The application of Optical Coherence Tomography (OCT) to the study of painted surfaces

Project leader: David Saunders

Department: Conservation and Scientific Research

Project start: Summer 2006
End date: Summer 2009

External partners:

Dr Haida Liang, Nottingham Trent University:
http://www.ntu.ac.uk/research/school_
research/bns/staff/24819.html
Professor Adrian Podoleanu, University of Kent:
http://www.kent.ac.uk/physical-sciences/research/aog/profiles/podoleanu.html
Marika Spring, The National Gallery, London

Project funded by:

2006-2009 Leverhulme Trust Research Project Grant (£126,480) Project title: ‘Application of a new non-invasive technique (Optical Coherence Tomography) to paintings conservation’
http://www.leverhulme.ac.uk/

Description:

The project investigates the application of a technique called 'optical coherence tomography (OCT) to the non-invasive analysis of painted surfaces. OCT is a technique that has been developed for in vivo examination of the retina, so is completely non-invasive and non-destructive. In a preliminary study, the method was used to look at paint and varnish layer structures of paintings. The aim of the new project is to refine the technique and extend the range of painted objects to include works on paper or textile substrates and wall paintings.

The museum partners will provide surrogate or non-accessioned/registered material for the experimental stage of the project; in the latter part of the project, the OCT equipment will be brought to London and used to investigate objects in the British Museum and National Gallery collections. Should the project be successful, a follow-on project might develop new equipment tailored to the needs of museum collections, rather than the medical applications which currently dominate.

Objectives:

To find the best method of obtaining quantitative measurements of optical parameters, such as refractive index, thickness, absorption and scattering coefficients, of the paint and varnish layers using OCT;

To conduct a study of how the paint and varnish material of historic paintings or painted objects age in terms of their optical properties;

Use OCT to assist practical conservation, address various issues in conservation and art historical or historical  studies;

To find the optimum wavelength bands in the visible to near infrared spectrum (400-2500 nm) for a dedicated OCT system for the conservation of painted objects;

Modify an existing OCT system to include a movable arm, with versatile interface optics for direct use on valuable objects in museums and galleries;

To add a second wavelength channel to the existing OCT system to explore the potentials of spectral measurements;

To specify a design for an ideal OCT system tailored for museum use.

More information:

A description of the use of the technique can be found in an article by some of the project participants in Optics Express (2005) Vol. 13, Issue 16, pp. 6133-44.

http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-16-6133

Two PhD students have been recruited to move the project forward at the universities of Kent and Nottingham Trent. Surrogate materials have been produced to evaluate the usefulness of the technique and to test the strengths and weakness of current OCT systems. The optical; properties of these surrogates have been analyzed and three papers are in draft.

An initial project meeting was held in Munich in September 2006 (where several of the partners were attending the International Institute for Conservation Congress).

In December 2006, the full team met at the British Museum to assess progress and plan for next year.

During 2007, the plan is to complete the assessment of paint samples and surrogate materials; prepare samples for accelerated ageing tests and begin accelerated ageing; redesign the OCT optics and mechanical support to allow delicate artefacts to be scanned safely and examine the penetration of various wavelengths of radiation through paint layers.

A promising area for the use of OCT is the examination of the deterioration of glass. Some initial studies have shown that OCT is able to map, non-destructively, the ‘halos’ of deterioration in pitted glass, offering a complementary technique to the taking of samples for analytical study.

Publications:

H. Liang, B. Peric, M. Hughes, A. Podoleanu, D. Saunders, M. Spring, ‘The Potentials of Multi-wavelength Optical Coherence Tomography as a New Technique for Art Conservation’, in SPIE International Symposium on Optical Metrology , Munich, June 18-21 2007, (SPIE 2007) (forthcoming)

H. Liang, B. Peric, M. Hughes, A. Podoleanu, D. Saunders, M. Spring, ‘Non-invasive imaging of subsurface paint layers with optical coherence tomography’, (Institute of Conservation, London, 2007) (forthcoming)

Images (from top):

• The OCT microscope in use to examine a fragment of early seventeenth glass (inset)