Analysis of scratch visibility on polymeric surfaces using 3D roughness measurement and the bidirectional reflectance distribution function (BRDF)

The evolution of visual aspect of polymers with wear is a fundamental problem in numerous industries. In this article, we study the visibility of scratches. Fifty parallel micro-scratches are performed on three samples differing only by their values of gloss. Scratches are much more visible on glossy samples than on mat ones, in agreement with previous works. To interpret these results, a bidirectional reflectance distribution function (BRDF) model is developed to compute the way a surface scatters light based on a roughness measurement described by a distribution of microfacet orientations. In this paper, this BRDF model is applied to investigate the influence of the scratches on the light scattering properties of a surface. Roughness measurements are performed on the surfaces before and after scratching. Then, the BRDFs are computed and compared to each other. The visibility of scratches strongly depends on how much they modify the roughness slope distribution function of the surface. Depending on the initial gloss of the surface, this leads to more or less contrast. Finally, a visibility criterion is proposed based on this contrast effect.