Visible and infrared imaging spectroscopy of paintings and improved reflectography
© Delaney et al. 2016
Received: 11 November 2015
Accepted: 28 February 2016
Published: 16 March 2016
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.
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.
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).
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.
KeywordsImaging spectroscopy Reflectance spectroscopy Pigment mapping Infrared reflectography
The conservation of a painting requires an understanding of the materials (pigments and binders) and construction methods used by the artist. There are numerous analytical methods used today that provide a wealth of detailed information about the materials used as well as the stratigraphy of the paint layers. These methods, such as SEM–EDS, high-performance liquid chromatography, and transmission-mode mid-IR spectroscopy, all require dispersed samples (scraping from the paint surface) or paint cross-sections. While advances in technology and the development of new protocols by conservation scientists have minimized the sample size needed, the taking of samples is still required. Even though these methods provide the most detailed chemical information about the materials used, they cannot be used across the entire painted surface because the number of samples allowed is limited.
One method conservators use to gather information across an entire painted surface is infrared reflectography. Infrared reflectography is the use of infrared radiation to penetrate below the surface of paint and visualize compositional paint changes and preparatory sketches . The resulting images are called infrared reflectograms and can also provide information on the evolution of the composition and construction methods. Infrared reflectography is often implemented such that the reflected light is collected over a wide spectral range (i.e. broadband) in the near-infrared (NIR, 750–1000 nm) and shortwave infrared (SWIR, 1000–2500 nm). The amount of chemical information that can be obtained in this situation is quite limited because the gray–scale values in the reflectogram are an average of the reflected light intensity over such a large spectral range . In comparison to broadband reflectance imaging, the narrowing, and shifting of the spectral band used to examine works of art (through the use of either optical filters or a spectrometer), changes the optimal contrast of the paint, and preparatory drawings may be better visually separated from other paint layers .
In order to actually identify chemically distinct materials such as pigments (both organic and inorganic) and paint binders, conservators and conservation scientists have become interested in adapting chemical imaging methods to acquire information on the macroscale. Chemical imaging methods can also be used to supplement and extend the observations made from site–specific analyses and guide the selection sites for sampling. Most of the imaging–based methods have been developed from successful, site–specific, in situ analysis methods that do not require samples. These include reflectance [2–4], luminescence [4, 5], and x-ray fluorescence (XRF)  spectroscopies. Reflectance and luminescence spectroscopies provide information about electronic transitions [5, 7–13]. Reflectance spectroscopy in the SWIR provides information about vibrational transitions, which are mostly overtones and combination bands whose fundamental transitions occur in the mid-IR. These features are often related to functional groups like hydroxyl (−OH) and carbonate (−CO3) . Absorption features from organic materials like the paint binding media, including the amide groups of proteins and lipid bands of oils and waxes, can also be observed  and even used to map their spatial distribution [15, 16]. The spectral resolution needed for identification and mapping varies across the spectral regions. In the visible range, many of the electronic transitions from pigments (i.e., blues, greens and earths) are slowly varying with wavelength, implying that a low spectral resolution can be used. However, to discriminate between pigments having sharp transition edges, such as red and yellow semiconductor pigments, a resolution of <5 nm may be necessary. In the SWIR region, the required resolution should be <10 nm in order to resolve the narrow vibrational bands associated with some binding media [14–16]. X-ray fluorescence, in contrast, provides information on the elemental composition of materials and allows one to infer the possible colorants present . Since many of the pigments used are inorganic materials, the use of XRF spectroscopy in conservation is widespread.
Efficient macroscale imaging of paintings using line-scanning imaging spectrometers [2–4, 15, 16] or filter–based imaging systems [5, 10, 18] has allowed conservation scientists to collect reflectance and luminescence images of paintings in the visible (400–750 nm), NIR, and SWIR. Using highly sensitive focal plane arrays, the images can be acquired under lighting conditions suitable for examination of paintings and even illuminated manuscripts [15, 18]. Collection rates are typically on the order of a few minutes to scan an area of 640 by 640 spatial pixels in 150–300 spectral bands [2, 4, 15, 16]. In contrast, XRF imaging spectroscopy has been implemented using a raster scanning method, which provides detailed information about the elements used and in some cases the way they are layered within a work of art. These XRF scans can take many tens of hours to complete depending on the size of the painting .
