Reflectance imaging spectroscopy, the collection of images in narrow spectral bands, has been developed for remote
sensing of the Earth. In this paper we present findings on the use of imaging spectroscopy to identify and map artist
pigments as well as to improve the visualization of preparatory sketches. Two novel hyperspectral cameras, one
operating from the visible to near-infrared (VNIR) and the other in the shortwave infrared (SWIR), have been used to
collect diffuse reflectance spectral image cubes on a variety of paintings. The resulting image cubes (VNIR 417 to 973
nm, 240 bands, and SWIR 970 to 1650 nm, 85 bands) were calibrated to reflectance and the resulting spectra compared
with results from a fiber optics reflectance spectrometer (350 to 2500 nm). The results show good agreement between
the spectra acquired with the hyperspectral cameras and those from the fiber reflectance spectrometer. For example, the
primary blue pigments and their distribution in Picasso's Harlequin Musician (1924) are identified from the reflectance
spectra and agree with results from X-ray fluorescence data and dispersed sample analysis. False color infrared
reflectograms, obtained from the SWIR hyperspectral images, of extensively reworked paintings such as Picasso's The
Tragedy (1903) are found to give improved visualization of changes made by the artist. These results show that
including the NIR and SWIR spectral regions along with the visible provides for a more robust identification and
mapping of artist pigments than using visible imaging spectroscopy alone.
Site specific, in situ techniques such as X-ray fluorescence (XRF) and Raman spectroscopy are commonly used to
identify pigments on illuminated manuscripts. With both techniques, spectra are usually acquired on visually identified
sites thought to be representative of the pigments and mixtures used for the illumination. Such visual inspection may not
always ensure an adequate representation of the pigment diversity. Here we report on the application of multispectral
(MSI) visible/infrared reflectance and luminescence imaging spectroscopy, along with fiber optics reflectance
spectroscopy (FORS) to help determine and map the primary pigments in a late 14th century miniature on vellum,
attributed to Niccolo da Bologna and representing the birth of John the Baptist. XRF analyses of visually selected sites
found elements consistent with azurite, ultramarine, vermillion, lead white, "mosaic gold" and yellow earth pigments.
Visible/infrared FORS analyses confirmed these assignments and showed evidence for the use of organic dyes. The
spectral analysis of the MSI-reflectance images gave distribution maps for these pigments (i.e., regions of azurite,
ultramarine, vermillion) along with some indication of pigment layering not identified visually. The luminescence image
gave a probable map of the organic dye(s). Images acquired in the near- and shortwave-infrared (NIR and SWIR, 750 to
2400 nm) revealed preparatory sketches and illumination techniques. These results show, like those of a prior study
carried out on another 14th century Italian miniature, that the combination of low light multi-spectral imaging
spectroscopy with FORS provides improved in situ mapping and identification of pigments on illuminated manuscripts.
Qualitative UV-fluorescence of varnishes is commonly used to locate repaints on paintings or to specify the homogeneousness of a varnish layer. Photographers can now use flash UV-lamps coupled with a CCD camera to obtain colour images of the fluorescence of paintings, unveiling thus both interest and difficulty to interpret these colours.
Starting from this point of view, UV-fluorescence spectra appear to be a potential technique to characterize the nature of varnishes and, if possible, their state of degradation. This identification will be non-invasive, without contact, obtained in real time and workable in situ, as the identification of pigments or dyes by reflectance spectrometry which is already done in our group. The last goal will be to realize both identifications with the same device.
Emission fluorescence spectra are implemented with the Jobin-Yvon Fluorolog-3, providing an incident wavelength laying between 200 and 850 nm. The emission spectra are implemented with an optical fiber linked to a Jobin-Yvon spectrometer HR460 and a multi-channel CCD detector.
In a first step, popular, fresh, raw resins used between the XVI th and the XIX th century, as mastic, dammar and sandarac, have been used to prepare varnishes films with different solvents. The fluorescence spectra of these films have been carried out at different excitation wavelengths to build databases. After having tested the coherence, the limits and the accuracy of the method, we suggest different applications of our method. A synthesis of the results will be presented to characterize each varnish by their fluorescence spectra.
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