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

Lead Research Organisation: Nottingham Trent University
Department Name: School of Science & Technology

Abstract

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.

Planned Impact

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.

Publications


10 25 50
 
Description Optical Coherence Tomography (OCT) is an imaging method based on a fast scanning Michelson interferometer. Originally designed for biomedical applications such as in vivo examination of the eye, it is capable of non-invasive 3D imaging of subsurface microstructure. In previous projects, we were able to demonstrate the potential application of OCT to art conservation, art history and archaeology. 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.
The project has significantly improved the capabilities of OCT through increasing the depth resolution and the probing depth
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 on paintings for art historical research.
Increasing the depth resolution involves broad spectral band light sources and increasing the probing depth needs sources at longer wavelength than conventionally used in biomedical imaging. However, for a given bandwidth source, the shorter the wavelength the better the resolution, and therefore to achieve this, two state-of-the-art OCT systems have been built so that when used in conjunction, they would aim to match the information from conventional invasive microscopic examination of sample cross-sections (tiny samples removed from objects):
1. An 800nm OCT optimised for imaging at high depth resolution: targeted towards applications where highest resolution is required but the material is relatively transparent, such as imaging of multiple thin varnish layers on painting surfaces, gel layers on degraded glass or glazes on the surface of ceramics and enamels. An ultra-high resolution OCT at 810nm with depth resolution of 1.2 microns in varnish or glass has been developed and shown to be able to resolve thin varnish layers at similar resolution to invasive microscopic examinations currently employed in museums. The development of an ultra-high resolution OCT is challenging as it requires stable ultra-broad band light sources as well as specialist optical components and image processing techniques to achieve both high resolution and high sensitivity images. As the instrument is a Fourier domain OCT using a fast camera in the spectrograph, it can collect 50 virtual cross-section images in one second or a 4mm x 4mm x 1.6mm volume in 10 seconds. Imaging of National Gallery paintings has shown the OCT to produce superior images of varnish and glaze layers than invasive optical microscopic examinations of sample cross-sections.
2. A 2-micron OCT optimised for deeper penetration suitable for imaging cultural heritage objects consisting of highly scattering materials. This would allow improved imaging of highly scattering or absorbing paint layers and underdrawings beneath them (at higher transverse resolution than conventional infrared imaging), as well as pigmented objects such as coloured enamels and glass with opacifiers. The development of long wavelength OCT requires the development of novel broadband sources at such wavelength. Three types of novel sources, a superfluorescent fibre source, a swept (tuneable) laser source and a supercontinuum source were developed. OCTs at 1950nm have been developed and shown to reveal layer structures of both highly scattering paint such as Titanium white and highly absorbing paint such as Prussian blue. These were not possible with shorter wavelength OCTs.
Exploitation Route Improved capability of OCT will encourage the adoption of OCT for non-invasive imaging in the heritage field for conservators and for investigating technological aspects of an object/painting.
Beyond the heritage field, non-destructive testing in industrial applications, biomedical imaging and optical sensing are obvious areas which can benefit from this research.
Sectors Agriculture, Food and Drink,Healthcare,Manufacturing, including Industrial Biotechology,Culture, Heritage, Museums and Collections,Pharmaceuticals and Medical Biotechnology
URL http://www.ntu.ac.uk/apps/research/groups/3/home.aspx/project/144023/overview/the_next_generation_optical_coherence_tomography_(oct)_for_art_conservation
 
