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Abstract
Terahertz (THz) spectroscopy and imaging are emerging techniques for non-invasive analysis. THz waves can penetrate opaque materials and fingerprint spectra appear as those in infrared bands. Various THz imaging systems have been developed to detect concealed weapons, illegal drugs, and defects in materials. Several attempts have been made to analyse artworks by reflection and/or transmission. To encourage the activities, we have developed a spectral database of painting materials including traditional mineral pigments. Time domain reflection imaging uses THz pulses that propagate in specimens, and in this technique, pulses reflected from the internal boundaries of the specimen indicate the internal structure non-destructively. Examples of THz spectra of various art materials and their mixtures, and some imaging results including the first ever non-invasive cross-section image of a tempera masterpiece by Giotto were introduced. These results prove that THz imaging can yield useful information for the art conservation science. Introduction Terahertz (THz) spectroscopy and imaging are emerging techniques in the field of optics research and they have been used as solutions in security problems; drug detection in envelopes is one such application [1-4]. THz imaging can be performed either by the transmission or reflection of THz waves and the internal structure can be observed non-invasively. Moreover, the 3D internal structure can be obtained if necessary. There have been several attempts to analyse paintings using THz spectroscopy [5]. We have proved that THz spectroscopy can distinguish pigments with same colours [6], and spectra of binders are also discussed [7, 8]. Jackson et al. applied THz imaging in mural painting and determined the difference in reflection depending on pigments [9]. There are some studies that detect pencil drawing (graphite) in a paper block [10]. We have also applied character recognition [11]. Recently, the non-invasive cross-section measurement of oil on canvas model painting was carried out as a trial measurement of hidden painting [12]. This paper compares the transmission and reflection imaging of model paintings prepared carefully in the traditional manner. Finally, the first ever THz imaging results of a tempera masterpiece "Polittico di Badia" by Giotto, now part of the permanent collection of the Uffizi Gallery, are introduced. We believe that the THz spectroscopy should be considered as a complementary non-invasive measurement method in the art conservation science research field, which commonly uses radiation ranging from X-rays to infrared region for analysis. Terahertz spectroscopy and imaging Terahertz (THz) waves (frequency: 0.1 to 10 THz; 3 to 300 cm–1; wavelength: 30 µm to 3 mm), which exist between radio waves (electronics) and light waves (optics), are perfectly non-invasive, and can penetrate opaque materials. As in infrared bands, fingerprint spectra of substances appear in THz frequency region, and the spectral features depend on molecular and intermolecular behaviour. THz waves had not been used due to the lack of stable sources, referred to as “THz gap”, as shown in Figure 1. In addition, spectral databases have not been constructed, and thus only specialists, such as space physicists, use the THz spectroscopy to analyse their materials of interest. We have developed a spectral database of more than 250 art materials, including pigments, dyes, binders, and materials used for conservation activities [13], to encourage the use of THz spectroscopy. Figure 2 shows examples of THz images which can suggest possible applications to cultural heritage. The image of a red pepper [14] in Figure 2 (a) proves that THz can observe internal structure of non-metal substances, and a drying leaf in Figure 2 (b) [15] suggests that THz can monitor the water content during the conservation process of historic objects. THz-Fourier Transform Spectroscopy (THz-FT) Transmission and reflection spectra of gas, liquid or solid specimens can be obtained by conventional Fourier Transform Spectroscopy (FTIR) systems with far-infrared option. Most of commercially available systems covers the frequency range from 0.5 THz to 15 THz, and often require specimens to be under vacuum or in nitrogen to avoid strong absorption by water vapour in THz region. Figure 3 shows a THz-FT system with a ceramic or a mercury lamp source, a silicon beam splitter [16, 17], and a triglycine sulphate (TGS) detector. In this system, a sample is placed between an air gap of 10 mm for transmission and on the window for reflection. Since other parts are enclosed in vacuum, the influence of water vapour becomes negligible, and specimens such as powders and liquids can be measured without special preparation. Similar to the case in the mid-infrared region, total reflection spectroscopy (ATR) is also commercially available. Since the THz frequency region corresponds to the molecule and intermolecular behaviour, it has been used in pharmaceutical industries to distinguish crystal polymorphs [18]. The spectra of art materials obtained by THz-FT systems will be discussed in the next chapter. Although systems are available, there is no commercial spectral library for THz spectroscopy as mentioned above. Therefore, users must have reference materials in the research field of interest so as to analyse unknown specimens.
