ID-7-IN-5763 Optical Coherence Tomography (OCT) – a novel tool for examination of artworks

Optical Coherence Tomography (OCT) is an originating from medicine diagnostic non-contact and non-invasive technique of optical sectioning. OCT employs the interferometry of light of high spatial but low temporal coherence to reveal locations of reflecting interfaces and scattering centres within internal structure of the examined object. Spectral domain version of the OCT method (SdOCT) is especially fast and sensitive. The OCT technique offers a micrometre-level in-depth resolution and therefore is well suited for investigation of fine sub-surface details of structures which absorb infrared light moderately such as varnishes, glazes and underdrawings of paintings on canvas (Fig. 1a), reverse painting on glass (Fig. 1b), glazes on porcelain and faience, jade, historic glass (Fig. 1c), and many others.
Images obtained by OCT are usually presented in convenient manner of cross-sectional views, similar to microscopic photographs of cross-sections of samples collected from the object. However, for better readability, images are usually stretched in in-depth direction as indicated by scale bars in figures and shown in false colours where cold colours indicate areas of low scatter whereas warm ones indicate areas of high scatter. Black areas correspond to either non-scattering media like air above the object or regions not reachable by the probing light - below the surface of the first opaque layer (e.g. the paint). The major advantage of using OCT is in the complete non-invasiveness of the technique (intensity of light used for examination is of order of single miliwatts), very fast data collection, and no need for any preparation of the object. Therefore the examination may be repeated as many times as necessary in many places, thus making the obtained results much more representative than obtained from sample collection.
In this contribution firstly a brief introduction to the technique will be presented and essentials of the construction of the OCT instruments will be given. Then using examples from our practice it will be shown how the OCT technique may be used to examine the structure of artworks, trace former conservation attempts and be useful for monitoring some restoration procedures. All results shown have been obtained with a spectral domain portable high resolution OCT system built by the authors especially for examination of objects of art in situ. This instrument uses IR radiation from the range 770 nm – 970 nm and its axial (in-depth) resolution is 3 μm in air (2.2 μm in varnish) whereas the lateral one is switchable between 7 μm and 13 μm. The area possible to be examined in one data collection is considerably large as for the microscopy and equals 5 x 5 mm² and 17 x 17 mm² respectively for both available lateral resolutions.