Bi-directional characteristics of leaf reflectance and transmittance: Measurement and influence on canopy bi-directional reflectance

Leaf reflectance and transmittance are often considered as lambertian in radiative transfer models. Progress in canopy reflectance modelling forces to take into account the non isotropic behaviour of of reflectance and transmittance. Using the PARCINOPY ray tracer, we simulated the reflectance of a maize canopy. We compared the reflectance for canopy made with purely lambertian leaves, to that made of leaves with an additional specular component. Results show that a significant increase of around 0.03 reflectance; value is observed in the specular direction for red and near infrared, For any view angle, a 0.01 increase in the multiple scattering reflectance is also induced for the near infrared. This confirms the importance of taking into account actual leaf bi-directional characteristics. A goniometric system was developed to measure in the red and near infrared the directional distribution oi reflectance and transmittance over the whole hemispheres for a range of incidence angles. Measurements of a selection of leaf types show a generally strong specular feature, even for hairy leaves. The surface component of reflectance does not vary significantly with the wavelength. Conversely, the volume scattering is obviously strongly dependant on wavelength, and appears to be not lambertian. Transmittance shows also a non lambertian behaviour, with a peak transmittance in the incidence direction. It is concluded that modelling of the leaf BRDF is still an important issue for an accurate simulation of canopy reflectance.