10.1117/2.1201209.004464 Statistical analysis of laser-based spectroscopic data elucidates painting materials Austin Nevin and Iacopo Osticioli Combining multivariate statistical analysis, Raman spectroscopy, and laser-induced breakdown spectroscopy helps to identify and classify the material constituents of artworks. The analysis and discrimination of painting materials (such as pigments, binding media, varnish, and composite materials like alloys and ceramics) is critical to establishing the origin and value of the art and artifacts that contribute to cultural heritage. But analysis is also complex because of the vast range of inorganic and organic compounds found in paintings. The need for portable instrumentation for investigating art and artifacts has spurred interest among scientists in constructing appropriate new devices for studying complex materials. These devices in turn have led to the development of novel laser-based instrumental methods for investigating and monitoring changes in the condition or degradation of artworks. 1 However, we still face challenges regarding in situ system implementation and interpretation of results. Moreover, integrating the efforts of engineers and analytical scientists is crucial for optimizing the entire research endeavor. To this end, mathematical analysis of complex spectral data has proved very useful for classifying data. Raman spectroscopy and laser-induced breakdown (LIB) spectroscopy (also known as laser-induced plasma spectrosco- py) have been used to analyze paint 2, 3 and other materials, 4, 5 providing complementary molecular and atomic information that enables identification of original and nonoriginal materi- als in works of art. Subsequently applying statistical methods to the spectra makes it possible to classify samples. 6, 7 Raman and LIB spectroscopy are usually performed using a variety of instruments equipped with different optics and lasers. 3 We recently built a combined setup capable of acquiring both types of spectra. 2, 6 Detecting weak Raman signals (from pigments) often necessitates working in the dark with a continuous laser Figure 1. Instrumental setup using a laser, mirror (M 1 ), beamsplit- ter (BS), two lenses (L1 and L2), and a notch filter (NF). S: Sample. P: Polarizer. M, D, C: Monochromator, detector, and computer. source, whereas a pulsed laser is needed to generate a plasma for LIB spectroscopy. Operating in darkness is possible in the laboratory but complicated when working directly on paintings and monuments. Our approach focuses on engineering a compact, combined instrument based on second-harmonic (532nm) emission of a pulsed nanosecond Nd:YAG (neodymium-doped yttrium aluminum garnet) laser, an intensified gated detector, and suit- able optics (see Figure 1). A key aspect of this approach is to assure controlled ablation of material in LIB spectroscopy, and the collection of sufficient Raman signal from the same spot using the same laser source, monochromator, and detector. We have explored statistical analysis of recorded spectra using data- bases of reference materials and multivariate statistical analysis of groups of spectra following the selection of suitable regions of interest in the Raman or LIB spectra (based on the position of atomic emission lines or specific vibrations). We tested the instrument on synthetic and natural ultra- marine pigments, which are characterized by different trace materials. 8 In addition, we combined depth profiling of mul- tilayered samples using ablation (LIB spectroscopy) followed Continued on next page