Open issues in hyperspectral imaging for diagnostics on paintings: when high spectral and spatial resolution turns into data

redundancy

Costanza Cucci, Andrea Casini, Marcello Picollo, Marco Poggesi, Lorenzo Stefani Istituto di Fisica Applicata “Nello Carrara” del CNR, Via Madonna del Piano 10, I-50019

Sesto Fiorentino, Italy

ABSTRACT

Hyper-Spectral Imaging (HSI) has emerged in the last decade as one of the most promising technologies for diagnostics and documentation of polychrome surfaces. Despite the fact that presently HSI is a well-established technique for non-invasive investigations on paintings, a number of technological issues remain open and are still topics for on-going studies. In particular, it is known that high spatial resolution is a crucial parameter for obtaining high quality images, whereas the possibility to identify pictorial materials strictly depends on the spectral resolution and on the extent of the spectral region investigated. At the same time, by increasing the sampling rates in both the spatial and spectral dimensions, the size of the data-set will be enlarged and the acquisition times will be lengthened. As a consequence, a good compromise between the acquisition of high-quality data and their application should always be reached, taking into account the specific purposes of the HSI application. The above questions are discussed in the present work, which illustrates two applications of the latest version of a hyperspectral scanner designed at IFAC-CNR for the digitization of artworks. The prototype has recently been upgraded, with new visualization software as well as mechanical and optical improvements. This high performance system operates in the 400-1000nm spectral range, with a spectral resolution of about 2-3 nm and a spatial sampling of 0.1 mm over areas of about 1 m2. Three case-studies are presented, which highlight the importance of both high spatial and high spectral sampling rate in hyperspectral imaging. Two of the examples reported focus on the full exploitation of the spatial resolution: the first one is a study performed on a small painting, dated from the eighteenth century and belonging to the Uffizi Gallery in Florence; the second case-study refers to the valuable “Carrand diptych” (14th century) from the Bargello Museum in Florence. The last application, instead, shows the crucial importance of a high spectral resolution to identify selected pigments in the oil-painting "Ritratto di Maffeo Barberini", dated around 1596-1600, which has recently been attributed to Caravaggio.

Keywords: Hyper-spectral imaging, imaging spectroscopy, reflectance spectroscopy, non-invasive diagnostics

1 INTRODUCTION An imaging system is commonly considered multispectral (MSI) if its spectral sensitivity range is divided into several contiguous bands whose bandwidths are comprised between one hundred and ten nanometers. Instead bandwidths between ten nanometers and one nanometer define hyperspectral imagers (HSI). These systems, originally developed for space-borne and airborne remote sensing, have been increasingly applied in conservation science from 2005. During the past two decades, MSI has indeed become a common technique for the investigation of paintings, as it provides relevant information on morphological features (drawings, color) and it can distinguish different paintings materials [1]. Despite a relatively simple and compact implementation as staring band sequential cameras, MSI systems provide spectral data that are poorly resolved and often are inadequate for material identification. Moreover, the colorimetric analysis is one of the most relevant applications of multispectral data, which are exploited to provide high quality color reproductions for documentation and archiving purposes. In this case, ten nanometers bandwidths or narrower are recommended for color measurements, to avoid both metamerism errors and the need of color calibration with test charts [2].

O3A: Optics for Arts, Architecture, and Archaeology III, edited by Luca Pezzati, Renzo Salimbeni, Proc. of SPIE Vol. 8084, 808408 · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.889460

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In recent years, different studies have been performed aimed at improving the instrumental performances of imaging systems for diagnostics and documentation of artworks, and valuable results have been achieved thanks to the recourse to the technology borrowed from portable terrestrial HSI imagers [3-4]. These systems offer high performances in terms of spatial and spectral sampling rates of the imaged areas, thus significantly increasing the informative content and the usability of the data-set acquired.

