Table of contents

The Proceedings published in this special issue of Metrologia are from New Developments and Applications in Optical Radiometry IV, also known as the NEWRAD '92 Conference. The conference was held from 5–7 October 1992 in Baltimore, Maryland, USA, and was organized through the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center in Greenbelt, Maryland. The 1992 Conference was the fourth in a series of occasional international meetings held to integrate the activities of the space radiometry community with those of the laboratory metrology community.

The location of Maryland was chosen to bring the meeting back to the hemisphere of the Americas where the first of the series was held. NASA/Goddard was chosen as the sponsoring organization for this meeting because of the number and diversity of space measurement programmes which are managed there, and because of its proximity to the United States National Institute of Standards and Technology.

The general organization of the meeting included sessions on the Earth Observing System (which I chaired), the Shuttle Atlas I Mission and UV Calibrations (chaired by E Hilsenrath, NASA/Goddard), Cryogenic Radiometry (N Fox, NPL), Detector Radiometry (T Quinn, BIPM), Space-based Applications (J Metzdorf, PTB) and Ground-based Applications (C Cromer, NIST). Thirty-eight papers were presented orally, and poster sessions were also provided. Most of the presentations are represented in these Proceedings. The fifth meeting in this occasional series is scheduled for 19–21 September 1994 in Berlin. The Conference institutional host is the PTB, and the convener is Dr Joachim Fischer, Secretary.

The success of the meeting derived principally from the individual contributions of the presenters and the interest and attention of all the participants. The success of these Proceedings is attributable to the individual authors, the guest editors and the referees of the papers. Each paper was subjected to a critical review which was organized by the chairman of the session in which the paper was presented. The work of the guest editors and referees is gratefully acknowledged.

The drawing together of the papers in Metrologia format was carried out most capably by Dr David Blackburn and his staff. Without his help and encouragement the review process may never have converged to produce this publication.

An anecdotal history of the evolution of satellite remote sensors for making Earth observations in the infrared, visible and ultraviolet spectral regions during the past three decades at the National Aeronautics and Space Administration (NASA) is presented together with an intercomparison of the earlier scanning remote sensors and the Moderate Resolution Imaging Spectroradiometer (MODIS) of the Earth Observing System (EOS) project. Developments in the pre-launch and in-flight radiometric calibration systems and techniques for these satellite remote scanners, sounders and television imagers are highlighted.

The EOS Multi-angle Imaging SpectroRadiometer (MISR) will study the ecology and climate of the Earth through acquisition of global multi-angle imagery. The MISR employs nine discrete cameras, each a push-broom imager. Of these, four point forward, four point aft and one views the nadir. Absolute radiometric calibration will be obtained pre-flight using high quantum efficiency (HQE) detectors and an integrating sphere source. After launch, instrument calibration will be provided using HQE detectors in conjunction with deployable diffuse calibration panels. The panels will be deployed at time intervals of one month and used to direct sunlight into the cameras, filling their fields-of-view and providing through-the-optics calibration. Additional techniques will be utilized to reduce systematic errors, and provide continuity as the methodology changes with time. For example, radiation-resistant photodiodes will also be used to monitor panel radiant exitance. These data will be acquired throughout the five-year mission, to maintain calibration in the latter years when it is expected that the HQE diodes will have degraded. During the mission, it is planned that the MISR will conduct semi-annual ground calibration campaigns, utilizing field measurements and higher resolution sensors (aboard aircraft or in-orbit platforms) to provide a check of the on-board hardware. These ground calibration campaigns are limited in number, but are believed to be the key to the long-term maintenance of MISR radiometric calibration.

The CERES instrument has three radiometers which operate in the short-wave (0,3 μm to 5 μm), the long-wave (8 μm to 12 μm) and total broadband (0,3 μm to >200 μm) spectral regions. The long-wave calibration plan for CERES provides for absolute radiometric calibration of the two CERES sensors operating in the long-wave portion of the spectrum. Ground calibration is carried out in the TRW Radiometric Calibration Facility (RCF), a cryogenic vacuum chamber with the capability to simulate radiometrically both cold space and Earth viewing conditions. Absolute calibration is made using a temperature-based radiometric scale created by a black-body radiance standard controlled at temperatures from 200 K to 320 K. In May 1992, the CERES radiometric test model (RTM), a developmental but functionally flight-like instrument, was calibrated at TRW in the RCF. This was the first end-to-end long-wave calibration of a CERES sensor and provided a test and initial validation of the long-wave calibration plan and procedures. The gain, offset, out-of-field response and spectral response were determined for each sensor. The end-to-end long-wave calibration plan is described and the RTM calibration and validation results are presented.

