Showing papers in "Metrologia in 2000"

Argument for a direct realization of the quantum metrological triangle

Abstract: The quantum metrological triangle experiment, which is under development at the Bureau National de Metrologie-Laboratoire Central des Industries Electriques (BNM-LCIE), consists of applying Ohm's law directly to the quantities related to the single-electron tunnelling (SET) effect, the ac Josephson effect (JE) and the quantum Hall effect (QHE). The goal of this experiment is to test, at a significant level of uncertainty of about 1 part in 108, the coherence of the constants involved in these three quantum phenomena: the Josephson constant KJ, expected to correspond to the ratio 2e/h (where e is the elementary charge and h the Planck constant); the von Klitzing constant RK, in relation to the quantum resistance h/e2; and a new constant QX, defined here as the estimate of e. Moreover, realization of the metrological triangle experiment, combined with the experiment being developed at the NIST aimed at charging a capacitor by means of a SET pump, will give information liable to be taken into account in future adjustments of the fundamental constants without requiring any hypothesis regarding the physical phenomena involved. The combination of these two experiments should yield a new value of RK in ohms of the International System of Units (SI). The broad outlines of our experimental set-up are given, along with the expected uncertainties in both the short and long terms.

NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources

Abstract: A laser-based facility has been developed to provide high-flux, monochromatic, Lambertian radiation over the spectral range 0.2 µm to 18 µm. The facility was designed to reduce the uncertainties in a variety of radiometric applications, including irradiance and radiance responsivity calibrations. The operational characteristics of the facility are discussed and the results of detector responsivity calibrations over the spectral range 0.406 µm to 0.920 µm are presented.

Accurate determination of the spectral responsivity of silicon trap detectors between 238 nm and 1015 nm using a laser-based cryogenic radiometer

Abstract: The Physikalisch-Technische Bundesanstalt has further improved and extended its spectral responsivity scale based on the laser-operated radiation thermometry cryogenic radiometer. Five laser wavelengths in the ultraviolet (UV) (238 nm to 400 nm) and another fifteen in the visible and near-infrared (NIR) (400 nm to 1015 nm) almost cover the entire spectral range where silicon photodiodes can be used in air. The lasers employed are a Kr+ laser, an Ar+ laser with optional intra-cavity frequency-doubling, and a continuously tuneable Ti:sapphire ring laser. The total relative standard uncertainty of the spectral responsivity at the laser lines is u = 10-4 in the visible and NIR below 950 nm and u = 10-3 in the UV. With an improved model for interpolation between the laser wavelengths, a continuous scale of spectral responsivity has been established in the wavelength range 400 nm to 1015 nm.

Calibrations of filter radiometers for determination of atmospheric optical depth

Abstract: Atmospheric optical depths are determined by relating ground-based measurements of direct solar radiation to the extraterrestrial value, I0, that a filter radiometer would read outside the atmosphere. Usually I0 is determined by the Langley extrapolation technique from a high-altitude site, where clear and highly stable atmospheric conditions may be found. Alternatively, I0 can be measured in situ from a stratospheric balloon experiment. We have employed both methods and found agreement to better than 1 %. Filter radiometers tend to change over time, especially when used operationally outdoors. Absolute calibrations in the laboratory are used to monitor the radiometric stability of filter radiometers at the Physikalisch-Meteorologisches Observatorium Davos (PMOD/WRC, Switzerland). A spectral calibration facility based on a calibrated trap detector from the Physikalisch-Technische Bundesanstalt (PTB, Germany) is used to relate the filter radiometer to an accurate and long-term traceable standard. An FEL-lamp-based standard, previously used for several years, was compared with the new trap standard via a filter radiometer at four wavelengths between 368 nm and 862 nm and revealed a systematic difference of the order of 5 %. The link between radiometric and I0 calibration is the value of the extraterrestrial solar spectrum at the filter radiometer wavelengths which can be determined from these two calibrations and compared with published values.

SURF III - An Improved Storage Ring for Radiometry

Abstract: The National Institute of Standards and Technology (NIST) operates the newly upgraded Synchrotron Ultraviolet Radiation Facility (SURF III) mainly as a light source for radiometry. SURF III provides continuum radiation from the far-infrared to the soft X-ray spectral range and has its peak output in the extreme ultraviolet. SURF III is a circular-orbit, weak-focusing (single dipole magnet) storage ring, a feature which is advantageous if the synchrotron radiation output is calculated. We report the improvements achieved during a recent upgrade from SURF II to SURF III and our strategy to accurately determine the magnetic flux density, radio frequency (RF), beam current, and beam size, which are the parameters necessary to characterize the source completely.