One of the drawbacks is that the combination of high spectral with high spatial resolution imaging unavoidably entails huge data amounts and prolonged acquisition times, thus rising several questions related to data-handling and processing and to the practical consequences of long measurement sessions. The typical sizes of high definition HSI imaging data-set can be as large as several hundred gigabytes. Although this is not much of a problem for current storage means, troubles may arise from throughput limits, during the acquisition phase, in data formatting and also in data editing, which may result to be very long. Moreover, complications may rise by the limitations set to virtual memory management of the commercial software and operating systems, which in turn limits the display of very large images. Large data amounts also mean long times in running processing algorithms. The run-times of some widely used statistical algorithm, such as principal components analysis (PCA), tend to exponentially grow with data amount [5]. Consequently, in many cases it is advisable to evaluate the possibility of processing appropriate spatially sub-sampled data sets by retaining the full spectral resolution. In other cases the available full spatial resolution can be required to extract the needed information, and limits have to be set to the dimensions of the imaged area in order to lighten the data-processing and shorten the elaboration times. The advantages and drawbacks of using highly sampled data-set have therefore to be evaluated case by case, by considering the final aim of the specific study.

The above issues are discussed in the present work, and three selected case studies are reported which highlight different uses of high spatially and spectrally resolved HSI data.

2 EXPERIMENTAL 2.1 Equipment

The first prototype of a hyperspectral scanner specifically developed at IFAC-CNR for non-invasive diagnostics on artworks and museum objects [6] has been improved and updated over the last five years, in order to address the issues related to the on-site practical applications. The rationale of the work performed and the main technical aspects tackled are reported in the following.

To go beyond the early experiences carried on with multispectral Vidicon® cameras [7-9], the two primary features to be improved were both spectral and spatial resolution. Namely, the wish that spectral resolution should be close to that of fiber-optic spectrometers (~2 nm), while the spatial resolution should be competitive with the best imagers used for infrared reflectography and color imaging. To reach these objectives different instrumental design and technical solutions have been proposed and tested by different research groups. Some laboratories had implemented imagers based on a sensor array or matrix that scans the image plane of a large format camera so to compose a mosaic image so large that would be very expensive to be obtained by means a single area array [9,10]. This image-plane scanning approach had also been adopted by Lumiere Technology for a MSI camera developed under the aegis of the European CRISATEL project [11]. This system was a multi-spectral evolution of Jumbo-ScanTM, a planetary scanner for the fast digitization of large documents. MSI Jumbo-ScanTM implemented the spectral analysis by means of thirteen band-pass optical filters sequentially put in the optical path before a very long linear-array CCD, at each scan step. The illuminator consisted in two sweeping light-line projectors whose motion was synchronized with that of the linear CCD in order to keep the illuminated region confined around the scan line. In this way, exposure of the non viewed parts of the painting to light was avoided. Other models of scanners were based on the object-plane scheme, such as those devised by INO in Florence since 1990 [12]. In this case the scanning head was moved in a plane parallel to the painting and close to it. In this way, the illumination confinement was a simple consequence of the joint assembly of the light projectors with the scanning head. Illumination confinement is particularly important in HSI systems, where the light power density must be balanced with the number of bands used if the same quality has to be maintained in each band image. To this aim, in designing a HSI scanner, an object-scanner scheme is preferable to an image-plane one, because it avoids sophisticated external illumination systems, seemingly critical to keep aligned. The main disadvantage in this case is a bulkier and heavier mechanical structure.

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The HSI scanner developed at IFAC-CNR consists of an orthogonal pair of linear motion actuators which move a compact line-spectrograph (Specim V10E) in a ~1 m2 vertical-plane. With respect to the first implementation [4], the present version mounts an enhanced version of the spectrograph, with really negligible geometrical deformations, and additional filters for the compensation of internal stray-light. The system operates in the 400-900 nm spectral range. Indeed, due to a combination of the poor infrared response of the camera with the insufficient rejection of the order-blocking filter of the spectrograph, the upper wavelength limit is set at 900 nm, beyond which the spill out of the second order blue wavelengths are not negligible with respect to the first order infrared one.

Figure 1. Colorimetric errors on LabsphereTM diffuse color standards (CIE 1976 L*a*b*, D65/10°).

