Modification of the volumetric scattering in the spectral BRDF microfacet model based on measurements of dicotyledonous leaves.

Bidirectional reflectance distribution function (BRDF) microfacet models play an essential role in studying leaves. Diffuse reflection is generally integrated with volumetric scattering in traditional models. However, research shows the necessity of separately calculating volumetric scattering from diffuse reflection. As such, common volumetric scattering was added to the traditional model in this work, and the trend of volumetric scattering was obtained. Nevertheless, the trend contradicts the experimental data. According to the experimental data, the reflectance becomes higher in the absorption band of 600-700 nm at large incident angles. The layer-by-layer optical analysis of the leaves revealed that light is specularly reflected at large incident angles, which leads to weak volumetric scattering. On this basis, incident angle and wavelength variables were added to volumetric scattering. In addition, a new spectral BRDF microfacet model aligning with the experimental data and encompassing the spectral variable, to our knowledge, was established. Additionally, good fitting results for four leaves were found. The volumetric scattering term and its coefficients better reflected the changes in the proportion of volumetric scattering. This study provides insights into the volumetric scattering of leaves and enables more accurate remote sensing.