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Remote multispectral imaging with PRISMS and XRF analysis of Tang Tomb Paintings
Abstract
PRISMS (Portable Remote Imaging System for Multispectral Scanning) is a multispectral/hyperspectral imaging system designed for flexible in situ imaging of wall paintings at high resolution (tens of microns) over a large range of distances (less than a meter to over ten meters). This paper demonstrates a trial run of the VIS/NIR (400-880nm) component of the instrument for non-invasive imaging of wall paintings in situ. Wall painting panels from excavated Tang dynasty (618- 907AD) tombs near Xi'an were examined by PRISMS. Pigment identifications were carried out using the spectral reflectance obtained from multispectral imaging coupled with non-invasive elemental analysis using a portable XRF.
... Scientific methods used in the examination of easel paintings have evolved over the last 50 years toward a totally noninvasive way of inspection by introducing ever more devoted techniques for chemical analyses of pigments and binders thanks to multispectral imaging [1][2][3][4]. In reflectography, the InGaAs-based detectors, sensitive in short-wave infrared (SWIR, 0.78-1.70 ...
... The improvements of the detectors in mid-wave infrared (MWIR) and long-wave infrared (LWIR) coupled with fast digital postprocessing using infrared thermography (IRT) on artworks [13][14][15] have eventually changed in recent years, giving IR inspection of paintings new chances of application in conservation projects. Relevant works have been proposed to the scientific community considering the issues of long-distance inspection [4,12,16] as well as pictorial film detachments [15]. Finally, the use of statistical analysis of thermal image sequences after short thermal pulse stimulation has shown to be helpful in both diagnosis of pictorial layers and underdrawings detection when traditional SWIR reflectography fails [16][17][18]. ...
Reflectographic analyses applied on paintings can be performed using cameras equipped with different detectors with different abilities in detecting and visualizing underdrawings, repainting, restorations, and other nonvisible information. In this research, the results obtained through thermographic imaging followed by statistical imaging postprocessing methods have been compared with those obtained with traditional reflectographic methods in the short-wave infrared range. The comparison has been performed studying the thermal sequence after a single pulse of light with a different spectrum of ad hoc mock-ups. Results showed that for limited cases, signal-to-noise ratio seems to be more relevant in obtaining reliable images of underdrawings with respect to the effect of optical absorption of visible light by painting layers.
... A linear gamma correction 12 is used to ensure that the intensity values are the same in the saved file as the corresponding pixel response on the camera sensor. Dark Frame Correction (when an image is captured with the shutter closed to record any noise from the camera sensor which is then subtracted from the original to improve signal to noise ratio) is sometimes used [23], [112], [132][133][134][135]. ...
Although multispectral imaging (MSI) of cultural heritage, such as manuscripts, documents and artwork, is becoming more popular, a variety of approaches are taken and methods are often inconsistently documented. Furthermore, no overview of the process of MSI capture and analysis with current technology has previously been published. This research was undertaken to determine current best practice in the deployment of MSI, highlighting areas that need further research, whilst providing recommendations regarding approach and documentation. An Action Research methodology was used to characterise the current pipeline, including: literature review; unstructured interviews and discussion of results with practitioners; and reflective practice whilst undertaking MSI analysis. The pipeline and recommendations from this research will improve project management by increasing clarity of published outcomes, the reusability of data, and encouraging a more open discussion of process and application within the MSI community. The importance of thorough documentation is emphasised, which will encourage sharing of best practice and results, improving community deployment of the technique. The findings encourage efficient use and reporting of MSI, aiding access to historical analysis. We hope this research will be useful to digitisation professionals, curators and conservators, allowing them to compare and contrast current practices.
... Multispectral reflection imaging and ultraviolet (UV) fluorescence are techniques widely used in archaeology to study wall paintings, since multispectral and false-colour imaging can provide qualitative information on the material used and help in the assessment of their state of conservation (Lu et al. 2009;Lange et al. 2011;Liang et al. 2011;Amadori et al. 2014). Three-dimensional multispectral techniques have also been proposed (Liang et al. 2014;Grifoni et al. 2018). ...
This paper presents the results of a multispectral reflectometry and ultraviolet (UV) fluorescence study on the entrance wall of the Tomb of the Blue Demons in the Necropolis of Monterozzi (450–430 bc) in Tarquinia (Viterbo, Italy), which is a UNESCO site. The technique is based on the acquisition of a multispectral set ranging from 10 to 25 images, acquired at wavelengths between 370 and 1120 nm, with a spectral width of 50 nm per image, using both halogen lamps for visible and infrared images and high‐purity UV diodes for fluorescence. Blind‐source separation algorithms were then applied to the whole image set to extract from the resulting images the details not otherwise visible in the single spectral images. The multispectral technique presented was tested and improved upon over the last decade on ancient Etruscan and Roman wall paintings and, because of its quickness and cost‐effectiveness, it can now be proposed as a powerful tool for the study of poorly conserved archaeological wall paintings and a preliminary diagnostic survey before any future conservation intervention on them.
