Optical Coherence Tomography (OCT) is an optical interferometric technique developed mainly for in vivo imaging of the eye and biological tissues. In this paper, we demonstrate the potential of OCT for non-invasive examination of museum paintings. Two en-face scanning OCT systems operating at 850 nm and 1300 nm were used to produce B-scan and C-scan images at typical working distances of 2 cm. The 3D images produced by the OCT systems show not only the structure of the varnish layer but also the paint layers and underdrawings (preparatory drawings under the paint layers). The highest ever resolution and dynamic range images of underdrawings are presented and for the first time it is possible to find out non-invasively on which layer the underdrawings were drawn.

It is current practice to take tiny samples from a painting to mount and examine in cross-section under a microscope. However, since conservation practice and ethics limit sampling to a minimum and to areas along cracks and edges of paintings, which are often unrepresentative of the whole painting, results from such analyses cannot be taken as representative of a painting as a whole. Recently in a preliminary study, we have demonstrated that near-infrared Optical Coherence Tomography (OCT) can be used directly on paintings to examine the cross-section of paint and varnish layers without contact and the need to take samples. OCT is an optical interferometric technique developed for in vivo imaging of the eye and biological tissues; it is essentially a scanning Michelson's interferometer with a 'broadband' source that has the spatial coherence of a laser. The low temporal coherence and high spatial concentration of the source are the keys to high depth resolution and high sensitivity 3D imaging. The technique is non-invasive and noncontact with a typical working distance of 2 cm. This non-invasive technique enables cross-sections to be examined anywhere on a painting. In this paper, we will report new results on applying near-infrared en-face OCT to paintings conservation and extend the application to the examination of underdrawings, drying processes, and quantitative measurements of optical properties of paint and varnish layers.

Scientific examinations of paintings are routinely carried out in major galleries and museums to assist in conservation treatment and as part of technical or art historical examinations. Care is taken to examine the paintings nondestructively as far as possible. However, in order to study the paint and varnish layers, it is still currently necessary to take tiny samples of a painting to examine the cross section of a small area of the painting under a microscope. In an attempt to solve this problem we evaluate the potential of optical coherence tomography (OCT) in providing high resolution information about paint layers. Two OCT systems have been assembled, operating at 850 nm and 1300 nm, each using two single mode in-fiber couplers. Both systems can produce A (reflectivity profile in depth), T (lateral reflectivity profile), B (cross section image) and C-scans (constant depth image). Using superluminiscent diodes, a depth resolution better than 9 microns is achieved. We present results of applying OCT to sample panels and paintings. We show that infrared OCT is capable of non-destructive examination of paintings in 3D, which shows not only the structure of the varnish layer but also the paint layers. The OCT images present better microscopic tomography of the surface of the varnish and paint layers than any system currently employed in the examination of paintings. OCT could also be used for accurate measurement of the thickness of the varnish layer on a painting.

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