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Contrast in Terahertz Images of Archival Documents—Part I: Influence of the Optical Parameters from the Ink and Support

Authors
  • Bardon, Tiphaine1
  • May, Robert K.2
  • Jackson, J. Bianca3
  • Beentjes, Gabriëlle4
  • de Bruin, Gerrit4
  • Taday, Philip F.2
  • Strlič, Matija1
  • 1 UCL Faculty of the Built Environment, UCL Institute for Sustainable Heritage, The Bartlett, London, WC1H 0NN, UK , London (United Kingdom)
  • 2 TeraView Ltd, Platinum Building, St John’s Innovation Park, Cambridge, CB4 0DS, UK , Cambridge (United Kingdom)
  • 3 Capital Normal University, Department of Physics, Beijing, 100010, China , Beijing (China)
  • 4 National Archives of The Netherlands, The Hague, 2509 LM, The Netherlands , The Hague (Netherlands)
Type
Published Article
Journal
Journal of Infrared, Millimeter, and Terahertz Waves
Publisher
Springer US
Publication Date
Jan 17, 2017
Volume
38
Issue
4
Pages
443–466
Identifiers
DOI: 10.1007/s10762-016-0351-0
Source
Springer Nature
Keywords
License
Green

Abstract

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coefficients from various archival materials are assessed and their influence on contrast in terahertz images from historical documents is explored.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-transformed into spectral information, terahertz TD imaging in reflection mode can serve as a hyperspectral imaging tool. However, chemical recognition and mapping of the ink is currently limited by the fact that the morphology of the document influences more the terahertz spectral response of the document than the resonant behaviour of the ink.

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