Rainer Winkler

Metal (carbide)–carbon eutectics for thermometry and radiometry: a review of the first seven years

Since 1999, when the first high temperature fixed-points based on the metal–carbon eutectic phase transitions were realized, more than 60 papers have been published on this topic. Eutectic based fixed-points are already being considered as secondary reference points for the International Temperature Scale and have been introduced into industrial laboratories. This rapid progress has been possible through the combined effort of scientists around the world, from national metrology institutes, universities and industry. It has been proposed that these fixed-points should be officially adopted as a way to improve the realization and dissemination of temperature scales above the silver point. In radiometry, the availability of stable high temperature fixed-points will give greater flexibility and at some wavelengths the potential for greater accuracy for spectral radiance and irradiance scale realization. This paper summarizes the major progress in eutectic research so far.

Accurate radiometry from space: an essential tool for climate studies.

The Earths climate is undoubtedly changing; however, the time scale, consequences and causal attribution remain the subject of significant debate and uncertainty. Detection of subtle indicators from a background of natural variability requires measurements over a time base of decades. This places severe demands on the instrumentation used, requiring measurements of sufficient accuracy and sensitivity that can allow reliable judgements to be made decades apart. The International System of Units (SI) and the network of National Metrology Institutes were developed to address such requirements. However, ensuring and maintaining SI traceability of sufficient accuracy in instruments orbiting the Earth presents a significant new challenge to the metrology community. This paper highlights some key measurands and applications driving the uncertainty demand of the climate community in the solar reflective domain, e.g. solar irradiances and reflectances/radiances of the Earth. It discusses how meeting these uncertainties facilitate significant improvement in the forecasting abilities of climate models. After discussing the current state of the art, it describes a new satellite mission, called TRUTHS, which enables, for the first time, high-accuracy SI traceability to be established in orbit. The direct use of a ‘primary standard’ and replication of the terrestrial traceability chain extends the SI into space, in effect realizing a ‘metrology laboratory in space’.

Fourth World Radiometric Reference to SI radiometric scale comparison and implications for on-orbit measurements of the total solar irradiance

We report the fourth World Radiometric Reference (WRR)-to-SI comparison. At the National Physical Laboratory we compared three transfer pyrheliometer instruments in power mode with the SI radiometric scale. Compared with the three previous comparisons, we improved the experiment by operating the transfer instruments in vacuum. At the Total solar irradiance Radiometer Facility (TRF) located at the Laboratory for Atmospheric and Space Physics (LASP) in Boulder, we repeated the power comparison of one of the transfer instruments. The TRF also allowed the comparison and characterization of this instrument in irradiance mode. Using the WRR comparisons performed in Davos, we find that the WRR is 0.34% higher than the SI scale. Comparing irradiance mode calibrations with power mode calibrations reveals that previous estimates of stray light of PMO6-type radiometers were very low. The instrument calibrated at TRF was integrated in the space experiment PREMOS on the French satellite PICARD and carries the first vacuum irradiance calibration to space.

A Monte Carlo method for uncertainty evaluation implemented on a distributed computing system

This paper is concerned with bringing together the topics of uncertainty evaluation using a Monte Carlo method, distributed computing for data parallel applications and pseudo-random number generation. A study of a measurement system to estimate the absolute thermodynamic temperatures of two high-temperature blackbodies by measuring the ratios of their spectral radiances is used to illustrate the application of these topics. The uncertainties associated with the estimates of the temperatures are evaluated and used to inform the experimental realization of the system. The difficulties associated with determining model sensitivity coefficients, and demonstrating whether a linearization of the model is adequate, are avoided by using a Monte Carlo method as an approach to uncertainty evaluation. A distributed computing system is used to undertake the Monte Carlo calculation because the computational effort required to evaluate the measurement model can be significant. In order to ensure that the results provided by a Monte Carlo method implemented on a distributed computing system are reliable, consideration is given to the approach to generating pseudo-random numbers, which constitutes a key component of the Monte Carlo procedure.

Effect of polytetrafluoroethylene (PTFE) phase transition at 19°C on the use of Spectralon as a reference standard for reflectance.

Sintered polytetrafluoroethylene (PTFE) is highly reflective and is widely used as a reference standard in remote sensing, radiometry, and spectroscopy. The relative change in output flux from a PTFE integrating sphere over the room temperature phase transition at 19°C has been measured at a monochromatic wavelength of 633 nm as 1.82±0.21%. The change in output flux was attributed to a small change of 0.09±0.02% in the total hemispherical reflectance of PTFE, caused by a change in its material density as a result of the phase transition. For the majority of users, this small change measured in total hemispherical reflectance is unlikely to impact significantly the accuracy of PTFE flat panel reflectors used as reference standards. However, owing to the multiple reflections that occur inside an integrating sphere cavity, the effect is multiplied and remedial action should be applied, either via a mathematical correction or through temperature stabilization of the integrating sphere when high accuracy (<5%) measurements of flux, irradiance, or radiance are required from PTFE-based integrating spheres at temperatures close to the phase transition at 19°C.

Direct Solar Irradiance measurements with a Cryogenic Solar Absolute Radiometer

The World Radiometric Reference (WRR) is an artefact based reference for Direct Solar Irradiance (DSI) measurements. The WRR is realized by a group of electrical substitution radiometers, the World Standard Group (WSG). In recent years, a relative difference of about -0.3% between the International System of Units (SI) scale and the WRR scale was observed with the SI scale being lower. The Cryogenic Solar Absolute Radiometer (CSAR) aims for i) providing direct traceability of DSI measurements to the SI system, ii) reducing the overall uncertainty of DSI measurements towards 0.01% and for iii) replacing the WSG in future. The latest SI-WRR intercomparisons performed with CSAR revealed a relative difference of -0.29% ± 0.064% (k = 1) between the SI and the WRR scale, a result that agrees well with previous findings. The uncertainty of corrections for the window transmittance results currently in the largest contribution to the total uncertainty for the CSAR measurements. The formal transition from the WRR t...

Traceable Radiometery Underpinning Terrestrial- and Helio- Studies (TRUTHS): Establishing a climate and calibration observatory in space

TRUTHS (Traceable Radiometry Underpinning Terrestrial-and Helio- Studies) is a proposed small satellite mission to enable a space-based climate observing system capable of delivering data of the quality needed to provide the information needed by policy makers to make robust mitigation and adaptation decisions. This is achieved by embedding trust and confidence in the data and derived information (tied to international standards) from both its own measurements and by upgrading the performance and interoperability of other EO platforms, such as the Sentinels by in-flight reference calibration. TRUTHS would provide measurements of incoming (total and spectrally resolved) and global reflected spectrally and spatially (50 m) solar radiation at the 0.3% uncertainty level. The calibration scheme components and the route to SI-traceable Earth-reflected solar spectral radiance and solar spectral irradiance are described.