Development of advanced micro-SORS for Heritage Science

The development of two micro-SORS prototypes and their optimization for Heritage Science applications are the result of the research carried out over the last few years at CNR-ISPC Raman Laboratory in collaboration with the Rutherford Appleton Laboratory and the University of Cincinnati [1, 2]. First, a commercial benchtop micro-Raman instrument has been modified for increasing depth sensitivity and 3D mapping capabilities; micro-SORS can be deployed using three different modalities (defocusing, internal beam-steered and point-like) to fit specific application effectively. Painted layer sequences and diffusion of conservation or decay products were investigated using defocusing for obtaining an average distribution of compounds and point-like for increasing the contrast between external and inner portions of the materials; extremely thin layers (10-15 µm) required small spatial offsets provided by internal beam-steered modality. Different imaging/mapping modalities (conventional, StreamLine and StreamHR) were coupled with micro-SORS variants paving the way for studies of high-resolution molecular distribution of compounds hidden by external opaque layers in art objects, as in the case of hidden letters in sealed or closed documents. Secondly, an in-house portable micro-SORS prototype was developed to enable conventional and micro-SORS measurements in situ. This device represents a technological evolution of existing commercial portable Raman since the detection of Raman signals of the surface and subsurface is achieved by using a micrometric linear fiber bundle to conserve the offset information on the detector, permitting simultaneous acquisition of Raman photons emerging from the surface and subsurface in separate spectra in analogy to approaches also used in macro-SORS spectroscopy [3]. The system is particularly well suited to non-invasive analyses in museum collections, archaeological or conservation sites, where vibrations could be an issue, since no additional mechanical movements are required for setting spatial offset. Designs and applications to case studies in Heritage Science will be discussed.