Reflectance Spectroscopy as a Novel Tool for Thickness Measurements of Paint Layers

Abstract

A major challenge in heritage science is the non-invasive cross-sectional analysis of paintings. When low-energy probes are used, the presence of opaque media can significantly hinder the penetration of incident radiation, as well as the collection of the backscattered signal. Currently, no technique is capable of uniquely and noninvasively measuring the micrometric thickness of heterogeneous materials, such as pictorial layers, for any painting material. The aim of this work was to explore the possibility of extracting stratigraphic information from reflectance spectra obtained by diffuse reflectance spectroscopy (DRS). We tested the proposed approach on single layers of ten pure acrylic paints. The chemical composition of each paint was first characterised by micro-Raman and laser-induced breakdown spectroscopies. The spectral behaviour was analysed by both Fibre Optics Reflectance Spectroscopy (FORS) and Vis-NIR multispectral reflectance imaging. We showed that there is a clear correlation between the spectral response of acrylic paint layers and their micrometric thickness, which was previously measured by Optical Coherence Tomography (OCT). Based on significant spectral features, exponential functions of reflectance vs. thickness were obtained for each paint, which can be used as calibration curves for thickness measurements. To the best of our knowledge, similar approaches for cross-sectional measurements of paint layers have never been tested.

A Performance Comparison of 3D Survey Instruments for Their Application in the Cultural Heritage Field

“…For all the instruments used in this work, no data sheet specifies the axial resolution value. To assess the instrument performance in quantifying this parameter, we extracted a set of profiles from the point clouds acquired on the acrylic paint mock-up, the thickness of which is known [ 26 ] ( Figure 7 a).…”

“…A complete characterisation of the latter can be found in ref. [ 26 ]. The canvas samples are labelled from 1 to 3 for increasing thickness: canvases 1 and 3 (thinnest and thickest) are mounted onto a wooden frame whose dimensions are 24 cm × 34.5 cm × 1 cm ( Figure 1 b), with canvas 2 (intermediate thickness) mounted onto a wooden frame 31 cm × 31 cm × 2 cm in size ( Figure 1 c).…”

Using MA-rFTIR Mapping as a Tool to Assess the Efficacy of Cleaning Treatments and to Aid in the Restoration Activities of Paintings

“…To address these limitations, researchers are exploring two main directions: (1) combining multiple techniques, such as point-by-point analyses with imaging or mapping devices [21,22] and (2) developing portable instruments for imaging or mapping that convert point-by-point analyses into comprehensive techniques [23][24][25].…”

Non-invasive stratigraphic analyzes of gelatine-based modern painting materials with linear and nonlinear optical methods

“…x-rays, NIR, or THz) is the primary condition to obtain adequate stratigraphic information. Methods such as confocal x-ray fluorescence (XRF) [6], optical coherence tomography (OCT) [7], THz imaging [8], reflectance spectroscopy [9], photoacoustic [10], confocal Raman microspectroscopy (CRM) [11] and micro-spatially offset Raman spectroscopy (SORS) [12] have proved powerful for this purpose. Still, some of these methods are time-consuming and/or require complex calculations, taking into consideration different optical phenomena.…”