In vivo spectroscopy and internal optics of leaves as basis for remote sensing of vegetation

Abstract

In vivo reflection spectra of intact bean leaves (Phaseolus vulgaris) were measured between 400 and 800 nm under remote sensing conditions (illumination with white light, detection of a narrow angle of the reflected light) using the VIRAF spectrometer. The leaves with colours from yellow to green were chosen at different times during light-induced greening. The colours of the leaves were characterized by the chromaticity coordinates according to CIE 1931 calculated from the reflection spectra. The influence of the absorption of chlorophyll—the main pigment of green leaves—on the reflection spectrum of leaves is outlined. The shape of the in vivo reflection spectra is interpreted taking into account (a)the formation of pigment-protein complexes, (b) the sieve effect and the detour effect, as well as (c) the reflection, refraction and scattering of light inside the leaf tissue. Reflection signals at several distinct wavelengths and their ratios as well as the inflection point of the reflection rise from the far red towards the near-infrared were checked for linear correlation with the chlorophyll content per leaf area. The normalized difference vegetation index (NDVI) exhibited a relatively bad correlation with the chlorophyll content. The best correlation was found for the logarithm of the ratio of the reflection signals at 800 and 550 nm. We suggest that this parameter is used, in combination with the detection of the inflection point, for remotely-detecting vegetation and for the estimation of its chlorophyll content.