Limitations of the Welch-Satterthwaite approximation for measurement uncertainty calculations

Abstract: The Welch-Satterthwaite approximation is used to estimate an effective degrees of freedom for a probability distribution formed from several independent normal distributions for which only estimates of the variance are known. The International Organization for Standardization (ISO) Guide to the Expression of Uncertainty in Measurement recommends its use for the calculation of expanded uncertainties (the ISO term for confidence intervals) for uncertainties formed from several distributions. Although it is well recognized that this formula is an approximation, counter-intuitive results may be obtained when a variance estimate of the dominant distribution has a small number of degrees of freedom; the calculated confidence limits are sometimes found to decrease with increases in the contributing uncertainties.

Controlling ac losses in quantum Hall effect devices

Abstract: When measured with ac at kilohertz frequencies the quantized Hall resistance (QHR) of a quantum Hall effect (QHE) device is usually found to be current- and frequency-dependent. This is a limitation on its use as a quantum impedance standard. We develop a model for the principal ac losses arising in the QHE device and show how they are responsible for the observed QHR current and frequency coefficients. We believe that losses are mainly caused by dissipative ac charging of the device along its edges. Charging is induced by the passage of the Hall current and by capacitive coupling between an edge and any nearby conductor maintained at an ac potential different to that of the edge, as for example at shield potential. The loss power is proportional to frequency and increases more rapidly than the square of the applied voltage or current. We model losses in terms of in-phase loss currents, which are a function of the amplitude of the ac charge reaching or leaving edges. The QHR frequency coefficient is zero only when the loss current for one portion of the high-potential edge and that for a corresponding portion of the low-potential edge are equal and of opposite sign. We propose a simple method for approaching that balance condition: gates are located under the device edges and their ac potentials adjusted so that the QHR current coefficient, evaluated at a constant frequency, is zero. We report measurements of the residual QHR frequency coefficients obtained after adjustment for GaAs/GaAlAs devices of two different types. For five different devices of the most favourable type, the QHR frequency coefficients do not exceed ±2 parts in 108 per kilohertz.

Calibration of displacement sensors up to 300 µm with nanometre accuracy and direct traceability to a primary standard of length

Abstract: A new class of sensor has recently appeared: nanometre sensors. These sensors are characterized by nanometre or sub-nanometre resolution and an uncertainty of a few nanometres over a range of at least several micrometres. Instruments such as capacitive or inductive sensors, laser interferometers, holographic scales, and scanning probe microscopes belong to the class of nanometre sensors. Linearity errors and drift in the mechanical and electronic system limit the accuracy of all these sensors. In order to determine these errors in a traceable way, the instrumentation described in this paper was developed. The heart of the system consists of a Fabry-Perot cavity. One mirror of this cavity generates the required displacement. A so-called slave laser is stabilized to the cavity length. The frequency of this slave laser is compared with the frequency of a primary length standard. In this way the displacement is measured with a resolution of a few picometres, a range of 300 µm and an uncertainty of about 1 nm. Experiments confirm the performance of this instrument and show typical deviations of the probe systems investigated.

Spectral irradiance measurements of tungsten lamps with filter radiometers in the spectral range 290 nm to 900 nm

Abstract: A method of measuring the absolute spectral irradiance of quartz-halogen-tungsten lamps is described, based on the known responsivity of a filter radiometer, the components of which are separately characterized. The characterization is described for the wide wavelength range essential for deriving the spectrum of a lamp, from 260 nm to 950 nm. Novel methods of interpolation and measurement are implemented for the spectral responsivity of the filter radiometer. The combined standard uncertainty of spectral irradiance measurements is less than 1.4 parts in 102 from 290 nm to 320 nm (ultraviolet B) and 4 parts in 103 from 440 nm to 900 nm (visible to near-infrared). As an example, the derived spectral irradiances of two lamps measured at the Helsinki University of Technology (HUT, Finland) are presented and compared with the measurement results of the National Institute of Standards and Technology (NIST, USA) and the Physikalisch-Technische Bundesanstalt (PTB, Germany). The comparisons indicate that the HUT spectral irradiance scale is between those of the NIST and the PTB in the wavelength range 290 nm to 900 nm. The long-term reproducibility of the spectral irradiance measurements is also presented. Over a period of two years, the reproducibility appears to be better than 1 part in 102.

