Optical Coherence Tomography for Non-destructive Investigations of Structure of Objects of Art

In this contribution the Optical Coherence Tomography (OCT), as a novel tool for non-invasive structural imaging of selected objects of cultural heritage, will be reviewed. This technique relies on multispectral interference of infrared light and therefore is well suited for investigation of transparent and semi-transparent structures. The technique originates from medical imaging and has been present in conservation science since 2004. Until now it has been successfully utilized in imaging of varnishes, glazes and underdrawings of paintings, glazes on porcelain and faience, structure of archaic jades, stained and archeological glass, parchment and recently for revealing the surface details of varnished punchwork. Authors of this paper are active in this field from the very beginning and will report their own results obtained with OCT technique. The physical background of the OCT method will be shown in an accessible manner. Then advantages and disadvantages of various modalities of the technique will be discussed. However, the major emphasis will be laid on applications. Examples of imaging of the layers of varnish and semi-transparent glazes of paintings will be presented. The thickness of these layers may be directly measured with OCT in completely nondestructive, quick and convenient way as many times as necessary. The application of these images for real-time monitoring of conservation treatments as well as for authentication of signature will be shown. Another important and perspective application of OCT is examination of stained and archeological glass. We use this technique for nondestructive evaluation of in-depth range of atmospheric corrosion and in our contribution we present results obtained from various samples of stained glass. INTRODUCTION Non-invasive methods for examination of artworks have been in focus of interest of conservators and art historians since over a century – X-rays were used for inspection of underneath layers of paintings (Bridgman 1974) shortly after being discovered. At present many other methods, also often originating from medicine, are used for non-invasive examination of objects of art. In addition to standard radiography, 3D imaging with an aid of computed tomography (CT) is used in examination of various art objects successfully. However the resolution offered is not sufficient for examination of paintings. Among other methods X-ray fluorescence (Woll, Bilderback et al. 2005), neutron-induced autoradiography (Taylor, Cotter et al. 1975), high-energy proton-induced X-ray emission (PIXE) (Griesser, Denker et al. 2000) and most popular IR reflectography should be mentioned (Boutaine 2006). Unfortunately, these methods lack in-depth resolution: although they permit identification of certain components of the object, their precise location within the piece remains unknown. This disadvantage is significant especially in examination of objects composed of thin layers e.g. easel paintings. The optical sectioning offers resolution in micrometer range due the short wavelength of radiation used. Confocal laser scanning microscopy (CLSM) enables obtaining threedimensional images of the internal structure of oil paintings even with submicron resolution. 1 9th International Conference on NDT of Art, Jerusalem Israel, 25-30 May 2008 For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=65