## 5 Advancements in CH Conservation: The Role of Terahertz Technology
### THz Imaging for Art and Archeology
successfully captured transmission images of various materials, including timber, concrete, and ceramic tiles [173; 174]. They developed a compact transmission imaging system using a GaAs tunnel injection transit time (TUNNETT) diode as the source and a Schottky barrier diode as the detector, operating at room temperature and 0.2 THz. Lower frequencies (0.01-0.5 THz) allow penetration of several centimeters into opaque materials while overcoming scattering, achieving a spatial resolution of 1-2 mm [168]. In contrast, higher frequencies (0.5-13 THz) are effective for examining historic architecture and archaeology, including wall paintings and structures [168]. These different wavelengths promise to be useful for inspecting defects, detachments, degradation, water damage, and biological infestations in the conservation of both structural and decorative architecture. [PERSON] et al. [175] utilized a compact free-electron laser (FEL) operating at 150 GHz to generate reflection images of paintings with sub-wavelength resolution [175]. The wood panel painting, featuring gold leaf and natural pigments, was partially covered by a 0.5 mm layer of gesso. Signals were detected using room-temperature Schottky diodes connected to waveguide directional couplers. [PERSON] et al. have developed a highly compact and portable THz camera capable of capturing real-time images of art objects at an impressive acquisition rate of 60 frames per second [176; 177; 178]. They measured real-time water diffusion across the paper in ten seconds, achieving a spatial resolution of about 0.3 mm. Integrating a multifrequency source with a real-time THz camera enables immediate material mapping in the field, enhancing the rapid identification of key areas, monitoring restoration processes like diffusion or drying, and observing dynamic deterioration such as sub-surface crack progression during repairs. _Broadband Time-Domain Techniques._ THz time-domain systems use ultrafast femtosecond laser pulses to measure wide spectra and provide imaging capabilities. Spectral information is obtained via Fourier transformation of the time-domain signal, revealing the amplitude and phase of the transmitted or reflected THz signal. This approach provides a direct method for determining the complex optical constants of materials. In transmission, the primary pulse propagates through the material, while in reflection, it is associated with the first surface reflection. In both scenarios, the resulting time-domain signal captures signatures from internal reflections occurring at interfaces within layered materials, each separated by their time-of-flight. By gating the time-domain signal, it becomes possible to isolate reflections from a specific layer, facilitating imaging and spectroscopy of concealed layers [165]. [PERSON] et al. were pioneers in utilizing transmission THz-TDS to image canvas paintings [178]. They analyzed temporal signatures to detect paint layer variations and used spectral information to examine differences in paint composition and the canvas. [PERSON] et al. demonstrated in [176; 177] that spectral signatures can effectively extract distinct information about parchment, ink, and stains of medieval manuscripts. This is achieved through component spatial pattern analysis applied at each pixel, as illustrated in [178]. [PERSON] et al. [165] were pioneers in employing reflection THz-TDS to examine the 8B graphite underdrawing of a water-based painting on plaster of Paris. The paint's transparency and graphite's reflectivity were affected by layer density, thickness, and the refractive index contrast with the plaster substrate. [PERSON] et al. successfully measured the thickness of the underdrawings beneath a canvas painting using reflection techniques. Their findings highlight the advantages of THz-TDS compared to traditional methods such as X-ray radiography and infrared reflectometry for this analysis [179; 180; 181].