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After the adoption of the new mise en pratique of the definition of the metre in 1992, a wide-ranging programme of laser comparisons at lambda 633 nm, the most used of the recommended wavelengths, was carried out involving national laboratories and the Bureau International des Poids et Mesures (BIPM). During the period July 1993 to September 1995, six grouped and five bilateral comparisons were made involving thirty-five national laboratories. This report will comprise six different parts. The first part, this paper, describes the general conditions for such comparisons and the results of the unpublished bilateral comparisons, and in the five following parts, the detailed results of the grouped comparisons will be presented and discussed. Except for a few lasers which exhibited some malfunction, the frequency differences between the lasers being compared lay inside the standard uncertainty (2,5 parts in 10¹¹) given in the mise en pratique, once the lasers had been adjusted to have the recommended parameter values.

This second Northern European comparison between seven lasers exhibits closer results than the first 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 first comparison was again observed; partial explanations of the origin of this difference were found. This difference has been excluded from the calculation of the average. Typical frequency stabilities using Allan standard deviations of about 1,6 × 10^-11 and 1,7 × 10^-12 were observed on 1s and 100 s sampling times, respectively.

A laser control system is described which locks the frequencies of three He-Ne lasers (543 nm, 612 nm and 633 nm) to iodine transitions using a single external cell stabilizer. The system is fully automated and is now used at the CSIRO to provide three unmodulated frequency references for the CSIRO gauge block interferometers and for a 1 m length bar interferometer. The relative uncertainty of the three references is thought to be better than 2 × 10^-10.

We have stabilized a CO₂ laser on narrow saturated absorption resonances of CO₂ detected on the transmitted beam of an external Fabry-Perot cavity in the 30 THz spectral region. By using another CO₂ laser stabilized on a close OsO₄ line whose absolute frequency is precisely known, and measuring the beat frequency 4 with a precision of 100 Hz, we determined the absolute frequency of twenty CO₂ laser lines, with uncertainties between 100 Hz and 1 kHz mainly limited by the accuracy of the OsO₄ grid. The set of measurements reveal some discrepancies with data based on measurements performed with the widely used saturated fluorescence technique; we attribute these to pressure shifts of several kilohertz under the conditions of this last method.

The spectrum of the titanium monoxide molecule has been studied by means of a sub-Doppler experiment combining an effusive beam of TiO and a cw tunable dye laser beam. In the 560 nm to 565 nm range, 260 absorption lines have been recorded through the laser-induced fluorescence technique of detection. Because the absorption profiles are narrow, a sensitive program of data acquisition and analysis is used. These wave-numbers have been measured by reference to the absorption spectrum of the iodine molecule to an absolute uncertainty of 30 MHz (0,001 cm^-1). For the 140 lines of the c1-a1 (0-0) band of ⁴⁸Ti¹⁶O which have been identified, the rms error between measured and calculated wave-numbers is just 21 MHz (0,0007 cm^-1).

Recently developed microform measurement techniques have reduced the measurement uncertainties in the geometry of Rockwell diamond indenters. It is now possible to establish standard-grade Rockwell diamond indenters characterized by high geometric uniformity, high hardness performance uniformity, interchangeability and reproducibility. By using the standard indenters under different national standard machines and a standardized testing cycle, a world-wide unified Rockwell hardness scale could be established with metrological traceability, stability and reproducibility. Geometric measurements and hardness tests in five laboratories have shown that tightly controlled indenter geometry can significantly improve the consistency of Rockwell C hardness (HRC) measurements. These results support the feasibility of establishing a world-wide unified HRC scale with an expanded uncertainty of approximately 0,2 HRC and without significant bias with respect to an ideal scale.

A ring laser goniometer was investigated in different applications of precision angle measurement, such as the calibration of optical polygons and optical encoders, external angle measurement of rotating objects and inertial angle measurement by rotation of the goniometer itself. The results agree with standard methods of angle measurement to about 0,5 µrad (0,1 ). Further improvement in accuracy is expected mainly from the reduction of noise in the mechanical and electrical setup of the measurement system.

The results are reported from the calibration, using Thompson's method, of a 10 : 1 ratio transformer which will be used for setting up scales of capacitance at the Electrotechnical Laboratory, Tsukuba. The deviation of the ratio from the nominal value of 10 : 1 was measured as -1.37 × 10^-7 for the in-phase component and -7,22 × 10^-7 for the quadrature component, with the transformer operated at a frequency of 1592 Hz with an rms applied voltage of 100 V. The uncertainties of the measurement were estimated to be 5 × 10^-9 for the in-phase component and 1,2 × 10^-8 for the quadrature component.

A comparison of the absolute diffuse reflectance factor scales of the National Research Council (NRC, Canada) and the Physikalisch-Technische Bundesanstalt (PTB, Germany) for radiance factor measurements in the d/0 geometry was carried out in 1996. The spectral range of the comparison was 360 nm to 830 nm. The transfer standards exchanged were two matte opal glasses, one of which was similar to those used in a 1981 international comparison between the NRC, the NBS, and the PTB. The current NRC/PTB agreement is within 0,15 % from 370 nm to 830 nm, which is well within the combined relative standard uncertainties for the two laboratories. These results are generally in good agreement with those of the 1981 exchange but are significantly improved for wavelengths below 420 nm, where discrepancies of up to 0,5 % were previously reported at 390 nm. The improvement at shorter wavelengths is largely due to improvements in the uniformity of the radiance of the sphere wall and out-of-band blocking of the interference filters used for wavelength separation.

The decade series 10-5-2-2-1 and 10-5-3-2-1 have been studied at the Korea Research Institute of Standards and Science (KRISS) to find designs which give minimum variances for up to forty-eight weighings. For the calibration designs used for both submultiples and multiples of the unit of mass, it was found that the variances decrease smoothly without maxima or minima as the number of weighings increases. Improved designs also produced smaller variances than those currently used to calibrate multiples of the unit of mass.

A water triple point (WTP) cell maintained for the past three years at the Instituto Nacional de Tecnología Industrial (INTI, Argentina) in a totally frozen state, was compared on three consecutive Mondays with a WTP cell from the Instituto Centroamericano de Investigación y Tecnología Industrial (ICAITI, Guatemala). The latter was prepared by letting it freeze slowly and completely over a period of one month in an ice bath at 0 °C, under the influence of the 0,01 °C temperature difference. Preparation an measuring techniques are described. Since measurements were performed using four polarizing currents (5 mA, □2 × 5 mA, 10 mA, and □2 × 10 mA), results are compared using two, three, and four currents, in order to extrapolate to zero power.

The main results were the following. By comparing the temperatures measured on the same day, the WTP temperatures realized using the two cells differed by less than ±24 µK (±5 µK when using only three currents for extrapolation). These differences do not depend on the long-term stability of the measuring system. By comparing the temperatures measured over the two-week period, the maximum temperature difference measured within a single cell was 129 µK (82 µK when using only three currents for extrapolation). These differences reflect the long-term instability of the measuring system, but should have a limited effect on precision thermometry.