Abstract
Abstract. Most methods for retrieving foliar content from hyperspectral data are well adapted either to remote-sensing scale, for which each spectral measurement has a spatial resolution ranging from a few dozen centimeters to a few hundred meters, or to leaf scale, for which an integrating sphere is required to collect the spectral data. In this study, we present a method for estimating leaf optical properties from hyperspectral images having a spatial resolution of a few millimeters or centimeters. In presence of a single light source assumed to be directional, it is shown that leaf hyperspectral measurements can be related to the directional hemispherical reflectance simulated by the PROSPECT radiative transfer model using two other parameters. The first one is a multiplicative term that is related to local leaf angle and illumination zenith angle. The second parameter is an additive specular-related term that models BRDF effects. Our model was tested on visible and near infrared hyperspectral images of leaves of various species, that were acquired under laboratory conditions. Introducing these two additional parameters into the inversion scheme leads to improved estimation results of PROSPECT parameters when compared to original PROSPECT. In particular, the RMSE for local chlorophyll content estimation was reduced by 21% (resp. 32%) when tested on leaves placed in horizontal (resp. sloping) position. Furthermore, inverting this model provides interesting information on local leaf angle, which is a crucial parameter in classical remote-sensing.
Highlights
Hyperspectral remote-sensing has proven to offer a great potential for extracting vegetation optical properties at different scales, ranging from leaf level to canopy level
Because hyperspectral measurements are affected by bi-directional reflectance distribution function (BRDF) effets, it is of great importance to take into account all these sources of variability to relate remote-sensing data to leaf physiological processes in a proper way
The PROSPECT model for leaf directional hemispherical reflectance has been adapted to the close-range remotesensing case by adding two parameters describing the specular component and incident angle, which is related to leaf angle and illumination zenith angle
Summary
Hyperspectral remote-sensing has proven to offer a great potential for extracting vegetation optical properties at different scales, ranging from leaf level to canopy level. In the solar emission spectrum (i.e., between 400 and 2500 nm), various parameters are known to have an influence in some specific wavelength ranges At leaf level, these parameters characterize the leaf internal structure and biochemical composition through absorption and scattering processes, e.g., chlorophyll, water and dry matter contents (Curran, 1989; Jacquemoud and Baret, 1990; Feret et al, 2008). Canopy reflectance models such as PROSAIL must be applied to mixed pixels (containing both soil and leaf materials), for which effects of leaf composition, canopy structure, soil properties and viewing/illumination angles are integrated into a single spectrum They are well suited for airborne and satellite-borne hyperspectral measurements as well as ground-based spectroradiometric measurements, for which.
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