# Selected Applications of Terahertz Pulses in Medicine and Industry
## 5 Terahertz Imaging and Spectroscopy in Cultural Heritage
### Terahertz Imaging  
The two principal methods of imaging using coherent terahertz radiation are CW and pulsed, and both have been considered in art restoration and other studies. The advantages of CW are lower costs and simplicity, due to the absence of the femtosecond pulsed laser; however, the image obtained is two-dimensional (2D). Frequency dependent generation is typically obtained via beating two infrared laser diodes mixed in a semiconductor emitter. By scanning the output frequency of one of the laser diodes, the frequency of the CW terahertz can be altered up to about 3 THz. Imaging at a fixed frequency with higher power can also be obtained by terahertz quantum cascade lasers, resonant tunnelling diodes or cascaded Gunn devices. However, the image obtained is 2D, whereas the pulsed technique enables a 3D image to be obtained in which the time dependence of reflection is converted to a 3D spatial dependence of features. Although the depth resolution can be of the order  
Figure 4: High throughput sampling system for measuring porosity of tablets. of microns for larger objects, the use of CW tomographic techniques, such as imaging at a range of incident angles, can yield meaningful data. [PERSON], [PERSON] and colleagues [86] found that image reconstruction methods produced CW images of nested Russian dolls, (Matryoshkas). Three-dimensional reconstruction has also been based on Gunn oscillators as CW sources and projection techniques were utilised, in which a lens focuses the terahertz beam at a fixed point that varies in depth as the sample is rotated [87]. However, the resolution of distance is just below a millimetre, as compared to microns for a pulsed technique. Frequency modulated continuous wave, FMCW, has also been used with similar poor resolution. As discussed below, experimental work has shown that inks vary in their reflective (refractive index) properties and when reflections are weak, 3D imaging is very difficult [88] and requires high sensitivity that is not present in most CW systems to date. It may be possible to use a high intensity CW source, such as a quantum cascade laser or Gunn diode, tuned to the resonant absorption of the ink for imaging, but this is yet to be demonstrated in art restoration applications. In addition to depth information, surface topological features can also be investigated, which are very relevant to the cleaning and restoration of art. In recent times, authenticity, determined by 3D imaging, has grown in importance with the increase in fraud and the need to identify stolen art. Such features include cracks that can be visually observed, but whose depth can be ascertained with terahertz pulses. Uneven surfaces have also been found in an investigation of Neolithic wall paintings [89]. A demonstration of the importance of terahertz imaging was provided by an investigation of a painting of renowned artistic beauty \"Sacrifice to Vesta\" painted by the Spanish artist [PERSON] in 1771. The investigators established a deterioration in the canvas that was not visibly apparent, and also found the artist's signature, which could not be observed visually or with X-rays. It was thought that originally, the painting was covered with a protective layer of varnish that had darkened with time, and thus rendering the signature unobservable, but the signature was clear with terahertz [90]. In general, terahertz imaging can reveal drawings under the painting or earlier versions, which were then hidden during repainting. Early work that showed that writing on folded paper could be observed through an envelope stimulated imaging studies of documents [91]. [PERSON] et al. [92] produced a table showing that the reflectivity and refractive indices of various inks can vary considerably. When the refractive indices of the ink and paper/parchment are similar, imaging is of poor or negligible quality. Sepia ink gives poor quality, whereas lamp black, carbon black and graphite give good contrast.