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PRISMS: A portable multispectral imaging system for remote in situ examination of wall paintings
Abstract
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.
... The spectral range was extended to 1550 nm within the EU MUSIS project (Multispectral terahertz, infrared, visible imaging and spectroscopy, 2008-2012, FP7-ICT STREP) [89]. Liang et al. [90,91] developed a MSI/HSI PRISMS (Portable Remote Imaging System for Multispectral Scanning) system specifically designed for remote imaging of wall paintings. The device operates in a VIS/NIR (400-880 nm, 10 filters, FWHM = 40 nm in the VIS, 70 nm in the NIR) as well as in a SWIR (900-1700 nm, best spectral resolution = 10 nm) range. ...
... Therefore, a compromise between high-quality data and ease of data management should be reached. [36] The introduction of MSI/HSI in heritage science boosted an informatics research branch focused on pigment identification and mapping [38,[95][96][97], automated classification of data [98,99], materials characterization [100], virtual restoration [101][102][103], machine learning analysis [104], automatic registration and mosaicking [105], web visualization and archiving [90,91,106,107]. ...
The present paper focuses on the reflectance spectral imaging of painted
surfaces in the visible-near infrared spectral region (400–2500 nm). Other spectral
ranges and methods are mentioned, to contextualize the spectral investigation of works
of art.
... Based on experience with high resolution, precision colour and spectral imaging of easel paintings in studios, we have developed a portable colour and multispectral camera system that is light-weight, flexible and without any cumbersome mechanical structure for in situ high resolution colour and spectral imaging of wall paintings [2,3]. 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. ...
... PRISMS (Portable Remote Imaging System for Multispectral Scanning) is designed for the in situ, high resolution imaging of wall paintings or any painting hang at heights that is difficult to access without any mechanical structure such as scaffoldings [1,2]. PRISMS is portable and can operate from the ground level and achieve sub-mm resolutions of up to 25 pixels/mm. ...
... The wall painting was also imaged with a hyperspectral/multispectral imaging system -PRISMS (Portable Remote Imaging System for Multispectral Scanning) which was designed specifically for the in situ imaging of wall paintings 3,4 . High resolution multispectral/hyperspectral images (sub-mm resolution) spanning the wavelength range of 400nm to 1700nm can be obtained in situ from a distance of less than a meter to over ten meters. ...
... PRISMS is designed for the in situ imaging of wall paintings which can be found on the walls or ceilings of large architectural spaces such as cathedrals or in narrow spaces such as tomb corridors 3,4 . It is therefore designed to be flexible and able to image at high resolution (i.e. ...
Multispectral and hyperspectral imaging are efficient methods of measuring spectral reflectance at high spatial resolution. This non-invasive technique has been applied to the imaging of paintings over the last 20 years. PRISMS (Portable Remote Imaging System for Multispectral Scanning) was designed specifically for imaging wall paintings. Optical Coherence Tomography (OCT) is a low coherence interferometric technique capable of fast non-invasive imaging of subsurface microstructure. This paper shows the first application of in situ OCT imaging of a wall painting. The combination of PRISMS and OCT gives information on the varnish and paint layer structure, pigment identification, the state of degradation of the paint and varnish layers and informing curators on the painting schemes and techniques.
... This type of imaging system is based on the selection of specific wavelengths and the acquisition of black and white images, in order to underline the presence of non-homogeneous regions in the surface This technique provides non-invasive mapping and classification of materials with different chemical and optical properties. It is mentioned here, because it is very interesting, but there are no known applications that have allowed to characterize organic binders [30]. • Fiber optic reflectance spectroscopy (FORS). ...
Wall painting realized using organic binders is the oldest form of parietal painting and precedes the birth of the affresco by about 20,000 years. This paper reports the results obtained from the main studies in the field of archaeological wall paintings. The attention was paid to the study of organic binders used for the application of the color, as well as on the instrumental techniques chosen to obtain such information. Different techniques can be used for the study of organic material in archeological paintings: non-destructive techniques, which can be applied directly in situ without sampling, and laboratory micro-invasive techniques for a more in-depth characterization. Among these, the chromatographic techniques represent a potential tool to acquire as much information as possible about chemical composition of binders.
... 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.