Uncertainty in SI-traceable measurements of amount of substance by isotope dilution mass spectrometry

Abstract: The use of isotope dilution mass spectrometry (IDMS) to make measurements of amount of substance that are traceable to the International System of Units (SI) depends on an analysis of possible sources of uncertainty. A comprehensive application is presented of the law of propagation of uncertainty to the two-way IDMS method, which allows sensitivity coefficients to be calculated and optimum regimes identified for carrying out IDMS measurements. This has allowed the effect of deviations from "exact matching" on the uncertainty of the measurement to be evaluated for the first time. Furthermore, the relationship between traceability and uncertainty has been investigated for the two-way IDMS method, which is shown to have the potential to be a primary ratio method and to provide traceability to the SI. Our analysis shows that proper consideration of correlation in the uncertainty calculation and appropriate experimental design can reduce the requirement for highly characterized spike reference materials.

Quantum efficiency and dead time of single-photon counting photodiodes: a comparison between two measurement Techniques

Abstract: An experimental set-up based on correlated photons has been developed at the Istituto Elettrotecnico Nazionale G. Ferraris (IEN) for the absolute measurement of the quantum efficiency and dead time of single-photon counting photodiodes. The overall accuracy is improved with respect to previously reported results, by using a thin and highly transmitting non-linear crystal. For comparison, a conventional set-up has been developed at 632.8 nm with a He-Ne laser, a trap detector and calibrated neutral-density filters. Measurements at different count levels have been performed with both techniques and results are compared for the quantum efficiency in the limit of zero count, η0, and for the dead-time measurement, tD. Contributions to the final uncertainty are analysed for both techniques. The results obtained are η0 = 0.3372 (standard uncertainty u(η0) = 0.0016) and tD = 33.8 ns (u(tD) = 2.0 ns) in the case of the two-photon technique and η'0 = 0.3376 (u(η'0) = 0.0036) and t'D = 31.7 ns (u(t'D) = 5.4 ns) in the case of the conventional technique.

Preamplifier limitations on the accuracy of Johnson noise thermometers

Abstract: A detailed analysis is presented of noise and input impedance in noise thermometer preamplifiers. The analysis highlights the advantages of cascode amplifiers and the disadvantages of amplifiers employing global feedback. Feedback gives rise to two significant errors, one arising from an increase in the input-noise current, the other from the modification of the amplifier input impedance. With care, the errors arising from thermal-noise sources associated with the preamplifiers can be kept below 1 mK. However the largest error associated with the preamplifiers, typically 10 mK to 30 mK and previously unnoticed, arises from the interaction of the input capacitance and the transmission line inductance. This effect negates the common practice of matching the RC time constants in noise thermometers.

Thermocouple observations of melting and freezing plateaus for metal-carbon eutectics between the copper and palladium points

Abstract: Melting and freezing plateaus were observed with type-R thermocouples for the metal-carbon eutectics Pd-C, Ni-C, and Fe-C. For Pd-C, no apparent difference between the melting and freezing temperatures was observed at a heating/cooling rate of 3 °C/min. For Ni-C, the difference was 0.4 °C, and for Fe-C, 1.6 °C. The freezing temperature for Fe-C showed dependence on cooling rate, and the difference between the melt and the freeze decreased to 0.5 °C, though still significant, at a heating/cooling rate of 0.1 °C/min. The melting temperatures, measured with type-R thermocouples, were 1491 °C, 1329 °C, and 1153 °C, respectively, for Pd-C, Ni-C, and Fe-C eutectics, which agreed to within 1 °C with the values previously measured by radiation thermometers or the literature values. The crucibles, being made from graphite, were able to withstand heating cycles without breaking. The three metal-carbon eutectics, with melting temperatures between the copper point and the palladium point, are potentially useful practical reference points for calibration and evaluation of high-temperature thermocouples.

Uncertainty calculations for the preparation of primary gas mixtures. Part 1: Gravimetry

Abstract: A generic method for the preparation of primary gas mixtures is described. This description forms the basis for a detailed analysis of all sources of uncertainty. Expressions are derived for the combined standard uncertainty in a single weighing cycle as well as for a transferred mass of gas. The expressions are rigorous, as they take into account all aspects, including dependencies between variables introduced by the preparation process. The strengths of these dependencies are evaluated in a generalized way, in order to find out whether the rigorous expressions can be simplified without limiting their validity under practical conditions. The model can be simplified by assessing the covariance between results of weighing cycles, as it can be demonstrated that this covariance will never become significant compared with the uncertainty in the result of a weighing.

