Visible and infrared imaging spectroscopy of paintings and improved reflectography

Abstract

Background: Imaging spectroscopy, the collection of spatially co-registered images in many contiguous spectral bands, has been developed for remote sensing of the Earth utilizing reflectance or luminescence. In this paper we summarize findings on the application of imaging spectroscopy to works of art in order to identify and map artists' pigments, improve the visualization of preparatory sketches, and identify non-original material. Methods: Two types of novel hyperspectral cameras, one operating from the visible to near-infrared (VNIR, 400-900 nm) and the other in the shortwave infrared (SWIR, 950-1700 nm), have been used to collect diffuse reflectance spectral image cubes from a variety of paintings. The resulting cubes (400-1700 nm, 310 spectral bands) were calibrated to reflectance and the resulting spectra compared with point-based spectra collected with a fiber reflectance spectrometer (350-2500 nm). In addition, a calibrated multispectral camera (600-950 nm, eight spectral bands) was used to obtain luminescence spectral image cubes after exciting paintings at blue wavelengths. Results: False-color images obtained from the SWIR imaging system are shown to give improved visualization of changes from paintings having large and complex reworkings, e.g. Pablo Picasso's The Tragedy (1903). False-color images and reflectance spectra allowed definitive identification of restoration in a late 13th century Sienese painting that cannot be seen by traditional infrared reflectography. Spectral image processing on the reflectance image cubes have been used to identify the primary pigments or pigment mixtures. For example, the primary pigments and their distribution in Picasso's Peonies (1901) was determined and compared with x-ray fluorescence (XRF) analysis and scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDS). Conclusion: The results show that the combination of both the SWIR reflectance data and the luminescence data provides a more robust ability to assign the pigments than with visible spectroscopy alone. Also highlighted is the utility of reflectance imaging spectroscopy in the SWIR region to better reveal specific paint changes, including changes in the drawings and markings made by the original artist, or changes made by a subsequent conservator such as a fill.