UV-IR image enhancement for mapping restorations applied on an Egyptian coffin of the XXI Dynasty

The use of non-invasive analytical techniques on cultural heritage objects usually faces the insurmountable limit of dealing with artworks subjected to more restorations. For painted surfaces, imaging techniques, such as infrared photography (NIR) and ultraviolet-induced visible fluorescence (UVIVF), allow to distinguish areas with different kind and period of interventions. This paper presents the application of such techniques on two wooden coffins of the 21st Dynasty with a complex restoration history, the analytical inspections deal with the issues occurring in the reconstruction of a fluorescence map that is influenced by on both the use of non-original pigments and the presence of protective varnishes. In uncontrolled laboratory conditions, the possibility to associate quantitatively and univocally the luminance recorded by the camera with a certain level of fluorescence can be very difficult. An algorithm has been developed to obtain a map with an evident relationship between fluorescence and diagnostic information. First, the pixels of the fluorescent areas presenting different reflectance in the visible image have been used to equalize the visible image. Then, the ratio of the UVIVF image to the equalized image has been calculated, highlighting the low and non-fluorescent areas. After the thresholding and the segmentation of the UVIVF image, the masks obtained have been overlaid on the NIR images to merge and to map the areas with supposed similar restoration materials. The results illustrated in this work are followed by a punctual spectroscopic campaign, where the information on the imaging analysis is linked to and completed by the study of the chemical compounds in the coffins.