The Next Generation of Optical Coherence Tomography (OCT) for Art Conservation - in situ non-invasive imaging of subsurface microstructure of objects

Scientific examination of works of art is essential for conservation, preservation and understanding of material change. Ideally non-invasive methods of examination need to be used. Optical Coherence Tomography (OCT) is a non-invasive, non-contact imaging technique designed for in vivo imaging of the eye and other biological tissues. OCT is a fast scanning Michelson interferometer capable of 3D imaging of subsurface microstructure. In 2004, the principal investigator led a collaboration pioneering the application of OCT to paintings. In the same year, two other groups also reported OCT examination of jade, ceramics and paintings. Apart from the non-invasive examination of the stratigraphy of paint and varnish layers, OCT has also been shown to be the most sensitive technique for revealing preparatory underdrawings beneath paint layers owing to its high dynamic range and depth selection capabilities. OCT has been used for dynamic monitoring of the wetting and drying of different varnishes, varnish removal using solvents, real time laser ablation of varnish layers and tracking of canvas deformation due to environmental changes. OCT has found application in the examination of ancient glass, enamel, ceramics, jade, faience and parchment. Our current research has shown that OCT has the potential to become a routine non-invasive tool in museums allowing cross-section imaging anywhere on an intact object where there are no other methods of obtaining subsurface information. OCT can go beyond qualitative imaging toward quantitative measurement of optical properties giving information on ageing processes and assisting material identification.

While current OCTs have shown potential in this field, they are optimised for biomedical applications. Some major limitations are: (i) lower depth resolution compared to conventional microscopic examination of paint cross-sections; (ii) limited probing depth through highly scattering paint. A depth resolution of less than ~4 microns is needed to resolve the thinnest varnish and paint layers, and the operation wavelength needs to be longer to increase the penetration depth. The depth resolution of OCT is proportional to the source bandwidth. OCT research in recent years has moved towards development of novel wideband sources.

OCT systems for biomedical applications are generally restricted to wavelengths between 800nm and 1300nm for the best compromise between water absorption and tissue scattering. However, the requirements of art conservation are very different; our recent survey of the transparency of historical artists' pigments has shown that a third of pigments in oil have >5 times improvement in transparency at wavelnegth of 2000nm compared with 900nm. Few OCT systems have been built beyond 1300 nm. The best resolution commercial OCT at any wavelength is around 6 microns. We propose to explore OCT systems at higher resolutions and at 2000nm wavelength using novel superfluorescent fibre sources, pushing the boundaries of OCT to match the information content given by the microscopic examination of sample cross-sections currently employed. This project intends to explore new problems in conservation and art history that the next generation OCT for art can help to solve and push the boundaries in near infrared OCT imaging for non-biological material. It will significantly improve the capabilities of OCT through increasing the depth resolution and penetration in order to reduce the need for sampling and enable the subsurface microstructure to be imaged on intact objects where sampling is not possible, encourage more frequent and thorough examination of the whole object for early warning of deterioration, improve the visibility and resolution of underdrawing for art historical research, better inform conservation strategy and create long term savings in the cost of conservation, and hence firmly establish OCT as a tool for non-invasive imaging in the heritage field.

The immediate beneficiaries of the research are the curators, conservators, conservation scientists, conservation management policy makers of the National Gallery (co-investigating institute) and the English Heritage (project partner) where the new OCTs will benefit the non-invasive technical examination of objects in their care even during the project. As the project aims to develop OCTs with the capability to match the information given by destructive techniques of microscopic examination of sample cross-sections, it has the potential to be widely used in museums for conservation and reduce the need for sampling and hence benefiting the general conservation community. The OCT system developed will be made available for continued collaboration in heritage science beyond the lifetime of the project. More art historical results ultimately lead to interpretation of the collection and engagement with the public. Beyond the heritage field, non-destructive testing in industrial applications is an obvious area which can benefit from this research. This project will develop a novel superfluorescent fibre source that is capable of generating very broadband emission in the two-micron wavelength regime. The source will provide unrivalled performance with continuous spectral coverage over the range ~1.7 - 2.2 micron with excellent power stability and will offer precise 'real-time' control over the shape of the emission spectrum. Hence, the source will benefit any potential OCT application where operating in the two-micron wavelength regime offers improved penetration into the medium under investigation. In addition to OCT, the source will have numerous applications in optical metrology, spectroscopy and sensing. The emission band of the superfluorescent source conveniently overlaps with overtone and combination absorption bands of many gases including methane and carbon dioxide. Thus, we anticipate that the source could find important applications in gas sensing and pollution monitoring. Potential beneficiaries include industrial manufacturers and users of optical systems targeting these applications and, ultimately, the wider public. A project website (linked to the investigators' and project partners' web sites) will be set up providing details of the project, the main results and other potential applications of the technology in a form suitable for the general public. This will be supplemented by external media releases. Research results will be published in conservation, archaeometry, optics and photonics journals. Presentation at conferences in a similarly broad range of subjects will help to disseminate the results to a wider audience.