The optical module of the system has been optimize to minimize the errors and distortions in the final image due to the non-planarity of canvas and surfaces defects of objects imaged. The front telecentric lens (Opto-Engineering) performs parallel projection within ~3 cm depth, which is crucial for close distance scanning in order to avoid parallax errors and to allow the exact splicing of adjacent scan-lines, even when the painting surface deviates from planarity. Telecentric depth tolerance is also useful to ease the preliminary operations in positioning the scanner with respect to the painting before scanning. Initial maneuvering is made fast and easy also by means of line pointers that, at overlap, mark the view-line on the panting. The lens-magnification is such that the line-spectrograph focuses a dispersed image of the 60 mm long view line-segment onto a 8.8 mm width interline-transfer CCD of an ORCA-ER camera (Hamamatsu). The scan movement is performed by adjacent vertical strips, slightly overlapped, at the constant speed of 1.5 mm/sec. That means that the ~1 m2 area can be scanned in less than three hours and a half. During each vertical scan the camera runs free at 16.7 frames per second in 2x2 binning mode. Its line-spectra images (672 x512 pixels, 16 bit) are written onto the PC disc through its IEEE-1394 serial output by the support of a buffering program, carefully designed to avoid any data loss.

The illumination module consists of a fiber-optic illuminator (Schott-Fostec) which focuses the light of a 3300 K QTH lamp onto a fiber-optic bundle terminating with a pair of light-lines with cylindrical lenses. These latter project their beams around the viewed line-segment, symmetrically at 45° with respect to the normal to the surface, in compliance with CIE standards for colorimetric measurements. The colorimetric accuracy has been tested on SpectralonTM diffuse color standards and the results are reported in Figure 1: were it not for the red and orange targets, the accuracy would be excellent.

The main cause of colorimetric errors is internal stray-light, which is roughly compensated only by a flat subtraction from all the wavelengths of the signal measured on the spectral channels below 400 nm, isolated from external light.

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The system operates with a spatial sampling rate of 11 points/mm (279 dpi). The overall spatial resolution evaluated at 50% of contrast reduction, is higher than 2 lines/mm at all the wavelengths in the 400-900 nm range. This is easily confirmed by looking at the plot in Figure 2, which reports the line profile on the 550 nm scan image of a reflection sinusoidal test pattern aligned along the scan direction (vertical). Sinusoidal fit of the 2 lines/mm profile and the calibration reflectance values give a contrast of 0.62, which tends to lower towards 900 nm even if it is still above 0.5. The resolution in the orthogonal direction (horizontal) is somewhat better because it is not affected by the scan motion blurring.

Figure 2. Line profile of a sinusoidal test pattern aligned with the scan direction (vertical) at 550 nm. At 2 cycles/mm

contrast is still > 50% of maximum.

Regarding the spectral resolution, the 512 spectral channels from 380 to 1000 nm give a full width at half maximum resolution of 2.55 nm, as can be seen in Figure 3, which reports the detail of a helium line of a spectral calibration lamp.

Figure 3. Response to a helium spectral lamp. FWHM of the Gaussian fit is 2.55 nm.

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Since excessive light exposure may damage the artworks under analysis, the scanner has been tested under this point of view in operative condition, and an experimental protocol to ensure the object safety has been set-up. In order to circumscribe any possible cause of damage the light exposures and local temperature increase have been measured under typical measurements conditions. During a scan at 1.5 mm/sec, the maximum illumination is ~ 16000 lux, with a UVA fraction of about 56 μW/lumen. The total light exposure results below 500 luxhr, which is a fair value with regard to photo-degradation risks [13]. Anyway, the high peak value suggests caution when dealing with light-sensitive and fading materials. Even if the infrared components of light beyond 1000 nm are cutoff by means of a special cold-filter, slight transient heating of the painting surface cannot be avoided. This problem has not been considered by other authors, even if thermography, after the first applications to wall paintings [14], has begun to be used to detect subsurface flaws in mobile paintings [15,16].

To the best of the authors’ knowledge, studies of heat diffusion in paintings aimed at predicting the possible strains in those inhomogeneous layered structures seem not to be available. For a precautionary measure a simple measurement of the magnitude of the transient warming during a scan was performed. A 1 mm thick black cardboard with nearly constant ~10% reflectance in the visible and near infrared regions was imaged with a FLIR thermo-camera, which recorded, for the maximum illumination of ~ 16000 lux, a temperature rise of ~4 °C in ~30 sec, followed by a slower exponential decay in ~80 sec. Though the evaluation seems pessimistic because of the very low reflectivity of the target and the low thermal conductivity of paper, the incompleteness of accurate information suggested the authors to avoid scanning paintings whose surface has not been carefully inspected by a conservator.

