Analytical Techniques: Optical Coherence Tomography

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Author: Jessica Ford
Editors: Anne Schaffer, Kari Rayner
Optical coherence tomography (OCT) is a non-invasive technique that can gain both surface and subsurface information about a painting based on the materials’ response to an infrared laser (Liang et al. 2008, Targowski et al. 2012). A broad band infrared laser is split into a scanning beam that is reflected off of the painting, and a reference beam measures the distance the reflected radiation has traveled, resulting in a monochromatic depth profile of the area scanned (Liang 2014). The depth of penetration is determined by how much the scanned material absorbs or scatters light (Cheung et al. 2015a, 2015b). 
A linear scan produces a single cross-section, and a square area scan produces a virtual cube. The cube can be viewed in cross-section from any angle and also in thin slices of depth parallel to the painting’s surface. The thin slices are especially useful for studying underdrawings in ultra-high resolution, and even differentiating between multiple layers of application. The cross-sections have been useful in distinguishing the full stratigraphy of old master paintings, including multiple coatings, glazes, paint layer, imprimatur, and ground (Spring et al. 2008, Cheung et al. 2015a). Coatings information such as varnish removal may also be studied (Nankivil et al. 2015). Study of modern and contemporary paintings may be hindered by the infrared-scattering qualities of many modern pigments, such as titanium white and cadmium pigments, which obscures viewing lower layers (Ford et al. 2016). However, recent development of a purpose-built instrument at Nottingham-Trent University can potentially overcome this limitation (Cheung et al. 2015b).
=References=
Cheung, C. S., M. Spring, and H. Liang. 2015a. Ultra-high resolution Fourier domain optical coherence tomography for old master paintings. Optics Express 23(8):10145-10157.
 
Cheung, C. S., J. M. O. Daniels, M. Tokurakawa, W. A. Clarkson Wa, and H. Liang. 2015b. High resolution Fourier domain optical coherence tomography in the 2 μm wavelength range using a broadband supercontinuum source. Optics Express 23(3): 1992–2001.
Ford, J., H. Liang, and C. S. Cheung. 2016. The Mellow Pad by Stuart Davis: a treatment and technical study. Paper presented at the AIC 44th Annual Meeting, Montreal.
Liang H., B. Peric, M. Hughes, A. Podoleanu, M. Spring, and S. Roehrs. 2008. Optical coherence tomography in archaeological and conservation science – a new emerging field. Proc. SPIE 7139, 713915.
Liang, H. 2014. Optical coherence tomography (OCT) for the non-invasive examination of paintings – present and future. Paper presented at ICOM-CC Painting and Scientific Research Working Group Joint Meeting, The non-invasive analysis of painted surfaces: scientific impact and conservation practice, Washington, DC.
Nankivil, D., A. DeCruz, and J. A. Izat. 2015. Observations on the use of OCT to examine the varnish layer of paintings. Paper presented at the AIC 43rd Annual Meething, Miami.
Spring, M., H. Liang, B. Peric, D. Saunders, and A. Podoleanu. 2008. Optical coherence tomography – a tool for high resolution non-invasive 3D-imaging of the subsurface structure of paintings. ICOM Committee for Conservation preprints. 15th Triennial Meeting, New Delhi. New Delhi: ICOM. 2: 916-923.
Targowski, P., and M. Iwanicka. 2012. Optical coherence tomography for structural examination of cultural heritage objects and monitoring of restoration processes – a review. Applied Physics A 106(2): 265–277.