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Comprehensive Remote Sensing || Solar Induced Chlorophyll Fluorescence: Origins, Relation to Photosynthesis and Retrieval
摘要： Photosynthesis has been key to Earth’s habitability, especially since the evolution of oxygenic photosynthesis with the accompanying rise in atmospheric O2 starting in the Proterozoic era, around 2.5 billion years ago. The conversion of light to chemical energy enables higher life forms and the presence of reduced carbon stocks and molecular O2 (accumulated products of photosynthesis) provided the fossil fuels for the initiation of our industrialized society. As a visible indicator of photosynthesis, fluorescence emanating from chlorophyll molecules attracted interest more than 100 years ago. In 1874, red fluorescence from a green living leaf was observed to be weaker than from a dilute chlorophyll solution (Müller, 1874), indicating an alternate fate for energy absorbed by chlorophyll in vivo. More quantitative studies of chlorophyll a fluorescence go back to observations by Hans Kautsky and A. Hirsch in 1931 at the Chemical Institute at the University of Heidelberg, Germany (Kautsky, 1931). They discovered the so-called Kautsky effect, a decrease in Chl fluorescence following illumination of dark-adapted leaves. Both studies already indicated an important aspect of chlorophyll fluorescence, namely, that its yield can vary. This effect is caused by variation in the balance of pathways that compete with fluorescence for quenching of the excited chlorophyll state. The porphyrin ring of the chlorophyll molecule is the center of light absorption and fluorescence. In a dilute chlorophyll solution, where fluorescence is the only route for de-excitation, the fluorescence lifetime (sF) is about 6 ns and the fluorescence yield (FF) approaches 1. In vivo, the excited chlorophyll state can be quenched by other processes, most notably photosynthesis itself but also quenching to heat; FF is lower (1%–5%) and the sF proportionally shorter. Fluorescence in itself is not a significant route for de-excitation of chlorophyll but changes in FF or sF can be used to study how absorbed energy is used. On the other hand, when FF is constant, fluorescence can provide information on the total amount of light absorbed, as the amount emitted will be proportional to the total amount of photosynthetically active radiation (PAR) that is absorbed by chlorophyll and associated pigments. Fig. 1 shows an example of a typical fluorescence spectrum as well as the absorbance spectrum by chlorophyll.
关键词： Fluorescence Yield,Solar Induced Chlorophyll Fluorescence,Chlorophyll,Photosynthesis,Remote Sensing
更新于2025-09-04 15:30:14
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A Method to Reconstruct the Solar-Induced Canopy Fluorescence Spectrum from Hyperspectral Measurements
摘要： A method for canopy Fluorescence Spectrum Reconstruction (FSR) is proposed in this study, which can be used to retrieve the solar-induced canopy fluorescence spectrum over the whole chlorophyll fluorescence emission region from 640–850 nm. Firstly, the radiance of the solar-induced chlorophyll fluorescence (Fs) at five absorption lines of the solar spectrum was retrieved by a Spectral Fitting Method (SFM). The Singular Vector Decomposition (SVD) technique was then used to extract three basis spectra from a training dataset simulated by the model SCOPE (Soil Canopy Observation, Photochemistry and Energy fluxes). Finally, these basis spectra were linearly combined to reconstruct the Fs spectrum, and the coefficients of them were determined by Weighted Linear Least Squares (WLLS) fitting with the five retrieved Fs values. Results for simulated datasets indicate that the FSR method could accurately reconstruct the Fs spectra from hyperspectral measurements acquired by instruments of high Spectral Resolution (SR) and Signal to Noise Ratio (SNR). The FSR method was also applied to an experimental dataset acquired in a diurnal experiment. The diurnal change of the reconstructed Fs spectra shows that the Fs radiance around noon was higher than that in the morning and afternoon, which is consistent with former studies. Finally, the potential and limitations of this method are discussed.
关键词： Fluorescence Spectrum Reconstruction (FSR),solar-induced chlorophyll fluorescence (Fs),Spectral Fitting Method (SFM),Fraunhofer Line Discriminator (FLD),hyperspectral remote sensing,Singular Vector Decomposition (SVD)
更新于2025-09-04 15:30:14
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[IEEE IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Valencia, Spain (2018.7.22-2018.7.27)] IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Influence of Cosine Corrector and Uas Platform Dynamics on Airborne Spectral Irradiance Measurements
摘要： Measuring solar-induced chlorophyll fluorescence from small-sized Unmanned Aircraft Systems (UAS) can potentially fill the scaling gap between ground-based and airborne/space-borne observations. These measurements require well calibrated, high-spectral resolution spectroradiometers and precise measurements of vegetation radiance and incoming solar irradiance. Here we present a system equipped with a spectroradiometer with a split optical path that measures incoming irradiance through a cosine corrector/diffuser. The objectives of this study are to characterise cosine corrected solar irradiance measurements with regard to sensor homogeneity and possible offset from an ideal cosine response. We further suggest a methodology to calculate a corrected zenith angle that accounts for changing sensor orientation due to pitch, roll and heading of the UAS platform during flight. We found that the cosine corrector is sufficiently homogeneous, thus measurements are independent of UAS heading. The response follows the cosine law for zenith angles, however, the sensor significantly underestimated irradiance for zenith angles >10?, with overall cosine errors ranging from 0.991 to 1.229. Yet, typical in-flight platform pitch and roll angles produce a zenith angle offset of up to 6? in low wind conditions. Cosine sensor measurements corrected for the zenith angle offset and the cosine error resulted in a 1.7% change in irradiance.
关键词： irradiance cosine corrector,spectroradiometer,sun zenith angle,solar-induced chlorophyll fluorescence,unmanned aircraft system
更新于2025-09-04 15:30:14