Multivariate approach-based system for the automated interpretation of spectraapplication to pigments identification through raman spectroscopy in art analysis

Resumen

The application of spectroscopic techniques is crucial for art historians and conservators who require knowledge of materials used in works of art (pigments, dyes, binders, additives, ...) in particular instances. In this sense, the knowledge of pigments which were in use on the ancient artists' palettes is fundamental to preserve the art works. In addition, this knowledge is important to determine correct conservation approaches, to study degradation processes or authenticity-related issues. For instance, the proper interpretation of molecular signatures from a vibrational spectroscopy gives valuable information about the materials used by the artists. In this regard, the spectral identification is one of the essential interpretations to be performed, which is generally carried out by visual comparison between the unknown spectra with an appropriate database of reference spectra. This identification approach while being simple and intuitive may turn out a complex task which usually requires an experienced analyst and inevitably introduces an element of subjectivity linked to the intervention of the investigator. Besides, these analyses can be limited due to interferences from other phenomena like noises or admixtures. This task is further complicated when the spectra are to be interpreted by a software system. Hence, the noise impact must be reduced to have an effective identification and a robust strategy for processing multi-component spectra needs to be implemented. Clearly, a fully-automated data processing system for a reliable spectral interpretation is of practical interest. Several automated methodologies were designed, developed and analysed in this Ph.D. Thesis for the purposes of art works analysis through Raman spectroscopy. In this sense, the usage of mathematical morphology together with p-spline fitting demonstrated to be a consistent combination in the application of data enhancement Raman spectra from artistic pigments. Besides, a generalised identification methodology to identify single- and multi- component spectra was developed. This identification method relies on automated spectral matching based on principal component analysis (PCA) and independent components analysis (ICA), being computationally efficient and conceptually simple. Moreover, a supervised classification methodology to automatically distinguish between Raman spectra showing small differences was developed. According to predefined reference training sets, the classification method is able to classify unknown Raman spectra relying on PCA and multiple discriminant analysis (MDA). Both the identification and classification methodologies successfully work using a single spectral observation for the unknown Raman spectra, with no user intervention or previous knowledge of the analysed sample. The designed, developed and analysed automated methodologies for noise filtering and identification and classification of artistic pigments are integrated in a global system for the automated data interpretation of spectra from art works analysis implemented in this Ph.D. Thesis, namely PigmentsLab. This software platform together with the integrated methodologies can play a good auxiliary role in the analysts' endpoint interpretation, providing insight from the raw spectral measurements into pigments. The system implementation provides an easy-to-use software platform and straightforward to update when new spectral data become available. The robust, reliable and consistent results obtained on Raman spectra demonstrated the competitiveness of the implemented data processing solutions. The system has great potential as an accurate and practical method for the automated interpretation of Raman spectra for not only pigment analysis, but essentially for any material group.