Abstract
Multispectral imaging is a preliminary screening technique for the study of paintings. Although it permits the identification of several mineral pigments by their spectral behavior, it is considered less performing concerning hyperspectral imaging, since a limited number of wavelengths are selected. In this work, we propose an optimized method to map the distribution of the mineral pigments used by Vincenzo Pasqualoni for his wall painting placed at the Basilica of S. Nicola in Carcere in Rome, combining UV/VIS/NIR reflectance spectroscopy and multispectral imaging. The first method (UV/VIS/NIR reflectance spectroscopy) allowed us to characterize pigment layers with a high spectral resolution; the second method (UV/VIS/NIR multispectral imaging) permitted the evaluation of the pigment distribution by utilizing a restricted number of wavelengths. Combining the results obtained from both devices was possible to obtain a distribution map of a pictorial layer with a high accuracy level of pigment recognition. The method involved the joint use of point-by-point hyperspectral spectroscopy and Principal Component Analysis (PCA) to identify the pigments in the color palette and evaluate the possibility to discriminate all the pigments recognized, using a minor number of wavelengths acquired through the multispectral imaging system. Finally, the distribution and the spectral difference of the different pigments recognized in the multispectral images, (in this case: red ochre, yellow ochre, orpiment, cobalt blue-based pigments, ultramarine and chrome green) were shown through PCA false-color images.
Highlights
Pigments used as pictorial materials in Cultural Heritage are inorganic and organic compounds
We propose an optimized method for mapping inorganic pigments by using multispectral imaging combined with hyperspectral spectroscopy for the study of Vincenzo Pasqualoni’s wall painting at the Basilica of S
A Principal Component Analysis (PCA) on the hyperspectral data to evaluate the effective discrimination among the identified pigments; A PCA on a wavelength reduction of the hyperspectral data to evaluate if a limited spectral range, corresponding to the filters set of the multispectral system, was sufficient to discriminate the pigments; Acquisition of the multispectral images coupled with PCA to individuate all the areas in which the identified pigments are located
Summary
Pigments used as pictorial materials in Cultural Heritage are inorganic and organic compounds. During the creation of mural paintings, it is important considering that some pigments are not alkali-resistant and cannot be used in fresco, such as organic pigments, white and red lead, malachite, or azurite [3] Even if these materials are mixed with organic binders [4], they suffer from chemical alterations caused by the exposition to some environmental conditions, such as high moisture, capillary rise, light direct exposition, etc. To simplify multispectral systems, reducing the number of filters employed to the most significant ones, some methods have been proposed, trying to obtain similar performances for hyperspectral data [11,12]. A PCA on a wavelength reduction of the hyperspectral data to evaluate if a limited spectral range, corresponding to the filters set of the multispectral system, was sufficient to discriminate the pigments; Acquisition of the multispectral images coupled with PCA to individuate all the areas in which the identified pigments are located
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