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Integrated hyperspectral and time-resolved fluorescence imaging combined with statistical data analysis: diagnostic investigation of wall paintings

For the first time we have applied a nanosecond fluorescence lifetime imaging (FLIM) system together with a hyperspectral fluorescence imaging device during a diagnostic investigation of wall paintings. The FLIM apparatus is based on a pulsed laser emitting nanosecond UV pulses at 337 nm for fluorescence excitation and on a nanosecond time-gated image intensifier for acquiring fluorescence emission from the surface. Based on the exponential decay of fluorescence intensity immediately following excitation, FLIM can be used for measuring the fluorescence lifetime map of the surface, thus differentiating between different fluorophores in the field of view. The hyperspectral fluorescence imaging device is based on two filtered lamps emitting at around 365 nm coupled with a tunable liquid crystal filter and a a cooled CCD for collecting fluorescence images at selected different spectral bands of approximately 20 nm FWHM in the visible range. The apparatus can be used to measure the fluorescence spectrum at each pixel in the recorded image. The integration of the two fluorescence imaging techniques provides both spectral and lifetime information, therefore strengthening the differentiation between fluorescent materials on the surface of a painting. Following acquisition, images can be processed to highlight differences in fluorescence properties (lifetimes and spectra). However a novel approach has been further carried out based on the use of principal component analysis on the acquired images, which can effectively emphasize trends and similarities across the entire image. The portable apparatus has been applied to the monitoring of the condition of the wall paintings in the Baptistery of Colleggiata Church in Castglione Olona. Results will be presented and a final critical discussion of the technique will be given. INTRODUCTION Imaging techniques are well appreciated in conservation science. This is due to the fact that they can be used to analyze an artwork not in few selected points, but to visualize larger areas, thus taking account of the compositional heterogeneity of an artwork, which is especially relevant in paintings. Typical imaging techniques applied to the analysis of works of art range from X-ray imaging to thermography, UV-fluorescence and diffuse reflectance in the visible and in the near infrared. Among them, optical imaging techniques are especially attractive due to their simplicity, low cost and universal applicability to complex systems, including wall paintings and stone sculptures. UV fluorescence is typically used in conservation science for identifying the presence of heterogeneities, which can be ascribed to organic or inorganic fluorescent materials on artistic surfaces. In fact, many artist materials (tempera and oil binders, colorants, lakes, fluorescent modern pigments and varnishes) and restoration compounds (glues, coatings and fixatives) exhibit UV fluorescence [1]. Recently, portable point-like fluorescence spectrometers have been developed and effectively applied to the analysis of artistic surfaces, allowing the identification of specific compounds, which include modern fluorescent pigments [2], natural colorants present in wood and silk textiles [3] and natural varnishes [4]. A strong effort has been further devoted to transform fluorescence spectrometers in imaging devices, especially 1 9th International Conference on NDT of Art, Jerusalem Israel, 25-30 May 2008 For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=65