Abstract
The paper presents two procedures for the wavelength calibration, in the oxygen telluric absorption spectral bands (O2-A, λc = 687 nm and O2-B, λc = 760.6 nm), of field fixed-point spectrometers used for reflectance and Sun-induced fluorescence measurements. In the first case, Ne and Ar pen-type spectral lamps were employed, while the second approach is based on a double monochromator setup. The double monochromator system was characterized for the estimation of errors associated with different operating configurations. The proposed methods were applied to three Piccolo Doppio-type systems built around two QE Pros and one USB2 + H16355 Ocean Optics spectrometers. The wavelength calibration errors for all the calibrations performed on the three spectrometers are reported and potential methodological improvements discussed. The suggested calibration methods were validated, as the wavelength corrections obtained by both techniques for the QE Pro designed for fluorescence investigations were similar. However, it is recommended that a neon emission line source, as well as an argon or mercury-argon source be used to have a reference wavelength closer to the O2-B feature. The wavelength calibration can then be optimised as close to the O2-B and O2-A features as possible. The monochromator approach could also be used, but that instrument would need to be fully characterized prior to use, and although it may offer a more accurate calibration, as it could be tuned to emit light at the same wavelengths as the absorption features, it would be more time consuming as it is a scanning approach.
Highlights
Fluorescence, the radiant flux emitted by chlorophyll molecules after excitation by photons, is the most directly-measurable reporter of photosynthetic efficiency and a key indicator of the health and carbon fixation of photosynthesizing organisms [1,2]
This paper investigates alternative methods of wavelength calibration
The aim of our study was to identify the best practice related to wavelength calibration of high-resolution spectrometer systems across the telluric O2-A and O2-B bands using two different methods
Summary
Fluorescence, the radiant flux emitted by chlorophyll molecules after excitation by photons, is the most directly-measurable reporter of photosynthetic efficiency and a key indicator of the health and carbon fixation of photosynthesizing organisms [1,2]. A synoptic view of Earth vegetation health will become available when the ESA Fluorescence Explorer (FLEX) mission, scheduled for launch in 2022, becomes operational. This mission will fly in tandem with Sentinel 2 and 3 and enable both the Sun-induced fluorescence (SIF) and Earth surface radiance to be measured simultaneously [5,6,7]. Reliable short-term and long-term system characterization, calibration and validation of the spectrometer system used for ground and near ground calibration and validation science activities [8,9,10,11] need to be considered to ensure accurate and replicable results. One of the m3aoinf 19 strategies used to separate the effects of vegetation reflected and emitted fluxes is by exploiting the chaacnrogsess athcreossps ethcteraolxryagnegnetethllautriccoarbressoprpontidosn tloineths:eogxaysgechne-Ami(sOtr2y-Aa)n, dhapvhinygsicthsefucenndtarme wenatvaelsle. nTghtehse atli6n8e7s namreanndotoxnyegceesns-aBri(lOy 2e-Bve) nhlayvidnigsttrhibeucteendtraecwroasvseltehnegtfhulal t r7a6n0g.6e nomf [t1h2e,1s3p].ecTtrhoemseemteresthboedisng recqauliibrerattheed,uasnedofthheiigrhinstpeencstirtayl artessoolmuteiownasvsepleenctgrtohms emtearys (bfeultlo-wo ildotwh thoalbf-emoafxpimraucmtica(lFWusHe M[29) ]o. fIn leassddthitaionn0,.3thnemem) [i1s3si,1o4n].linTehse dnoeendotfonrohrmigahl-lryesmolauttcihonthsepewcatrvoemleentgetrhssfocrormreesapsounrdininggfltuoorOe2sc-Aenacend wOith2i-nB tahbessoerptetilolunrilcinbeas.nAdsnoatnhderthoepteifofnecitss toof uinseacacuturanceiaebsleanmdounnoccehrrtoaminattioesr hsyasstebmee.nThheigsehlsigyhstteemds, byw[h15ic–h17c]a, nanbdethperongereadmfomrewdeltlo-caelmibirtaltiegdhat nadt cshpaercaicfitcerwiseadveslyesntgemthss iisnemmpinhiamsiusemd isntetphse EofSA0.0F5LEnXm, redpeoprtenfodrinmgisosniotnhesecloecntfiiognur[a3t]i.oTnhs,eciamnpbaecttuofneddiffteorepnrtosvpidecetrFoWmHetMersFbWetHwMeesno
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