Description The optical coherence tomography systems and laser sources developed in this project has been applied to a wide range of fields: 1. Industrial paint formulation and their drying properties; 2. Enables the fundamental understanding of the film and droplet drying process through dynamic monitoring using the OCT systems that has a wide ranging impact on industry, e.g. inkjet printing 3. Non-destructive testing, inline quality control from the food industry, pharmaceutical to the electronics industry 4. Monitoring of the liquid surfaces generated by electro-optics effects for tuneable liquid optical components with benefits to the optics industry 5. Monitoring the formation of biofilms and transdermal drug delivery 6. Gas sensing & pollution monitoring using the laser sources developed 7. useful non-invasive tool for conservators during conservation treatment 8. benefits for collection care and informing conservation decisions - for intact objects where sampling is not an option, OCT will be the only method of examining the subsurface microstructure for early warning of deterioration 9. dynamic imaging to monitor drying of varnish, rate of water transport through rocks and effectiveness of conservation treatment 10. high contrast imaging of underdrawing for art history 11. multi-surface micro-profilometry - study of tool marks for understanding manufacturing process Apart from museum applications, we have taken OCT to the field to solve practical problems for heritage organisations in the UK and internationally. Imaging of the Byward tower 14th century wall painting at the Tower of London has generated considerable interest from the heritage, optics communities to the general public. New Scientist and NTU have separately recorded videos of the event and have published these online. As a result over a dozen websites now feature articles and videos about this application. SPIE (an international optics and photonics society) featured an outreach article in SPIE NEWSROOM on the results of this field work. As a result we were invited to image wall paintings at a UNESCO world heritage site, the Mogao caves near Dunhuang, on the ancient Silk Road in the Gobi Desert. OCT imaging has not only revealed previously unknown underdrawings but was also able to separate the underdrawings from the final sketches. The development of the long wavelength OCT in this project has encouraged the development of 2-micron optical components such as optical fibre couplers at Gooch & Housego plc. (UK industrial partner on the project) and the development of 2-micron balanced photo-detectors at Thorlabs (optics component company). The demands from art conservation and art history have resulted in innovation in UK and international industries. Detectors and optical components at 2-microns are increasingly sought after in various applications from LiDAR to gas sensing. Funding from this project has enabled these two companies to make the first step in their development. Application in cultural heritage gives them high profile. Beyond the heritage field, non-destructive testing in industrial applications is an obvious area which can benefit from this research. In addition to OCT, the sources 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. The sources are very useful for spectroscopic studies and when used in high power can be used for material processing.
First Year Of Impact 2012
Sector Agriculture, Food and Drink,Electronics,Healthcare,Manufacturing, including Industrial Biotechology,Culture, Heritage, Museums and Collections
Impact Types Cultural,Societal,Economic
 
Description AHRC CDP with BM
Amount £75,000 (GBP)
Funding ID AH/R001413/1 
Organisation Arts & Humanities Research Council (AHRC) 
Sector Public
Country United Kingdom of Great Britain & Northern Ireland (UK)
Start 10/2017 
End 10/2020
 
Description AHRC CDP with National Gallery
Amount £75,000 (GBP)
Funding ID AH/R00174X/1 
Organisation Arts & Humanities Research Council (AHRC) 
Sector Public
Country United Kingdom of Great Britain & Northern Ireland (UK)
Start 10/2017 
End 10/2020
 
Description Application of Imaging Science to the Interdisciplinary Study of Wall Paintings along the Silk Road
Amount £24,000 (GBP)
Funding ID RW10-10 
Organisation Research Councils UK (RCUK) 
Department RCUK-China
Sector Public
Country China, People's Republic of
Start 11/2010 
End 12/2010
 
Description BASF OCT
Amount £3,000 (GBP)
Organisation BASF 
Sector Private
Country Germany, Federal Republic of
Start 08/2015 
End 11/2015
 
Description Bank of America funding for conservation of Stuart Davis' painting at Brooklyn Museum
Amount £10,000 (GBP)
Organisation Bank of America 
Sector Public
Country Unknown
Start 02/2016 
End 12/2016
 
Description Contract research from MPI
Amount £159,898 (GBP)
Organisation Max Planck Society 
Department Max Planck Institute
Sector Public
Country Germany, Federal Republic of
Start 12/2016 
End 02/2018
 
Description EU COST COSCH visiting postdoc
Amount € 2,280 (EUR)
Organisation European Science Foundation (ESF) 
Department European Cooperation in Science and Technology (COST)
Sector Public
Country European Union (EU)
Start 11/2015 
End 11/2015
 
Description EU H2020 INFRADEV
Amount
Funding ID 739503 
Organisation European Commission (EC) 
Department Horizon 2020
Sector Public
Country European Union (EU)
Start 02/2017 
End 01/2020
 
Description EU Laserlab
Amount £3,000 (GBP)
Organisation Laserlab-Europe 
Sector Public
Country Sweden, Kingdom of
Start 01/2016 
End 12/2016
 
Description GPE OCT
Amount £1,300 (GBP)
Organisation GPE Germany 
Sector Private
Country Germany, Federal Republic of
Start 06/2015 
End 07/2015
 
Description Innovation Fellowship
Amount £10,000 (GBP)
Organisation European Commission (EC) 
Department European Regional Development Fund (ERDF)
Sector Public
Country European Union (EU)
Start 01/2014 
End 06/2014
 
Title Long wavelength Optical Coherence Tomography at 2 microns 
Description Long wavelength OCT developed at 2 microns in the AHRC/EPSRC project has enabled deeper penetration into the layers. 
Type Of Material Improvements to research infrastructure 
Year Produced 2014 
Provided To Others? Yes  
Impact Ability to see through highly scattering paint layers and other materials with low water content and make these layers more discenable 
 
Title Ultra high resolution OCT 
Description Ultra high resolution OCT developed in the project enables much improved visualisation of the thin varnish layers on paintings that is not possible even with the conventional invasive method of taking a sampling and viewing the mounted cross-section under an optical microscope. 
Type Of Material Improvements to research infrastructure 
Year Produced 2014 
Provided To Others? Yes  
Impact paint layers on illuminated manuscript visualised non-invasively. It was not possible to image the paint layer structures on illuminated manuscripts previously and little is known about this. 
 
