2 WAYS OF ASSESSING ABSORBENCY OF FRESH LEAVES FROM NEAR-INFRARED REFLECTANCE SPECTROSCOPY

In the standard way of measuring fresh leaf absorbance (log1/R) using laboratory equipment, the leaf generally needs to be put on a reflecti ve background. Leaf transmissivity can be deduced from modelling the p ropagation of light within this bi-component sample and its subsequent reflectance or absorbance. The contribution through the leaf of the e nergy emanating from this background is highest near 1000 nm. A model of radiation transfers inside a fresh leaf put upon a reflective backg round, provides a simulation of the subsequently measured reflectance and allows an index of transmissivity T to be calculated. This index t akes into account possible optical differences between the two leaf su rfaces. Tests carried out using near-infrared reflectance spectroscopy at 1000 nm are presented in relation to a structural (leaf mass per a rea) and a chemical (chlorophyll content) variable: the index of trans missivity and the related absorbance (log1/T) may provide an improveme nt in the predictive character of standard leaf absorbance (log 1/R) s pectra. Some problems involved in fresh leaves spectroscopy are examin ed.