title: Determination of thickness of thin turbid painted over-layers using micro-scale spatially offset Raman spectroscopy.
abstract: We present a method for estimating the thickness of thin turbid layers using defocusing micro-spatially offset Raman spectroscopy (micro-SORS). The approach, applicable to highly turbid systems, enables one to predict depths in excess of those accessible with conventional Raman microscopy. The technique can be used, for example, to establish the paint layer thickness on cultural heritage objects, such as panel canvases, mural paintings, painted statues and decorated objects. Other applications include analysis in polymer, biological and biomedical disciplines, catalytic and forensics sciences where highly turbid overlayers are often present and where invasive probing may not be possible or is undesirable. The method comprises two stages: (i) a calibration step for training the method on a well characterized sample set with a known thickness, and (ii) a prediction step where the prediction of layer thickness is carried out non-invasively on samples of unknown thickness of the same chemical and physical make up as the calibration set. An illustrative example of a practical deployment of this method is the analysis of larger areas of paintings. In this case, first, a calibration would be performed on a fragment of painting of a known thickness (e.g. derived from cross-sectional analysis) and subsequently the analysis of thickness across larger areas of painting could then be carried out non-invasively. The performance of the method is compared with that of the more established optical coherence tomography (OCT) technique on identical sample set.This article is part of the themed issue 'Raman spectroscopy in art and archaeology'.
authors: Consiglio Nazionale delle Ricerche, Istituto per la Conservazione e la Valorizzazione dei Beni Culturali (ICVBC), via Cozzi 53, 20125 Milano, Italy conti@icvbc.cnr.it.
authors: Claudia
authors: http://orcid.org/0000-0002-5379-7995
authors: C
authors: Conti
authors: Consiglio Nazionale delle Ricerche, Istituto per la Conservazione e la Valorizzazione dei Beni Culturali (ICVBC), via Cozzi 53, 20125 Milano, Italy.
authors: Marco
authors: M
authors: Realini
authors: Consiglio Nazionale delle Ricerche, Istituto per la Conservazione e la Valorizzazione dei Beni Culturali (ICVBC), via Cozzi 53, 20125 Milano, Italy.
authors: Chiara
authors: C
authors: Colombo
authors: Consiglio Nazionale delle Ricerche, Istituto per la Conservazione e la Valorizzazione dei Beni Culturali (ICVBC), via Cozzi 53, 20125 Milano, Italy.
authors: Alessandra
authors: A
authors: Botteon
authors: Consiglio Nazionale delle Ricerche, Istituto per la Conservazione e la Valorizzazione dei Beni Culturali (ICVBC), via Cozzi 53, 20125 Milano, Italy.
authors: Moira
authors: M
authors: Bertasa
authors: Consiglio Nazionale delle Ricerche (CNR), Istituto Nazionale di Ottica and LENS, Università di Firenze, via Nello Carrara 1, 50019 Sesto Fiorentino, Italy.
authors: Jana
authors: J
authors: Striova
authors: Consiglio Nazionale delle Ricerche (CNR), Istituto Nazionale di Ottica and LENS, Università di Firenze, via Nello Carrara 1, 50019 Sesto Fiorentino, Italy.
authors: Marco
authors: M
authors: Barucci
authors: Central Laser Facility, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, Harwell, Oxford OX11 0QX, UK pavel.matousek@stfc.ac.uk.
authors: Pavel
authors: P
authors: Matousek
keywords: cultural heritage; non-invasive; optical coherence tomography; spatially offset Raman spectroscopy; thickness