Direct traceability of the portable QASUME irradiance scale to the primary irradiance standard of the PTB

The portable Quality Assurance of Spectral UV Measurements in Europe (QASUME) irradiance scale has been directly compared to the primary irradiance standard (blackbody BB3200pg) of the Physikalisch-Technische Bundesanstalt (PTB) using the travelling spectroradiometer QASUME. The average blackbody spectral irradiance scale based on three different temperatures (2940 K, 2997 K and 3059 K) was on average 0.7% greater than the QASUME irradiance scale based on the average of three low-power lamps. This is fully consistent with the expanded uncertainty (k = 2) of 4% in the ultraviolet (UV) wavelength range and 2.6% above 400 nm of the QASUME irradiance scale. If these blackbody measurements are used to establish directly the portable QASUME irradiance scale, then its expanded uncertainty (k = 2) is substantially reduced to 2.5% in the UV and 2.0% above 400 nm.