Terahertz (THz) imaging is an emerging technique for non-invasive analysis. Since THz waves can penetrate opaque
materials, various imaging systems that use THz waves have been developed to detect, for instance, concealed weapons,
illegal drugs, and defects in polymer products. The absorption of THz waves by water is extremely strong, and hence,
THz waves can be used to monitor the water content in various objects.
THz imaging can be performed either by transmission or by reflection of THz waves. In particular, time domain
reflection imaging uses THz pulses that propagate in specimens, and in this technique, pulses reflected from the surface
and from the internal boundaries of the specimen are detected. In general, the internal structure is observed in crosssectional
images obtained using micro-specimens taken from the work that is being analysed. On the other hand, in THz
time-domain imaging, a map of the layer of interest can be easily obtained without collecting any samples. When realtime
imaging is required, for example, in the investigation of the effect of a solvent or during the monitoring of water
content, a THz camera can be used.
The first application of THz time-domain imaging in the analysis of a historical tempera masterpiece was performed on
the panel painting Polittico di Badia by Giotto, of the permanent collection of the Uffizi Gallery. The results of that
analysis revealed that the work is composed of two layers of gypsum, with a canvas between these layers. In the paint
layer, gold foils covered by paint were clearly observed, and the consumption or ageing of gold could be estimated by
noting the amount of reflection.
These results prove that THz imaging can yield useful information for conservation and restoration purposes.