Scientific Examination of Art Modern Techniques in Conservation and Analysis (2005) / Chapter Skim
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Multispectral Imaging of Paintings in the Infrared to Detect and Map Blue Pigments--John K. Delaney, Elizabeth Walmsley, Barbara H. Berrie, and Colin F. Fletcher
Multispectral Imaging of Paintings in the Infrared to Detect and Map Blue Pigments--John K. Delaney, Elizabeth Walmsley, Barbara H. Berrie, and Colin F. Fletcher
Pages 120-136
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to separate and support identification of these blue pigments in situ and to map their distribution. To test this, visible and infrared cameras were equipped with spectral band filters to allow collection of multispectral images of test panels and 1Conservation Division, National Gallery of Art, 2000-B S Club Drive, Landover, MD 20785 2 The Genomics Institute of the Novartis Research Foundation, 10675 John Jay Hopkins Drive, San Diego, CA 92121 120
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Multispectral imaging of two paintings by Vincent van Gogh provided reflectance spectra consistent with the presence of the blue pigments Prussian blue, cobalt blue and ultramarine and gave information on the distribution of the pigments in the works by utilizing spectral band ratio images and false color composites. The identification of the pigments was confirmed using air-path X-ray fluorescence spectroscopy and energy dispersive spectrometry.
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The ability to acquire high fidelity visible reflectance spectra (<5 nm band pass) of entire paintings is beyond the resources of conservation laboratories.
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Diffuse Reflection Spectra of Pigments and Paints Diffuse reflectance spectra of seven dark blue pigments, in pressed powder form and from the test panels, were collected using either a Nicolet System 510
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For each test panel or painting, a set of 8 to 15 images was captured. In the multispectral image sets of the paintings, the paint test panels and Spectralon reflection standards were included in each image.
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RESULTS AND DISCUSSION Diffuse Spectra of the Blue Pigments and Paints The six blue pigments have similar reflectance spectra in the visible region with few features. However, the spectra demonstrate larger and more varied changes in reflectance in the reflective infrared (0.7-2.5 microns)
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of the blue paint test panels (Figure 2) were obtained using spectral band pass filters.
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Multispectral Imaging of Paint Samples To examine the extent to which diffuse reflectance spectra of pigments can be modeled by using imaging cameras available to conservators, multispectral images of the paint swatches were collected (Figure 2)
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This shows that these blue pigments can be distinguished from each other using only the multispectral images. This ability exists despite the fact that the measurements on the powdered pigment samples were performed using an integrating sphere, whereas the imaging of the panels and the works of art were performed with a narrow collection solid angle.
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The lack of large reflectance changes in some of the darker and brighter blue regions of the visible image suggests the use of a different blue pigment there. To more confidently assign the regions identified in the band ratio image of La Mousmé, multispectral images in 11 spectral bands from the visible to the SWIR were collected from an 8 × 10-inch section of the painting.
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Reflectance spectra derived from the multispectral image cube of the painting itself support the assignment of Prussian blue to bright areas of the ratio
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of La Mousmé. The image-derived spectra of the bright regions in the ratio image that map to the dark blue areas of the visible image most closely resemble the reference spectrum of Prussian blue paint test panel and powdered pigment (Figure 4B)
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. The reflectance spectra of the dark blue outlining FIGURE 5 Multispectral image analysis of a detail of La Mousmé showing the use of two different dark blue pigments for outlining.
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The reflectance spectra derived from the image cube suggest that the dark outline of the buttonholes and the blue outlines around the spots on the girl's skirt were painted using ultramarine. Cobalt blue, a pigment that van Gogh used often, does not appear to have been used for painting La Mousmé.
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This study of two paintings demonstrates that multispectral imaging in the visible and NIR regions can be employed as a useful tool in the scientific examination of paintings. We have demonstrated that the optical properties of pigments in the infrared display diagnostic features which can be employed to assign and map pigments, and that these features can be detected using conventional imaging techniques, including modeling of reflectance spectra and ratioing images
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Jack Salisbury, who gave us access in 1994-95 to his laboratory at Johns Hopkins University in order to collect the diffuse reflectance spectra of the samples, and Mr. Dana D'Aria, who assisted us in the collection of the spectra; Raymond Rehberg, David L Clark, and Rollo E Black, of Eastman Kodak, who provided generous assistance using the Kodak thermal imager; Elizabeth Freeman, Kristi Dahm, Lucy Bisognano, and Laura Rivers, who helped with the image captures; Dr.
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11. The Complete Letters of Vincent van Gogh, 3 vols.
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