Lasers from 8 national metrological institutes (NMIs) were brought to the NIM in Bejing in November 2004 for the APMP.L-K11 regional comparison. The frequency difference between the f components of the R(127) 11-5 transition was measured for these lasers following the Protocol for BIPM.L-K11. The absolute frequencies of three of the participating lasers have been measured with the comb technique either before or after the comparison. The absolute frequencies for the other lasers were then deduced using the three lasers as a link. The results of these measurements are compiled in the present paper. The comparison reports, as communicated by each participant, are included as Appendices. The K11 concerns those wavelengths present in the list of recommended radiations in the MeP, which are used in the field of dimensional metrology. Typical examples would be the 633 nm, 612 nm, 543 nm and 532 nm iodine-stabilized standards but others may also become appropriate to include. The CCL also proposed to include absolute frequency measurements, matrix measurements as well as direct frequency heterodyne measurements in which only the difference in frequency between two standards is measured. Besides being a key comparison, K11 will not only provide reduced uncertainties for the frequencies listed in the MeP but also extend the ways in which participants can claim traceability to the definition of the metre to comply with the MRA and the related ISO/IEC 17025 [4]. In connection to the ongoing BIPM.L-K11 comparison a RMO comparison APMP.L-K11 was organized in NIM in Beijing in October 2004. 8 national or regional labs took part in the measurement campaign. These institutes are listed in Table 1. Measurements The measurements carried out are compatible with the protocol of K11. For these laser standards, all working at 633 nm, the f-component of the 127(R) 11-5 transition in iodine was measured being the reference component recommended in the MeP.
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