Application of THz-TDS imaging technology to the investigation of panel paintings

Terahertz (THz) Time Domain Spectroscopy (TDS) imaging systems have many applications as a new, non-invasive methodology for investigating various materials. In recent years, THz-TDS technology has also been proposed for the study of artworks in order to identify the different compounds that constitute multilayered painting systems. THz-TDS imaging has been performed in the reflection and/or transmission modes, depending on the objects being analyzed. In general, the internal structure of the analysed object is observed in cross-sectional images obtained by using micro-specimens taken from the work that is being analysed. Also, in THz-TDS imaging, a map of the layer of interest can easily be obtained without the need to collect any samples. Thus, in principle THz-TDS imaging makes it possible to fill in the information gap between two imaging diagnostic techniques, namely IR reflectography and X-ray radiography, because it provides useful data on the internal physical structure of non-metallic objects without touching the artworks investigated. This noninvasive cross-section image of the object is acquired by extracting the reflected pulse from a particular interface of two media with different refractive indexes. To summarise, THz- TDS imaging technique provides various types of information in the investigation of panel paintings, as follows: a) condition and appearance of the gilding and gold backgrounds; b) information about the chemical composition of some of the pigments; c) extent of surface deterioration; d) stratigraphic analysis of the various materials with the capability to isolate images of the individual layer of interest. The application of the above described imaging techniques to some specific case studies will be presented.