## 9 Conclusions and Perspectives  
The new technique developed for THz imaging measurements has shown its strengths, but also its weakness. The portable device proved to be useful for \"in situ\" measurements. Problems arising from the non-planarity of the sample surface were solved by utilizing a laser triangulation system to obtain the surface topology of the sample and to correct it with the use of various techniques. Depending on the size of the sample, it is possible to choose the right technique: for small objects, a complete, but more time-consuming technique can be used for performing multiple \"slice\" scans, while a faster scan that adjusts the probe distance in real time is preferred for large areas. Unfortunately, the non-planarity of subsurface structures cannot be corrected with this method, thus limiting the amount of information that it is possible to obtain from the measurements. For example, it is possible to detect any sort of defect that produces a distinct change in the refraction index inside the sample (similar to a detachment in a fresco, or a nail inside wooden materials), but it is difficult to determine precisely the depth of the discontinuity and to obtain optical information regarding internal defects. New measurements on mosaics also shown a limit in the ability of the system to correct the surface topology of the sample, when sharp angle are present on the surface. In such conditions conventional imaging technique would probably produce similar or better results, thanks to the high reflectivity of water. Differential measurement, to monitor time dependent changes in the samples would benefit from the phase sensitive technique though. Another practical limitation of the current device configuration is related to the long wavelength: at times, the penetration depth is too great for some of the objects. For example, after the new successful measurements on leather samples in the laboratory, \"in situ\" preliminary measurements showed that, in scanning leathers mounted on a wall, the non-planar surface of the wall under the leather affects the measurements to enable the obtaining of meaningful results involving the leather structure. For such measurements, a higher frequency would be necessary in order to perform \"in situ\" measurements. Fortunately leather wallpapers can be detached from the wall in order to perform tests in a laboratory environment, where the reflection from the substrate can be easily eliminated. Such a procedure is non-invasive and is currently utilized from the art conservators when the leather wallpapers have to be restored. According to the authors' experience, future developments for THz measurements in the field of art conservation could then proceed in different directions. For applications at low frequencies (<1 THz), an upgrade in the current system configuration is in progress and includes the use of a variable frequency source, in the 150-300 GHz range. The ability to electronically change the emission frequency makes this new configuration more flexible. For example, it might be possible to use the system in a configuration similar to the typical set-up of a geo-radar system in Frequency-Modulated Continuous-Wave (FMCW) operating mode. Phase sensitivity is useful for obtaining optical information about the sample, as demonstrated by the results of the study on the pigments described in Section 5; however, by moving towards higher frequencies, it is no longer necessary, since it is easier to find specific pigments that have transitions in this part of the THz region (above 1 THz). Therefore, it would be interesting to extend the present hyperspectral reflectometry systems toward THz frequencies. Such a system should include both a THz illumination system and a THz camera, to be coupled and assembled within the same mechanical system as the one used for IR reflectometry. The sensitivity of the THz cameras currently available on the market should be taken into account. Hence, a bolometric type camera should be the best option, due to its high sensitivity.