Reflectance Transformation Imaging is a technique that provides a digital and useful representation of an object through photometric and geometric local assessment of the surface. RTI technique consists in acquiring a sequence of images from a fixed observation position while varying the direction of the light source around the observed object. Thanks to a further reconstruction process, the continuous angular reflectance for each pixel can be computed from the set of discrete acquisitions and rendered interactively. Currently, the most used mathematical functions that allow this reconstruction from RTI’s acquisitions are Polynomial Texture Mapping (PTM), a method based on Hemispherical Harmonics (HSH) and most recently the Discrete Modal Decomposition (DMD). For these three approaches, a uniform spatial distribution of light sources is an implicit hypothesis. In practice, it is often not possible to achieve this uniform spatial distribution due to intrinsic limitations in systems or in the acquisition conditions. It is then necessary to take into account this nonuniformity in order to avoid artifacts that could alter modelling and subsequent visual rendering. To address this issue, we propose a methodology consisting in the estimation of the local density of the lighting directions used during RTI acquisition. These values are then used to generate a weight for each light position enabling to correct its contribution in the regression performed during the fitting.
The study and analysis of defects in wall paintings is possible by stimulated infrared thermography, this approach is of great interest to the scientific community responsible for the conservation and restoration of these works of art. The work presented consists in identifying the best way to stimulate and analyze the thermal signals measured on these works of art in order to reveal defects invisible to the naked eye such as internal decohesions. In the case of wall paintings, for example, optical effects associated with the presence of a pictorial layer can degrade this detection. To do this, we compared two excitation modes of an academic sample, excitation using conventional halogen sources emitting in the visible and near infrared and an excitation source emitting in the mid-infrared. We compared the two excitation modes and applied a postprocessing to these two experimental approaches: SVD. The results presented made it possible to observe the interest of infrared sources and the additional contribution made by post-processing of the SVD type.
Infrared thermography is a non-destructive testing technique that affects many areas. This technique of analysis is, for example, very interesting in the field of restoration and conservation of heritage works. The possibilities of active thermography can help in the early detection of defects in works of art and their characterization. In this work we will demonstrate that it is possible to detect old restorations in frescoes and murals by flash method. This new possibility offered by thermography will make it possible, for example, to verify the compatibility of a restoration with the original work, which can prevent the appearance of a defect and may allow the conservator to follow up on restoration. First we will show the feasibility of this approach on a laboratory specimen containing different types of clogging materials and then present results of analysis carried out in situ at one restoration sites that reveal the possibility of locating these restorations but also to characterize the pictorial technique used in this one.
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