Subsurface Raman Analysis of Thin Painted Layers

Abstract

Here we present, for the first time, an extension of spatially offset Raman spectroscopy to thin (tens of micrometers thick), highly turbid stratified media such as those encountered in paintings. The method permits the non-destructive interrogation of painted layers in situations where conventional Raman microscopy is not applicable due to high turbidity of the top layer(s). The concept is demonstrated by recovering the pure Raman spectra of paint sub-layers that are completely obscured by paint over-layers. Potential application areas include the analysis of paintings in art preservation and restoration avoiding the cross-sectional analysis used currently with this type of samples. The technique also holds promise for the development as a non-destructive subsurface tool for in situ analysis using portable instruments.

Raman spectroscopy: techniques and applications in the life sciences

“…Microscope-based SORS (μSORS) has also recently emerged as an application of the SORS concept at smaller length scales [396], for measuring thin multi-layered samples such as painted layers and plant seeds [397]. Experimentally, this was carried out in a slightly different manner to standard SORS, by defocusing the objective to collect light exiting the sample from larger offset positions.…”

Raman Spectroscopy of cultural heritage Materials: Overview of Applications and New Frontiers in Instrumentation, Sampling Modalities, and Data Processing

“…In recent years, the potential of SORS for the investigation of works of art has been explored in depth [95][96][97][98][99]. Because for art applications the layers to penetrate are much thinner (in the micrometer-size domain) than those for industrial or pharmaceutical applications (which tend to be in the millimeter-size domain), the method has been adapted, leading to the development of defocusing micro-SORS [96].…”

“…Because for art applications the layers to penetrate are much thinner (in the micrometer-size domain) than those for industrial or pharmaceutical applications (which tend to be in the millimeter-size domain), the method has been adapted, leading to the development of defocusing micro-SORS [96]. In this modality, two spatially offset measurements are collected, one at the surface, which is dominated by the spectral signature of the top layer, and one at a defocused distance obtained by lowering the sample from the microscope (i.e.…”

Determination of thickness of thin turbid painted over-layers using micro-scale spatially offset Raman spectroscopy

“…Recently, a new concept in Raman microscopy providing substantially higher penetration depths than conventional Raman microscopy has emerged-micro-SORS [1,2], building on earlier advances of its parent technique, spatially offset Raman spectroscopy (SORS) [3]. The most basic variant of micro-SORS is defocusing micro-SORS [1] (figure 1).…”

“…The most basic variant of micro-SORS is defocusing micro-SORS [1] (figure 1). This approach is not the most effective in existence because it does not involve fully separated laser illumination and Raman collection zones as in classical SORS.…”