Optical coherence tomography: a non-invasive technique applied to painting conservation
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 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.
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).
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.
