The CCPR K1.b key comparison of spectral irradiance from 200 nm to 350 nm was carried out to meet the requirements of the Mutual Recognition Arrangement by five participating national metrology institutes (NMIs). The comparison was arranged as a star comparison with deuterium lamp systems (DLS) as transfer standard. PTB (Germany) piloted the comparison and, by measuring all lamps, provided a link between participants' measurements. The participants were BNM–INM (France), NIST (United States of America), NPL (United Kingdom), NRC (Canada) and PTB (Germany).

The use of multiple lamps and multiple measurements ensured sufficient redundancy that all participants were adequately represented. All participants supplied detailed reports of their measurements including full uncertainty statements. All measurement results reported by the participants could be used for the intercomparison and no measurement was subject to rejection.

The comparison data were analyzed following the Guidelines for CCPR Comparison Report Preparation. A minor adaptation of the proposed analysis method allowed a clear separation between the pilot PTB and the participant PTB. This led to a Key Comparison Reference Value (KCRV) that does not require an absolute unit realization by the pilot.

Across the spectral region (200 nm to 350 nm) there were 16 wavelengths at which a comparison was made. These wavelengths in 10 nm steps were treated entirely independently and thus the report describes 16 comparisons. For the calculation of the key comparison reference values no participant had to be excluded and the weighted mean with cut-off has been supported by all participants. The unilateral degrees of equivalence (DoE) calculated for each participant are 96% consistent with their uncertainties at the k = 2 level and fully consistent within k = 3.

These results impressively demonstrate the substantial improvements in UV radiometry during recent years and the excellent calibration capabilities of all participants. The outstanding quality of the results was greatly supported by the availability of high quality transfer standards. For the first time the stability and reproducibility of the DLS allowed a reasonable direct comparison of UV spectral irradiance calibrations.

Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.

The final report has been peer-reviewed and approved for publication by the CCPR, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).