Reflectance hyperspectral imaging for colorimetric and spectroscopic studies: the analysis of an impressionist painting

In the last few years reflectance hyperspectral imaging (HSI) has been increasingly employed in the study of cultural heritage and its conservation. HSI systems acquire hundreds of images in narrow contiguous spectral bands in the visible (400-750 nm) and near infrared (750-2500 nm) spectral regions. These form a tridimensional data set called "cube-file": the two coordinates x and y are associated to the spatial information, while the third one to the spectral information. By manipulating this cube-file it is possible to obtain different kind of information: reflectance spectra, colorimetric data, and images at different spectral bands (or range). In particular, this article will focus on the combination of colorimetric and spectroscopic analyses for the study of artworks, with the final aim of highlighting the colorimetric differences between different pigments. To reach this goal, a 19th century easel painting was analyzed with a HSI system developed at the "Nello Carrara" Institute of Applied Physics. Starting from the cube-file, it was possible to extract the reflectance spectra for different areas of color and to calculate the colorimetric values for these pixels. Therefore, the materials were identified, and this allowed to compare the different shades of hues - obtained with different pigments - with their colorimetric values.