Type: 2D/3D Analysis
Optical Coherence Tomography (OCT) originates from diagnostic non-contact and non-invasive techniques in the medical field that can provide information on the internal, sub-surface structure of various objects and which can be successfully applied to artworks. OCT offers a micrometre level in-depth resolution and is well suited for the investigation of fine details of structures which moderately absorb infrared light, such as varnishes, glazes and underdrawings of easel paintings, reverse paintings on glass, glazes on porcelain and faience, jade, historic glass and amber amongst others.
Images obtained by OCT are usually presented in the convenient manner of cross-sectional views, similar to microscopic photographs of cross-sections of samples collected from the object. If necessary, information of an entire volume (a cube) may be collected by combining together 100 – 200 parallel cross-sections.
The major advantage of OCT is in the complete non-invasive nature of the technique (intensity of light used for examination is in the order of single milliwatts), rapid data collection, and no object preparation. Furthermore the number of measurements across the entire surface can be unlimited, rendering the obtained results much more representative.
The instrument (in the figure below) comprises a broad-band light source made up of super luminescent LEDs emitting in a band of 770-970 nm. The intensity of radiation at the object never exceeds 800 μW, and due to fast scanning is focused at any given spot on the object for 40 ms. The axial imaging resolution is 3 μm in air (2/nRμm in a media or refractive index nR), with an axial imaging range of 1.4 mm. The lateral resolution, in the standard configuration, is about 13 μm with a field of view of 17 x 17 mm2 and the distance to the object from the most advanced element of the device equal 43 mm. If necessary, the alternate configuration may be used, providing better resolution (ca 6 μm) but for the price of smaller field of view (5 x 5 mm2) and distance to the object (7.5 mm). The acquisition time of a volume information comprised of 100 cross-sections is ca 12 s.
All papers devoted to application of OCT to examination of artworks are listed at www.oct4art.eu.
- Targowski, M. Iwanicka “Optical Coherence Tomography: its role in the non-invasive structural examination and conservation of cultural heritage objects—a review” Applied Physics A 106(2), 265-277, (2012), DOI: 10.1007/s00339-011-6687-3,
- Targowski, M. Iwanicka, M. Sylwestrzak, E.A. Kaszewska, C. Frosinini “OCT structural examination of Madonna deiFusi by Leonardo da Vinci“ Proc. SPIE 8790 87900N, (2013), DOI: 10.1117/12.2021607
- Targowski, M. Iwanicka, B.J. Rouba, C. Frosinini, “OCT for Examination of Artwork” in: W. Drexler, G. Fugjimoto (Eds.) Optical Coherence Tomography. Technology and Applications, Springer, Cham Heidelberg New York Dordrecht London, 2015, pp. 2473- 2495.
MOLAB Poland: NCU