## 6 The Realization of a Portable Imaging Device
With the laser triangulation system, it is possible to obtain information regarding the surface distance of each pixel of the image and to correct it by using two different techniques. The first one employs laser data in order to perform a THz scan at a fixed distance from the sample by following the shape of the surface with the probe head in real time (Figure 14). The second technique makes use of multiple planar THz scans at different nominal distances from the sample but interpolates the data inside the data cube obtained with the theoretical model described above, to obtain virtual scanning surfaces at a constant distance from the sample (Figure 14). A schematic layout of the system is reported in Figure 15.  
The 1-mm step size does not correspond exactly to the real lateral resolution of the system: at 97 GHz, the wavelength is about 3 mm, so that the lateral resolution is limited by diffraction to about 1.5 mm. Moreover, despite the fact that the probe head is very close to the sample surface, radiation undergoes diffraction widening when launched toward the sample. Choosing a smaller step  
Figure 14: Methods for correcting the effect of surface topology: (**left**) surface following; (**right**) data cube interpolation. Figure 15: Layout of the new imaging system. for the raster scan results in an integration between data of adjacent pixels, thus producing a slight \"smoothening\" of the resulting image. With the use of this new device, a fresco painting on _tavella_ (51 cm \(\times\) 37 cm) depicting [PERSON] by [PERSON] (1655-1726) was analyzed at the Restoration Laboratories of the former \"Polo Museale della Toscana\", in Florence (Figure 16) [43]. The fresco of [PERSON]'s head belongs to the permanent collection of the Florentine San Marco Museum (inv. 1890 n. 8511) and was painted by [PERSON] in 1700-01 for the Giovanato chapel (also known as [PERSON] chapel) in the Convent of San Marco [44; 45]. The imaging scanner makes use of the laser triangulation system to align the axes of the scanner with the fresco surface and to determine its surface topology. These data were used in order to set the position of the motors in such a way as to make a collision of the probe head with the fresco, while moving it very close to the fresco surface (about 1-mm distance), mechanically impossible. For the sake of the same \"safety\" concerns, instead of following the shape of the fresco surface with the probe head, it was decided to utilize the second technique previously described, which consisted of performing multiple scans at different \"nominal\" distances from the painting's surface. Data from these scans were assembled for the purpose of building a data cube. Data regarding the surface shape, measured through the laser triangulation system, were used to interpolate the values in the data cube, using the simple diffraction-based theoretical model described above, in order to obtain a series of \"virtual surfaces\" that lie \"parallel\" to the real sample surface (Figure 14). Such virtual surfaces are not affected by the phase error due to the non-planar artwork surface, and since these surfaces are the result of an interpolation with a theoretical model, it is possible to obtain reflectivity data also for surfaces that lie very close to the artwork surface, where experimental data are not available. By using this technique, measurements were performed in different areas of the artwork. The analysis of the data revealed clear patterns that did not correspond to any visible feature of the painting. THz image patterns are most likely to be related to structures that are below the outermost paint layers, thus producing a phase variation that is much greater than the one induced by the pigment difference on the artwork surface. The non-planarity of the fresco surface was mathematically corrected, but the _tavella_ used as a support for the painting could not be considered perfectly planar, and hence might have produced artifacts that could not be corrected as was done for the artwork surface.