Automatic identification of varnish wear on historical instruments: The case of Antonio Stradivari violins

In the field of cultural heritage, UV-induced fluorescence (UVIFL) photography is extensively applied to the study of artworks. In the case of historical musical instruments (e.g. violins), this technique allows seeing important details that usually cannot be detected under visible light, such as retouching, different paint and varnish coats or worn areas of the superficial varnishes. The interpretation of UVIFL images, even when performed by expert people, may be very complex, taking into account the chemical and physical modifications undergone by the analyzed instruments during the centuries. The aim of this work is the development of a new tool able to help experts by automatically detecting the presence of worn areas on the surface of violins. The proposed algorithm is based on a specific combination of thresholding and mathematical morphology designed to detect some characteristic fluorescence colors. The system discriminates different wear levels, finds their position on the surface and computes their percentage respect to the total area. To validate our approach a collection of UVIFL images of Stradivari's violins held in the “Museo del Violino” in Cremona (Italy) was considered. The analyses of the UVIFL images taken on the back plates of the instruments provide results that are in agreement with the naked eye segmentation and classification performed by expert people (e.g. violin makers, restorers and the curator of the museum).