FFT-based imaging processing for cultural heritage monitoring

Conservation of precious metallic works of art requires an in situ monitoring of the conditions of the objects and of the protecting coatings employed for their restoration. In this paper a simple diagnostic tool based on digital photography and 2D-FFT imaging processing is proposed that is used for monitoring the corrosion process occurring on metal surfaces. The proposed approach has been tested in laboratory on a set of silver reference samples which have been coated with SiO_x thin films deposited via plasma and submitted to a tarnishing test. The obtained results show that the proposed technique permits to overcome most of the problems usually encountered when using conventional image processing based on amplitude and colors that require maintaining standard lighting and artifacts conditions, that are not easily achievable in museums. Although the proposed solution is not able to provide details about protective coating failure and the corrosion mechanism, its results can be correlated to the tarnishing susceptibility and, therefore, this technique can be use to assess the stability of the artifacts and to put in evidence the beginning of dangerous localized corrosion phenomena, that require immediate restoration.