The CCPR K6 key comparison on spectral regular transmittance was carried out in the framework of the CIPM Mutual Recognition Arrangement by 15 laboratories: IFA (Spain), KRISS (Republic of Korea), LNE-IMN (France), NIST (USA), NMC-A*STAR (Singapore), NMi (The Netherlands), NMIA (Australia), NMIJ/AIST (Japan), NPL (United Kingdom), NRC (Canada), MIKES (Finland), MSL (New Zealand), PTB (Germany), SMU (Slovakia) and VNIIOFI (Russia), with the LNE-INM acting as the central and reporting laboratory.

The aim of the comparison was restricted to checking the accuracy of the radiometric scale of spectrophotometers, using coloured glass filters with nominal transmittance of approximately 92%, 56%, 10%, 1% and 0.1%. The wavelengths of measurement were 380 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm and 1000 nm. The comparison was a star type comparison with the samples provided by the pilot laboratory and sent out to all the participants at the same time.

The report describes in detail the measurements made at the LNE-INM and summarizes the reports submitted by the participants. The comparison of the results of measurements carried out by the pilot laboratory before and after the circulation of the filters points out that the stability of most of the filters was rather poor. Moreover, for the coloured glass filters B to E, the drift was wavelength dependent. As a consequence, for many results, the uncertainty due to the drift of the filters during the comparison is the major cause of uncertainties, much larger than the measurement uncertainty of the measurements reported by the laboratories.

Measurement results from participants and their associated uncertainties were analysed in this report according to the Guidelines for CCPR Comparison Report Preparation, but with some adjustment for taking into account that transmittance measurements have unit 1. Across the spectral region (380 nm to 1000 nm) there were 8 wavelengths at which a comparison was made. These were treated entirely independently and thus the report describes 8 comparisons for each type of filter. The key comparison reference values of the comparison and the degrees of equivalence of the participants are based on weighted means with cut-off and the removal of very few values (4 values out of 600) considered as outliers.

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).