Broadband terahertz imaging of documents written with lead pencils (original) (raw)
Thin layered drawing media probed by THz time-domain spectroscopy
Dry and wet drawing materials were investigated by THz time-domain spectroscopy in transmission mode. Carbon-based and iron-gall inks have been studied, some prepared following ancient recipes and others using current synthetic materials; a commercial ink was studied as well. We measured the THz signals on the thin films of liquid inks deposited on polyethylene pellicles, comparing the results with the thick pellets of dried inks blended with poly-ethylene powder. This study required the implementation of an accurate experimental method and data analysis procedure able to provide a reliable extraction of the material transmission parameters from a structured sample composed of thin layers, down to a thickness of a few tens of micrometers. THz measurements on thin ink layers enabled the determination of both the absorption and the refractive index in an absolute scale in the 0.1–3 THz range, as well as the layer thickness. THz spectroscopic features of a paper sheet dyed by using one of the iron-gall inks were also investigated. Our results showed that THz time-domain spec-troscopy enables the discrimination of various inks on different supports, including the application on paper, together with the proper determination of the absorption coefficients and indices of refraction.
THz time-domain spectroscopy: investigation of thin layers of inks.
We develop an original experimental procedure and a comprehensive method of data analysis to measure the optical parameters of drawing media in the THz spectral range. The method based on the THz – Time Domain Spectroscopy is applied to recover material parameters on drawing inks deposited on polyethylene pellicles with thicknesses down to tens of micrometers. In particular, we investigated three commercial inks: a red ink (based on cochineal carmine), a blue ink (indigo) and a black ink (irongall). The implemented experimental procedure and numerical method enable us to extract the absorption coefficient and refractive index spectra in an absolute scale, opening the possibility to investigate the material parameters in the THz range with a complete quantitative study.
Drawing materials studied by THz spectroscopy
ACTA IMEKO
THz time-domain spectroscopy in transmission mode was applied to study dry and wet drawing inks. In specific, cochineal-, indigo- and iron-gall based inks have been investigated; some prepared following ancient recipes and others by using synthetic materials. The THz investigations have been realized on both pellet samples, made by dried inks blended with polyethylene powder, and layered inks, made by liquid deposition on polyethylene pellicles. We implemented an improved THz spectroscopic technique that enabled the measurement of the material optical parameters and thicknesses of the layered ink samples on absolute scale. This experimental investigation shows that the THz techniques have the potentiality to recognize drawing inks by their spectroscopic features.
Papyrus imaging with terahertz time domain spectroscopy
Applied Physics A, 2010
Terahertz time domain spectroscopic imaging (THz-TDSI) is a non-ionizing, non-contact and nondestructive measurement technique that has been recently utilized to study cultural heritage artifacts. We will present this technique and the results of non-contact measurements of papyrus texts, including images of hidden papyri. Inks for modern papyrus specimens were prepared using the historical binder, Arabic gum, and two common pigments used to write ancient texts, carbon black and red ochre. The samples were scanned in reflection at normal incidence with a pulse with a spectral range between 0.1 and 1.5 THz. The temporal analysis of the signals provides the depth of the layer, and their frequency spectra give information about the inks.
Journal of Infrared, Millimeter, and Terahertz Waves
This study aims to objectively inform curators when terahertz time-domain (TD) imaging set in reflection mode is likely to give well-contrasted images of inscriptions in a complex archival document and is a useful non-invasive alternative to current digitisation processes. To this end, the dispersive refractive indices and absorption coefficients from various archival materials are assessed and their influence on contrast in terahertz images from historical documents is explored. Sepia ink and inks produced with bistre or verdigris mixed with a solution of Arabic gum or rabbit skin glue are unlikely to lead to well-contrasted images. However, dispersions of bone black, ivory black, iron gall ink, malachite, lapis lazuli, minium and vermilion are likely to lead to well-contrasted images. Inscriptions written with lamp black, carbon black and graphite give the best imaging results. The characteristic spectral signatures from iron gall ink, minium and vermilion pellets between 5 and 100 cm −1 relate to a ringing effect at late collection times in TD waveforms transmitted through these pellets. The same ringing effect can be probed in waveforms reflected from iron gall, minium and vermilion ink deposits at the surface of a document. Since TD waveforms collected for each scanning pixel can be Fourier
Terahertz Imaging for Paper Handling of Legacy Documents
Sensors
Despite predictions of the paperless office, global demand for printing and writing paper remains strong, and paper appears to be here to stay for some time. Not only firms, but also governments, libraries, and archives are in possession of large collections of legacy documents that still must be sorted and scanned. In this study, terahertz-based techniques are demonstrated to address several routine tasks related to the automated paper handling of unsorted legacy documents. Specifically, we demonstrate terahertz-based counting of the number of sheets in unconsolidated paper stacks, as well as locating stapled documents buried in paper stacks.
Terahertz imaging for analysis of historic paintings and manuscripts
2008 33rd International Conference on Infrared, Millimeter and Terahertz Waves, 2008
Through the application of terahertz spectroscopy and imaging to the analysis of art, we have developed a spectral database of art materials containing more than 200 spectra of various pigments and binders. With this database, a red ink used in a thirteenth-century manuscript was identified; the results were verified by X-ray fluorescence elements mapping. Terahertz transmission spectral component analysis successfully distinguished stains on the red ink. Reflection imaging also can be used to distinguish materials. Similar techniques can be applied to construct images of specific materials on objects not only in art conservation science but also in various other industries.
Application of terahertz spectroscopy for character recognition in a medieval manuscript
IEICE Electronics Express, 2008
Terahertz (THz) technology is a focus of attention in research on applied optics. We have applied THz spectroscopy and THzimaging method to text recognition of a medieval manuscript made from sheepskin. Based on the database which contains more than 200 spectra of art materials, the red ink on the manuscript was estimated as Cinnabar. The red ink text and stains on the text were successfully distinguished by the THz-Time Domain Spectroscopy (TDS) imaging with a component spatial pattern analysis. THz spectroscopy can be used as a non-invasive analysis method for conservation science of cultural properties.
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.
Terahertz spectroscopy applied to the analysis of artists’ materials
Applied Physics A-materials Science & Processing, 2010
Terahertz (THz) spectroscopy and imaging have been actively studied in these decades. THz waves (0.1–10 THz) are non-invasive, can penetrate opaque materials, and can be used to obtain fingerprint spectra whose characteristics depend upon molecular and intermolecular behavior. Unlike the mid-infrared region, no commercial spectral library is available for the THz region. Consequently, a spectral database of artists’ materials was developed in order to include THz spectroscopy among conservation science techniques. Most pigments and some synthetic polymers have characteristic fingerprint spectra in the THz region. With this technique most of these materials used in paintings can be identified alone or as paint, a combination of pigments and binders. Although the meaning of the spectral features has so far not been fully explained, previous studies on optical materials suggest that phonon absorption and the behavior of hydrogen bonds contribute to their spectral features. Examples of THz spectra of various art materials are discussed on the basis of measurements obtained using a conventional Fourier transform system.