Terahertz imaging for non-destructive evaluation of mural paintings (original) (raw)
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Application of Terahertz Imaging Methodologies for the Investigation of Paintings
2011
In recent years, Terahertz (THz) Time Domain Spectroscopy (TDS) imaging techniques have been proposed as a non-invasive means of studying artworks in order to identify the different compounds which constitute multilayered painting systems. In the present paper, reflectance THz-TDS images were acquired by using a portable THz-TDS imaging system, T-RayTM4000 by Picometrix, which was placed in front of the painting at a distance of approximately 20 mm. The intensity of the reflection is reported as gray scale, with the highest magnitude appearing as white/high gray levels and the lowest as black/low gray levels. The reported THz-TDS measurements were taken on a laboratory panel painting, on a tempera panel painting by Giotto from the Uffizi Gallery, and on the mural painting of the Takamatsuzuka tomb (ca. 700 CE), Japan. (C.E. = “common era”, a politically correct way of referring to western calendar dates without basing on the Christian calendar.) INTRODUCTION Terahertz (THz) Time Dom...
Inspection of panel paintings beneath gilded finishes using terahertz time-domain imaging
Scientific analysis of panel paintings could provide key information to art historians and conservators about the composition and condition of the constituent layers. Knowledge of the structure, stratigraphy, and condition of the subsurface layers of these objects is highly relevant to conservation measures, when stability problems such as delamination or internal cracking are considered. Terahertz time-domain imaging (THz-TDI) in reflection geometry is capable of highlighting interfaces between layers in a stratigraphic buildup, and thus makes structural information available in the form of subsurface 3D images and stratigraphic images (B-scans). The technique is contactless and non-invasive, and has been successfully applied to investigation of panel paintings. Regarding gilded panel paintings, the extremely high reflectivity of metals at submillimeter wavelengths generally precludes the transmission of electromagnetic waves through metallic films. Therefore, imaging of subsurface features below gilded finishes may be regarded as Very challenging. A small but non-negligible direct transmission through metal films occurs if the film thickness is of the order of the skin depth of the metal in the terahertz range, due to the skin effect. The thickness of gold leaf, a fraction of a micrometer, matches the skin depth of gold in the terahertz frequency range covered by THz-TDI devices. We therefore investigated and imaged subsurface features of panel paintings through gilded finishes with THz-TDI. Subsurface layers of three gilded panel paintings (two contemporary tempera panel replicas and one fourteenth-century icon) have been successfully imaged behind gold finishes with THz-TDI.
Examination of Painting on Metal Support by Terahertz Time-Domain Imaging
Two paintings on metal support have been imaged by terahertz time-domain imaging (THz-TDI) in a reflection setup and the X-ray radiographs were also recorded. The study was performed for testing the terahertz radiation (THz) as an imaging method alternative to X-ray radiography, which suffers several limitations in imaging paint layers on metal support. While the information regarding the paint layers of the paintings was almost lost in the records provided by the X-ray radiography, THz-TDI demonstrates the ability to provide important information about them, despite the presence of the underlying metal.
Terahertz Time-Domain Imaging to Guide a Conservation Intervention on a Stratified Easel Painting
Journal of Infrared, Millimeter, and Terahertz Waves, 2018
Very few real easel paintings have been scanned by means of terahertz time-domain imaging (THz-TDI) up to now. In this study, four different areas of an eighteenth century easel painting have been scanned by THz-TDI with the aim of proving useful information to the conservators for its imminent restoration treatment, owing to the complexity of its structure. Despite the unevenness of the layers composing the painting (lining canvas, original canvas, patching materials, and paint layers), the recorded THz dataset has been processed so that the THz images of the single layers composing the multilayered object have been clearly obtained. Relevant information about the painting structure, the presence of structural defects, overpainting, and the existence and extension of the original remains could have been determined. Infrared photography and X-ray radiography have been used for integrating the collected THz data and comparing the different outputs.
