DISENTANGLING ISOTROPIC FLUORESCENCE FROM THE CANOPY DIRECTIONAL REFLECTANCE USING BRDF MODELS

摘要

Remote sensing of sun-induced chlorophyll fluorescence (SIF) has recently attracted major interest as the SIF is directly linked to the photosynthesis. Nevertheless, SIF retrievals from the observed apparent radiance via the most common fraunhofer line discrimination (FLD) method are significantly affected by the directional reflectance. This study investigates the effects of directional reflectance on SIF estimates by in-situ multi-angular spectral measurements of lawn grass. Results show that the SIFs in both O_2-B and O_2-A atmospheric absorption bands vary greatly with view angles. Consequently, a new retrieval strategy based on kernel-driven BRDF (Bidirectional Reflectance Distribution Function) models was proposed. We initially demonstrate that the presented approach has the potential to eliminate the directional effects, producing the isotropic SIFs that are considered more closely related to vegetation physiological variations, and it would better comply with the FLD implied assumption of lambertian surface reflection. This study may further promote the understanding of the emission mechanism of remotely sensed SIFs.