An Alternative Phase-Sensitive THz Imaging Technique for Art Conservation: History and New Developments at the ENEA Center of Frascati

Abstract

In recent years, THz imaging techniques have been used in several fields of application. At the beginning of the century, the low availability of powerful THz sources was one of the limiting factors to the advancement of THz technology. At the ENEA center in Frascati, two Free Electron Lasers (FELs) operating in the THz spectral region were available at that time, making it possible to exploit all the features of THz imaging. In this paper, we will describe an alternative THz imaging technique, developed over 15 years of studies at the ENEA center of Frascati, and its application in the art conservation field, reporting the latest results of such studies on the optical properties of pigments in the GHz-THz region of the spectrum, on the possibility of detecting biological weeds under mosaic tiles and on the THz analysis of ancient leather wallpapers. This alternative technique was first developed in the framework of a bilateral collaboration between Japan and Italy, the THz-ARTE Project, which involved NICT (Tokyo), NNRICP (Nara), ENEA (Frascati) and IFAC-CNR (Florence). Most of the THz imaging techniques at that time were based on THz-Time Domain (THz-TD) devices. In the paper will be described how this alternative technique is able to measure the phase of the reflected radiation, thus providing information on the optical properties of the materials under study, such as mural paintings and mosaics. This makes it possible to detect the presence of hidden artworks, additional elements under paint layers, and dielectric materials. To describe the potential and the limits of this alternative imaging technique we will start from a description, in chapter 2, of the first THz imaging setup at the ENEA center of Frascati, based upon a THz Free Electron Laser. A description of the theoretical principle underlying this technique will be given in chapter 3. The first results in the field of art conservation are summarized in chapter 4, while the new results of a systematic study on the optical properties of pigments are reported in chapter 5. In chapter 6 the realization of a portable THz imaging device, and its application “on site” for the analysis of frescoes are shown. The success of this prototype lead to the identification of different types of artworks as possible targets to be studied. New results about the ability of detecting water, and possibly the water content of biological weeds, under mosaic tiles are described in chapter 7, while new experimental measurements on Leather Wallpapers, both “in situ” and in a laboratory environment, are discussed in chapter 8. A final analysis on the pro and the cons of this alternative imaging technique and on its possible utilization with the developed prototype is carried out in chapter 9, together with the considerations on possible future developments and its potential use as an extension of other imaging techniques.

Recent advances and applications to cultural heritage using ATR-FTIR spectroscopy and ATR-FTIR spectroscopic imaging

“…64 More detailed information can be acquired by examining the stratigraphy of paintings. Non-invasive approaches such as Optical Coherence Tomography (OCT), 65 Nuclear Magnetic Resonance (NMR), 66 Terahertz imaging, 67 confocal XRF 68,69 and micro-Spatially Offset Raman Spectroscopy (micro-SORS) 69 can be performed to investigate and characterise the layer successions, but they are not able to provide the two-dimensional distribution of compositional information of the materials. ATR-FTIR spectroscopy can be combined with multivariate techniques to analyse spatial and compositional information of the stratigraphy of a painting.…”

Self-Referenced Method for Geometrical Distortion Removal in THz Time-Domain Reflection Imaging

“…The majority of these methods have limited application to THz spectroscopy, as the problem is addressed considering the shift in sample position, under the requirement of optically flat object, or using elaborated mathematical approaches or specific instrumental components to remove it [13] [27] [28] [29]. For example, in reflection imaging application, an approach that has been proposed for solving spatial distortion entails the use of laser triangulation system for either precisely adjusting in real-time the distance between THz head and the object, following the shape of the surface, or by conducting multiple planar scans and interpolating them with theoretical model for obtaining virtual scanning surfaces [30]. With both acquisition modes, the time needed to complete the point-to-point correction is long and it requires the use of laser and precise coupling between the two instruments, which is challenging to obtain.…”

Study of the edge distortion effect in thermographic detection in semi-transparent Cultural Heritage

“…For this reason, non-destructive analyses are crucial for the identification and characterisation of such hidden precious elements, without compromising the book structure, aimed to the comprehension of the manuscript and for palaeographic purposes. Such an issue has been successfully investigates by the use of several imaging techniques [1][2][3], among which Pulsed Thermography (PT) has proven to be an efficient tool for the investigation of multi-layers structures [4][5][6], such as ancient manuscripts [7][8][9]. In particular, PT allows the detection of sub-surface features beneath the end papers of ancient book-bindings, providing the in-depth mapping of the investigated object and also indications of reused manuscripts.…”