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Electronic Imaging & Signal Processing
Optical coherence tomography: a non-invasive technique applied to painting conservation
By applying near-infrared en-face OCT to paintings, we can examine the optical properties of the varnish and paint layers, drying processes, and the underdrawings.
7 February 2006, SPIE Newsroom. DOI: 10.1117/2.1200601.0067
Scientific examination of easel paintings is carried out routinely in major galleries and museums to assist in conservation treatment and as part of technical or art-historical examinations. Current practice requires removing tiny samples from a painting to mount and examine in cross-section under a microscope, in order to study the paint and varnish layers. This is necessary to identify the pigments and media, their method of application, the composition of the superimposed layers, signs of deterioration, and previous alteration. However, this technique is invasive and conservation practice and ethics limit sampling to a minimum (normally samples are smaller the 1mm) taken from areas along cracks and edges of paintings, which are often not representative of the whole painting.
Increasing emphasis is now being placed on non-destructive and non-invasive methods of analysis and preventive conservation. Macrophotography, ultraviolet-fluorescence and raking-light imaging provide information primarily about the conditions of the surface of a painting;1 x-radiography is employed to examine the structure of the support of a painting; 2 whilst infrared reflectography is used to study the preparatory drawings or underdrawings beneath the painted layers that would otherwise be invisible to the eye. 3 These last two techniques reduce the 3D information of a painting into 2D, thus losing the detailed information perpendicular to the painting plane. Confocal microscopy, which is in theory a non-destructive and non-invasive technique, is potentially hazardous due to the close working distance (a few millimeters) required for high-depth-resolution imaging. 4
Recently, we demonstrated (along with others) that near-infrared OCT can be used directly on paintings to examine the cross-section of paint and varnish layers without contact or the need to take samples from anywhere on a painting. 5–7 OCT is basically a fast, high-resolution, 3D-scanning Michelson's interferometer, developed specifically for non-invasive examination of the eye and other biological tissues. It uses the low coherence interferometry principle, for which the depth resolution is given by the width of the source spectrum and coherence properties play an essential role. To achieve high depth resolution, short-coherence-length or wide-band sources are required. In comparison with confocal microscopy, OCT can give double the penetration depth in highly-scattering samples such as paint layers because it takes advantage of the coherence properties of light and registers only coherent signals. OCT systems also have the benefit of a comfortably-remote working distance, typically about 2cm. The optical configuration of the OCT system uses two single-mode directional couplers with a superluminescent diode as the wide-band source. 7 An en-face OCT takes images in planes parallel to the painting surface, one after another in depth: 8–9 an asset for the examination of paintings.
We have shown that non-invasive imaging of cross-sections can be achieved by placing a painting about 2cm in front of the OCT objective. 5–7Figure 1 shows the cross-sectional image of a hand-painted panel obtained with an 800nm en-face OCT. The varnish layer, smalt (a pigment containing cobalt glass) layer, and ground layer are clearly distinguishable
Figure 1. A cross-section of a hand-painted panel clearly shows the varnish, smalt, and ground chalk layers. The cross-section was obtained using 800nm en-face optical coherence tomography (OCT).
Figure 2 shows an example in which the OCT cross-sectional image was able to reveal an old varnish layer underneath a new one. The cross-section of point A of the 50-year-old test painting shown in Figure 2 (a), is shown in Figure 2 (b): here we can see that part of the painting is covered with a thicker (new) varnish layer on top of an old one.
Figure 2. (a) The 50-year-old test painting. (b) The OCT image of point A shows a clear distinction between the old and new varnish layers.
In addition, we demonstrated that en-face OCT is capable of high-dynamic-range and high-resolution imaging of underdrawings. 7 Because interferometers register only coherent signals, only the back-scattered light from the layer that matches the reference path length within the coherence length will be detected. Back-scattered light from the other layers is automatically filtered out. As a result, depth-selected en-face OCT images of underdrawings are clearer than conventional infrared images (see Figure 3).
Figure 3. Four views of a 1cm by 1cm section of painting showing different layers. (a) Color image of paint layer on top of underdrawings. The paint layers consist of two layers of lead-tin-yellow paint over underdrawings of quill pen made using bone-black ink with a gum medium. (b) A near-infrared vidicon image of the painted square. (c) A near-infrared image taken with a camera using an indium-gallium-arsenide detector. (d) The corresponding 1300nm OCT image taken at the depth of the underdrawings.
We recently obtained non-invasive measurements of the refractive index (n) of varnish layers using OCT by either measuring the ratio of the optical path difference (OPD) in the medium and the OPD in ir, or using a combined focus and OPD tracking method that allows he measurement of the varnish n anywhere on a painting.10
Optical coherence tomography is a powerful non-invasive technique for probing into the depth of paintings, providing 3D infrared image cubes that can not only show the structure of the paint and varnish layers, but also reveal the underdrawings and their depth positions. This method allows cross-section examination over the entire painting. We are now extending our studies to quantitative measurements of the refractive index of varnish layers as well as studying the drying process of varnish.
Haida Liang and Sophie Martin-Simpson
School of Biomedical & Natural Sciences, Nottingham Trent University
Dr. Haida Liang is Senior Lecturer in Physics at the School of Biomedical & Natural Sciences, Nottingham Trent University. She was previously at the Scientific Department of the National Gallery London, the Physics Department of the University of Bristol and Service d'Astrophysique, Commissariat a l'Energie Atomique(Saclay) She has written a number of papers for SPIE conferences.
Martin-Simpson is a PhD student at the School of Biomedical & Natural Sciences. She is working on the application of optical methods to art conservation. She obtained BSc (Hon) and MSc in Physics with Astrophysics from the University of Bristol.
Applied Optics Group University of Kent at Canterbury
Prof Podoleanu is professor of biomedical optics and head of the Applied Optics group at the University of Kent at Canterbury He was chairman of the Romanian Chapter of the SPIE. He has organised and chaired sessions for the SPIE Photonics North Symposium, SPIE "Coherence Domain Optical Methods in Biomedical Science and Clinical Applications" conference, and written numerous papers for various SPIE conferences.
Dept. of Conservation, Documentation and Science, The British Museum
Dr. Saunders is Keeper of Conservation, Documentation and Science Department of the British Museum. He was previously at the Scientific Department of the National Gallery London He has chaired session in the SPIE “Optical Methods for Arts and Archaeology” conference and has written a number of papers for this and other SPIE conferences.
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