lamps are calibrated within the expanded calibration uncertainty (k = 2) of 1.2%.