The chemical imaging techniques discussed above are types of imaging spectroscopy or hyperspectral imaging. In this paper we show the application of imaging spectroscopy to study paintings on canvas and wood panels. The first example uses imaging spectroscopy in the visible and SWIR regions to map and help identify the primary colorants used in Pablo Picasso’s Peonies. In the next examples, imaging spectroscopy in the SWIR helps provide detailed information concerning the underdrawings and paint changes in heavily re-worked paintings, and what areas are likely original material in paintings that have undergone restoration prior to modern conservation practice.
Reflectance Spectroscopy - Imaging and Point-based Systems: The hyperspectral reflectance imaging cameras and processing methods have been previously described  and are only briefly summarized here. The visible–to–near–infrared (VNIR) imaging spectrometer (SOC-730VNIR, Surface Optics Corporation, CA) is a slit–based hyperspectral sensor that uses a transmission grating spectrometer (Specim Corp, Finland). The spectral range is 417–973 nm with a spectral sampling of 2.25 nm giving 240 spectral band images. A rotating scan mirror is used to sweep out one spatial dimension of the hyperspectral image cube, giving a final image size of 640 by 640-spatial pixels by 240 spectral image bands. An integration time of 120 ms is used, giving an acquisition time of 77 s per cube. The first SWIR imaging spectrometer used for the two Picasso paintings is also a slit–based, scanning system that uses a grating spectrometer for wavelength dispersion (SOC-720SWIR, Surface Optics Corp). A 640 by 512 pixel InGaAs focal plane array (640SDV, Sensors Unlimited, NJ) is used as the detector array. The spectral range of the detector is from 960 to 1810 nm with a 10 nm spectral sampling, giving 85 spectral band images. An integration of 33 ms per line gives an acquisition time of 22 s per image cube. The second SWIR hyperspectral camera is similar, although a more sensitive InGaAs array (640HSX-1.7RT, Sensors Unlimited, NJ) was used along with a spectrometer having 3.4 nm sampling, giving 209 spectral bands from 967 to 1680 nm. A large white Spectralon® panel is used to flatfield the image cubes, and two in-scene black and white standards are used in all cases to calibrate the image cubes to reflectance. For the Picasso paintings studied, illumination was done using 750-W tungsten halogen lamps often employed for infrared reflectography [1, 4], and 125-W lamps were used for the Sienese painting. The cubes are spatially registered together using a rotation and translation mapping function whose parameters are defined by a series of manually determined tie points. This calibration and registration process yields a spectral image cube having usable information from 420 to 1660 nm in 310 spectral band images. The SWIR image cube was adjusted in gain to match the VNIR image cube at 960–970 nm. While this corrected the majority of the offsets, some small deviations can be observed, likely due to small residual errors from scattered light.
To determine a basis set of reflectance spectra that describe the components present (i.e. endmembers), the image cube was processed using hyperspectral algorithms developed for the analysis of remote sensing data  and found useful for analyzing image cubes of paintings and manuscripts [4, 15, 16, 18]. Because of the large number of reflectance spectra in the image cube, multivariate statistics were used to reduce the dimensionality of the data. To do this, principal component (PC) analysis was used to determine the diversity of the image cube, convex geometry was used to find the spectrally pure pixels, and then these pixels were manually clustered in PC space. Once the endmember spectra were determined, the spectral mapping algorithm was used to map each endmember. The algorithm for the mapping determines the angle between each endmember spectrum and the spectrum of each spatial pixel in the image cube. Using a threshold angle determined from examination of the histogram of angle values, a thresholded map is constructed to show where spatial pixels have reflectance spectra that well match that of the reference endmember spectrum. Using reference reflectance spectral databases of known artist pigments and previously identified spectral features of artist pigments, the endmember spectra were assigned to pigments when possible.
Fiber optics reflectance spectroscopy (FORS): FORS was performed on single points using a fiber spectroradiometer (Field Spec3, ASD Inc.) operating from 350 to 2500 nm . The spectral resolution was <3 nm in the VNIR range and <10 nm in the SWIR. The sampled area was approximately 3 mm in diameter, and the acquisition time for each spectrum was shorter than 4 s. Light levels were kept below approximately 4000 lx for these measurements. The spectrometer was calibrated using a white Spectralon® standard.
