# Selected Applications of Terahertz Pulses in Medicine and Industry
## 5 Terahertz Imaging and Spectroscopy in Cultural Heritage
### Terahertz Spectroscopy  
In a similar manner to other spectroscopic studies, such as pharmaceutical products, by investigating inter-molecular forces, terahertz offers additional, and often simpler to interpret, results compared with the intra-molecular determinations of infra-red. This can render spectral identification less ambiguous, particularly in reflection mode. Two main areas of research are studies of paints/inks and parchments, including canvas and skins. These can be complicated by the failure of some inks to produce a terahertz signature, such as Prussian blue and sepia compared to carbon black and graphite, which do have a signature. Many pigments are based on powders and granulated materials where the granular size is comparable to terahertz wavelengths, causing the radiation to scatter. Both Mie and Rayleigh scattering mechanisms are active and interfere strongly, obscuring the actual spectra. As has been shown, the summation of frequency dependent absorption, or reflection, plots taken across a sample area, which are of order of a wavelength apart, results in the integrated random structure becoming insignificant, allowing the spectra to be observed clearly [93]. By utilising the dependence of scattering on particle size [94],studies of Japanese paintings have exploited the change in brightness by the variation in the particle size of the pigments. Many of the pigments found in heritage studies have polymorphs, and so overlaps of spectroscopic structures can impede identification, although the spectral distribution of the structure remains more favourable than that found with infra-red. Pigments in general have much narrower absorption lines than dyes and binders, which allows considerable data to be assembled. For example, it has been shown that cobalt based pigments have two principal components, cobalt blue and cobalt yellow, (aureolin), as obtained from the sharp resonant absorption lines. Similarly, different forms of calcium carbonate have multiple uses and the different polymorphs can be distinguished from their spectra. Thickness variations present a problem in extracting spectroscopic data and are particularly prevalent in the examination of paintings with the different manner of the brush strokes. Far-infrared (THz) Fourier transform systems have been useful in the study of semiconductors and associated phonon spectra, as well as studies of pigment and binder components of inks. Although subject to the disadvantage of using incoherent radiation, these systems can be used for higher frequencies than the coherent time domain spectroscopy, which is best below 5 THz. However, the bulk of spectral features are found below this frequency. There are exceptions, for example lead white, which shows structures up to 7.5 THz. The identification of binders helps determine age, and many pigments have been identified in works of art, such as cinnabar, (HgS) and argument (As\({}_{2}\)S\({}_{3}\)). Some do not have a characteristic signature but rather a broad absorption, which renders identification difficult. Pigments, such as sinopia, have no terahertz structure and a more difficult analysis based on chemometric techniques and principal component analysis has to be employed [95]. Often, these are based on the differential of the broadband absorption as a function of frequency, particularly applicable to those binders and organic dyes that lack absorption features. Much sampling is performed in reflection when samples are too thick for transmission to occur. Murals have been investigated with terahertz imaging and the strength of reflection from the paint surfaces has been found to differ between different pigments. This allows for spectroscopic identification to occur even if different pigments appear to be the same colour to our eyes in the visible spectrum [96]. The inks that were used for documents prior to the invention of the fountain pen make use of iron gall, which contains iron sulfate and is well-known for its possible corrosive action on documents.