This second Northern European comparison between seven lasers exhibits closer results than the ® rst comparison. A standard uncertainty of 5 kHz was obtained against 15 kHz in 1990, while the frequency average of the group of lasers was about the same with 3,9 kHz, using the BIPM4 laser as reference. A large frequency difference on a laser observed during the ® rst com parison was again observed; partial explanations of the origin of this differenc e were found. This differenc e has been excluded from the calculation of the average. Typical frequency stabiliti es using Allan standard deviations of about 1,6 10 ±11 and 1,7 10 ±12 were observed on 1 s and 100 s sampling tim es, respectively. * are the characteristics of the M 1 m irror located on th e iodin e cell sid e of the lasers. ** are th e characteristics of th e M 2 m irror located on the gain tube sid e of the lasers. * T he m odulatio n w idth is alw ays give n in M H z peak-to -peak. ** E xterna l powe r of the laser.

103-133 (2003)]. This determination was made by beat frequency method between a femtosecond laser comb and an iodine-stabilized He-Ne laser. The difference between the mean frequency of the sixth components of the standard laser and those of CCL recommended values for the same components was found to be 6.557 kHz.

The results of an international comparison of iodine-stabilized He-Ne lasers at λ 633 nm, made by beat-frequency techniques, between the CEM (Spain), the IPQ (Portugal) and the BIPM are presented. Four lasers took part in the ten-day period of measurements, and were compared once or twice a day. The frequency differences between the lasers were all less than 6 kHz and the highest standard uncertainty of the mean of the frequency differences between two lasers was 7 kHz. These results lie inside the standard uncertainty given in the mise en pratique of the definition of the metre. Parameters which affect the frequency of the lasers, such as modulation width, iodine pressure and laser power, were also investigated.

Lasers from four national metrological institutes (NMIs) were brought to the BIPM in November 2005 and May 2006 as part of the BIPM.L-K11 ongoing key comparison initiated by the Comité Consultative des Longueurs (CCL) 11 th meeting in 2003. The absolute frequency of the f component of the R(127) 11-5 transition was measured for these lasers following the Protocol for BIPM.L-K11. The results of these measurements are compiled in the present paper. The comparison reports, as communicated by each participant, are included as Appendices.

Recently, a scientific comparison of flatness measuring instruments at European National Metrology Institutes (NMIs) was performed in the framework of EURAMET. The specimen was a well-polished optical surface with a maximum measurement aperture of 150 mm in diameter. Here, we present an evaluation concept, which allows the determination of a mean flatness map taking into account different lateral resolutions of the instruments and different orientations of the specimen during measurement. We found that all measurements are in agreement with the mean flatness map within the uncertainty intervals stated by the participants. The aim of this scientific comparison is to specify an appropriate operation and evaluation procedure for future comparisons.