Non-Destructive Investigation of Paintings with THz-Radiation

Nowadays the investigation of paintings is mainly done by means of X-ray radiography or ultra violet and infrared reflectometry. The latter is a non-invasive technique since the very low photon energies of the infrared radiation precludes damage to the paint by breaking molecular bonds as it is known to occur with UV-radiation.
Another advantage of infrared reflectometry appears when it is extended to picosecond pulses in the THz frequency band: X-ray radiography provides only an additive signal from absorption by all paint layers, which may be sufficient for revealing older painting hidden below the surface but does not provide information on the sequence of paint layers. However, THz time domain spectroscopy has the potential for providing depth information from the paint layers. In addition to this, the substances which the paint consist of can be identified by their absorption lines in the frequency spectrum of the reflected ps-THz-pulses.
Particularly evident is the potential of the THz technique for the investigation of wall paintings where X-ray radiography cannot be applied. Conventional infrared reflectometry is unsuitable for detecting hidden paintings when the cover layer is thick as it often is the case with wall paintings. THz-radiation is able to penetrate thicker layers and become partly reflected from them, thereby retrieving useful information from the depth.
The present contribution provides first results of transmission measurements on paintings in the frequency band up to 5 THz. It becomes evident that the different paints show a significantly different THz transmission. First THz transmission images of paintings give rise to the hope that hidden wall pictures can be made visible using a reflection geometry.