Figure 1. Terahertz Gap, an illustration of the THz band;
Figure 2. Examples of THz imaging. (a) Internal structure of a red pepper (K. Kawase, "Terahertz Imaging", Optics and Photonics News, Oct. pp. 34-39, 2004), (b) a drying leaf from freshly cut to after 48 hours (B. B. Hu and M. C. Nuss, " Imaging with terahertz waves", Optics Letters, Vol. 20, No. 16, pp. 1716-1718, 1995); Figure 3. An example of THz-FT system (JASCO, Japan); Figure 4. Schematic diagram of time domain spectroscopy.
THz-Time Domain Spectroscopy (THz-TDS)
THz-TDS is similar to the time domain reflectometry (TDR) that has been widely used to detect objects in soil. Figure 4 shows a schematic diagram of the THz-TDS system that applies a short THz pulse generated by various types of THz sources to a specimen and detects the transmission or reflection pulse by various types of detectors [4]. Commercially available systems often use a photo-conductive antenna with a femtosecond fibre laser as a THz source and a photoconductive detector that covers the frequency range from approximately 0.1 THz to 3 THz. The optical delay line is used to control the sampling time in one pulse, i. e., the point in the wave form, and by collecting signals from each point, the output signal can be reconstructed as a pulsed shape. Since the output signal includes information on the amplitude and phase, the refractive index can be obtained directly. Figure 5 shows an example of the THz-TDS system TPS Spectra 3000 of Teraview (Cambridge, UK) [19]. ATR systems have been recently developed and are used to characterise hydrogen state in solutions [20-22]. The advantage of TDS can be found in imaging techniques, i.e., either by transmission or reflection. In particular, in the case of reflection imaging, a THz pulse can penetrate into an object, and reflection pulses generated inside the specimen can provide useful information on obtaining the internal structure, as shown in Figure 6. In other words, non-invasive cross-section images can be obtained by using THz-TDS, which is often referred to as THz tomography. Moreover, the 3D internal structure can be obtained by scanning the surface using this technique. Pencil drawings in an envelop or a bunch of paper were clearly observed by several systems [23]. Figure 7 shows a transportable system T-Ray™ 4000 of Picometrix (Michigan, USA) [24] used for the analysis of Giotto’s tempera painting. The results will be introduced in the last paragraph of THz imaging section. There are other imaging systems with various sources and detectors. For example, an imaging system with a compact-free electron laser and waveguides developed by Gallerano et al. at ENEA (Ente per Nuova tecnologia, Energie e Ambiente, Frascati, Italy) has been used in an Italy-Japan collaboration project THz-ARTE (Terahertz Advanced Research TEchniques for non-invasive analysis in art conservation) [25]. A brief description of the imaging system and an example of the imaging result is introduced in THz imaging section. Real-time THz imaging system The THz imaging system using THz-TDS depends on the scanning system, so that a certain time of measurement, for example 20 minutes for an area of 200 mm x 200 mm, is required. Recently, real-time imaging systems have been proposed [26, 27]. A portable system using a quantum cascade laser as a source and a micro-bolometer array as a detector has been also developed [28, 29]. They can be applied to monitor conservation process, such as water content level of drying objects, because THz is very sensitive to water. Figure 8 shows an example of the THz camera system developed by NICT and NEC (Tokyo, Japan).