Ultra-precise thermal expansion measurements of ceramic and steel gauge blocks with an interferometric dilatometer

Abstract: Linear thermal expansion coefficients (LTECs) of two kinds of ceramic gauge block (seven in all) and steel gauge blocks (four in all) were measured in the range -10°C to 60°C with an optical heterodyne interferometric dilatometer. The dilatometer has an uncertainty and reproducibility of the order of 1 × 10-9 K-1 in LTEC measurements. LTECs of (9.229 ± 0.011) × 10-6 K-1 were obtained for four 2% Al2O3 partially stabilized zirconia (PSZ) gauge blocks; (9.381 ± 0.003) × 10-6 K-1 for three 99.9% PSZ gauge blocks; and (10.702 ± 0.064) × 10-6 K-1 for four steel gauge blocks. These results are discussed in relation to the compositions and production batches of the gauge blocks.

International comparison of two iodine-stabilized frequency-doubled Nd:YAG lasers at λ = 532 nm

Abstract: An international comparison of two I2-stabilized frequency-doubled Nd:YAG lasers at λ = 532 nm has been performed between the Istituto di Metrologia "G. Colonnetti" (IMGC)/Politecnico di Milano (PoliMI) and the Physikalisch-Technische Bundesanstalt (PTB). Both lasers used saturated absorption of the P(54) 32-0 iodine transition as a frequency reference. The signals for stabilizing the lasers were obtained using the frequency modulation spectroscopy technique with an external iodine cell. A minimum value for the relative Allan standard deviation of 7 × 10-14 was reached after an integration time of 40 s. The hyperfine line separations were measured by the beat-frequency method and compared with the frequency values recently reported by Ye and co-workers. Three different iodine cells were investigated. The range of frequency differences for the iodine cells IMGCQU and PTB1/98 between the two lasers was from -9 kHz to 7.3 kHz with a mean of 1.2 kHz and a standard deviation of 3.5 kHz. Taking into account the measured frequency offsets of the different iodine cells, the evaluated frequency repeatability of the optical standards was within 2.5 kHz.

Towards an uncertainty budget in quantum-efficiency measurements with parametric fluorescence

Abstract: The aim of this paper is a critical investigation of some correction factors in the quantum-efficiency calibration of photodetectors, in the photon-counting regime, by means of correlated photons generated through a parametric down-conversion process inside a non-linear crystal. An analysis is given of the main correction factors found in a real experimental situation, with different techniques for performing coincidence counts.

Correlated colour temperature - uncertainty and estimation

Abstract: Correlated colour temperature (CCT) of a source is estimated from position in CIE 1960 (u, v) colour space in relation to the black-body locus. In this paper, the methods of the ISO Guide to the Expression of Uncertainty in Measurement are applied to derive analytical expressions for uncertainty in u and v chromacity coordinates and CCT, including correlation effects. Uncertainty in CCT is found by fitting the form of the black-body locus and relevant derivatives. Examples are given for calibration of four typical sources of spectral distributions. Accurate CCT is determined by a search technique. An alternative method using fitted functions to solve for CCT using the geometry in (u, v) space is presented. An uncertainty in CCT of a few kelvin can be achieved.

Radiometry in photovoltaics: calibration of reference solar cells and evaluation of reference values

Abstract: The key quantity in the calibration of solar cells and photovoltaic modules is the short-circuit current of the device generated by a reference solar radiation with 1 kW m−2 total irradiance and with reference solar spectral irradiance distribution. Thus, (i) double spectral mismatch between test and standard device and between test and reference solar spectrum; and (ii) the calibration transfer from low to high irradiance level (non-linearity effects) may affect the calibration value. An overview of primary and secondary calibration procedures is given, focusing on the primary calibration methods applied by the four qualified laboratories establishing the World Photovoltaic Scale (WPVS) traceable to the International System of Units (SI). The differential spectral responsivity method of the Physikalisch-Technische Bundesanstalt (PTB) is described in more detail. Finally, different approaches are illustrated for the evaluation of a reference value, including the WPVS, based on the results for twenty transfer/travelling standards of the international comparison among eleven participants that was carried out to establish the WPVS.