Pre-flight and in-flight radiometric calibration plans are described for a multispectral optical imager of high spatial resolution for remote sensing of land surfaces and clouds from orbit, which will be launched in 1998 on NASA's EOS-AM1 spacecraft. The instrument named ASTER consists of three radiometers for three separate spectral regions, the visible and near-infrared radiometer (VNIR), the shortwave infrared radiometer (SWIR), and the thermal infrared radiometer (TIR). Absolute radiometric accuracy to better than 4% is required for the VNIR and SWIR radiance measurements, and from 1 K to 3 K, depending on the temperature region, from 200 K to 370 K for the TIR temperature measurements. The basic approach to in-flight calibration is to introduce a reference beam at the front end of the radiometer to calibrate the whole system in orbit. Thus the ASTER instrument is installed with internal on-board calibration units which include, as reference sources, incandescent lamps for the VNIR and SWIR and a black-body radiator for the TIR. Calibration reliability of the VNIR and SWIR is enhanced by a dual system of on-board calibration units and high stability halogen lamps. A ground calibration system of spectral radiances traceable to fixed-point black bodies is used for the pre-flight VNIR and SWIR calibrations.

The SSBUV is part of the National Plan for monitoring the stratosphere. Ozone monitoring from space employs backscatter ultraviolet-type instruments on NOAA operational, NASA research and foreign environmental satellites. The SSBUV provides calibration data for these instruments using nearly coincident observations of the Earth's ultraviolet albedo from a series of Space Shuttle flights. The SSBUV also measures the middle ultraviolet solar irradiance as part of the ozone measurement. A major requirement for the SSBUV is that its calibration be known to about 1% from one flight to the next. To achieve this, the SSBUV conducts a rigorous calibration program involving multiple standard sources and detectors. The SSBUV has now flown four times, over a thirty-month period beginning in October 1989. This paper deals with the calibration and radiometric stability before and after each of four Shuttle flights and is an update to the results reported after the first SSBUV flight. For the more recent flights, laboratory calibration precision continues to be better than 1%. Instrument sensitivity continued to degrade by 1% to 3% after each flight, and is wavelength dependent. Measurements of the solar irradiance in the wavelength region 200 nm to 400 nm have a precision of 1% to 2% from one flight to the next. Comparison of the SSBUV and the NOAA-11 SBUV-2 solar irradiance measurements indicates that the SBUV-2 instrument is degrading less than 1% per year at wavelengths longer than 300 nm and by as much as 6% per year at wavelengths shorter than 250 nm.

We report results of an investigation of a lamp for spectral calibration that is based upon molecular fluorescence. An argon ion laser is used to excite the B state of molecular iodine and the resulting fluorescence provides a series of lines that cover the wavelength range 515 nm to 1 340 nm. The intensities of the lines are fixed with respect to one another and are appropriate for determining the relative spectral responsivity of an optical instrument. We discuss a specific example that demonstrates the salient features of this lamp.

Characteristics of three diffuser materials used in the radiometric calibration of the SBUV-2 are compared. Data on reflectance, BRDF, uniformity and stability are presented and their effect on calibration discussed.

Historically, flat-plate diffusers of measured angular scattering properties are illuminated by NIST standards of spectral irradiance to provide a calculated spectral radiance standard in the wavelength region 250 nm to 340 nm for space-borne SBUV-2 instruments used to determine long-term changes in stratospheric ozone. Correspondingly, space-borne large-aperture scanning radiometers used to measure spectroradiometric properties of Earth scenes in the red and near-infrared regions have used internally illuminated integrating spheres. The spectral radiance of the aperture of an integrating sphere is calculated from its spectral irradiance which is referenced to NIST standards of spectral irradiance. Spectral radiance calibration constants in the region 250 nm to 340 nm derived using the flat-plate diffuser and spherical integrator techniques agree to within 1% in the wavelength region from 250 nm to 340 nm. The results presented are for the radiometric calibration of the SBUV-2 flight model number two which is the next instrument scheduled for flight.