Description 2micron (wavelength) optical components 
Organisation Gooch & Housego
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Private 
PI Contribution optical fibre couplers, AOTF at 2microns for the development of 2 micron OCT
Start Year 2010
 
Description EU COST COSCH visiting postdoc 
Organisation University of Vigo
Country Spain, Kingdom of 
Sector Academic/University 
PI Contribution Provide expertise in the optical properties of paint, pigment identification, provide OCT including the two OCTs developed in the AHRC/EPSRC project to study the effect of particle size on paint. Provide expertise in non-invasive spectroscopic techniques.
Collaborator Contribution mineralogy, geology
Impact This is multidisciplinary involving geology, optics and heritage science
Start Year 2015
 
Description EU LaserLab project on OCT and non-linear microscopy 
Organisation Foundation for Research and Technology Hellas (FORTH)
Country Greece, Hellenic Republic 
Sector Public 
PI Contribution A systematic study of historic artists' paint imaged by OCT including the two OCTs developed in the AHRC/EPSRC project to measure their properties for comparison with non-linear microscopy in order to find out the complementarity and pros and cons of the two imaging modality. Provide expertise in OCT instrumentation, image analysis and optical properties of historic artists' paint
Collaborator Contribution Provide the expertise on non-linear microscopy
Impact Imaging data of paint samples have been collected with the various OCTs and the non-linear microscope in Greece An abstract is submitted to the LACONA conference to be held in September 2016 Prof Liang gave a talk on OCT and its applications at FORTH. members of the team including a research fellow and a PhD student visited the laser and imaging labs at FORTH.
Start Year 2016
 
Description OCT imaging of pharmaceutical glass coatings 
Organisation GPE Germany
Country Germany, Federal Republic of 
Sector Private 
PI Contribution Expertise in Optical Coherence Tomography, instrumentation. Using the UHR OCT developed in the AHRC/EPSRC project for the pilot study.
Collaborator Contribution prepared samples and design mechanical device to speed up measurements
Impact The disciplines involved are Physics (optical instrumentation), mechanical Engineering, Pharmaceutical packaging. Proof of concept solution to a quality control problem in manufacturing.
Start Year 2015
 
Description OCT inspection of industrial paint 
Organisation BASF
Country Germany, Federal Republic of 
Sector Private 
PI Contribution Providing the two OCTs developed in the AHRC/EPSRC project to inspect car paint and other polymer thin films, providing expertise in OCT instrumentation and image interpretation
Collaborator Contribution bringing various teams in BASF to take an interest in OCT applications to their analytical problems, coming up with samples and appropriate problems to solve.
Impact Pilot study conducted and data collected Prof Liang was invited to BASF in Ludwigshafen to give a talk on OCT applications. The disciplines involved are Physics (optics) and Chemical Engineering and material science Discussions are ongoing for longer term collaborations in various areas of interest at BASF
Start Year 2015
 
Description OCT investigation of Ancient Egyptian Faience 
Organisation British Museum
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Public 
PI Contribution collaboration with the British Museum to study the manufacturing methods of ancient faience
Start Year 2007
 
Description OCT of modern polymer sculpture with the Smithonian 
Organisation Smithsonian
Country United States of America 
Sector Academic/University 
PI Contribution OCT expertise
Collaborator Contribution conservation knowledge and samples
Impact Physics, Conservation
Start Year 2014
 
Description UHR OCT imaging of Stuart Davis's paintings 
Organisation Brooklyn Museum
Country United States of America 
Sector Academic/University 
PI Contribution Expertise in OCT instrumentation and its application to the examination of paintings. Providing the UHR OCT developed in the AHRC/EPSRC for scanning paintings. Expertise in multimodal non-invasive investigation of paintings developed in the recent AHRC project.
Collaborator Contribution Conservation and Curatorial expertise in the American Modernist painter, Stuart Davis's painting and painting techniques
Impact Data has been collected for a number of Stuart Davis' paintings An abstract has been submitted to the American Conservation Institute conference to be held in May 2016. A paper is planned following the conference. A demonstration and lecture was given by Prof Liang at Brooklyn Museum during the week of OCT scanning of the Stuart Davis painting the 'Mellow Pad' and other related paintings at Brooklyn Museum. Conservators, Conservation Scientists, curators and students in the New York region attended the workshop. This is an interdisciplinary project involving curators, conservators and physicists.
Start Year 2016
 