... To overcome these limitations, in recent years the possibility of implementing imaging spectroscopy in 'remotesensing mode' in CH applications has been explored. A pioneer work by Liang et al. demonstrated the usability of a multispectral imaging system for the remote investigation of wall paintings [9]. In subsequent years, further studies demonstrated the feasibility of using HSI avionic instrumentation in the CH field, by borrowing technology and knowledge from the remote-sensing applications. ...
Hyper-Spectral Imaging (HSI) is nowadays a well-established technology in the field of Cultural Heritage (CH), being acknowledged as a highly effective tool for accomplishing both non-invasive diagnostics and documentation of different typologies of polychrome artworks. Until recently, in CH field HSI technique has been mainly applied for investigating polychrome surfaces of small or medium size objects in museum collections, such as easel paintings, illuminated manuscripts or paper based artifacts. Conversely, the outdoors applications of HSI on large mural surfaces, wall paintings, frescoes and archeological assets are still mostly unexplored. This is due to the fact that the HSI systems designed for CH applications usually work at short-distance from the targets and can cover only limited size surfaces. To overcome this limitation, and to extend the applications of HSI to investigating mural paintings in external contexts, cloisters, ceilings, or assets in archaeological contexts, a remote sensing approach, based on the use of readapted avionic sensors, has been considered and tested in-field.
This work illustrates the results of the measurements campaigns carried out in the archaeological site of Pompeii (Italy) by means of remote sensing HSI technology. To this purpose, the high-performance prototype SIMGA, an avionic hyperspectral imager developed by Leonardo Company for earth-surface observations, was employed for the first time in an archeological context, within the framework of the wider project “Great Pompeii Project”. HSI measurements were performed on different types of wall surfaces, including mural paintings and mural inscriptions. On the wall paintings, the acquired HSI data proved to be effectively usable for identification of most of pictorial materials and for mapping their distributions. The presence of gypsum as marker of degradation phenomena could be detected and mapped. On mural inscriptions, the HSI data could be suitably processed to enhance some vanishing traits, hardly perceivable by visual inspection. The proposed approach provided evidence that HSI is a promising tool to retrieve some faded parts of partially lost texts in archeological contexts.
... In the visible range, Liang et al. (at Nottingham Trent University) developed a remote system for the analysis of, especially, mural paintings. They describe in several publications their remote sensing multi-spectral instrument PRISMS, which can acquire 3D information and multispectral information (9 bands) in parallel with 80 μm as spatial resolution at 10 m [39,40]. This system has also been adapted for close range imaging (replacing the telescope by a lens and the tilt stage by motorized linear stages) and recently used to investigate the Selden map of China, an early seventeenth century map of Asia. ...
This paper provides an overview over the application of scanning macro-XRF with mobile instruments for the investigation of historical paintings. The method is compared to synchrotron based macro-XRF imaging and Neutron Activation Auto-Radiography. Full-Field XRF imaging instruments, a potential future alternative to scanning macro-XRF, and confocal XRF, providing complementary depth profiles and developing into a 3D imaging technique itself, are described with the focus on investigations of historical paintings. Recent developments of X-ray radiography are presented and the investigation of cultural heritage objects other than paintings by MA-XRF is summarized. In parallel to XRF, hyperspectral imaging in the visible and range has developed into a technique with comparable capabilities, providing insight in chemical compounds, where XRF imaging identifies the distribution of elements. Due to the complementary nature of these techniques the latter is summarized. Further, progress and state of the art in data evaluation for spectroscopic imaging is discussed. In general it could be observed that technical capabilities in MA-XRF and hyperspectral imaging have reached a plateau and that with the availability of commercial instruments the focus of recent studies has shifted from the development of methods to applications of the instruments. Further, that while simple instruments are easily available with medium budgets only few groups have high-end instrumentation available, bought or in-house built.
... PRISMS is a multispectral/hyperspectral imaging system specifically designed to image wall paintings in situ: see Figure 1(a). 1,2 The technique measures the spectral reflectance characteristic of a material. Additionally, PRISMS gives qualitative images in various spectral bands in the visible/near infrared (NIR; 400-900nm)-see video 3 -and the short-wave IR (900-1700nm) such that spectra for millions of pixels can be obtained simultaneously. ...