2.2 Data

Following geospatial HSI usage, each scan data can be recorded into a single file according to a format suitable for prompt access to the information of interest. In the present case the main interest is spectral analysis, thus the band interleaved by pixel (BIP) data order was adopted. This data order was the same as the common raster scan pixel order in a bitmap image, but with each pixel value replaced by the spectrum in the same position. Such organization privileges spectrum retrieval and makes display fast even if the file is very large.

The application program for spectrum display and other basic interactive functions is non-commercial customized software, developed at IFAC-CNR laboratories. The application software has been developed within the Microsoft environment, and this sets some limitations to the display of larger images. The present version of the program cannot display images larger than ~6000x6000 pixels, beyond which sub-sampled data or limited areas of the original image have to be considered. One way to overcome these limitations might be to use progressive resolution coding.

Full spatial resolution is usually required for detecting under-drawings on the infrared images, either alone or in combinations with those in the visible. Infrared false color images are also a common tool to enhance the presence of retouches and to differentiate pigments which appear similar in the visible. Nevertheless, this approach cannot be considered rigorous and accurate for the study of materials distribution on the pictorial surface. Indeed, a precise and reliable study of materials can only be done on the basis of the whole spectral information. Usually, 279 dpi comes out to be a redundant spatial density for the variability of the materials of almost all the paintings, except in the case of manuscripts and illuminates. This fact makes it possible to work on five-time spatially sub-sampled data, without losing information. Under these circumstances images are perfectly manageable by the hand-made software.

Spatial sub-sampling is also necessary to reduce the execution time of some algorithms, especially those based on statistical analysis, like PCA [5]. Anyhow, much of the classifying work may be done by means of faster algorithms. Sometimes, even a simple approach, like a least squares fit of the spectra in a narrow sub-band with a parabolic curve, is sufficient to form maps of absorption. Obviously such a simplistic method must be cautiously used.

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Another helpful algorithm for the HSI data elaboration is SAM (spectral angular mapping) [17,18], which calculates the angular difference between each spectrum and a reference one, with the spectra thought as vectors in the hyperspace of wavelengths. This method is relatively fast but not immune to false result: all the spectra laying on a cone surface with the reference spectrum give the same difference value.

3 CASE STUDIES 3.1 Small painting by Gaspare Lopez (1650-1732) and the Carrand dyptich (14th century)

Clearly, the high spatial resolution of the scanner is valuable for studying the reflectance spectra of details.

A set of still-life and landscape small format paintings have been studied on the basis of the HSI data acquired by means of the hyperspectral scanner. The paintings belong to the Galleria degli Uffizi and were painted by the Neapolitan painter Gaspare Lopez, also known as “Gasparo of the flowers”, in the early XVIII century with oil on copper technique (Figure 4). In this case, due to the fineness of details, the full spatial resolution of the acquired data has been exploited to gain a deep insight into the artist technique and also to extract information on the palette. In Figure 5 the spectra of two points at a distance of about 2.5 mm of each other are reported. Point 1 spectrum suggests a mixture of Prussian blue and lead white, while point P2 can be identified as lead white mixed with cinnabar.

Another example of the use of the full spectral resolution of the hyper-spectral data refers to the study carried out on the diptych (Parisian school of the 14th century) of the Carrand collection, from the Bargello Museum in Florence.

Figure 4. Oil on copper panel by G. Lopez (10 cm x 12 cm). Figure 5. P1: Prussian blue with lead white, P2: cinnabar.

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The detail reported in Figure 6 represents the left valve, around a horizontal line of the architectural motif. The line appears to be painted in a copper-based green (P1) on the damask pattern, painted in gold, lapis lazuli and red lake (P2) (Figure 7). But occasionally the underlying background transpires and as a result a mixed spectrum was recorded (P3). The three points fall within a 4 mm x 4 mm boundary and each spectrum is averaged on a 0.27x0.27 mm2 area (3 points x 3 points). The hint of the presence of the copper based green in the line was given by a SAM map of the spectra that, in the 600-880 nm interval differ less than 6° from a reference spectrum taken on a green coat.

Figure 6. Carrand diptych, detail (6.1 cm x 6.4 cm). Figure 7. P1: copper green with yellow. P2: red lake. P3: composite spectrum.