Description Ultra-high resolution imaging of paint layers on illuminated manuscripts at the Fitzwilliam museum 
Organisation The Fitzwilliam Museum
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution sample taking on manuscripts are generally not allowed therefore very little is known about the paint layer structure on illuminated manuscripts.
Collaborator Contribution spectroscopic measurements and curatorial knowledge
Impact We are analysing the data and preparing for a number of publications. this is multidisciplinary involving physics, chemistry, curatorial and conservation.
Start Year 2014
 
Description imaging of wall painting at the UNESCO site of Mogao caves 
Organisation Dunhuang Academy
Department Institute of Conservation
Country China, People's Republic of 
Sector Academic/University 
PI Contribution OCT imaging of ancient Silk Road wall paintings, separating the preparatory sketches and final sketches
Collaborator Contribution archaeology and conservation knowledge
Impact publications and talks at international conferences and invited seminars internationally
Start Year 2011
 
Title Supercontinuum laser source at 2 microns 
Description Ultra-broad band Supercontinuum laser source at 2 microns 
Type Of Technology Systems, Materials & Instrumental Engineering 
Year Produced 2013 
Impact Improved axial resolution for OCT at 2 microns 
 
Title Ultra-high resolution OCT 
Description A fast ultra high resolution Fourier domain Optical coherence tomography with axial resolution ~1 microns in paint and varnish. 
Type Of Technology Systems, Materials & Instrumental Engineering 
Year Produced 2012 
Impact Can resolve thin varnish layers on paintings with high contrast non-invasively that is not possible with conventional destructive technique of viewing a sample cross-section through a optical microscope. 
URL http://www.ntu.ac.uk/apps/research/groups/3/home.aspx/project/144023/overview/the_next_generation_op...
 
Title swept source laser at 2 microns 
Description swept source laser at 2 microns for OCT 
Type Of Technology Systems, Materials & Instrumental Engineering 
Year Produced 2013 
Impact enables fast OCT and potential for spectroscopy 
 
Title two micron Fourier domain OCT 
Description a Fourier domain OCT in the 2-micron wavelength regime 
Type Of Technology Systems, Materials & Instrumental Engineering 
Year Produced 2013 
Impact It can penetrate highly scattering material . The first OCT in 2 microns with axial resolution of ~10 microns. 
 
Description BOSCH visit 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact BOSCH senior managers from their packaging division visited our LAB.
Year(s) Of Engagement Activity 2016
 
Description International Institute of Conservation - International Training Centre for Conservation on non-destructive analysis in conservation 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact 32 young conservation scientists attended the lectures
Year(s) Of Engagement Activity 2016
 
Description Media interest - New Light for Old Mater Paintings 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact A press release was written by Optics Society of America about our publication in Optics Express on Ultra-high resolution Fourier domain optical coherence tomography for old master painting.

The press release was picked up by Reuters, Science World Report, AZO optics, The Engineer, Bloomberg Business and the Daily Mail among other publications.
Year(s) Of Engagement Activity 2015
URL http://www.osa.org/en-us/about_osa/newsroom/news_releases/2015/new_light_for_old_master_paintings/
 
Description TEDx talk (Nottingham) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact talk sparked interest and realisation

Realisation on the strong link between disciplines.
Year(s) Of Engagement Activity 2014
 
Description new scientist 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact video generated further enquires

invitation for collaboration or give talks
Year(s) Of Engagement Activity 2010
 
Description talk at Historic England 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Talk was focussed on OCT applications in Archaeology
Year(s) Of Engagement Activity 2015
 
Description talk given at BASF 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact introduced OCT technology to BASF including discussions on the data taken previous for a BASF funded pilot project
Year(s) Of Engagement Activity 2015
 
Description visit from Historic England survey section 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact head of survey from Historic England visited our lab and future plans were made for collaboration
Year(s) Of Engagement Activity 2016
 
Description workshop at Brooklyn Museum 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact A talk about OCT and its applications in Heritage Science, a demo involving one of the OCTs developed in the AHRC/EPSRC project
Year(s) Of Engagement Activity 2016