... PRISMS is designed for portable, flexible and versatile remote imaging, consisting of modular components: (1) for imaging at distances >3–4 m, a telescope (focal length 1250 mm and aperture diameter 90 mm) is used; (2) for close range imaging at distances <3–4 m, lenses are used; (3) for imaging in the range 400–900 nm (VIS/NIR), interference filters are used with CCD detectors (Liang et al., 2007); and (4) for imaging in the 900–1700 nm range (SWIR), an imaging AOTF spectrograph and an InGaAs detector are used (Liang et al., 2010). This paper will concentrate on 3D spectral imaging with the VIS/NIR multispectral imager, which is a simple, low budget instrument consisting of a filter wheel with 10 filters and a CCD camera. ...
PRISMS (Portable Remote Imaging System for Multispectral Scanning) is designed for in situ, simultaneous high resolution spectral and 3D topographic imaging of wall paintings and other large surfaces. In particular, it can image at transverse resolutions of tens of microns remotely from distances of tens of metres, making high resolution imaging possible from a fixed position on the ground for areas at heights that is difficult to access. The spectral imaging system is fully automated giving 3D topographic mapping at millimetre accuracy as a by-product of the image focusing process. PRISMS is the first imaging device capable of both 3D mapping and spectral imaging simultaneously without additional distance measuring devices. Examples from applications of PRISMS to wall paintings at a UNESCO site in the Gobi desert are presented to demonstrate the potential of the instrument for large scale 3D spectral imaging, revealing faded writing and material identification.
This paper presents a novel multimodal remote sensing setup to analyze the complex stratigraphy of historical wall paintings at distances of order 10 m. The proposed method enables comprehensive investigation of the chemical composition of multilayer paint stratigraphy by combining standoff laser-induced breakdown spectroscopy for elemental profiling with noninvasive standoff Raman spectroscopy and visible and near-infrared (400–900 nm) reflectance spectral imaging for depth-resolved complementary material characterization from a range of distances with instruments and operators located on stable ground. Following proof-of-concept laboratory tests, the feasibility and effectiveness of this standoff analytical approach is demonstrated through field analysis of a whitewashed historical wall painting, successfully identifying at least seven distinct layers from a distance of 7 m. The remote sensing method presented here can also be applied to other scientific and industrial domains to characterize the chemical composition of layered materials at a distance.
There is growing interest in bringing non-invasive laboratory-based analytical imaging tools to field sites to study wall paintings in order to collect molecular information on the macroscale. Analytical imaging tools, such as reflectance imaging spectrometry, have provided a wealth of information about artist materials and working methods, as well as painting conditions. Currently, scientific analyses of wall paintings have been limited to point-measurement techniques such as reflectance spectroscopy (near-ultraviolet, visible, near-infrared, and mid-infrared), X-ray fluorescence, and Raman spectroscopy. Macroscale data collection methods have been limited to multispectral imaging in reflectance and luminescence modes, which lacks sufficient spectral bands to allow for the mapping and identification of artist materials of interest. The development of laboratory-based reflectance and elemental imaging spectrometers and scanning systems has sparked interest in developing truly portable versions, which can be brought to field sites to study wall paintings where there is insufficient space or electrical power for laboratory instruments. This paper presents the design and testing of a simple hyperspectral system consisting of a 2D spatial spot scanning spectrometer, which provides high spectral resolution diffuse reflectance spectra from 350 to 2500 nm with high signal to noise and moderate spatial resolution (few mm). This spectral range at high spectral resolution was found to provide robust chemical specificity sufficient to identify and map many artists’ materials, as well as the byproducts of weathering and conservation coatings across the surface of ancient and Byzantine Cypriot wall paintings. Here, we present a detailed description of the hyperspectral system, its performance, and examples of its use to study wall paintings from Roman tombs in Cyprus. The spectral/spatial image processing workflow to make maps of pigments and constituent painting materials is also discussed. This type of configurable hyperspectral system and the imaging processing workflow offer a new tool for the field study of wall paintings and other immovable heritage.
Chinese scroll painting is one of the oldest continuous artistic traditions in the world. These paintings are characterized by brushwork, pigment category and painting style. In order to nondestructively investigate the techniques of large-scale Chinese scroll paintings, an automatic hyperspectral scanning system, composed of a hyperspectral camera, a halogen light source, an automatic scanning platform, data processing software and a reference spectral library, was set up. This system was applied to the study of Portrait of Bazalibudala Arhat, an important Chinese scroll painting, along with macroscopic X-ray fluorescence technique. As a result, sketches and outlines of the patterns were detected. Pigments identified included azurite, malachite, white lead, cinnabar, ochre, gold and carbon black and were mapped to the painting. Furthermore, quantitative maps of pigments were obtained, which provided a vital clue in investigating the painters’ craft practice for the use of pigments.