3.2 Portrait of Maffeo Barberini by Caravaggio (1571-1610)

The portrait of Maffeo Barberini is a canvas painting (122 cm x 93 cm) from the end of the 16th century belonging to a private collection in Florence (Figure 8). During the careful restoration of the painting for the exhibition “Caravaggio e Caraveggeschi” held in Florence in 2010, the canvas was recognized as an authentic work by Caravaggio. In that circumstance the painting underwent also several non invasive diagnostic analyses, such as x-ray fluorescence (XRF) and fiber optic reflectance spectroscopy (FORS) in the near ultraviolet, visible and near infrared regions. After the cleaning and before the inpainting procedure the painting was imaged by IFAC-CNR hyperspectral scanner. The final HSI data (~106 Gbyte) were obtained by stitching two slightly overlapped frames.

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The infrared images appeared quite dark: it seemed that some black pigment had been mixed with iron oxides and lead white in the preparatory layer to reinforce its dark shades. The shapes of the reflectance spectra of the red tones suggest the use of cinnabar, confirmed by the detection of mercury by XRF, but they also revealed the use of lake pigments, generally weak except for the flowers. That may be seen in Figure 9, which shows in detail the interval of two small absorption bands, typical of lake pigments [19]: the logarithmic scale relatively enhances the tiny signature on the spectra of the red fold of the cape, where the lake had been used as glazing layer. Both the least-square-error parabolic fit and the spectral angle mapping (SAM) algorithms were applied.

Figure 8. Portrait of Maffeo Barberini by Caravaggio. Figure 9. Absorption bands, typical of lakes.

In Figure 10 the 2nd degree coefficient of the parabola that fits the local spectrum in the 514-541 absorption sub-band of lake is reported as a grey level of the corresponding pixel: the brighter the tone, the stronger the concavity of the band. As the parabolic fit gives a positive signal to any concavity, the chosen band is the farther from the shoulder of cinnabar in order to decrease the strong “false” response due to its concave ascent. The intense brightness on the upper flower, a pink carnation, is in agreement with the abundant presence of lake (Figure 9). Also the lower bright levels of the two red carnations below are somewhat consistent with the more feeble signature of lake, here used as a glaze layer as on the red fold of the vest. The brightness of the lead-tin yellow centers of the jasmines and white roses is a false signal, instead, due to the general concave ascent of their spectra in the chosen sub-band. The same type of false signal is given by the flesh tone, where the mixture with lead white makes the cinnabar spectrum rise earlier, with a concave trend. Nonetheless, the results of SAM, a method which is likely the divergence algorithm more commonly used in HSI data analyses, may not be completely reliable.

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Figure 10. 2nd degree coefficient of the parabolic fit in the

514-541 nm sub-bands. Figure 11. Spectral angle differences <10° with the upper

flower spectrum in the 500-600 nm interval.

Figure 11 reports the map of the angle difference below 10° of all the spectra with that of the upper carnation in the 500-600 nm range (both the lake sub-bands): the darker the tone, the smaller the difference. The low difference (dark tone) on the petals of the white flowers is unexpected: the modulation impressed on the reference spectrum by the lake signature is likely too small to weigh enough against the trend similarity that lead white exhibits with the reference spectrum in this narrow interval. The resemblance still indicated at 10° begins to disappear below 6° of divergence, threshold below which the false links with the vase, the collar, and some parts of the incarnate are yet reported. Where the attention drawn by the SAM image is not deceived is on the moiré cape: on the dark regions lake is definitely present, with different strength in various areas of the breast, homogeneously on the right shoulder of the personage, where even the tones of the visible image appear reddish (Figure 8). In this case-study, the high performance of the hyper-spectral scanner in terms of spectral resolution turned out to be crucial for permitting to carry out an accurate analysis of materials distribution.

ACKNOWLEDGMENTS

The authors wish to thank Cristina Acidini (head of the “Soprintendenza Speciale per il Polo Museale Fiorentino”), the Director of the Uffizi Gallery, Antonio Natali, the Director of the Restoration Laboratories of the “Soprintendenza Speciale per il Polo Museale Fiorentino”, Magnolia Scudieri, and the conservators Elena Prandi and Marina Ginanni, for enabling the authors to perform investigations on the painting by Lopez and the Carrand Diptych. Authors would also like to acknowledge the restorer Muriel Vervat for her support during the investigation of the Caravaggio painting.

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