Including of specular component in a BTDF or BRDF assessment based on digital imaging

Bi-directional Transmission (or Reflection) Distribution Functions, commonly named BTDFs (and BRDFs), are essential quantities to describe any complex fenestration system in details. They are defined as the ratio of the luminance diffused from a surface element in a given direction (after transmission or reflection), and the illuminance incident on the sample. However, these functions are capable of describing the regular (specular) as well as the diffuse components of emerging light, and their mutual knowledge is necessary to assess a glazing or shading system’s optical performances properly. Although the analytical expression of a BT(R)DF differs whether it is related to regular (specular) or diffuse light, a simultaneous assessment of the two components can be achieved under certain conditions, presented in this paper. They are thereafter analyzed for the particular data acquisition procedure developed for a novel type of bi-directional photogoniometer, based on digital imaging.