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Spectral emissivity (SE) measurement uncertainties across 2.5 – 14 μm derived from a round-robin study made across international laboratories

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Mary Langsdale, Martin Wooster, Jeremy Harrison, Michael Koehl, Christoph Hecker, Simon Hook, Elsa Abbott, William Johnson, Alessandro Maturilli, Laurent Poutier, Ian Lau, Franz Brucker

Original languageEnglish
Article number102
Pages (from-to)1-37
Number of pages37
JournalREMOTE SENSING
Volume13
Issue number1
Early online date30 Dec 2020
DOIs
Accepted/In press21 Dec 2020
E-pub ahead of print30 Dec 2020
Published1 Jan 2021

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Abstract

Information on spectral emissivity (SE) is vital when retrieving and evaluating land surface temperature (LST) estimates from remotely sensed observations. SE measurements often come from spectral libraries based upon laboratory spectroscopic measurements, with uncertainties typically derived from repeated measurements. To go further, we organised a ‘round-robin’ inter-comparison exercise involving SE measurements of three samples collected at seven different international laboratories. The samples were distilled water, which has a uniformly high spectral emissivity, and two artificial samples (aluminium and gold sheets laminated in polyethylene), with variable emissivities and largely specular and Lambertian characteristics. Large differences were observed between some measurements, with standard deviations over 2.5 – 14 μm of ± 0.092, 0.054 and 0.028 emissivity units (15.98%, 7.56% and 2.92%) for the laminated aluminium sheet, laminated gold sheet and distilled water respectively. Wavelength shifts of up to 0.09 μm were evident between spectra from different laboratories for the specular sample, attributed to system design interacting with the angular behaviour of emissivity. We quantified the impact of these SE differences on satellite LST estimation and found that emissivity differences resulted in LSTs differing by at least 3.5 K for each artificial sample and by more than 2.5 K for the distilled water. Our findings suggest that variations between SE measurements derived via laboratory setups may be larger than previously assumed and provide a greater contribution to LST uncertainty than thought. The study highlights the need for the infrared spectroscopy community to work towards standardized and inter-laboratory comparable results.

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