The need exists for a very stable, portable radiometer which can transfer readings from an absolute measuring cryogenic radiometer operating in a vacuum chamber to other laboratories. This will allow electro-optical instruments operating in either air or vacuum to be calibrated relative to the single absolute measuring cryogenic standard. It will also allow characterization of vacuum chamber windows by comparing measurements made in front of and behind the windows. To perform these functions the transfer standard must have good radiometric sensitivity and long-term repeatability in both air and vacuum. It must be reasonably compact and portable and must operate over a wide range of ambient temperatures. Ideally, the transfer standard is linear over a large dynamic range with direct readout in watts or watts/cm² or watts/cm²·sr and has a flat spectral response from UV to far-infrared wavelengths. Such a transfer standard radiometer utilizing a dualcavity detector is described. The dynamic range, radiometric accuracy and long-term stability of the transfer standard radiometer is presented. Stability data of better than 0,001% per day in air is shown.

This paper presents the results from a comparative study of solar total irradiance data from the Earth Radiation Budget Experiment (ERBE) on board the Earth Radiation Budget Satellite (ERBS), and the Solar Maximum Mission Active Cavity Radiometer Irradiance Monitor-1 (SMM/ACRIM), for the overlap period 25 October 1984 to 5 July 1989. Both measurements used active-cavity radiometers (ACR). For days when measurements are available from both satellites, the mean difference in the irradiance values is 2,18 W·m^-2. The data show good agreement both in the details of the day-to-day variations and in the long-term trends. The irradiance variations observed in the measurements track the solar activity cycle quite well. The amplitude of the solar cycle related irradiance variation is 0,1%. For the period October 1984 to July 1989, a correlation coefficient of 0,781 is obtained for the two data sets. The correlation becomes stronger with the increase in solar activity, but breaks down for low solar activity periods. Independent studies of ERBS/ERBE and Nimbus-7/ERB; SMM/ACRIM and Nimbus-7/ERB also show high correlation between the data sets during high solar activity periods.

The Solar Ultraviolet Radiation and Correlative Emissions (SOURCE) mission is intended to advance our ability to specify the spectral irradiance of the Sun in the extreme ultraviolet (EUV) wavelength range through simultaneous, radiometrically accurate measurements of the solar EUV spectral irradiance and measurements, including EUV and visible images, of solar parameters that are correlated with the EUV flux. The data will be used in combination with empirical modelling to develop and validate a more accurate system of proxy, or surrogate, indices for the solar EUV flux.

We describe two recent applications of cryogenic Electrical Substitution Radiometers (ESRs), which can provide significantly improved accuracy and sensitivity in the calibration of solar-terrestrial remote sensing instruments. In the first, a portable UV filter radiometer was calibrated at 257 nm using a cryogenic ESR and an intensity-stabilized laser. This filter radiometer was then used to measure the UV irradiance at 257 nm of deuterium lamps and of the SURF II synchrotron. Our results indicate that the accuracy of these measurements is three to eight times better than presently achieved with calibrations of these source standards, at this wavelength. Improvement of UV calibrations is critical to the success of NASA remote sensing of ozone, and of related solar UV irradiance variations. In the second application, a fast cryogenic ESR was developed for use at TRW as a spectrally flat transfer standard, to provide radiometric calibrations of the CERES spectroradiometer with accuracies of better than 1% in the short-wave band and 0,5% in the long-wave and total bands. The CERES instrument is designed to provide improved measurements of the Earth's radiation budget for climate studies.

Two independent methods of measurement were used to determine the absolute spectral responsivity and external quantum efficiency of light-trapping silicon photodiode packages. These trap packages were calibrated first by the NIST High Accuracy Cryogenic Radiometer at laser wavelengths of 633 nm and 442 nm. They were also measured in the NIST Spectral Comparator Facility with working standards traceable to a 100% quantum efficient radiometer (QED-200). The two sets of measurements agree to better than 0,1% at 633 nm and 0,25% at 442 nm.

The PTB primary standards for radiant power in Braunschweig are the German national standard for detector calibrations within the spectral range of 200 nm to 2,5 μm. This paper presents the progress in development and optimization of a high accuracy measurement facility using a cryogenic radiometer as the primary standard. The following features are highlighted: method of dynamic power substitution; temperature dependence of the absorptance of the cavity; error reduction by distance-error-free calibration transfer; and concept of window-correction-free transfer.

