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The visual aspect of products meant for the general public is becoming an equally important criterium for their commercial success as their technical qualities and their cost. In most cases, the aesthetic quality of products is visually assessed by experienced people whose judgement is considered as a reference.This procedure has two major disadvantages: i) This visual estimation is entirely related to individual sensitivity and cannot be quantified. Only a comparative analysis is possible and requires a sufficient range of reference samples which are indispensable for this evaluation. ii) This visual evaluation concerns the global evaluation of the appearance and cannot dissociate with entire precision the roles of different parametres such as the color, the luminosity, the form and the shine of the surface. .. It is therefore rather difficult to point out possible modifications that could be brought to the process of fabrication in order to improve the aesthetic quality of the products. To be freed from the restrictions of visual inspection, the industrial world tries to endow itself with systematic procedures and instruments for physical measurement that evaluate the visual aspect of the surfaces. The study that we propose here belongs to this field of research, and applies more particularly to the measuring of the brightness of plastic pieces the surface of which presents an undulation of less than a millimetre deep (skin profile). In the first part of this study, we shallpropose a systematic analysis of visual aspect which four different and independant criteria defines. We shall explain why traditional methods for the measurement of scattering cannot be applied to such wavy surfaces. The material BRDF (Bidirectionnal Reflection Distribution Function) must be estimated from the analysis of their roughness by the means of established scattering models, such as that of Beckmann and Spizzichino. Secondly we shall introduce the procedure that we have developed in order to analyze the visual aspect of plastic samples, through confocal microscopy and image processing. A low magnification analysis allows one to measure the skin contours of the surfaces and to infer the local surface orientation which determines the exact values of the incidence and observation angles. A second analysis, with high magnification and an adequate image processing software, enables the extraction of the microroughness responsible for the scattering, and one can then evaluate the scattering properties of the material. Finally,the combination of the properties of scattering and of the local orientation of the surface allows the prediction of the degree of visibility of the skin profile of the surface. The statistical distribution of the measured parametres is also taken into account.
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