Two ways of assessing absorbance of fresh leaves from near-infrared reflectance spectroscopy
In the standard way of measuring fresh leaf absorbance (logl/R) using laboratory equipment, the leaf generally needs to be put on a reflective background. Leaf transmissivity can be deduced from modelling the propagation of light within this bi-component sample and its subsequent reflectance or absorbance. The contribution through the leaf of the energy emanating from this background is highest near 1000 nm. A model of radiation transfers inside a fresh leaf put upon a reflective background, provides a simulation of the subsequently measured reflectance and allows an index of transmissivity T to be calculated. This index takes into account possible optical differences between the two leaf surfaces. Tests carried out using near-infrared reflectance spectroscopy at 1000 nm are presented in relation to a structural (leaf mass per area) and a chemical (chlorophyll content) variable: the index of transmissivity and the related absorbance (logl/T) may provide an improvement in the predictive character of standard leaf absorbance (log l/R) spectra. Some problems involved in fresh leaves spectroscopy are examined.