Bidirectional scattering of light from tree leaves

A laboratory goniometer consisting of a helium - neon laser (632.8 nm), vertical leaf holder, and silicon photovoltaic detector was used to measure the bidirectional scattering (both transmittance and reflectance) of red oak (Quercus rubra L.) and red maple (Acer rubrum L.). Three illumination angles were used, 0° (i.e., nadir), 30°, and 60°, and the scattering was recorded approximately every 10° in the principal plane. The scattering profiles obtained show the non-Lambertian characteristics of the scattering, particularly for the off-nadir illumination directions. The transmitted light was more isotropic than the reflected light. The reflectance of the adaxial side,of the leaf was greater than the adaxial side, although the transmittances of the two sides were about the same. Measurements taken during leaf senescence show that reflectance and transmittance increase as the leaf changes color from green to red to yellow. An equation is proposed as a simple representation of the scattering profiles.