## 5 Advancements in CH Conservation: The Role of Terahertz Technology  
This section highlights the significant advancements made in the application of THz technology for cultural heritage (CH) conservation science in recent years. These advancements include material spectroscopy, as well as both 2D and 3D imaging and tomographic analyses [151]. The most important imaging methods include technical photography [152; 153; 154; 155; 156; 157], ultraviolet photography [158], reflectance transformation imaging [159], infrared reflectography [160], multispectral imaging [161; 162; 163], and X-radiography [164]. Standard techniques for analyzing artworks encompass a range of advanced methods, including photographic examinations, infrared and ultraviolet imaging, X-ray analysis, and fluorescence techniques to identify metals and pigments. This technique combines material characterization with time-of-flight imaging to penetrate opaque materials, allowing for subsurface imaging of culturally significant artifacts. Unlike the mid-infrared region, which mainly reveals intramolecular information, THz spectral features are influenced by molecular and intermolecular interactions, weak bonds, and phonon absorptions. The spectra of various mineral and inorganic materials historically employed as pigments in artwork--such as cinnabar (HgS) and ornment (As\({}_{2}\)S\({}_{3}\)) [165; 166]--have been extensively studied. THz spectroscopy offers adjustable resolution, from micrometers to millimeters, facilitating in situ and field measurements without sample extraction [150]. Nondestructive imaging methods like X-rays and \(\gamma\)-rays are ionizing and require large setups. In contrast, THz radiation is non-ionizing, posing less risk to artifacts and humans [150]. This technology provides a safer, noninvasive tool for analyzing the construction and history of objects and buildings. THz spectroscopy can be performed in either frequency-domain spectroscopy (FDS) or time-domain spectroscopy (TDS). FDS uses tunable narrow-bandwidth sources, while Fourier transform infrared spectroscopy (FTIR) is more commonly used for analyzing cultural heritage materials. FTIR employs a broadband continuous wave source with a two-beam interferometric spectrometer covering 0.5 to 20 THz [150]. In contrast, TDS uses broadband-pulsed terahertz sources and coherent detection methods. In 2008 [167], it was demonstrated that THz imaging could penetrate up to 1 cm into plaster, leading to research on wall paintings covered with whitewash or thick layers. For this reason,  
Figure 15: _Superficie 538_, [PERSON], 1961 [73]. Credit [PERSON] et al. Distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license: http://creativecommons. org/licenses/by/4.0/ (accessed on 20 November 2024). The figure is made by one out of three images of the original figure; changes were made to capture.  
Figure 16 shows a detail from a 14 th-century icon that was analyzed using both infrared imaging and THz [166].  
THz imaging can identify structural issues in panel paintings by revealing different interfaces (wood, primer, paint) and showing paint flaking. It also helps to understand the role of the wood grain in paint cracks, which are documented through infrared RTI (Infrared Reflectance Transformation Imaging) [167]. This non-invasive technique enables the evaluation of visible paint cracks, determining their depth and extent, and visualizing the complex preparation layers involving canvas and gesso.