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.

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Advances in multispectral and hyperspectral imaging for archaeology and art conservation

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.

Multispectral and hyperspectral imaging technologies in conservation: current research and potential applications

We show terahertz reflection images of hidden paint layers in a painting on canvas and compare the results with X-ray Radiography and In-frared Reflectography. Our terahertz measurements show strong reflections from both the canvas/paint interface and from the raw umber/lead white interface, indicating sufficient refractive-index contrast. Our results show that X-rays cannot be used to image through the lead white pigment which effectively blocks the X-rays. Although Infrared Reflectography is capable of vaguely observing the hidden paint strokes from the canvas side, we show that only terahertz imaging is capable of providing information on the thickness of the hidden paint layers. Terahertz imaging is thus shown to be a powerful imaging method for art historians, conservators and conservation scientists.

/pdf/terahertz-imaging-of-hidden-paint-layers-on-canvas-4gtrhb162t.pdf

TeraHertz imaging of hidden paint layers on canvas.

Often, just micrometers below a painting’s surface lies a wealth of information, both with Old Masters such as Peter Paul Rubens and Rembrandt van Rijn and with more recent artists of great renown such as Vincent Van Gogh and James Ensor. Subsurface layers may include underdrawing, underpainting, and alterations, and in a growing number of cases conservators have discovered abandoned compositions on paintings, illustrating artists’ practice of reusing a canvas or panel. The standard methods for studying the inner structure of cultural heritage (CH) artifacts are infrared reflectography and X-ray radiography, techniques that are optionally complemented with the microscopic analysis of cross-sectioned samples. These methods have limitations, but recently, a number of fundamentally new approaches for fully imaging the buildup of hidden paint layers and other complex three-dimensional (3D) substructures have been put into practice. In this Account, we discuss these developments and their recent practical appl...

Photon-based techniques for nondestructive subsurface analysis of painted cultural heritage artifacts.

The development of advanced methods for non-destructive selective imaging of painted works of art at the macroscopic level based on radiation in the X-ray and infrared range of the electromagnetic spectrum are concisely reviewed. Such methods allow to either record depth-selective, element-selective or species-selective images of entire paintings. Camera-based ‘full field’ methods (that record the image data in parallel) can be discerned next to scanning methods (that build up distributions in a sequential manner by scanning a beam of radiation over the surface of an artefact). Six methods are discussed: on the one hand, macroscopic X-ray fluorescence and X-ray diffraction imaging and X-ray laminography and on the other hand macroscopic Mid and Near Infrared hyper- and full spectral imaging and Optical Coherence Tomography. These methods can be considered to be improved versions of the well-established imaging methods employed worldwide for examination of paintings, i.e., X-ray radiography and Infrared reflectography. Possibilities and limitations of these new imaging techniques are outlined.

/pdf/examination-of-historical-paintings-by-state-of-the-art-3uswi6rr68.pdf

Examination of historical paintings by state-of-the-art hyperspectral imaging methods: from scanning infra-red spectroscopy to computed X-ray laminography

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 multi-spectral imaging system for examining paintings

A New Multi-spectral Imaging System for Examining Paintings.

It is current practice to take tiny samples from a painting to mount and examine in cross-section under a microscope. However, since conservation practice and ethics limit sampling to a minimum and to areas along cracks and edges of paintings, which are often unrepresentative of the whole painting, results from such analyses cannot be taken as representative of a painting as a whole. Recently in a preliminary study, we have demonstrated that near-infrared Optical Coherence Tomography (OCT) can be used directly on paintings to examine the cross-section of paint and varnish layers without contact and the need to take samples. OCT is an optical interferometric technique developed for in vivo imaging of the eye and biological tissues; it is essentially a scanning Michelson’s interferometer with a ‘broadband’ source that has the spatial coherence of a laser. The low temporal coherence and high spatial concentration of the source are the keys to high depth resolution and high sensitivity 3D imaging. The technique is non-invasive and noncontact with a typical working distance of 2 cm. This non-invasive technique enables cross-sections to be examined anywhere on a painting. In this paper, we will report new results on applying near-infrared en-face OCT to paintings conservation and extend the application to the examination of underdrawings, drying processes, and quantitative measurements of optical properties of paint and varnish layers.

/pdf/optical-coherence-tomography-a-non-invasive-technique-3y9fo0g5ek.pdf

Optical coherence tomography: a non-invasive technique applied to conservation of paintings

The new SIRIS (Scanning InfraRed Imaging System) camera developed at the National Gallery in London allows high-resolution images of paintings to be made in the near infrared region (900-1700 nm). Images of 5000 × 5000 pixels are made by moving a 320 × 256 pixel InGaAs array across the focal plane of the camera using two orthogonal translation stages. The great advantages of this camera over scanning infrared devices are its relative portability and that image acquisition is comparatively rapid - a full 5000 × 5000 pixel image can be made in around 20 minutes. The paper describes the development of the mechanical, optical and electronic components of the camera, including the design of a new lens. The software routines used to control image capture and to assemble the individual 320 × 256 pixel frames into a seamless mosaic image are also mentioned. The optics of the SIRIS camera have been designed so that the camera can operate at a range of resolutions; from around 2.5 pixels per millimetre on large paintings of up to 2000 × 2000 mm to 10 pixels per millimetre on smaller paintings or details of paintings measuring 500 × 500 mm. The camera is primarily designed to examine underdrawings in paintings; preliminary results from test targets and paintings are presented and the quality of the images compared with those from other cameras currently used in this field.

/pdf/siris-a-high-resolution-scanning-infrared-camera-for-2qolavm0qx.pdf

SIRIS: a high resolution scanning infrared camera for examining paintings

A new camera – SIRIS (scanning infrared imaging system) – developed at the National Gallery in London allows high-resolution 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.

/pdf/a-new-camera-for-high-resolution-infrared-imaging-of-works-55pdqqfv9t.pdf

A new camera for high-resolution infrared imaging of works of art

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