Ultraviolet-induced Luminescence (UVL) is the property of some materials of emitting light once illuminated by a source of UV radiation. This feature is characteristic of some mediums and pigments, such as some red lakes, widely used for the realisation of works of art. On the one hand, UVL represents a like strike for a researcher in the cultural heritage field: in fact, UVL allows to characterise the state of conservation of the paintings and, in some cases, to recognize at glance some of the materials used by the artists. On the other hand, the contribution of UVL to the study of the artefacts is almost always limited to qualitative observation, while any speculation about the cause of the luminescence emission relies on the observer’s expertise. The aim of this paper is to overcome this paradigm, moving a step toward a more quantitative interpretation of the luminescence signal. The obtained results concern the case study of pictorial materials by Giuseppe Pellizza da Volpedo (1868–1907, Volpedo, AL, Italy) including his iconic masterpiece Quarto Stato (1889–1901), but the method has general validity and can be applied whenever the appropriate experimental conditions occur. Once designed an appropriate set-up, the statistical comparison between the acquisitions performed on Quarto Stato, on a palette belonged to the master, on drafts made by the author himself and on a set of ad hoc prepared samples both with commercial contemporary pigments and prepared with the traditional recipe, shed some light on which materials have been employed by the artist, where they have been applied and support some intriguing speculations on the use of the industrial lakes in the Quarto Stato painting.
X-ray fluorescence (XRF) spectroscopy has become an important materials characterization technique used by researchers studying cultural and archaeological artifacts. XRF is often a first choice for an initial materials investigation due to its nondestructive nature, swift setup, short acquisition times, portability, and ease of use. XRF analysis is noninvasive, and can be undertaken without contacting the artifact, making it also preferred from a conservation standpoint, and routinely adopted by heritage conservation practitioners. In particular, handheld, portable XRF instruments can be easily transported and deployed under most field scenarios and conditions. As such, XRF has secured its place in the cultural heritage/archaeological tool box. This chapter will present an overview of the role and use of XRF for the collection/acquisition of materials characterization data on cultural heritage and archaeological artifacts.
The near-infrared (NIR) region covers the transition from the visible spectral range to the mid-infrared (IR) region, spanning the wavelength range 750–2500 nm (13 300 to 4000 cm−1). Light in the NIR region was discovered in 1800 by Herschel. However, the earliest applications of near-infrared spectroscopy (NIRS) were only reported in the 1950s. The use of portable NIR spectrometers is relatively recent (1990s) owing to obvious technological constraints, but since its onset, the applications were immediately expanded to a vast array of fields. The staggering development of hardware and components of these past decades, combined with a miniaturizing trend of highly performant constituents and accessories, has enabled the development of state-of-the-art portable NIR equipment. Among the different components of portable NIR instruments, wavelength selectors and detectors are the most important in the instrument performance and key factors when selecting the appropriate instrument for a specific purpose. This technology is now used in monitoring processes and quality control studies, among others, at the forefront of such innovative fields and industries such as agriculture and food products, art conservation and cultural heritage studies, pharmaceutical industry, and clinical applications to name just a few.
Keywords:
near-infrared spectroscopy;
portable spectrometers;
optical spectroscopy;
high-throughput chemical analysis;
field monitoring
The construction and examination of test panels is an ad hoc procedure, necessary for every spectral imaging study of paintings. Despite the common features, almost every scientific team follows a different way of construction. Furthermore, many of these approaches are not adequately documented in the relevant papers. Failure to use common language and practice leads to confusion about properties of materials and paint layers that have been overall examined by the scientists, as well as the validity of the results and their exploitation in several conservation applications. The present theoretical approach points out the need for common protocols for the construction of test panels and draws general principles as a flow chart on which they should be based.
Multispectral imaging has been applied to the field of art conservation and art history since the early 1990s. It is attractive as a non-invasive imaging technique because it is fast and hence capable of imaging large areas of an object giving both spatial and spectral information. This paper gives an overview of the different instrumental designs, image processing techniques and various applications of multispectral and hyperspectral imaging to art conservation, art history and archaeology. Recent advances in the development of remote and versatile multispectral and hyperspectral imaging as well as techniques in pigment identification will be presented. Future prospects including combination of spectral imaging with other non-invasive imaging and analytical techniques will be discussed.
