Abbott, E.

Donaldson-Hanna, K.

Harrison, J.

Hecker, C.

Hook, S.

Koehl, M.

Langsdale, M.

Maturilli, A.

Poutier, L.

Wooster, M.

English

Infrared surface spectral emissivity is a fundamental property of terrestrial materials, important in a host of environmental applications ranging from mineral mapping to land cover change determination, and in particular the remote sensing of land surface temperature (LST). Surface spectral emissivity has also been used to inform the chemical and physical composition determination of terrestrial and planetary materials. Multiple laboratories have now developed facilities for determining the surface spectral emissivity of samples from reflected infrared radiation measurements (making use of Kirchhoff’s Law), with the results often made publicly available such as in the ECOSTRESS (Ecosystem Thermal Radiometer Experiment on Space Station) spectral library. However, until now there has been relatively little work conducted in evaluating the uncertainties in these types of measurements and the consistency of measurements made in different laboratories. Here we intercompare surface spectral emissivity measurements in a ‘round-robin’ exercise using identical samples at 7 different international laboratories. During this exercise, 4cm-1 spectral resolution emissivity spectra were determined for three samples: distilled water, which has a uniformly high spectral emissivity, and two artificial samples constructed from gold and aluminium sheets laminated in polyethylene (PE), which show respectively both Lambertian and specular characteristics along with widely varying emissivity features across the 2.5 – 14 μm region. Here we present first results from the round-robin intercomparison exercise and discuss the implications for the uncertainty of surface spectral emissivity values determined in the laboratory and for the satellite LST products and LST field validation studies that rely on them

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A ‘Round-Robin’ inter-comparison of spectral emissivity measurements made in different international laboratoriesMary Langsdale1,2*, Martin Wooster1,2, Jeremy J. Harrison2, 3, Michael Koehl4, Christoph Hecker5, Simon Hook6, Elsa Abbott6, Alessandro Maturilli7, Kerri Donaldson-Hanna8, Laurent Poutier9 , Ian C. Lau10, Franz Brucker11Affiliations at bottom. *Corresponding author: mary.langsdale@kcl.ac.ukParticipants from 8 international laboratories in Europe, US andAustralia2 artificial samples (aluminium + gold sheets laminated inpolyethylene) were measured, with specular and Lambertiancharacteristics respectively13 different instruments and setups were used in their standardsetups, all measuring directional hemispherical reflectance (𝜌) over2.5 – 14 μm at 4 cm-1 and 8 cm-1.Emissivity (𝜀) spectra were calculated from each reflectance spectrausing Kirchhoff’s Law (𝜀 = 1 − 𝜌). Spectra were then convolved toASTER bands 10 – 14 (centered at 8.3 μm, 8.7 μm, 9.1 μm, 10.6μm and 11.3 μm respectively) to get band-specific emissivities foreach participant measurement. Corresponding LSTs werecalculated as in Guillevic et al. (2018), assuming a surfacebrightness temperature (BT) of 300K, sky temperature of 260K andnear-surface observation (with negligible atmospheric transmissivityor path radiance effects).Affiliations1Department of Geography - King's College London, 2 NERC National Centrefor Earth Observation (NCEO), 3Department of Physics and Astronomy,University of Leicester, 4Optosol GmbH, 5Department of Earth SystemsAnalysis - University of Twente, 6 NASA - Jet Propulsion Laboratory, 7GermanAerospace Centre (Deutsches Zentrum für Luft- und Raumfahrt;DLR), 8Department of Physics - University of Oxford, 9ONERA - The FrenchAerospace Laboratory, 10CSIRO, Commonwealth Scientific and IndustrialResearch Organisation, 11 Fraunhofer Institute for Solar Energy System ISEConsistency of measurements• For both samples, there are absolute emissivity differences evident in the measured spectra across the full wavelength range (Fig. 1) with mean emissivity differences for Samples 1a and 2a over 2.5 – 14 μm of 0.44 and 0.77 respectively. Each measurement technique in this plot is described in the online methods.• Spectral shape is generally consistent between participant measurements for the Lambertian sample (Sample 2a), with the exception of those made at Oxford and CSIRO using the handheld Agilent FTIR (Fig. 1)Impact of differencesIntroduction1Method2Results3Conclusions4WANT MORE INFO?Further information on participant setups and measurement techniques is available athttps://tinyurl.com/y3tabkhn and through the QR code.Background• Emissivity (the ratio of thermal radiation emitted by a body to that froma perfect blackbody) is essential for remote sensing of land surfacetemperature (LST) among other applications• It is wavelength-dependent and commonly measured in laboratorieswith Fourier Transform InfraRed (FTIR) spectrometers, with someresults available in online spectral libraries e.g. the ECOSTRESS(formerly ASTER) spectral library (Baldridge et al., 2009)• Despite wide use of these spectral libraries, there has been relativelylittle work comparing the results from different laboratoriesObjectives1. To investigate the consistency of measurements made in differentinternational laboratories2. To assess the impact of any differences from these types ofmeasurements on LST evaluationFig. 2: A subset of the participants’ measurements of the specular sample showing the wavelength shifts over 10 – 11 μmFig 1: All participants’ emissivity measurements for both samples over 2.5 – 14 μm with ECOSTESS and ASTER thermal band locations shown.• Wavelength shifts of up 0.08 μm to are evident in the specularsample spectra (Sample 1a), particularly in the 10 – 11 μm spectralregion (Fig 2). These shifts are not observed with the sample withLambertian properties (Sample 2a).Fig. 3: Predicted LSTs (K) calculated assuming surface BTs of 300K and sky temperature of 260K and the measured emissivities for both samples convolved to ASTER TIR bands 10 – 14. See Fig. 1 for ASTER waveband locations.• Emissivities convolved to ASTER bands for both samples differed byover 0.10 for most bands, with greater range for the specular sample(Sample 1a). Calculated LSTs (Fig. 3) using these emissivities ranged byover 3.5 K in each band, with a maximum difference of 21.3 K (Sample1a, Band 13). This corresponds with the area of wavelength shifts andincreased atmospheric effects.• A round-robin investigation into laboratory-based spectral emissivity measurements was conducted.Eight international laboratories compared measurements of two specially prepared test samples with specularand Lambertian properties respectively• Spectral emissivity differences were observed over the 2.5 – 14 μm range for both samples between theparticipant measurements• Wavelength shifts (~0.08 μm) were also observed in the specular sample measurements• The impact of these differences were assessed by calculating the equivalent LST of a near-surfaceobservation with surface BT of 300K, sky temperature of 260K and emissivity equal to the measuredemissivities convolved to ASTER bands 10 - 14• Calculated LSTs ranged by at least 3.5K, with the variability wavelength dependent and with greatervariability observed for the specular sampleReferencesBaldridge et al. 2009 Rem. Sens. Env. 113, 711-715Guillevic, et al, 2018, Land Surface Temperature Product Validation Best Practice Protocol Version 1.1 doi:10.5067/doc/ceoswgcv/lpv/lst.001

A ‘round-robin’ inter-comparison of 2.5 – 14 μm surface spectral emissivity measurements made in different international laboratories, and its implication for emissivity measurement uncertainty

https://ris.utwente.nl/ws/files/329216681/hecker_rou_pos.pdf

A ‘round-robin’ inter-comparison of 2.5 – 14 μm surface spectral emissivity measurements made in different international laboratories, and its implication for emissivity measurement uncertainty

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