## 6 The Realization of a Portable Imaging Device  
A new research project (THz-ARTE) based off a previous scientific collaboration [36] was funded in 2013 by the Italian Ministry of Foreign Affairs and International Cooperation (MAECI), within the framework of an Italy-Japan bilateral agreement. The project's main objective was to test the potentiality of the newly developed THz technique and to produce a prototype of a THz 3D scanner, one that is specifically devoted to investigating mural paintings, to be used in situ. Such a device would make it possible to detect \"objects\" (defects, picture layers, water damages, substrate structure, any changes in the refraction index, etc.) under the surface of the sample, thus avoiding all the problems related to the use of X-rays radiation: due to the low photon energy, THz radiation is non-ionizing. Therefore, it is not necessary to comply with all the safety requirements related to the use of X-rays devices, ones that complicate the logistics of \"in situ\" measurements. ENEA, IFAC-CNR and NICT were the main partners of this new project [41]. The first task of the project was to identify a suitable source, since the FEL can obviously not be used for \"in situ\" measurements. A 70 mW IMPATT diode was selected as a source at 97 GHz and a Shottky diode with 200 V/W responsivity was chosen as a detector  
Figure 11: **(a)** Panel painting mock-up with holes (side view); **(b)** THz scan over the hole indicated by the arrow (top view). Figure 10: THz image of an ancient wood fragment: **(a)** fragment with identification of the scanned area; **(b)** visible image; **(c,d)** THz images taken at different distances. Performances of the system were tested by means of a low-cost scanner prototype that was based upon the mechanical structure of a 3D printer (Figure 12). The results of the measurements carried out with the new scanner were comparable to those previously obtained with the FEL (Figure 13) presented in the previous section. Once the first results, which demonstrated the performances of the device, were obtained, a new prototype was constructed [42]. This new prototype demonstrated better performances in terms of speed, precision of the motor stages, and the dimensions of the scanning area. The three high-speed motorized linear stages, assembled in a X-Y-Z configuration, enabled measurements on vertical (XY) surfaces covering an area up to \(500\times 300\) mm\({}^{2}\). The Z-stage has a travel range of \(100\) mm. The IMPATT diode oscillator described above was fitted to the Z stage of the scanner together with a probe head, thus making it possible to change the distance between the probe head and the sample surface. All three mechanical stages had a position accuracy of better than \(1\) um. A custom control software was designed in order to maximize the signal acquisition and the scanning speed. The scanning time of the entire  
Figure 12: First prototype of a THz 3D imaging scanner. Figure 13: Comparison of the imaging performance of the FEL imaging device (**a**) and of the new imaging prototype (**b**). Size of the FEL THz image: \(50\times 40\) mm\({}^{2}\); size of the IMPATT THz image: \(60\times 60\) mm\({}^{2}\). \(500\times 300\) mm\({}^{2}\) area with 1-mm resolution was about 40 min. THz radiation was transported toward the sample using a WR10 open-ended waveguide and launched in free space. The radiation reflected from the sample was partially injected back into the same waveguide and was measured by means of a 3-dB directional coupler and a Shottky diode with a 200 V/W responsivity. A laser triangulation system was added to the probe head in order to measure precisely the shape of the sample surface. In fact, the phase variation that produced the modulation of the reflected signal depended on both the sample distance and the sample optical properties. If the sample is not planar, and that is the usual situation when working on artworks, the morphological information of the object under investigation is combined with the optical one.