TeraHertz imaging of hidden paint layers on canvas
Optics Express, 2009
We show terahertz reflection images of hidden paint layers in a painting on canvas and compare the results with X-ray Radiography and Infrared Reflectography. Our terahertz measurements show strong reflections from both the canvas/paint interface and from the raw umber/lead white interface, indicating sufficient refractive-index contrast. Our results show that X-rays cannot be used to image through the lead white pigment which effectively blocks the X-rays. Although Infrared Reflectography is capable of vaguely observing the hidden paint strokes from the canvas side, we show that only terahertz imaging is capable of providing information on the thickness of the hidden paint layers. Terahertz imaging is thus shown to be a powerful imaging method for art historians, conservators and conservation scientists.
The potentials of the Terahertz Time-Domain Imaging (THz-TDI) technique for a non-invasive inspection of panel paintings have been considered in detail. The THz-TD data acquired on a replica of a panel painting made in imitation of Italian Renaissance panel paintings were processed in order to provide insights as to the limits and potentials of the technique in detecting different kinds of underdrawings and paint layers. Constituent layers, construction techniques, and anomalies were identified and localized by interpreting the extracted THz dielectric stratigraphy. The scientific analysis of paintings is a well-established practice for obtaining information on the state of preservation of artifacts and for gaining insight into the creation process. In the field of conservation science, the analytical methods are usually divided up according to the degree to which they are invasive. Hence, the first classification of analytical techniques is made among those which require physical sampling (invasive) and those which do not require J Infrared Milli Terahz Waves
Journal of Cultural Heritage, 2014
Over the past two decades, terahertz (THz) technology has gained in importance and visibility as an emerging technique to be applied in a variety of fields. Since 2006, noninvasive THz time domain spectroscopy (TDS) imaging technology has been proposed for the study of artworks. When applied to panel paintings, THz-TDS imaging makes it possible to fill in the information gap on these artworks between two diagnostic imaging 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. The resulting noninvasive cross-section image of the object is acquired by extracting the reflected pulse from a particular interface of two media that have different refractive indexes. In this paper, we used a portable THz-TDS imaging system to acquire THz images on a laboratory panel painting and on two artworks, namely the Badia Polyptych (c. 1300-1) by Giotto di Bondone in the Uffizi Gallery in Florence and the San Giovenale Triptych (1422) by Masaccio in the Masaccio Museum at the Pieve (Parish Church) of San Pietro at Cascia di Reggello, near Florence. This was the first application of THz-TDS on masterpieces that is part of conservation-restoration projects focussed on acquiring data on the materials and techniques used by the artists and the state of conservation of the paintings, as well as on testing the potentials of an application of the new emerging THz-TDS imaging technique for the art conservation field. In particular, the data acquired noninvasively on the panel paintings provided useful information on the internal structure of the paintings and on their conditions, as well as a tentative chemical composition of the works or some of their parts. Lastly, this paper offers some basic information on the application of the THz-TDS imaging technique to panel paintings.
Reflection Terahertz Time Domain-Imaging for Analysis of an 18th Century Neoclassical Easel Painting
Terahertz time domain imaging (THz-TDI) has been applied for imaging a hidden portrait and other subsurface composition layers of an 18th century (18C) easel painting by Nicolai Abildgaard, the most important 18C Danish neoclassical painter of historical and mythological subjects. For the first time, a real hidden portrait on easel painting has been imaged by THz-TDI, with an unexpected richness of details. THz C- and B-scans have been compared with images obtained by X-radiography and invasive cross-sectional imaging, leading to a deeper understanding of the strengths and limitations of this technique for art diagnostic purposes and defining its role among complementary tools for the investigation of art objects. We present a fast and effective method to separate single THz pulse reflections of interest from the entire signal across the image, adapted for uneven surfaces typically encountered in practical applications of the technique. Interfaces between layers of the painting have been successfully imaged, contributing substantially to the understanding of the structure of the painting
Abstract Characterization of subsurface features of wall paintings is important in conservation and technical art history as well as in building archaeology and architecture fields. In this study, an area of the apsidal wall painting of Nebbelunde Church (Rødby, Denmark) has been investigated by means of terahertz time-domain imaging (THz-TDI). Subsurface structures have been detected at different depths inside the lime-based plaster of the wall painting until approximately 1 cm from the surface. The surface morphology of the buried structures has been 3D imaged in detail, providing a substantial contribution in their characterization.
Terahertz imaging systems: a non-invasive technique for the analysis of paintings
2009
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.