Different types of photodiode have been characterized with respect to uniformity, linearity, stability of responsivity and temperature coefficient at several wavelengths between 257 nm and 799 nm based on cryogenic radiometry and intensity stabilized lasers. The Hamamatsu S1337 photodiode was found to be the best suited for use as a high accuracy transfer standard for both the visible and UV spectral regions. For low-level radiant power measurement, this kind of photodiode can be calibrated via a cryogenic radiometer with a total uncertainty as low as ±0,02% in the visible and ±0,25% in the UV.

The Cryogenic Solar Absolute Radiometer (CSAR) is a new primary radiometric standard for use in space and traceable to the International System of Units (SI). The CSAR will be capable of measuring the total irradiance of the Sun with an uncertainty of 0,01%, and a resolution of 0,001%, more than a factor of ten improvement over existing instruments. The standard is at present under development and is part of a proposal, Absolute Radiometric Measurements in Space (ARMS), which has been submitted to the European Space Agency in response to a Call for Opportunities for Columbus Pre-Cursor Flights. The CSAR is currently on the provisional payload of EURECA III.

A technique is presented for realizing spectral irradiance using a large-area, high temperature, uniform, black-body source and filter-radiometers that are calibrated using a High Accuracy Cryogenic Radiometer. The method will be studied by calibrating irradiance lamps with this new technique and comparing the results with those obtained by the method currently employed at the National Institute of Standards and Technology (NIST). Progress to date and preliminary results are presented. The ultimate goal of the programme is to reduce the measurement uncertainties in the spectral irradiance scales that are made available to industry by calibrating deuterium and tungsten-halogen irradiance lamps.

We have measured the electrical and radiometric properties of a cryogenic bolometer/amplifier which is intended for use as a transfer standard in an infrared spectral comparator facility under development at the NIST. We have found its sensitivity to be similar 20 pW, instability <1%, responsivity similar 1,7 × 10⁶ V/W, spatial nonuniformity <1% and nonlinearity <1% over five decades. These results show that this device can be used for its intended purpose as an IR detector transfer standard.

The paper describes the properties of a new large area InGaAs photodiode with near unity internal quantum efficiency. Measurements of linearity, spatial uniformity and dark current of the photodiodes are presented and compared with measurements on germanium photodiodes. The paper also describes the performance of a detector constructed from an arrangement of three of these photodiodes and shows how it has a calculable responsivity from 980 nm to 1 640 nm. It also describes how such a detector can be used as a sensor for a fibre-optic power meter with an uncertainty of around 0,1%.

A brief history of the development of quantum-efficient detectors and their incorporation into light-trapping configurations is presented. Various light-trapping schemes are surveyed. A new refractive configuration is described which yields the largest field-of-view reported to date.

The differential spectral responsivity method has been used to determine the linearity of large-area Ge detectors near room temperature. The nonlinearity caused by using a traditional transimpedance amplifier is discussed and the applicability of a bootstrapping technique to compensate for the low shunt resistance of the photodiode is shown. The detectors were found to be linear up to a saturation limit which is nearly independent of wavelength. The temperature dependence of the saturation level is also described.

The goal of the Low Background Infrared (LBIR) calibration facility at the NIST is to provide the infrared user community with a measurement base for both broadband and spectral radiometric calibrations in a low background environment. The standard detector used in this facility is the Absolute Cryogenic Radiometer (ACR). The ACR is an electrical substitution radiometer which is capable of measuring between 20 nW and 100 μW of radiant power. By using an optical trap detector, an intercomparison with the ACR is made as part of a programme to monitor the long-term stability of the ACR. A comparison is also made between the optical trap detector and an in-house trap detector that was calibrated using the High Accuracy Cryogenic Radiometer (HACR).

The errors associated with modelling the quantum efficiency of high quality silicon photodiodes in the ultraviolet using the semiconductor device-modelling program PC1D are discussed. Uncertainty spectra have been calculated for these errors, which are mainly a result of the application of basic solid-state models and parameters in the extreme case of ultraviolet absorption. The more critical parameters have been identified and, where possible, corrections have been made to account for them. The overall uncertainty associated with quantum efficiency modelling is found to be well below 0,2% for detection of radiation from 400 nm to 300 nm, and below 1% for radiation down to 250 nm.

Fatigue effects in germanium photodetectors were observed and studies carried out to determine the main components of these effects. Irradiance level, spectral distribution of the incident radiation and irradiated surface on the detector all play an important part in the existence and magnitude of fatigue effects. The time needed to recover the initial characteristics was also studied.

