New vegetation indices for remote measurement of chlorophylls based on leaf directional reflectance spectra

Directional reflectance (R) spectra from 380 to 780 nm for nadir illuminated leaves of four different plants (croton, Codiaeum variegatum; spotted eleagnus, Eleagnus pungens Maculata; Japanese pittosporum, Pittosporum tobira and Benjamin fig, Ficus benjamina Starlight) were acquired at a viewing angle of 30° from the nadir direction. Chlorophyll-a and -b content of leaves covered a range of 1–60 and 0.5–21 μg/cm2, respectively. In contrast with previous results from hemispherical reflectance measurements, directional reflectance data does not correlate well with chlorophyll concentration. This is mainly due to the external reflectance (RE) at the leaf epidermis, caused by the mismatch of the refractive index at the air–epidermis and epidermis–inner layer boundary. The external reflectance can be identified with the blue flat reflectance between 380 and 480 nm. The inner reflectance (RI), obtained by subtracting the external reflectance from the measured spectra, was found to be linearly related to the logarithm of the chlorophyll content. Good fitting of the log (Chl) versus RI(λ) curves were obtained for RI in the green band (around 550 nm) and close to the inflection point in the red edge (around 700 nm). The coefficient of determination, r2, of curve fitting improved (up to 0.97) when the normalised inner reflectance NRI(λ)=RI(λ)/RI(λ0), with λ0≥750 nm, was used instead of the absolute reflectance. The best indices for Chl, Chl-a and Chl-b determination were RI542/RI750, RI706/RI750 and RI556/RI750, respectively. However, since the content of Chl-a relative to Chl-b was almost constant for the plants investigated, the two last indices must be further validated on leaves with a high variability in the Chl-a:Chl-b ratio. The error in the determination of chlorophyll content was found to be of the order of 10%. This value was lower than those obtained by applying the vegetation indices previously suggested. Therefore, the normalised inner reflectance in the green and in the red edge represents a more suitable index for the chlorophyll determination than those up to now used.