Nigel P. Fox

Achieving Climate Change Absolute Accuracy in Orbit

The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission will provide a calibration laboratory in orbit for the purpose of accurately measuring and attributing climate change. CLARREO measurements establish new climate change benchmarks with high absolute radiometric accuracy and high statistical confidence across a wide range of essential climate variables. CLARREOs inherently high absolute accuracy will be verified and traceable on orbit to Systeme Internationale (SI) units. The benchmarks established by CLARREO will be critical for assessing changes in the Earth system and climate model predictive capabilities for decades into the future as society works to meet the challenge of optimizing strategies for mitigating and adapting to climate change. The CLARREO benchmarks are derived from measurements of the Earths thermal infrared spectrum (5–50 μm), the spectrum of solar radiation reflected by the Earth and its atmosphere (320–2300 nm), and radio occultation refractivity from which...

A Cryogenic Radiometer for Absolute Radiometric Measurements

The adoption in 1979 of a new definition of the candela, which permitted a detector-based approach to the realization of the unit, has emphasized the importance of high-accuracy absolute radiation detectors. This paper describes a new electrical-substitution absolute radiometer operating at 5 K, based on a standard commercial helium cryostat, which has been developed at NPL for optical radiant-power measurements. The principal advantages of operating the radiometer at liquid helium temperatures are that the detector can be a large, highly absorbing cavity (absorptivity 99.998%) and that exact equivalence of electrical and radiant heating can be clearly demonstrated. It will be the primary absolute detector for a new realization of the candela at NPL and will provide the basis for the development of a wider range of radiometric and spectroradiometric standards. The use of the radiometer to measure the radiant power of an intensity-stabilized laser source at the milliwatt level with an uncertainty of 4 parts in 105 is also described, together with its application in the determination of the responsivity of transfer-standard detectors, such as silicon photodiodes.

Photometry, radiometry and 'the candela': evolution in the classical and quantum world

The metrological fields of photometry and radiometry and their associated units are closely linked through the current definition of the base unit of luminous intensity—the candela. These fields are important to a wide range of applications requiring precise and accurate measurements of electromagnetic radiation and, in particular, the amount of radiant energy (light) that is perceived by the human eye. The candela has been one of the base units since the inception of the International System of Units (SI) and is the only base unit that quantifies a fundamental biological process—human vision. This photobiological process spans an enormous dynamic range of light levels from a few-photon interaction involved in triggering the vision mechanism to a level of more than 1015 photons per second that is accommodated by the visual response under bright daylight conditions. This position paper, prepared by members of the Task Group on the SI of the Consultative Committee for Photometry and Radiometry Strategic Planning Working Group (CCPR WG-SP), reviews the evolution of these fields of optical radiation measurements and their consequent impact on definitions and realization of the candela. Over the past several decades, there have been significant developments in sources, detectors, measuring instruments and techniques, that have improved the measurement of photometric and radiometric quantities for classical applications in lighting design, manufacturing and quality control processes involving optical sources, detectors and materials. These improved realizations largely underpin the present (1979) definition of the candela. There is no consensus on whether this radiant-based definition fully satisfies the current and projected needs of the optical radiation community. There is also no consensus on whether a reformulation of the definition of the candela in terms of photon flux will be applicable to the lighting community. However, there have been significant recent advances in radiometry in the development of single-photon sources and single-photon detectors and the growth of associated technologies, such as quantum computing and quantum cryptography. The international acceptance of these new quantum-based technologies requires improved traceability and reliability of measurements at the level of a few photons. This review of the evolution of the candela and the impact of its possible reformulation might lead, in the future, to a reformulation in terms of quantum units (photons). This discussion is timely since redefinitions of four of the other SI base units are being considered now in terms of fundamental constants to provide a more universally realizable quantum-based SI system. This paper also introduces for the first time a fundamental constant for photometry.

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’.

Establishing the Antarctic Dome C community reference standard site towards consistent measurements from Earth observation satellites