The paper discusses automated spectral linearity measurements using both absolute and relative methods. Results of measurements on typical silicon and germanium photodiodes are presented. These measurements show the agreement between the two methods. They also illustrate some interesting linearity properties: wavelength dependence, detector-to-detector variation, current versus current-density dependence, geometrical effects, and effect of reverse bias. In particular, it is shown that some silicon and germanium detectors must be underfilled with radiation to avoid edge effects. These cause nonlinearities which are generally small (1% level) but, in certain types of thermoelectrically cooled germanium detectors, can be very large (8% to 10%).

We have carried out high resolution spatial measurements of the surface reflectance and responsivity of single and trap-configured silicon photodiodes at laser wavelengths between 325 nm and 780 nm. The results indicate that the spatial nonuniformity of quantum efficiency for a given photodiode varies with laser wavelength and internal structure. For radiometric quality diodes this nonuniformity of response leads to uncertainties in detector measurements which can be reduced to 0,03% or better.

Progress in characterizing two bolometric absolute radiometers is described. Spatial variation in responsivity over the receivers is less than 0,2%, variation in responsivity between the four bolometers is less than 0,3% and their response relative to a photodiode was linear to within experimental uncertainty. Observations of noise in the radiometer heating power are reported and discussed.

A new radiometric method which employs quantum properties of radiation and does not require the use of standard radiation sources and receivers has been suggested. This method uses the process of three-photon down conversion in non-central-symmetrical media to measure the quantum efficiency of photodetectors.

A national standard of low background irradiance from 1 W/m² to 10^-3 W/m² has been created at the Research Institute for Optophysical Measurements (VNIIOFI, Moscow). The standard is based on a cryogenic radiometer and a low temperature black body (LTBB) variable in temperature from 80 K to 300 K.

The current misuse of the term intensity in physics and optics is deplored. Means of remedying the situation are suggested.

The Working Group on Air UV Radiometry of the Comité Consultatif de Photométrie et Radiométrie has analysed the present state of spectral radiometry in the air ultraviolet. The recommendations for future work given by the working group based on this analysis are presented.

The experiment SOVA (SOlar VAriability) on board EURECA (EUropean REtrievable CArrier) contains two types of radiometers sunphotometers for measuring the total and spectral irradiance at five wavelengths between 335 nm and 862 nm. Besides the direct observation of variations of the solar irradiance, these measurements will also be used for the observation of solar oscillations. Preliminary results are presented from the observations of total and spectral irradiance during the first five months of the mission (August to December 1992).

On board the ESA/NASA space mission SOHO (Solar and Heliospheric Observatory) there will be several VUV telescope/spectrometer systems to observe the solar disk and its corona in the wavelength range 15 nm to 160 nm. To determine the absolute spectral sensitivity of these instruments prior to launching, transfer source standards have been developed. These transfer standards consist of a high-current hollow cathode source combined with collimating optics. One standard uses a concave mirror at normal incidence for the wavelength range 50 nm to 160 nm and the other standard uses Wolter type II grazing incidence optics for the range 16 nm to 80 nm. These transfer source standards are calibrated against the electron storage ring BESSY as a primary radiometric source standard.

The stabilities of type 715, T-1, tungsten calibration lamps were evaluated aboard three spacecraft platforms during the period 1984 to 1989. In the 0,2 μm to <5,0 μm broadband spectral region, the emitted radiant power from each lamp was measured using shortwave thermistor bolometers and active-cavity radiometers. The radiant power was found to be constant to ±1%. The responses of filter and silicon photodiode monitoring systems used to evaluate the lamps were found to degrade as much as 6% during the first two years in orbit.

The ultraviolet spectrometers on the Voyager spacecraft are radiometrically stable and presently capable of making long-term, precise measurements of the solar spectral irradiance between 50 nm and 170 nm. If the detection efficiencies of these spectrometers were confirmed through a dedicated program of underflights, and if a campaign of regular Voyager observations of the Sun was instituted, some of the needs for timely, long-term solar EUV flux data could be efficiently met in this decade

The system of radiometric and photometric standards at the PTB is presented, emphasizing traceability, cross-linking, and future developments. Traceability from the national standards to the respective reference standards of calibration laboratories of the German Calibration Service (DKD) is also included. The future realization of the detector-based spectral irradiance scale resulting in a reduced uncertainty is described.

