Terahertz time-domain imaging (THz-TDI) and spectral-domain optical coherence tomography (SD-OCT) are two techniques capable of providing 3D datasets from which depth profiles and cross-sectional images of an object can be derived. They are novel photonics technologies of particular relevance to the field of heritage science, for which the comprehension of the stratigraphic structure of a cultural heritage object may help in the understanding of its artistic technology and state of preservation. The differences in imaging depth, field of view, and axial/lateral resolutions of the two imaging techniques provide different but complementary information of the same scene. Through the use of multimodal image fusion, the user will benefit from access to a detailed comparison of the information content of the two different datasets and images. To carry this out, we have developed a first data processing chain with the aim of representing THz-TDI and SD-OCT cross-sectional images in a unique image grid and space, in which the different datasets are dimensionally consistent.

In consequence of the flooding of the Loing river in 2016, some of the storage areas for the Musée Girodet were submerged, with 3000 artworks affected. Following this, the painting Le Jeune Trioson (nineteenth century, attributed to the school of Anne-Louis Girodet De Roucy Trioson) developed severe blanching in the varnish, over the dark background in particular. An initial non-contact investigation with a spectral-domain optical coherence tomographic system operating in the near-infrared range (830–1020 nm) gave valuable insights into the development of this alteration. This detected a change in the scattering properties of the blanched varnish compared to the non-blanched one. The depth at which the modification occurred has been clearly identified, together with different stages of alteration corresponding to the severity of the visible blanching. This information obtained non-invasively is useful for guiding a later targeted sampling campaign and for informing treatment planning for varnish removal.

Varnish removal from easel paintings is a restoration procedure that is regularly undertaken by cultural heritage conservators. In very few cases, traditional methods (chemical and mechanical) do not allow them to reach the goal of a perfectly controlled and selective cleaning. UV laser ablation has been considered since the 1990s to overcome these limitations, but its application to real cases is far from common practice. This study proposes a calibrated procedure based on ablation by an ns UV Nd:YAG laser at 266 nm combined with optical coherence tomography for micrometric laser varnish removal from inhomogeneous and highly photosensitive pictorial layers. The use of this wavelength for varnish removal, not adapted according to the literature, is discussed again after the beam shaping leading to a homogeneous and controlled intensity distribution. The innocuity is controlled by laser-induced luminescence, and some solutions for the transparency loss of the residual varnish following ablation are proposed.