Realization of the unit of luminous intensity at the HUT

Abstract: A description is presented of an upgraded trap-detector-based realization of the units of luminous intensity (candela) and illuminance (lux) at the Helsinki University of Technology (HUT). The realization is accomplished using a reference photometer, a light source and a distance-measurement system. A thorough characterization is presented of the reference photometer, consisting of a reflection trap detector, a temperature-controlled V(λ) filter and a high-precision aperture. The maintenance of the units is described. An updated uncertainty budget of the realization is given. Two of the three main uncertainty components of our earlier realizations have been significantly decreased. The uncertainty analysis indicates a relative expanded uncertainty of 2.2 × 10-3 for the realization of the candela and 1.8 × 10-3 for that of the lux. The HUT has participated in three international measurement comparisons, whose results are reviewed. According to the results, the HUT candela deviates by +4.0 × 10-3 from the candela of the Swedish National Testing and Research Institute with an expanded uncertainty of 10-2, -2.7 × 10-3 from that of the National Physical Laboratory (UK) with an expanded uncertainty of 5.6 × 10-3 and -3.3 × 10-3 from the world mean with an expanded uncertainty of 5.9 × 10-3.

Spectral characterization of Ge trap detectors and photodiodes used as transfer standards

Abstract: This paper presents the results of investigations of the spectral responsivity in terms of radiant power within the infrared spectral range. Large-sized InGaAs photodiodes of 10 mm × 10 mm and an InGaAs trap detector were studied within the spectral range 500 nm to 1900 nm. The spectral responsivity of this reflection trap detector with three photodiodes is compared with that of a single InGaAs photodiode (photodiodes all of the same type). The temperature effect on the spectral responsivity is measured within the temperature range 10 °C to 30 °C. The results, which are important for calibration of transfer standards for optical communication, are discussed in terms of spectral quantum efficiency.

New spectral radiance scale from 220 nm to 2500 nm

Abstract: The Physikalisch-Technische Bundesanstalt (PTB) has developed a spectral comparator facility for accurate spectral radiance measurements. As a primary standard source, a high-temperature black body (HTBB) is operated in the temperature range 1500 K to 3200 K. The black-body temperature is determined by two methods: monochromatic radiation thermometry relative to the freezing point of gold; and absolute radiometry with filter radiometers calibrated against a cryogenic radiometer. For these purposes, the HTBB is operated in radiance and irradiance modes at the same time. This paper presents an uncertainty analysis of the new spectral radiance scale embodied by gas-filled tungsten strip lamps from 220 nm to 2500 nm. Such lamps are currently used as transfer standards for the Consultative Committee for Photometry and Radiometry (CCPR) comparison of spectral radiance (CCPR-S1).

Primary radiometric quantities and units

Abstract: Methods used for the establishment and dissemination of primary radiometric quantities are reviewed together with the performance and use of filter radiometers, which are dominating advances in optical radiation measurement. The possibility is also discussed of using many of these advances in other metrological sectors, such as in thermal metrology to improve the realization and dissemination of the kelvin.

Quantum efficiencies of gold and copper photocathodes in the VUV and X-ray range

Abstract: The quantum efficiency, η, of gold and copper photocathodes has been measured in the photon-energy range from 50 eV to 10 keV. The experiments were performed with monochromatized synchrotron radiation at the radiometry laboratories of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage rings BESSY I and BESSY II. Relative standard uncertainties of η lower than 0.036 were achieved using a cryogenic electrical-substitution radiometer as the primary detector standard. The samples were cleaned, stored and examined in an ultra-high-vacuum chamber. The quantum efficiency of cleaned gold and copper was found to be reproducible within 2 parts in 102 to 3 parts in 102. A simple model assumes to be proportional to the product of absorption coefficient, μ, and photon energy, hν. The relative course of the absorption coefficient was also measured with relative standard uncertainties below 0.04. As proportionality was only locally fulfilled, a more sophisticated model for quantum efficiency was developed.

The propagation of uncertainty on interpolated scales, with examples from thermometry

Abstract: The propagation of uncertainty on measurement scales that are based on polynomial interpolation is described. Reformulation of the interpolation in terms of Lagrange polynomials, which are orthogonal over the set of measured variables, provides the key mathematical simplification. Consequently, with many uncertainty calculations the correlation terms do not need to be carried, and the effects of the various sources of uncertainty are easily visualized. Indeed, once a user is familiar with Lagrange interpolation, both the interpolation equation and the uncertainty equation can often be written down by inspection without the need for any intermediate calculation. The method is applied to both the International Temperature Scale of 1990 (ITS-90) and multiwavelength radiation thermometry to highlight the advantages of the Lagrange approach and to illustrate some of the advantages and disadvantages of interpolated scales. Several means of estimating the additional uncertainty arising from interpolation error are also discussed.