Figure 5. Example of THz-TDS system: TPS Spectra 3000 (Teraview, Cambridge, UK);
Figure 6. Schematic diagram of time domain tomography;
Figure 7. Example of THz-TDS imaging system: T-Ray™ 4000 (Picometrix, Michigan, USA);
Figure 8. A THz real time imaging system (NICTNEC, Tokyo, Japan).
Spectra of pigments, binders and mixtures The spectra of several inorganic pigments in the THz region, including most of the important pigments in historic paintings, such as cinnabar (HgS) and orpiment (As2S3) [30], were already observed in 1969 by using the very first THz-FT system. We have developed a database of art materials by using THz-FT and THz-TDS systems with more than 250 specimens as mentioned before. The specimens were placed on a cyclo-olefin polymer (Zeon Corporation, Japan) plate which has high transmission in the THz frequency range. The data was calibrated automatically by measuring a reference of the plate. Pigments were applied using painting oil of which main component is purified petrol that was more than 99% transparent in the entire frequency range in this work. A binder or a mixture specimen was applied directly on a cyclo-olefin polymer. Figure 9(a) shows the transmission spectra of four white pigments. All spectra have particular features and they can be easily distinguished in the THz region. Since calcite is generally used as a body pigment, the spectra often appear in other colours. Figure 9(b) shows the transmission spectra of several binders. Oil and natural resins are rather transparent and Venetian turpentine has characteristics of oil and resin. The spectra of Beva© includes the characteristics of polyvinyl acetate (PVAc) and natural resin. The transmission spectra of mixtures are examined by using a blue pigment (cobalt blue) and four binders. Having observed spectra of 20 cobalt blue pigments from around the world, only two types of spectra appeared, shown in Figure 10 (a). The cobalt blue (A) was used to mix with binders whose spectra are shown in Figure 10 (b). Figure 10 (c) proves that spectra of mixture of a pigment and binders reflect each spectrum before mixing. These results suggest that THz transmission spectroscopy is useful to identify mixtures if the database is properly developed, although performing quantitative analysis in a real painting would be difficult because materials change in time. Transmission can be of two types: one is a simple straight transmission through a specimen and the other is a double transmission as a reflection from the metalized, i.e., gold or silver finished area. Figure 11 shows an example of madder lake. Specific peaks appeared at the same frequency in both spectra of transmission and spectra of reflection from the gold base. Figure 12 shows the reflection and transmission spectra of three pigments. Transmission spectra are often used to identify the absorption lines. A low value in transmission can either mean absorption or reflection. The spectral features of cinnabar and lead white shown in Figures 12(a) and 12(b) prove that these materials are reflective. On the contrary, almost no reflection was observed in verdigris, as shown in Figure 12(c); the lines of low transmission are considered to be due to absorption. Although the reflection measurement is strongly affected by the surface conditions, including the thickness of the paint and brush stroke, traditional paintings such as tempera on wood with a smooth surface can be analysed in the reflection mode. Figure 10. THz transmission spectra of cobalt blue pigments and mixtures obtained by THz-FT system. (a) Cobalt blue pigments, (b) binders, (c) mixtures of cobalt blue (A) and binders. Figure 11. THz transmission and double reflection spectra of madder lake obtained by THz-FT system. Figure 12. Reflection and transmission spectra of three pigments, natural cinnabar, lead white and verdigris by FT-IR system. Figure 13. THz transmission and reflection imaging using a model painting specimen. (a) Visible image, (b) spectra of natural and artificial ultramarine pigments, (c) spectra of lamp black and two binders, (c) spectra of lead and zinc white pigments. The frequency band used for imaging was also indicated.