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 design and performance characteristics of a novel Acousto Optic Tunable Filter (AOTF) are presented. Particular attention has been paid to the reduction of optical side lobes, maximising the light throughput and achieving efficient wideband RF matching of a device for use in hyperspectral imaging systems. Conventional AOTFs are known to yield an optical pass band with side lobes at unacceptable levels of ~-10dB relative to the transmission peak. It is known that shaping the acoustic beam ("apodisation") can suppress the side lobe transmission of the AOTF and improve its imaging capabilities. Results of a novel electrode apodisation pattern are presented, reducing sidelobes to ~-25dB. This produces an AOTF which is capable of being placed in a diffraction limited optical system and introduces negligible amounts of image degradation. The large transducer area (associated with the large optical aperture) and acoustic impedance mismatch between the AO substrate (TeO2) and transducer (LiNbO3) pose a challenge in achieving wideband RF performance. Acoustic mismatch between substrate and transducer has been addressed by the introduction of a special acoustic matching layer in the bond. The layer reduces dispersion in the transducer impedance easing broadband matching. The transducer has a low (
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.
In this article we describe the experimental setup of a multispectral image acquisition system consisting of a professional monochrome CCD camera and a tunable filter in which the spectral transmittance can be controlled electronically. We have performed a spectral characterisation of the acquisition system taking into account the acquisition noise. To convert the camera output signals to device-independent data, two main approaches are proposed and evaluated. One consists in applying regression methods to convert from the # camera outputs to a device-independent colour space such as CIEXYZ or CIELAB. Another method is based on a spectral model of the acquisition system. By inverting the model using a Principal Eigenvector approach, we estimate the spectral reflectance of each pixel of the imaged surface. Keywords: Liquid Crystal Tunable Filter, spectral reflectance, multispectral imaging, spectral match, metamerism 1. INTRODUCTION Already in 1853 Grassmann stated that three variable...
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.
This paper describes VIPS, an dimage processing system developed by the authors in the course of the EU-funded projects VASARI (1989-92) and MARC (92-95). VIPS implements a fully demand-driven dataflow image IO system. Evalutaion of library functions is delayed for as long as possible. When evaluation does occur, all delayed operations evaluate together in a pipeline, requiring no space for storing intermediate images and no unecessary disc IO. If more than one CPUT is available, then VIPS operations will automatically evaluate in parallel, giving an approximately linear speed-up. The evaluation system can be controlled by the application programmer. We have implemented a user-interface for the VIPS library which uses expose events in an X window rather than disc output to drive evaluation. This makes it possible, for example, for the user to rotate an 800 MByte image by 12 degrees and immediately scroll around the result.
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.
A new camera - SIRIS (scanning infrared imaging system) - developed at the National Gallery in London, UK allows highresolution images to be made in the near infrared region (900-1700 nm). The camera is based on a commercially available 320 × 256 pixel indium gallium arsenide area array sensor. This relatively small sensor is moved across the focal plane of the camera using two orthogonal translation stages to give images of c. 5000 × 5000 pixels. The main advantages of the SIRIS camera over scanning infrared devices or sequential image capture and mosaic assembly are its comparative portability and rapid image acquisition - making a 5000 × 5000 pixel image takes less than 20 minutes. The SIRIS camera can operate at a range of resolutions, from around 2.5 pixels per millimetre over an area of up to 2 × 2 m to 10 pixels per millimetre when examining an area measuring 0.5 × 0.5 m. The development of the mechanical, optical and electronic components of the camera, including the design of a new lens, is described. The software used to control image capture and to assemble the individual frames into a seamless mosaic image is mentioned. The camera was designed primarily to examine underdrawings in paintings; preliminary results from test targets and paintings imaged in situ are presented and the quality of the images compared with those from other cameras currently used for this application.
Spectral imaging technology, which records simultaneously spectral and spatial information about an object, was initially developed for remote sensing and has since been successfully applied to other areas of research. Although relatively new to the field of conservation, this non-invasive method of investigation has already provided promising results in the analysis of paintings and written documents, the characterization of materials and digital documentation. This article reviews the published literature relating to the application of multispectral and hyperspectral imaging for the study and conservation of works of art and presents some new perspectives offered by this innovative and fast-developing technology.
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.
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.
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.
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