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The potential of Optical Coherence Tomography (OCT) as an online monitoring tool for the conservation treatment of artworks and historical objects is well recognised. OCT gives non-invasive micron scale measurements on the stratigraphy of transparent and turbid multi-layered structures such as the paint and varnish layers on Old Master Paintings, enabling the monitoring of the removal of degraded varnish coatings during the cleaning process. In previous work we highlighted the difficulties in distinguishing glaze layers on paintings (coloured, translucent layers containing pigments with refractive indices very close to the binding medium) from those of aged varnish due to similarities in their scattering properties which gave similar appearance in OCT images. We proposed a method to extract the spectra from the translucent layers to distinguish the glazed layer from the varnish layers, mitigating the possibility of accidental removal of glaze layers by over-cleaning. The method extracts the spectral features of a semi-transparent layer using a combination of spectral imaging (400-850 nm) and OCT images from a region with relatively uniform paint composition. The OCT image cube provides the thickness measure of the translucent layers and when coupled with the knowledge of the spectral information of the surface layer, the aged varnishes and glaze layers could be distinguished. This method, however, relied on the accurate alignment of the two separate imaging systems with vastly different spatial resolutions which makes spatial registration difficult, affecting the crucial correlation between the spectra and layer thickness. In this work, we propose a combined multimodal instrument that collects both the spectral image and OCT images from the same region of a painting through the same objective. A number of design options are considered based on the modification of an in-house developed high-resolution OCT system. Preliminary proof of concept results is demonstrated on painting mock-ups.
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