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The uniform diffuse component represents the nondirectional portion of the reflected intensity distribution. It corresponds to Lambertian reflection and typically arises from subsurface scattering (for the case of plastics and other nonmetals) or multiple surface scattering (for the case of metals). For nonmetals, the term can be significant, requiring experimental measurements of the directional-hemispherical reflectivity to complete the light reflection model [HE91]. For many metals with shallow roughness slopes, on the other hand, the multiple surface scattering is very small, and the uniform diffuse component may be neglected.
The uniform diffuse component dIr,ud is therefore a function of wavelength λ and material. The illustration below shows how it behaves for common materials. The larger semicircle corresponds to 100% Lambertian reflection of the incident beam.
Figure 14: Interactive display of the uniform diffuse term
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