The Atmospheric Optical Calibration System (AOCS) was developed at the National Renewable Energy Laboratory for making real-time, terrestrially based measurements of selected optical properties of the atmosphere. The original application of the AOCS was to provide measurements of atmospheric optical conditions pertinent to outdoor photovoltaic (PV) device performance measurements. It addresses the dependence of outdoor PV test results on the characteristics of the incident solar radiation that are dependent on these constantly changing atmospheric optical conditions. This paper describes the AOCS, its calibration and its application to PV testing. Additional applications related to the measurement of atmospheric optical properties are also mentioned.

A review is presented of recent claims on the effects of partial coherence in radiometric measurements, particularly in relation to spectral irradiance intercomparisons. It is shown that for practical sources typically used in radiometric laboratories, effects claimed to be due to partial coherence are equivalent to those of diffraction and interference associated with the use of extended sources. The relative importance of these effects may then be estimated using existing literature on diffraction corrections in radiometry. Estimates of the importance of coherence may be made from comparison of coherence dimensions with aperture sizes; where small apertures are used, coherence theory may be required to superimpose correlated fields.

Two spectral responsivity scales have been established at the National Physical Laboratory, UK. High quantum efficiency silicon photodiodes have been used to establish a continuous spectral responsivity scale for the spectral region 400 nm to 920 nm. The scale can be disseminated by the NPL with an uncertainty of ±0,1%. Pyroelectric detectors fitted with a reflecting hemispherical cavity have been used to establish a spectral responsivity scale for the spectral region 1 μm to 20 μm. This scale has an uncertainty of less than ±1,6%.

The performance of high power carbon dioxide and free electron lasers depends upon the quality of mirrors used in them. A precise characterization of mirrors is required, and this is achieved by employing reference standards. Standards for the calibration of reflectance scale for the infrared region are not available. The present work describes the development of a specular reflectance standard at glancing angles of incidence from 10° to 70° for the 2,5 μm to 25 μm infrared region. Whereas aluminium and gold mirrors have been used as high reflectance value standards, single and double surface polished silicon and germanium have been employed as standards at low reflectance values.

At the PTB radiometric scales are compared which are based on electrodynamics (electron storage ring BESSY), on thermodynamics (black body of known temperature T₉₀ of the International Temperature Scale) and on a thermal detector in combination with the electrical substitution principle (cryogenic radiometer). The uncertainty contributions of the different input parameters for establishing the independent radiometric scales are analysed in detail. According to this analysis spectral radiant powers in the visible are realized or expected to be realized (black body) with uncertainties (1 σ level) of 0,10% (BESSY), of 0,07% (black body, λ>650 nm) and of 0,007% (cryogenic radiometer). Filter radiometers calibrated at the three independent primary standards are used as transfer instruments for the comparison. The measured values of the responsivities of the filter radiometers are estimated to be uncertain by 0,11% (BESSY), 0,08% (black body, λ>650 nm), and 0,03% (cryogenic radiometer). Preliminary experimental results of the comparison between BESSY and the cryogenic radiometer are in agreement with the uncertainty analysis.

The 1994 Conference on Precision Electromagnetic Measurements will be held Monday 27 June to Friday 1 July 1994 in Boulder, Colorado, USA. The purpose of the bienennial meetings of the CPEM is to exchange information on a wide range of topics on precise electromagnetic measurements. These topics include:

• Advanced instrumentation including new sensors and measurement methods

• Automated measurement methods

• Dielectric and antenna measurements

• Direct current and low-frequency measurements

• Fundamental constants and special standards

• Laser, optical fibre and optical electronic measurements

• RF, microwave and millimetre-wave measurements

• Superconducting and other low-temperature measurements

• Time and frequency measurements

CPEM'94 will be extended to five days to provide for added special sessions on the fundamental physical constants. The CODATA Task Group on Fundamental Constants plans to carry out the next least-squares adjustment of the fundamental constants in 1995 with a cutoff date for input data that is shortly after CPEM'94. Thus, CPEM'94 provides a particularly timely forum for discussion of results important to the 1995 adjustment.

The conference language will be English. Authors are requested to submit an abstract and summary by 18 January 1994. If the paper is accepted, the author will be notified and encouraged to prepare a full paper for IEEE Transactions on Instrumentation and Measurement.

Conference Chairman

Donald B Sullivan, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80303, USA

Technical Programme Committee Chairman

Allen C Newell, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80303, USA

For further information, please contact Gwen E Bennett, Conference Secretary, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80303, USA Tel: (01) 303-497-3295, Fax: (01) 303-497-6461, Telex: 910 940 5906.