Two Non-invasive Techniques for the Study of the Surface of Cultural Heritage Objects: Optical Measurement of Micro-roughness and Backscattering Spectro-colorimetry

THE GONIO-SPECTROCOLOR METER An instrument dedicated to colour measurement, notably on varnished paintings, was developed at the C2RMF, a dozen years ago, at the initiative of Michel Menu, with the help of Alain Chiron and Françoise Viénnot from the Photobiology Laboratory of the MNHN[1]. One of the objectives was to reduce the influence of specular scattering due to the protective varnish. The solution of this problem relies on the use of backscattering mode. The principle is to use for colour measurement the fraction of scattered light which follows the same path as the incident light. For practical reasons, we use an angle of 22° in our set-up. Since the first tests, much progress has been made and today, on the basis of our results, this backscattering geometry and other functionalities are offered on commercial instruments such as those from STIL. Examples chosen to illustrate this technique were accomplished with the last prototype.