X-ray Fluorescence (XRF) Spectroscopy: XRF analyses were performed with a Röntec ArtTAX μXRF spectrometer equipped with a rhodium x-ray tube, helium flush and capillary optic lens with a diameter of 60 μm. The x-ray tube voltage was 50 kV, the current was 200 μA, and the integration time was 200 s. XRF data were interpreted using traditional methods, that is, the presence of pigments was inferred from the elements detected, the appearance of the locations examined, and the attribution of the work of art.
Luminescence Imaging Spectroscopy: As previously described [4, 5], an illumination and multispectral camera system assembled in–house, operating from 650 to 950 nm, was used to obtain image cubes calibrated to relative radiance. Excitation illumination was done using two Kodak Ektagraphic III AT carousel slide projectors (OSRAM 300 W EXP bulbs) equipped with F/D 2.5, 152 mm Buhl optical lenses set up at 45 degrees from the normal of the painting at a distance of approximately 3.6 m. The excitation bandpass was 380–520 nm defined by 50 mm2 Kron/Cousins B astronomy filters inserted into the two slide projectors. The multispectral camera system consists of a low-noise, 4-megapixel Si CCD camera (QImaging Retiga 4000R), outfitted with an F/D 2, 25 mm effective focal length hyperspectral lens (Coastal Optics). Spectral interference band pass filters (FWHM 40 nm, Andover Corporation) were used to create the image cube of seven spectral bands. An in-scene calibration method was used to convert the image intensities to relative spectral radiance, using a cadmium pigment target whose spectral emission had been previously measured using the ASD spectroradiometer.
Results and discussion
(A) Mapping and identification of the pigments in Picasso’s Peonies
Spectral endmember #5 (spectrum denoted with a black line in Fig. 1c) maps to the white flowers and was found to lack the narrow absorption feature at 1447 nm characteristic of the hydroxyl band of lead white [4, 9]. This indicates that zinc white or titanium white were possibly used. FORS spectra taken at sites in the white flowers have a sharp reflectance transition edge at 380 nm, which is consistent with zinc white and not lead or titanium white . This was confirmed by XRF measurements that showed the presence of Zn in the white flowers  (Fig. 3b).
(B) Optimal visualization of paint changes and sketches in Picasso’s The Tragedy
Similarly, the ‘dying horse’, which is under the final composition and is reminiscent of earlier Picasso bull-fight scenes , is optimally visualized at ~1350 nm (Fig. 6b). Here the maximum reflectance difference is less than 5 %, and the optimal spectral range is narrower. The false–color image shows not only the paint used to model the horse, but also the drawing lines used to delineate the horse’s features. Finally, at approximately 1500 nm, all the paint is penetrated and the extensive drawings of caricatures drawn directly on the wood panel are best visualized (Fig. 6c). In addition, the wood grain can be seen clearly near 1600 nm, indicating that no ground is present on the panel. Prior broadband infrared images revealed many of these features, however, they were difficult to discern because of the low signal–to–noise ratio of the detector and the multitude of images present . Therefore, the combination of narrow band, false–color composite images and being able to view the reflectance spectra improves the ability to emphasize features of interest when compared to the earlier broadband methodology where a single infrared reflectogram image was produced.
(C) Mapping conservation fill below the surface of a late 13th century panel painting
These results from reflectance and luminescence imaging spectroscopy show the ability of in situ methods to map and identify many of the primary pigments used in paintings. This pigment identification method is made more robust when combined with site–specific analysis methods such as XRF. 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. Imaging spectroscopy is one of several emerging mapping and imaging tools likely to play an important role in advancing our understanding of polychrome works of art.
JKD, MT, JZ, PR, and KD collected the reflectance and fluorescence imaging spectroscopy data and completed data analysis. KM and MT completed XRF data collection and analysis. The manuscript was written and edited by JD, KD, and PR. All authors read and approved the final manuscript.
The authors thank Elizabeth Walmsley, Ann Hoenigswald, and Joanna Dunn (NGA) for technical advice on interpreting the infrared images and Dr. Roy Littleton and Tyler Anderson (NVESD) for technical support during data collection. The authors acknowledge financial support from the National Science Foundation (award 1041827), and J.K.D. acknowledges support from the Andrew W. Mellon and the Samuel H. Kress Foundations.
The authors declare that they have no competing interests.
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