DIFFUSE-REFLECTANCE MODEL FOR SMOOTH DIELECTRIC SURFACES

A reflectance model that accurately predicts diffuse reflection from s mooth inhomogeneous dielectric surfaces as a function of both viewing angle and angle of incidence is proposed. Utilizing results of radiati ve-transfer theory for subsurface multiple scattering, this new model precisely accounts for how incident light and the distribution of subs urface scattered light are influenced by Fresnel attenuation and Snell refraction at a smooth air-dielectric surface boundary. Whereas simil ar assumptions about subsurface scattering and Fresnel attenuation hav e been made in previous research on diffuse-reflectance modeling, the proposed model combines these assumptions in a different way and yield s a more accurate expression for diffuse reflection that is shown to a ccount for a number of empirical observations not predicted by existin g models. What is particularly new about this diffuse-reflectance mode l is the resulting significant dependence on the viewing angle with re spect to the surface normal. This dependence on the viewing angle expl ains distinctive properties of the behavior of diffuse reflection from smooth dielectric objects, properties not accounted for by existing d iffuse-reflection models. Among these properties are prominent diffuse -reflection maxima effects occurring on objects when incident point-so urce illumination is greater than 50 degrees relative to viewing, incl uding the range from 90 degrees to 180 degrees, where the light source is behind the object with respect to viewing. For this range of incid ent illumination there is significant deviation from Lambertian behavi or over a large portion of most smooth dielectric object surfaces, whi ch makes it important for the computer vision community to be aware of such effects during incorporation of reflectance models into implemen tation of algorithms such as shape-from-shading, A number of experimen tal results are presented that verify the proposed diffuse-reflectance model.