Masters Student from University of Reading

AG 1, AG 2, AG 3, AG 4, AG 5, SWS, RG1, MMCI   Info

Public Audience

English

90 Minutes

Saarbrücken

Terahertz radiation is one of the most important unexploited areas of the electromagnetic spectrum that lies between microwaves and infrared radiations. This gap is called as the Terahertz gap. THz radiations provide a non-invasive, non ionizing and non destructive solution to determine the internal layers of the optically opaque materials. It has wavelengths shorter than the wavelengths in the microwave region of the electromagnetic spectrum, with an associated improvement in spatial resolution, but is long enough to be free from Rayleigh scattering present in infrared radiation applications. THz waves penetrate through diverse materials, sensitive to polar substances and through those invisible to other imaging modalities. Terahertz imaging is still an immature technology and holds great potential as a mainstream imaging technique. In terahertz pulsed imaging, every terahertz `pixel' consists of 29 – 212 time samples, and some form of parametric extraction needs to be carried out before an image can be formed.
Terahertz research entered the field of cultural art and heritage conservation in 1998. Recent advances in generating and detecting, THz radiation have made investigation of art works quite possible. It is the combination of material characterization, time of flight imaging and the preparation in optically opaque materials that gives rise to applications for subsurface imaging. The more powerful non-destructive forms of radiation used in imaging, like x-ray, gamma-ray, proton and neutron, are ionizing and they destroy the DNA in artifacts that can yield important information; they also affect dating techniques which means that irradiated samples cannot be subsequently dated. Visible range spectroscopy was also used to identify pigments but visible range was not broad enough to identify all the pigments unless they were properly diluted with white pigments. Therefore, in terms of conservation, it is essential to employ non-destructive and non-invasive techniques to investigate art objects including paintings, murals, coloured sculptures or furniture.
In this research work, THz pulsed imaging is examined, together with the use of novel signal and image processing methodologies for spectral analysis of the reflected beam that would provide information to differentiate the obscured pigments and visualise the obscured paintings of the archaeological site at Ḉatalhöyűk, Turkey. Paintings were practised in Ḉatalhöyűk throughout the life of its settlement and a full range of pigments like red, brown, yellow, blue, azurite, green malachite, cinnabar were used in these paintings. The paints were applied on the walls and after the painting served its purpose, the wall was covered with white plaster and was repainted later with a new painting. Thus the walls contained obscured paintings embedded within several layers of plaster for years. These paintings were of much significance as it was evident that the pattern inside were repeated throughout the layers of the walls. A signal and image processing routine has been established to extract image features from a real world terahertz imaging data set in order to image the portions of Neolithic paintings. This data was collected during the first ever use of terahertz imaging in a measurement campaign to Turkey in the summer of 2011. The research is mainly focused on optimised denoising and deconvolution techniques and using of several image enhancement and segmentation algorithms. It aimed to move beyond the conventional data analysis techniques and use the combination of various methodologies to analyse the interaction of THz radiation with natural pigments used in these wall paintings to locate a predicted brick pattern through the stratographic layers of plaster. Traditional terahertz data analysis has proven unsuccessful at sub-surface imaging of these paintings due to the effect of the uneven surfaces of the walls. For the first time ever, an image processing technique, based around Gaussian beammode coupling has been used. Other new techniques such as centroid imaging and synthetic aperture are also used. The synthetic aperture technique used in RADAR technologies has also been used for the first time on Terahertz images to improve the spatial resolution and it gave quite convincing results. The research also explored the application of various imaging modalities which were never previously been applied to the terahertz images like homomorphic filtering, phase preserved image denoising, adaptive thresholding and edge detection. All these image processing algorithms showed satisfactory results and they successfully located the pigmented regions in the wall section.

Aaron Alsancak

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