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Atkinson, PS 
ORCID: https://orcid.org/0000-0002-2398-8424,
2024.
A multi-modal instrument combining optical coherence tomography and microscopic spectral imaging for cultural heritage applications.
PhD, Nottingham Trent University.
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Abstract
The potential of Optical Coherence Tomography (OCT) as a non-invasive tool for the monitoring of conservation treatments and investigation of sub-surface microstructures in artworks and historical objects is well recognised. This is thanks to its ability to provide micron scale three dimensional measurements of the stratigraphy of turbid and/or transparent multi-layered structures such as the varnish layers, glaze and paint layers seen in Old Master Paintings. However, it can be challenging to distinguish semi-transparent paint and glaze layers on paintings from layers of aged and degraded varnish due to similarities in their scattering properties within OCT images. This problem can limit the use of OCT as a tool to assist with the cleaning of degraded varnish from paintings.
This work demonstrates a method to extract the spectral characteristics from such translucent layers using spectral imaging to allow pigment identification and the means to distinguish glaze layers from varnish layers. To do so, segmentation of the layer structure is performed, and a Beer-Lambert relationship is used to extract extinction coefficient spectrum associated with the layer by fitting the layer thickness against reflectance per spectral channel. The segmentation of OCT is also demonstrated to be advantageous in the analysis and interpretation of layer stratigraphy; providing access to additional information such layer thickness and interface topography mapping, adaptive windowed enface projections, and refractive index corrected data sets.
Previous attempts combining spectroscopic techniques with OCT have suffered from alignment issues and vastly different system resolutions. Thus, to facilitate this combined approach, a multi-modal instrument was developed combining filtered illumination based spectral imaging in the visible to near infrared (VIS-NIR) with a spectral domain OCT centred in the shortwave infrared (SWIR) at 1350 nm. The system produces OCT volumetric data sets, with axial and transverse resolutions of approximately 3 µm (in varnish/paint) and 5.5 µm respectively, co-registered with reflectance spectral image cubes with lateral and spectral resolutions of 5.3 µm and 10 nm respectively. Even without the extraction of the layer spectral features, co-registration of data sets is demonstrated to be beneficial in the interpretation of data from the complementary modalities.
The instrument was successfully employed on both mock-up paints and historical paintings from the National Gallery in London to address both conservation and art historical questions, demonstrating the potential for online assistance to conservation treatment and decision making. Additionally, the instrument shows promise for applications outside of the context of online conservation of historical paintings with its utilisation in the analysis of 19th and 20th century paintings, historical wall paintings, 16th - 18th century enamels, and archaeological objects such as the Herculaneum and Nash papyri’s.
| Item Type: | Thesis |
|---|---|
| Creators: | Atkinson, P.S. |
| Contributors: | Name Role NTU ID ORCID |
| Date: | February 2024 |
| Rights: | This work is the intellectual property of the author. You may copy up to 5% of this work for private study, or personal, non-commercial research. Any re-use of the information contained within this document should be fully referenced, quoting the author, title, university, degree level and pagination. Queries or requests for any other use, or if a more substantial copy is required, should be directed in the owner(s) of the Intellectual Property Rights. |
| Divisions: | Schools > School of Science and Technology |
| Record created by: | Jeremy Silvester |
| Date Added: | 22 May 2025 14:07 |
| Last Modified: | 22 May 2025 14:07 |
| URI: | https://irep.ntu.ac.uk/id/eprint/53642 |
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