# Terahertz Inspection of Buildings and Architectural Art
## 2 Terahertz Uses in Architectural Art
### Wall Paintings
[29] used a reflective pulsed THz medical imaging system based on direct detection to acquire imagery of painted plaster samples in order to assess the ability of the system to image the Byzantine wall paintings at the Enkleistra of St. Neophytos in Paphos (Cyprus). The walls of the Enkleistra feature several painting phases spanning from the XII to the XVIII century. The various painting phases are separated from each other by a thin plaster layer or a lead white paint. For the research, they prepared several wall painting mock-ups of both fresco and secco techniques. The two relevant samples were secco samples painted in the shape of a cross respectively with cinnabar and minimum and later obscured by lead-white and chalk (minimum). A stark contrast between the paintings and surrounding plaster material was observed in the THz pictures for both the pigments used. Despite the strong scattering of THz radiation by a covering chalk layer, the THz image of the minimum based paint obscured by the chalk reveals the major part of a four-arm cross while apart, one of the four arms, was not clearly imaged possibly due to an uneven application of the paint and/or uneven surface of the plaster. By utilizing the system's confocal-like features, they acquired multiple THz images of the cinnabar based paint obscured with lead-white at increasing focal point locations (z values). Only partial images of the cross were visible at each z value, yet, a more complete THz image of the cross was obtained by combining the multiple THz z-scans, demonstrating how this method serves to better visualize hidden layers that are not perpendicular to the THz signal. The study on replicas demonstrate the feasibility of an in-situ investigation of the Byzantine paintings at Enkleistra. Reflection terahertz time-domain method has been used to obtain important information about plaster and internal structure of wall paintings, helping in determining their manufacturing and conservation state. [PERSON] et al. [30] used a time-domain THz system to examine the plaster layer condition of the mural paintings of the Takamatsuzuka tumulus (Nara, Japan, end of VII beginning of VIII century). The paintings decorated the walls and ceiling made by tuff blocks and they are covered by few millimeters of plaster. For conservation reasons, in 2007 the stone chamber was disassembled and transported to a controlled environment space located near the Tumulus. The concerns were about the change of the hygrometric conditions from the old to the new location and the related reduced water content of the plaster, which caused material weakening and layer detachments because of its contraction. From the THzmesaurements, the areas where the plaster layer was well attached to the substrate, the reflection at the boundary of the plaster, and tuff stone could not be observed in the THz B-scans probably due to the similar refractive indices of both materials and to the absorbance of the reflected pulse. However, other regions displayed in the B-scans showed small reflected THz pulses displayed beneath the superficial one, indicating that the plaster layer had detached from the tuff stone, creating an air gap in between them. THz C-scans representing the absolute intensity integrated over increasing time intervals showed the subsurface air gap distribution within the scanned area. The detachment locations that were extracted in this manner were consistent with the ones recognized using other techniques by the restorers. Therefore, the terahertz imaging technique was proven to be a suitable method for noninvasive investigation of the conditions of a plaster layer. [PERSON] and [PERSON] [31] demonstrated the possibility to three-dimensionally (3D) visualize the surface profile of building elements buried inside the wall plaster by means of THz time-of-flight plot (ToF) and the volume rendering technique. An extended area of the apsidal Medieval wall painting of Nebbelunde Church (R\(\phi\)dby, Lolland, Denmark) was scanned with a THz time-domain system.