THz imaging
Reflection and transmission imaging of model specimens The THz transmission and reflection images in this section were obtained by the T-ray™ 4000 system of Picometrix (Michigan, USA). Figure 13(a) is a Mondrian-like specimen with various paints. The spectra of the pigments and binders are also shown in Figure 13 (b), (c), (d). Black spots were painted with lamp black with PVAc, and others were painted with a mixture of petrol and linseed oil. Artificial ultramarine is almost transparent, similar to cadmium red, whereas natural ultramarine exhibits low transmission. The black paint of lamp black and PVAc exhibits low transmission due to its synthetic binder. The white parts were painted using two pigments; lead white for the central area and zinc white for the top and bottom areas. Zinc white exhibits a considerably higher transmission than lead white for which the reflection is high. Figure 14(a) shows the transmission image in which natural ultramarine and lamp black appeared as black spots (low transmission), artificial ultramarine and cadmium red appeared white (high transmission), and the area of lead white became grey (relatively low transmission). The reflection image shown in 14(b) proves the high reflection from lead white (see Figure 12(b)), judging from the white area recognised in this figure. It is clear that the spectral features determine the imaging results, either by transmission or reflection. Figure 15(a) shows an oil painting specimen with organic dyes on the gold area (spectra are shown in Figure 15(b). Raw umber (face and hands) and ivory black (sleeves) are almost transparent in the frequency range of the imaging system. Since all area of this specimen, except the green background, was covered by gold before painting, the reflection from the gold should have been directly observed if the paints on the gold would not absorb reflected THz waves. As shown in Figure 15(c), dark spots and dots appeared corresponding to the area painted with stil de grain and madder lake. The areas painted by hatching with raw umber, such as face and hands, did not disturb the reflection from the gold underneath. Although the difference in transmittance between stil de grain/madder lake and raw umber in the measured frequency range is not too large as materials themselves, the amount of binder and thickness should have affected the transmission of reflected THz waves. Thus, it is important to construct the database including such parameters for practical applications of THz imaging to cultural heritage. Reflection imaging of specimens covered by plaster It is desired to detect hidden and plaster-covered paintings without causing damage to the paintings. A THz imaging system developed by ENEA with a compact-free electron laser (0.15 THz) and two WR6 directional couplers was used to demonstrate that THz can recognise different materials in the painting covered by a plaster. In this experiment, gypsum is used to cover the tempera model paintings. Figure 16(a) shows the visible image before and after the covering of a tempera model specimen, and Figure 16(b) shows a THz reflection image. Since gold is extremely reflective, the gold part was clearly observed under gypsum, even tiny pieces of “a missione” can be recognised. Not only metals but also reflective pigments such as cinnabar can be detected under gypsum. Figure 17 shows a tempera model specimen and a THz reflection image. The difference between the part painted with rose madder and that painted with cinnabar was clearly recognised. Further, in this example, a small piece of gold in the part painted with cinnabar stands out in the image. THz imaging analysis of the Polittico di Badia (Polyptych of Badia) We have used THz imaging for analysing Giotto’s masterpiece “Polittico di Badia” (c.a. 1300); this is the first application of THz imaging to a real tempera painting in a museum collection, which was under restoration in the Uffizi Gallery in Florence, Italy, until the end of February 2009. THz imaging was performed using Picometrix T-ray™ 4000. The frequency range used in this work was from approximately 0.5 to 1.2 THz, and it takes a scanning time of approximately 10 min to observe an area of 150 mm x 150 mm. The detection unit was placed in front of the painting at a distance of approximately 20 mm. The strength of reflection is displayed in grey scale, highest as white and lowest as black in the following figures. Figure 18 shows one of the observation parts in the polyptych. THz reflection from the gold is strong, existence of gold foil under paints is clearly observed at the outline of the head and wings of the Angel. The white paint is lead white that has the strongest reflection of all white pigments in the operating frequency range as shown in Figure 18 (b). The non-destructive cross-section image shown in the figure was obtained in the same area. As shown in Figure 18 (c) the layered structure of the painting is clearly observed on the basis of the reflection waveform in the time domain. In the case of medieval and early renaissance polyptychs, a gypsum layer was made directly on a base wood to flatten the carved wood base. A cloth was placed on the gypsum layer; subsequently, another gypsum layer was made as a preparation layer for painting. This technique had been used in the medieval era. The information obtained here using THz imaging would never be obtained by conventional methods. Time domain tomography can also easily obtain a map of the layer of interest [31]. If necessary, it is possible to construct a full three-dimensional model of the internal structure. These experimental results prove that THz rays can reveal the internal structure clearly as well as conditions of gold and pigments on the surface without damage in a relatively short time.
