Wednesday, 7 September 2011

3D THz Imaging of Heritage Objects

Dr. Gianluca Pastorelli from University College London, in collaboration with TeraView Ltd., is giving a talk at Lacona IX concference on 3D THz imaging of Heritage Objects. The talk will be given at the British Museum, London, at 12.50 on Friday 9th of September.
All measurements on heritage objects analysed were done at TeraView Ltd. using a Spectra3000.
More information on the conference schedule can be found here.

Summary of paper  "3D THZ Imaging of Heritage Objects"
7th September 2011

In the past few years, the way for Terahertz (THZ) technology into heritage science and conservation has been paved slowly but securely [1]. THZ radiation occupies the band between far-IR and microwaves (100 µm – 1 mm ~ 0.3 – 3 THz) and its recent advance is related to development of the necessary radiation sources and detectors.

The particular advantage of THZ radiation in comparison to other optical techniques is that it induces interactions between large molecules thus rendering itself particularly suitable for characterisation of macromolecular and supramolecular structures. THZ spectroscopy has been used for determination of glass transition temperature of polymers [2], has recently been shown to contain quantitative chemical information on polymeric materials [3] and spectral libraries have been built of a number of historically significant pigments [1].
In this work, we present a state-of-the-art THZ pulsed semiconductor-based imaging system developed specifically to monitor the thickness, uniformity and density of laminar and coated structures and to identify structural features such as cracks, dislocations and delaminations [4]. The emitter and receiver are based on a femtosecond pulsed laser system working in the range of 0.06 THz – 3 THz. The resolution of the system depends on the wavelength employed, and is typically in the range of several 10 µm.
The system operates on the principle of reflection of THZ pulses on (subsurface) phase boundaries, i.e. boundaries between materials with different refractive indices. Since both the emitter and the detector can be moved across a sample in the x-y direction, a 3D image of an object can be obtained.
Several case studies were selected to explore the capabilities of 3D THZ imaging, particularly plastic heritage objects. Using a degraded cellulose acetate (CA) negative, it was possible to image the typical delaminations observed as a consequence of advancing degradation. On other types of CA objects, depth distribution of cracks can be observed very effectively, while on polyurethane foam objects, the degradation process can be followed by imaging the cell shape and structure.
Due to its properties, combining a significant depth of penetration into opaque organic materials with a significant amount of chemical information, THZ imaging could significantly improve the present practice of structural and chemical monitoring of heritage objects of organic origin.

1. K. Fukunaga: Innovative Terahertz Spectroscopy and Imaging Technique for Art Conservation Science, e-Cons. Mag. no. 10, 2009, 30-42.
2. C. Jansen, S. Wietzke, O. Peters, M. Scheller, N. Vieweg, M. Salhi, N. Krumbholz, C. Jördens, T. Hochrein, M. Koch: Terahertz imaging: applications and perspectives, Appl. Opt. 49, 2010, E48-E57.
3. T. Trafela, M. Mizuno, K. Fukunaga, M. Strlič: THZ spectroscopy and chemometrics for quantitative characterisation of historic paper, LACONA IX, 2011.
4. L. Ho, R. Muller, K. C. Gordon, P. Kleinebudde, M. Pepper, T. Rades, Y. Shen, P. F. Taday, J. A. Zeitler: Monitoring the Film Coating Unit Operation and Predicting Drug Dissolution Using Terahertz Pulsed Imaging, J. Pharm. Sci. 98 (2009) 4866-4876.

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