Bi-directional reflectance distribution factor and gloss scales

Gloss is a visual attribute, which, as well as colour, provides qualitative information on the surrounding objects. The relevant physical quantity for gloss measurement is the BRDF that characterises the geometrical distribution of the reflected light on the sample. We hypothesise that the light reflection on a glossy sample is split in 2 parts. The volume diffusion and the surface reflection. We assume that the surface of the sample consists of tiny mirror facets the orientation of which is specific of the surface microstructure. Each facet is the source of specular reflection. With this hypothesis, we have calculated a theoretical model of the BRDF according to Fresnel laws. We have applied the model to black and white painted samples of various gloss indexes. In addition, we have acquired real measurements of the BRDF of the same samples by using a new device (the EZ-Contrast from Eldim) which gives the complete measurement of the luminance geometrical distribution in the hemisphere. The comparison of the measurements and the theoretical model shows that, for this series of samples, the facet hypothesis is reliable and makes it possible to model the real BRDF in all directions.