Characterization and calibration of broadband ultraviolet radiometers

Abstract: The band responsivities of radiometers for measuring ultraviolet irradiance or dosage in the UVA, UVB and UVC spectral regions are source-dependent and require spectrally weighted measurements for calibration. Currently, however, many industrial users lack knowledge of this and may use a radiometer originally calibrated using one particular type of source to measure UV radiation from a source of a different type. Worse still, when this radiometer is later sent for recalibration, the calibration laboratory could use a source of a third type. This problem results in considerable discrepancies, as high as 20 % to 50 %, in the calibration of UV radiometers, and consequent errors in actual measurements. Efforts are being made by the International Commission on Illumination (CIE) to standardize the calibration method and industrial uses of UV radiometers. This paper reviews the measurement principles of broadband radiometers and examines, both theoretically and experimentally, the effects of spectral distribution on the responsivities of several commonly used UV sources. The analysis agrees well with the experimental results. The methods of characterization and calibration of these radiometers are also discussed in detail.

Realization of a spectral radiance responsivity scale with a laser-based source and Si radiance meters

Abstract: A spectral radiance responsivity (SRR) scale derived from the spectral irradiance responsivity scale has been realized on the newly developed NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources (SIRCUS). The scales were directly derived from the primary standard cryogenic radiometer using transfer standard irradiance trap detectors and laser-based uniform sources. Silicon radiance meters of standards quality have been developed to hold the SRR scale. The radiance meters were characterized and optimized for size-of-source effects. The optimized meters were calibrated for SRR against the monochromatic uniform source. In addition to the SIRCUS calibrations, preliminary tests were also made on the Spectral Comparator Facility (SCF) of the NIST. These relative SRR tests were made in both imaging and non-imaging measurement modes. The results of the different SRR calibrations are compared and evaluated.

Magnetic flux density standard for geomagnetometers

Abstract: A high-precision standard has been established to create a dc magnetic field in the expanded geomagnetic range 10 µT to 110 µT. Over this range the systematic standard uncertainty of the total magnetic field is (0.003 − 0.03) nT, with a random standard deviation of 0.002 nT, inside a sphere 10 cm in diameter. The uncertainty of the magnetic-field direction is 1 second of arc. A new type of three-component coil system, with an enlarged working space and an advanced potassium magnetic-field stabilizer, has been designed. The standard is intended for the calibration of high-precision geomagnetometers.

Solar Total Irradiance Monitor (TIM)

Abstract: The Total Irradiance Monitor (TIM), an active-cavity solar radiometer, is to be launched in 2002 on the SORCE mission of the Earth Observing System (EOS). The relative uncertainty (1 σ) will be better than 10−4, i.e. 100 parts per million (ppm) (1 σ) with a noise level of < 1 ppm each 500 s. Sunlight passes through a shutter, a 50 mm2 aperture, and is then absorbed into a silver cavity blackened inside with nickel phosphorus. High thermal-conductivity diamond insulators at the electrical terminals help to localize thermal nodes. Four cavities are aligned side-by-side at the rear of a 2 kg heat sink. The flight "standard digital watt" is derived from a high-precision voltage and a pulse-width-modulator; this eliminates the need for a square root in the servo loop. We determine the irradiance from the in-phase sinusoidal component at the shutter frequency. This phase-sensitive detection allows more accurate characterization than traditional time-domain methods. We calibrate the aperture transmission integral over area against a chrome-on-quartz ruling, transferring by charge-coupled device (CCD) images. In flight, we recalibrate the servo-loop gain, pointing variations, and dark signal. We determine the equivalence ratio from models and by laser measurements of the cavity parameters.

The first key comparison of primary standard gas mixtures

Abstract: This paper reports the results of the first key comparison of primary standard gas mixtures (PSMs), held under the auspices of the Consultative Committee for Amount of Substance (CCQM). PSMs are (national) measurement standards for the realization of specific gas mixture compositions. This key comparison, registered at the Bureau International des Poids et Mesures (BIPM) as CCQM-K1.a-g, encompasses thirteen different gas mixture compositions. In total, 125 transfer standards were prepared and distributed among ten participating institutes. The results show that joint activities in the development and maintenance of PSMs lead to a relative agreement within 10-2 of the reference values for the results of the international comparison.