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 illuminate d leaves of four different plants (croton, Codiaeum variegatum; spotted ele agnus, Eleagnus pungens Maculata; Japanese pittosporum. Pittosporum tobira and Benjamin fig, Ficus benjamina Starlight) were acquired at a viewing ang le of 30 degrees from the nadir direction. Chlorophyll-a and -b content of leaves covered a range of 1-60 and 0.5-21 mug/cm(2), respectively. In contr ast with previous results from hemispherical reflectance measurements, dire ctional reflectance data does not correlate well with chlorophyll concentra tion. This is mainly due to the external reflectance (R-R) at the leaf epid ermis, caused by the mismatch of the refractive index at the air-epidermis and epidermis-inner layer boundary. The external reflectance can be identif ied with the blue flat reflectance between 380 and 480 nm. The inner reflec tance (R-1), obtained by subtracting the external reflectance from the meas ured spectra, was found to be linearly related to the logarithm of the chlo rophyll content. Good fitting of the log (Ch1) versus R-1(lambda) curves we re obtained for R-1 in the green band (around 550 nm) and close to the infl ection point in the red edge (around 700 nm). The coefficient of determinat ion, r, of curve fitting improved (up to 0.97) when the normalised inner re flectance NR1(lambda) = R-1(lambda)/R-1(lambda (0)), with lambda (0) greate r than or equal to 750 nm, was used instead of the absolute reflectance. Th e best indices for Ch1, Ch1-a and Ch1-b determination were R-542(1)/R-750(1 ), R-706(1)/R-750(1) and R-556(1)/R-750(1), respectively. However, since th e content of Chi-a relative to Ch1-b was almost constant for the plants inv estigated, the two last indices must be further validated on leaves with a high variability in the Ch1-a:Ch1-b ratio. The error in the determination o f chlorophyll content was found to be of the order of 10%. This value was l ower than those obtained by applying the vegetation indices previously sugg ested. 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. (C) 2001 Elsevier Science BY. All rights reserv ed.