Figure 14. THz transmission and reflection image of a model painting shown in Fig. 13, by Picometrix T-Ray™ 4000.
Figure 15. THz reflection imaging of an oil on wood model painting. (a) Specimen, (b) Spectra of colours on gold, (c) THz reflection image. Figure 16. THz reflection imaging of a tempera model painting covered by gypsum. (a) Specimen, before and after covering with gypsum. (b) THz reflection image after covering. Figure 17. THz reflection imaging using the FEL based THz imaging. (a) Visible image of tempera specimen, before and after covering with gypsum, (b) THz reflection image. Figure 18. THz reflection imaging and tomography of a part of the Polittico di Badia. (a)Visible, THz reflection images and their superposition. (b) Internal structure of the tempera painting observed non-destructively. Conclusions and prospectives We applied THz imaging to art analysis and confirmed that transmission and reflection imaging clearly identifies the difference in pigments; the images can provide information about the painting and the materials used for the original and subsequent restoration. The first ever THz imaging of the real tempera masterpiece of Giotto ensured that THz spectroscopy and imaging technique can provide useful information to conservators. The THz technology is still in its infancy, and discussion of metrology has begun recently [32, 33]. The meaning of peaks that appear in fingerprint spectra has not been theoretically explained, although a reference book introduces phonon absorptions of mineral substances which are often included in pigments [34]. However, we believe that the potential of being a non-destructive analysis tool is extremely high, judging from experimental results published so far around the world. Acknowledgements The author would like to thank Cristina Acidini (soprintendente per il Polo Museale Fiorentino), Antonio Natali and Angelo Tartuferi, Director and Curator of the Uffizi Gallery, respectively, who kindly gave permission for the analysis of the painting. The author expresses sincere thanks to the conservator Mr. S. Scarpelli and to Dr. M. Picollo of IFAC-CNR for useful discussions on art conservation. The author also thanks to Dr. I. N. Duling of Picometrix for the help of THz imaging with their T-Ray™ 4000, to Dr. G. P. Gallerano and his colleagues at ENEA for the use of images obtained by their FEL based system, and to Ms. M. Bokuda for specimen preparation and help during the experiment. Thanks are also due to Prof. Y. Ogawa of Tohoku University, Dr. S. Hayashi of Riken, Dr. N. Oda of NEC for discussions on THz spectroscopy and imaging, and to colleagues at NICT for everyday discussions and encouragement. References [1] Ed. G. Gruner, Millimeter and Submillimeter Wave Spectroscopy of Solids, Springer, Berlin, 1998 [2] D. M. Mittleman, M. Gupta, R. Neelamani, R. G. Baraniuk, J. V. Rudd, M. Koch, "Recent Advances in Terahertz Imaging", Applied Physics B, Vol. 68, 1999, 1085-1094. [3] D. M. Mittleman, Sensing with terahertz radiation, Springer, Berlin, 2003. [4] M. 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Kaori Fukunaga, PhD, is a research manager in the Applied Electromagnetic Research Center of the National Institute of Information and Communications Technology, Tokyo, Japan. Her current research fields include, deterioration analysis and test procedure development of polymer insulations, high frequency characteristics of dielectric materials, industrial applications of millimetre wave and terahertz technologies. She is a member of IEEE, International Institute for Conservation of Historic and Artistic Works, and Japan Institute of Electronics Packaging.
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