## 5 Advancements in CH Conservation: The Role of Terahertz Technology
### Terahertz 3D Imaging for Art and Archaeology  
Several methods exist for constructing 3D THz images, depending on the characteristics of the source. Narrow bandwidth methods, for instance, necessitate multiple projection angles along with back projection to create a spatial 3D reconstruction of the object. This approach is commonly referred to as terahertz-computed tomography (THz-CT). THz-CT is an imaging technique that has low spatial resolution. It is preferred over X-ray imaging for its non-interference with thermoluminescence dating and its sensitivity to organic materials. THz-CT employs narrow bandwidth sources, multiple projection angles, and back-projection methods. Broadband sources can represent time series for each pixel in the XY-plane, with depth along the Z-axis. THz-CT uses pulsed terahertz radiation for the 3D reconstruction of a sample's internal structure from a single projection via the time-of-flight delay of the terahertz pulse [168; 172]. Broadband time-domain systems excel at performing similar tasks. When broadband time-domain measurements are obtained in reflection geometry, they enable the creation of detailed 3D images of an object's internal structure. This is achieved by plotting the time series for each pixel in the XY-plane, with depth (Z) represented along one axis; the peaks in the data correspond to reflections from internal features [165]. _Narrowband Frequency-Domain Techniques._ [PERSON] et al. successfully demonstrated THz-CT of nested wooden Russian dolls, known as Matryoshkas, utilizing a system based on a continuous wave Gunn oscillator operating at 240 GHz [180; 181]. They employed three established X-ray CT methods to reconstruct cross-sectional images from transmission measurements taken at different angles, resulting in 3D representations of internal structures. [PERSON] et al. successfully recovered pencil characters from the 2 nd, 25 th, and 50 th pages of a 50-sheet stack of paper [182] using a filtered back projection technique known as tomosynthesis. This method utilized a frequency-multiplier continuous wave source operating at 540 GHz, paired with a Schottky diode detector. This technique is beneficial for imaging thin, wide, or flat objects, requiring only one-tenth the projection angles of traditional CT [182]. They addressed the layers of interest by scanning and rotating the sample plane within a range of \(-90^{\circ}\) to \(+90^{\circ}\)[182], capturing projections at arbitrary integer intervals. By shifting and combining the absorption images from each projection, they effectively diminished the prominence of overlapping layers. This approach achieved a 1.4 mm depth resolution between superimposed characters with five projections and provided better lateral resolution at smaller incidence angles. Further optimization of the beam profile and angle enhances its potential for imaging fragile ancient texts and wall paintings [182]. _Broadband Time-Domain Technique._ Pulsed THz radiation is less commonly used in computed tomography than continuous wave methods. [PERSON] et al. [183] successfully applied pulsed THz-CT imaging to larger archaeological artifacts, such as a clay vessel from the tomb of [PERSON] and [PERSON] in Thebes. The index and absorption spectra of clay were utilized to enhance and refine the surface image quality in the 3D reconstruction of a coin embedded beneath a clay tablet. This study highlights the potential of pulsed THz imaging for reconstructing images of objects within clay enclosures. Although THz-CT imaging may lack the spatial resolution of traditional X-ray methods, THz-CT has advantages, particularly in preserving valuable thermoluminescence dating results that X-ray irradiation can compromise. THz imaging offers enhanced sensitivity to low atomic number organic materials, making it a promising alternative for non-destructive analysis [184]. [PERSON] et al. [184] compared their continuous wave techniques with narrowband sources using a pulsed THz source to evaluate the internal reconstruction quality of a Teflon cylinder.