Establishing satellite measurement consistency by using common desert sites has become increasingly more important not only for climate change detection but also for quantitative retrievals of geophysical variables in satellite applications. Using the Antarctic Dome C site (75°06′S, 123°21′E, elevation 3.2 km) for satellite radiometric calibration and validation (Cal/Val) is of great interest owing to its unique location and characteristics. The site surface is covered with uniformly distributed permanent snow, and the atmospheric effect is small and relatively constant. In this study, the long-term stability and spectral characteristics of this site are evaluated using well-calibrated satellite instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Preliminary results show that despite a few limitations, the site in general is stable in the long term, the bidirectional reflectance distribution function (BRDF) model works well, and the site is most suitable for the Cal/Val of reflective solar bands in the 0.4–1.0 µm range. It was found that for the past decade, the reflectivity change of the site is within 1.35% at 0.64 µm, and interannual variability is within 2%. The site is able to resolve calibration biases between instruments at a level of ∼1%. The usefulness of the site is demonstrated by comparing observations from seven satellite instruments involving four space agencies, including OrbView-2–SeaWiFS, Terra–Aqua MODIS, Earth Observing 1 (EO-1) – Hyperion, Meteorological Operational satellite programme (MetOp) – Advanced Very High Resolution Radiometer (AVHRR), Envisat Medium Resolution Imaging Spectrometer (MERIS) – dvanced Along-Track Scanning Radiometer (AATSR), and Landsat 7 Enhanced Thematic Mapper Plus (ETM+). Dome C is a promising candidate site for climate quality calibration of satellite radiometers towards more consistent satellite measurements, as part of the framework for climate change detection and data quality assurance for the Global Earth Observation System of Systems (GEOSS).

Highly stable, monochromatic and tunable optical radiation source and its application to high accuracy spectrophotometry

An optical radiation source has been developed by coupling a dye laser to a small integrating sphere with an optical fiber. The radiant power from this source, which is monochromatic and spectrally tunable, has been stabilized to +/-0.02%. Nonuniformities in the emitted optical radiation field caused by speckle have been overcome by vibrating the fiber at ultrasonic frequencies. The source has been successfully used in a spectrophotometer to measure the transmittance of a large lens with an uncertainty of +/-0.01%, and the spectral responsivity of a filter radiometer with an uncertainty of +/-0.04%.

Absolute Spectral Radiometric Determination of the Thermodynamic Temperatures of the Melting/Freezing Points of Gold, Silver and Aluminium

Narrow band filter radiometers have been used to measure the spectral radiance of black bodies held at the melting/freezing points of aluminium, silver and gold with an accuracy of 0,04%. Descriptions are given of the optical components and the techniques used for their characterization. As the measurements are absolute, the thermodynamic temperatures of these points can be calculated directly from the Planck radiation law. The values obtained are: T (aluminium) = 933,509 ± 0,027 K, T (silver) = 1 235,009 ± 0,044 K and T (gold) = 1 337,330 ± 0,049 K. These values are analysed with reference to the International Temperature Scale of 1990 (ITS-90) and to the data that were used to derive the scale. The conclusion drawn is that, while the ITS-90 has a sounder thermodynamic basis than its predecessor (IPTS-68), the temperature intervals between the high temperature fixed points, which were thought to be well established, may need further investigation.

Traceable radiometry underpinning terrestrial- and helio-studies (TRUTHS)

The Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS) mission offers a novel approach to the provision of key scientific data with unprecedented radiometric accuracy for Earth Observation (EO) and solar studies, which will also establish well-calibrated reference targets/standards to support other EO missions. This paper presents the TRUTHS mission and its objectives. TRUTHS will be the first satellite mission to calibrate its EO instrumentation directly to SI in orbit, overcoming the usual uncertainties associated with drifts of sensor gain and spectral shape by using an electrical rather than an optical standard as the basis of its calibration. The range of instruments flown as part of the payload will also provide accurate input data to improve atmospheric radiative transfer codes by anchoring boundary conditions, through simultaneous measurements of aerosols, particulates and radiances at various heights. Therefore, TRUTHS will significantly improve the performance and accuracy of EO missions with broad global or operational aims, as well as more dedicated missions. The provision of reference standards will also improve synergy between missions by reducing errors due to different calibration biases and offer cost reductions for future missions by reducing the demands for on-board calibration systems. Such improvements are important for the future success of strategies such as Global Monitoring for Environment and Security (GMES) and the implementation and monitoring of international treaties such as the Kyoto Protocol. TRUTHS will achieve these aims by measuring the geophysical variables of solar and lunar irradiance, together with both polarised and unpolarised spectral radiance of the Moon, Earth and its atmosphere. Published by Elsevier Ltd on behalf of COSPAR.

The quantum candela : a re-definition of the standard units for optical radiation

The candela, the SI (système internationale) unit for optical radiation, has been one of the base units since the inception of the system. The latest definition was in 1979, when it was linked to the derived unit, the watt. Advances in optical technology and the needs of the communication sector suggest that it is timely that consideration be given to redefining the candela in terms of fundamental quantum optical entities, i.e. photons. Validation of this approach will require comparison against the most accurate conventional technique, cryogenic radiometry. A definition in terms of photon number and the requirements for demonstrating equivalence with existing techniques is discussed, together with new possibilities which would result from further improvements in accuracy. Work being carried out at the National Physical Laboratory (NPL) towards these goals is described, drawing on developments of photon-counting calibration techniques and low temperature measurements, and research into single photon sources and detectors.

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.