ABSTRACT We describe a new method,for non-invasive pigment identification by combining the spectral reflectance in the visible spectrum,with near infrared OCT cross-section images,of the subsurface layer structure. Keywords: optical coherence tomography, multispectral imaging, spectral imaging, pigment identification, scattering coefficient
The application of infrared spectroscopic imaging to non-destructive examination of works of art is described. Its advantages over infrared photography and reflectography are discussed. in particular its ability to provide spectroscopic information, which potentially allows identification of pigments, binders, and other materials. Near-infrared spectra of a selection of brown and black pigments are presented. Results are given of the application of infrared spectroscopic imaging to two works of art in different media: an ink drawing and an oil painting.
Hyperspectral imaging is a non-destructive optical analysis technique that can for instance be used to obtain information from cultural heritage objects unavailable with conventional colour or multi-spectral photography. This technique can be used to distinguish and recognize materials, to enhance the visibility of faint or obscured features, to detect signs of degradation and study the effect of environmental conditions on the object. We describe the basic concept, working principles, construction and performance of a laboratory instrument specifically developed for the analysis of historical documents. The instrument measures calibrated spectral reflectance images at 70 wavelengths ranging from 365 to 1100 nm (near-ultraviolet, visible and near-infrared). By using a wavelength tunable narrow-bandwidth light-source, the light energy used to illuminate the measured object is minimal, so that any light-induced degradation can be excluded. Basic analysis of the hyperspectral data includes a qualitative comparison of the spectral images and the extraction of quantitative data such as mean spectral reflectance curves and statistical information from user-defined regions-of-interest. More sophisticated mathematical feature extraction and classification techniques can be used to map areas on the document, where different types of ink had been applied or where one ink shows various degrees of degradation. The developed quantitative hyperspectral imager is currently in use by the Nationaal Archief (National Archives of The Netherlands) to study degradation effects of artificial samples and original documents, exposed in their permanent exhibition area or stored in their deposit rooms.
Image spectroscopy (IS) is an important tool for the noninvasive analysis of works of art. It generates a wide sequence of multispectral images from which a reflectance spectrum for each imaged point can be recovered. In addition, digital processing techniques can be employed to divide the images into areas of similar spectral behavior. An IS system designed and developed in our laboratory is described. The methodology used to process the acquired data integrates spectral analysis with statistical image processing: in particular, the potential of principal-component analysis applied in this area is investigated. A selection of the results obtained from a sixteenth-century oil-painted panel by Luca Signorelli is also reported.
A new multispectral system developed at the National Gallery is presented. The system is capable of measuring the spectral reflectance per pixel of a painting. These spectra are found to be almost as accurate as those recorded with a spectrophotometer; there is no need for any spectral reconstruction apart from a simple cubic interpolation between measured points. The procedure for recording spectra is described and the accuracy of the system is quantified. An example is presented of the use of the system to scan a painting of St. Mary Magdalene by Crivelli. The multispectral data are used in an attempt to identify some of the pigments found in the painting by comparison with a library of spectra obtained from reference pigments using the same system. In addition, it is shown that the multispectral data can be used to render a color image of the original under a chosen illuminant and that interband comparison can help to elucidate features of the painting, such as retouchings and underdrawing, that are not visible in trichromatic images.
An experiment was performed that compared conventional small-aperture and image-based reflection spectrophotometry of paintings. The imaging system used a liquid-crystal tunable filter, resulting in 31 spectral bands evenly sampled between 400 and 700 nm and ranging in bandwidth between 10 and 60 nm. The small-aperture spectrophotometer had a constant bandwidth of 10 nm. Test targets consisting of chromatic and neutral samples of various colors and spectral properties were used to derive a calibration transformation between the two technologies. Three paintings were analyzed: Saint Jerome Reading by Alvise Vivarini, Murnau by Alexej von Jawlensky and Pot of Geraniums by Henri Matisse, all from the collection of the National Gallery of Art, Washington, DC. Average colorimetric accuracy varied between 2.0 and 3.2 ΔE00 units and the average spectral accuracy varied between 1.0 and 2.1% spectral root-mean-square. Two drawbacks are that the imaging system has a high uncertainty at short wavelengths, and the spectral matches for samples with flat spectra are slightly worse than for other samples. Both limitations can be corrected by changes in lighting, the calibration target, and the method of deriving the transformation matrix. Nevertheless, the imaging system has the advantage of no moving parts and may not require image registration, making it well suited to perform scientific imaging of cultural heritage. Furthermore, the image-based spectra have sufficient accuracy for pigment identification and mapping.
