# Terahertz Inspection of Buildings and Architectural Art
## 2 Terahertz Uses in Architectural Art
### Wall Paintings
After converting the profile into a spherical coordinate system, deviations from the focal plane have been computed in the zenith, azimuthal, and orthogonal projection dimensions. The resulting propagation of the beam to wall surface and its deflection from the optimum line of reflection were calculated and the misalignments were used to evaluate the overlap integral describing the coupling between the displaced reflected beam and the detector, which has been, in turn, used to correct the data for the percentage of radiation deviated from the detector. [PERSON] et al. [27] studied the scattering effect caused by the mortar sand aggregates found in most of the wall paintings, whose presence prevents the THz radiation from penetrating deep into the wall. For the study, they created replicas of the wall paintings and scanned them with a terahertz time-domain system, such a study included a variety of styles of wall paintings, such as Pompeian, Etruscan, Byzantine, Buon Fresco, Sgraffiti, Egyptian, and Japanese. These kinds of wall paintings differ significantly in terms of materials used as support, preparation and paint layers. In addition to wide variations of the layer thicknesses and painting techniques. Significant results were obtained by analyzing THz B-scans. In all models, the THz reflections from the surface were more than ten times larger than those from internal boundaries. Yet, the THz reflection pulses were found above noise level in the layer, including large aggregate particles. Because the wavelength of the THz waves is comparable to the size of the sand particles (\(\sim\)1 mm), scattering in layers rich in sand becomes strong and prevents THz waves from propagating deep into them. Thus, THz B-scans obtained from replicas having large sand aggregates were not clearly representing the internal structure, owing to high scattering. Small echoes appearing on the recorded THz depth profiles have been related to changes of the layers' porosity or to interfaces between mortar with different aggregates. Since the first applications, THz time-domain imaging has been employed on wall paintings for inspecting their structure and conservation state as well as for visualizing hidden depictions. Historically, it is, in fact, not rare to find wall paintings that have been overpainted or covered over time, hiding the original representation, for aesthetic or conservation reasons. Concerning hidden paintings, analyzes of THz B-scans have been proficiently used to detect subsurface depictions and underdrawings. The results in this sense have been obtained by [PERSON] et al. [23] by THz scanning real wall paintings of different European Cathedrals, for instance, the Chartres Cathedral (France), the Riga Dome Cathedral (Latvia), and the Chartreuse du Val de Benediction (France) with a time-domain systems. Wall paintings of Chartres and Riga Cathedrals were covered by layers of plaster (Chartres) or lead white paint (Riga), while underdrawings sketched with sinopia were known to be under the paint layers of the paintings of the Church in Chartreuse du Val de Benediction. THz B-scans gave evidences about the presence of subsurface layers (Chartres and Riga), but also about subsurface layers losses (Riga). At Chartreuse, Sinopia underdrawing seemed to be detected in the THz B-scans at approximately 1 mm deep, together with the uneven stone surface upon which the wall plaster was applied, at depths calculated between 8-15 mm. [PERSON], [PERSON], [PERSON], and [PERSON] used a THz time-domain system to scan a XIX century wall painting, Trois homes armes de lances, in the Louvre's [PERSON] collection. As reported in a ScienceDaily article [28], [PERSON] was an Italian art collector and art historians who is believed to have painted the current depiction on a previous Roman fresco that was removed from its original location in Italy and entered his collection. The THz research team was able to image the previous ancient Roman fresco underneath the current depiction. [PERSON] et al.