Towards objective evaluation of the skin aspect: principles and instrumentation

Background/aims: The aim of this paper is to propose an objective procedure for the evaluation of the skin aspect. This work is based on light scattering phenomena and in-vivo measurements of the skin profile and of the skin Bidirectional Reflection Distribution Function (BRDF). Methods: We consider the basic mechanisms of light reflection and scattering by a surface to extract the key surface-parameters of aspect. It appears that the surface relief is determinant for the local surface orientation while the light scattering properties of the skin are responsible for the visibility of that skin relief. The first part presents the principles of skin-image formation on the retina and describes the actual roles of the BRDF and of the skin relief. The second part is dedicated to experimental measurements on the skin in vivo. The 3D relief is reconstructed by a fringe projection method and numerical processing. The angular distribution of light reflected by the skin is also measured by means of a different set-up. Results: The knowledge of both the skin relief obtained by 3D optical profilometry and the skin scattering behavior measured for regularly distributed set angle values, allows an objective aspect evaluation as well as a prediction of the skin images that an observer will receive. Conclusion: This paper presents both the principles and instrumentation elements suitable for the setting up of an objective evaluation procedure of the skin aspect.