We present a proto-type portable remote multispectral imaging system, PRISMS (Portable Remote Imaging System for Multispectral Scanning), that is light-weight, flexible and without any cumbersome mechanical structure for in situ high resolution colour and spectral imaging of large and inaccessible paintings such as wall paintings. This is the first instrument to be able to image paintings at inaccessible heights in situ from ground level to produce not only high resolution colour images but also multispectral images.
Wide-band near-infrared reflectography is a familiar imaging technique for conservators. For many decades it has been the most effective method for providing an extensive knowledge of underdrawings, retouchings and pentimenti. In this study, a narrow-band multi-wavelength imaging (image spectroscopy) system assembled around a lead oxide-lead sulphide (PbO-PbS) vidicon camera is described and applied to the investigation of paintings in the visible and near-infrared regions. A suitable methodology has been set up in order to make the measurements accurate and reliable. The technique has been applied to the detection of different pigments in the sixteenth-century panel painting 'La strage degli undicimila martiri' by Pontormo.
Describes the second-generation system used to detect and measure changes in the surface colour of paintings. It comprises a high-resolution monochrome digital camera mounted on a positioning system, which carries a filter set placed in front of the camera. Areas of the painting and of calibration charts are illuminated sequentially through the filters, and combined into a high-resolution colourimetric image of the whole painting. At times 10-20, or even 40 pixels (picture elements) per mm are required to give adequate resolution, and large paintings are dealt with by taking subimages and combining them electronically. Details of the equipment, calibrations, storage of data, etc. are given. Changes in surface texture after transport can also be measured.
Reflection imaging spectroscopy is a useful technique to remotely identify and map minerals and vegetation. Here we report on the mapping and identification of artists' materials in paintings using this method. Visible and infrared image cubes of Picasso's Harlequin Musician are collected using two hyperspectral cameras and combined into a single cube having 260 bands (441 to 1680 nm) and processed using convex geometry algorithms. The resulting 18 spectral end members are identified by comparison with library spectra, fitting by nonlinear mixing, and using results from luminescence imaging spectroscopy. The results are compared with those from X-ray fluorescence spectrometry, polarized light microscopy, and scanning electron microscopy-energy dispersive spectrometry (SEM/EDS). This work shows the potential of reflection imaging spectroscopy, in particular if the shortwave infrared region is included along with information from luminescence imaging spectroscopy.
We present a scanning device for multispectral imaging of paintings in the 380–2300 nm spectral range (32 VIS+14 NIR bands). The system is based on contact-less and single-point measurement of the spectral reflectance factor. Multispectral images are obtained by scanning the painted surface under investigation. At present the VIS and NIR modules work separately due to the lack of synchronization between them. Measurement campaigns were carried out on several paintings in situ and at the INOA Optical Metrology Laboratory located inside the Opificio delle Pietre Dure in Florence. We report herein on the measurements carried out on a few panel and canvas paintings. Multivariate image analyses (MIAs) were performed and the detected images were analysed by means of the conventional principal component analysis (PCA) and the K-nearest-neighbouring cluster analysis (KNN).
We have developed a computer controllable hyper-spectral imaging apparatus, capable of acquiring spectral images of 5 nm bandwidth and with 3 nm tuning step, in the spectral range 380-1000 nm. The critical component of the apparatus is the innovative imaging monochromator, which enables the tuning of the imaging wavelength. This module is coupled with a two-dimensional detector array composing a tunable wavelength camera system. Electronic controllers are employed for detector and monochromator synchronization and driving, while the system calibration, image processing and analysis are performed with the aid of specially developed software. The system records light intensity as a function of both wavelength and location. In the image domain, the data set includes a full image at each individual wavelength. In the spectroscopy domain, a fully resolved diffuse reflectance and/or fluorescence spectrum at each individual pixel can be recorded. The developed spatially resolved spectral acquisition system is ideal for the non-destructive analysis of heterogeneous materials such as objects of artistic and historic value. Experimental studies show its potential in assisting the identification and mapping of painting materials in situ. Furthermore, it was shown that it enables the recovery of erased-overwritten scripts in old manuscripts and the determination of proper spectral bands for the on-line monitoring of laser and non-laser cleaning procedures.
Spectral imaging technology, widely used in remote sensing applications, such as satellite or radar imaging, has recently gained importance in the field of artwork conservation. In particular, multispectral imaging in the near-infrared region (NIR) has proved useful in analyzing ancient paintings because of the transparency of most pigments and their varied reflectance changes over this spectral region. A variety of systems, with different detectors and filtering or dispersing technologies, have been implemented. Despite the recognized potential of multispectral NIR imaging, which provides information on both spectral and spatial domains (thus extending the capabilities of conventional imaging and spectroscopy), most of the systems currently used in art diagnostics have limitations. The technology is still in its early stages of development in this field.
