Abstract accepted for an oral presentation at the LACONA-X conference

Tiphaine Bardon from University College London (UCL), who TeraView are currently hosting, has had an abstract accepted for an oral presentation at the LACONA-X conference (Lasers in the Conservation of Artworks) held in the United Arab Emirates between the 9th and the 13th of June 2014, gathering both researchers, conservators and conservation scientists. 

The topic of the presentation will be "Contrast and readability in terahertz time-domain images of ink inscriptions: A comparative study of model and historical documents"


Tiphaine Bardon, Robert K. May, Bianca Jackson, Philip F. Taday, Gerrit De Bruin, Matija Strlič

Abstract

The terahertz range between 0.15 – 3 THz (i.e. 5 – 100 cm-1) has been made accessible due to the development of ultra-short pulse lasers, as the emission and reception of a radiation with such broad bandwidth in the high frequency domain rely on photoconductive semiconductors gated by femtosecond laser pulses.

Terahertz time-domain imaging can give in-depth contrasted images of each layer in a laminar structure, with a depth resolution of up to 30 μm and a penetration depth of a few mm in dry organic materials. The image contrast of a specific layer relies on the differences of absorption and dispersion of the terahertz pulse by the materials on each side of this layer. Documents are, in essence, laminar structures consisting of layers of support (paper or parchment), ink and air gaps. Terahertz time-domain imaging therefore represents an ideal tool for the study of historical documents, the condition of which may prevent their opening or excessive handling, provided that ink inscriptions and their surrounding support can be differentiated in the terahertz image.

This comparative spectroscopic and imaging study of various model inks on different supports demonstrates the independent and combined influence of the nature of both the ink and the support on the contrast of inscriptions seen in terahertz images. Lamp black pigment is found to intensively absorb terahertz radiation and, as a result, any inscription containing lamp black would show good contrast in terahertz images, regardless of the nature of the support. On the other hand, the refractive index of the red pigment vermilion varies greatly in the terahertz region and is similar to the refractive index of parchment at specific frequencies: the image contrast of vermilion inscriptions on parchment is therefore very frequency-dependent.

Other factors can influence the contrast in terahertz images of documents and are reported in the present work: the roughness of the support can induce scattering of the impinging terahertz radiation, the indentation left by a quill or a pen locally modifies both the surface profile and the density (and therefore the refractive index of the support), and the ink penetration into the support creates a refractive index gradient. Finally, good readability of ink inscriptions does not rely solely on the contrast of the image, but also on the spatial resolution of the imaging set-up with respect to the width of the ink inscriptions: spatial resolutions of different set-ups are discussed, together with the loss of spatial resolution when imaging deep into a stack of sheets, depending on the nature and thickness of the supports.

Improved interpretation of the contrast in terahertz images of historical documents is possible on the basis of these findings. As an example, terahertz imaging of an Ottoman document from the UCL Centre for Sustainable Heritage Historic Reference Material Collection, dating from 1870 AD, is discussed: the difference in contrast of two black inscriptions is discussed and corroborated by results of analysis with optical coherence tomography, attenuated Fourier-transform infrared spectroscopy and near-infrared spectroscopy.

Further information about the conference can be found on the conference website:

http://www.lacona10.org/