## 1 Introduction  
Artworks are continuously subjected to external influences, such as temperature and humidity variations, condensation-vaporisation cycli, biological attacks, (Ultraviolet (UV)) -radiation that lead both to internal and surface alterations and eventually damage [1]. In relation to the degradation processes, each layer of the artwork itself can be affected. The most important artworks (i.e., canvas paintings, mural paintings, panel paintings, inlay works, icons) can be considered as multi-layer structures, in which each layer fulfills a key role. Therefore, important remnants of the history of the artefact may be partially lost in time. At this point, the use of techniques which improve the readability of artefacts undetectable to the naked eye is highly desirable. Using radiation outside the visible range, different characteristics of the inspected artwork may be revealed according to the bandwidth in which images are acquired. Beyond the traditional diagnostic methods such as reflectography and thermography, selective multi-spectral analysis in the near-infrared region (IR-A) has recently been demonstrated to be a promising tool for investigating pictorial layers [2; 3]. Within this work, infrared radiation is divided according to the international Commission on Illumination (CIE) and sensor response scheme as presented in Table 1[4]. Non-destructive testing (NDT) methods are based on physical principles and, from their specific application, the necessary information is obtained to establish a diagnosis of the test object's integrity and quality. They help to establish the origin and period of manufacture of the piece of art [5]. Imaging data integration provides a multi-layered and multi-spectral representation of the painting that yields a comprehensive diagnosis, confirms or denies the anomalies individuation, by minimizing in the same time the ambiguities of information based on a single diagnostic method [6]. [PERSON] used a high-resolution IR-A camera to capture photographs (\"reflectograms\") of hundreds of paintings in over a dozen museums on three continents. In some cases, these IR-A reflectograms have provided new insights into the artists' creative process while creating the final images that are possible to see in the visible [7]. [PERSON] et al. studied an integrated approach based on traditional and innovative techniques (i.e., holographic interferometry, pulsed thermography (IR-C) and near-infrared reflectography (IR-A)) applied on poplar wood with a complex surface shape containing artificial defects positioned at several depths and linked to the influence of the support on the upper layers up to the external coating [8]. In [9], the authors applied high resolution IR-A reflectography and IR-C' imaging to characterize the superficial layer of a fresco \begin{table}
\begin{tabular}{c c c c c} \hline
**CIE Abbreviation** & **Wavelength** & **Spectral Response Abbreviation** & **Wavelength** & **Method** \\ \hline IR-A & 0.7–1.4 μm & Near-infrared (NIR) & 0.7–1.0 μm & Reflectography \\ IR-B & 1.4–3.0 μm & Short-wave infrared (SWIR) & 1.0–3 μm & Reflectography \\ IR-C’ & 3.0–5.0 μm & Mid-wave infrared (MWIR) & 3.0–5.0 μm & Active thermography \\ IR-C” & 7.0–1000 μm & Long-wave infrared (LWIR) & 7.0–14 μm & Active thermography \\ \hline \end{tabular}
\end{table} 
Table 1: Infrared radiation division schemes used within the manuscript, based on Commission on Illumination (CIE) and spectral response scheme. Note: the quotes are own modifications to subdivide in accordance with the spectral response scheme. and to analyse the stratigraphy of different pictorial layers. IR-C\" thermography in the 7-14 \(\upmu\)m band was also used to understand the nature of the support. [PERSON] et al.