Although digital multispectral imaging-particularly
ultraviolet-induced fluorescence imaging-is a very common
examination tool, its interpretation remains fraught with difficulties. Interpretation is strongly dependent on the capture
methodology, requires an understanding of the physical and chemical characteristics and interactions among materials in
artworks and is affected by data-analysis procedures.
The present research, which began with imaging of paint materials of known composition and proceeded to a range of
representative case studies, confirmed that fluorescence emissions by painting materials-such as organic binders or
colorants-are generally severely affected by the presence of absorbing non-fluorescing materials, such as inorganic
pigments. Application of a mathematical model based on the
Kubelka-Munk theory, resulted in the possibility of
distinguishing between real and apparent fluorescence emissions. Real emissions correspond to the presence of materials
which de facto exhibit fluorescent properties (typically organic binders and colorants), while apparent emissions relate to
the optical interactions among fluorescent materials and surrounding non-fluorescent materials (typically inorganic
pigments). Correction for the 'pigment-binder interaction' can also provide useful information on the presence of
materials whose fluorescence is almost obliterated by absorbing pigmented particles. Therefore, this image-processing
methodology can be used to characterise and reveal emissions that are dimmed or altered by re-absorption. This capacity
to reveal the presence of weakly fluorescing emitters has important conservation implications and informs the sampling
strategy for further analytical investigations.
Examples of the application of this data analysis to images made at the Grotto Site in Dunhuang, China, and at the
British Museum are presented.
Optical coherence tomography (OCT) is a fast scanning Michelson interferometer originally designed for in vivo
imaging of the eye. In 2004, our group along with two other groups first reported the application of OCT to art
conservation and archaeology. Since that time we have been conducting a project to investigate systematically the
potential of OCT as a new tool for non-invasive examinations of a wide range of museum objects and to design an OCT
optimised for in situ use in museums. Here we present the latest results from this ongoing project, which include the
determination of the optimum spectral windows for OCT imaging of paintings and painted objects executed using
traditional techniques, and non-invasive imaging of the subsurface stratigraphy of painted layers at multiple
wavelengths. OCT imaging in assisting spectral pigment identification and in measuring refractive indices of paint will
also be presented to illustrate the potential of the technique.
Scientific examinations of paintings are routinely carried out in major galleries and museums to assist in conservation treatment and as part of technical or art historical examinations. Care is taken to examine the paintings non-destructively as far as possible. However, in order to study the paint and varnish layers, it is still currently necessary to take tiny samples of a painting to examine the cross section of a small area of the painting under a microscope. In an attempt to solve this problem we evaluate the potential of optical coherence tomography (OCT) in providing high resolution information about paint layers. Two OCT systems have been assembled, operating at 850 nm and 1300 nm, each using two single mode in-fiber couplers. Both systems can produce A (reflectivity profile in depth), T (lateral reflectivity profile), B (cross section image) and C-scans (constant depth image). Using superluminiscent diodes, a depth resolution better than 9 microns is achieved. We present results of applying OCT to sample panels and paintings. We show that infrared OCT is capable of non-destructive examination of paintings in 3D, which shows not only the structure of the varnish layer but also the paint layers. The OCT images present better microscopic tomography of the surface of the varnish and paint layers than any system currently employed in the examination of paintings. OCT could also be used for accurate measurement of the thickness of the varnish layer on a painting.
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 "broad-band" 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 non-contact 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.
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.
We present results of applying low coherence interferometry to gallery paintings. Infrared low coherence interferometry is capable of non-destructive examination of paintings in 3D, which shows not only the structure of the varnish layer but also the paint layers.
Over ten years ago the National Gallery in London began a program to make digital images of paintings in the collection using a colorimetric imaging system. This was to provide a permanent record of the state of paintings against which future images could be compared to determine if any changes had occurred. It quickly became apparent that such images could be used not only for scientific purposes, but also in applications where transparencies were then being used, for example as source materials for printed books and catalogues or for computer-based information systems. During the 1990s we were involved in the development of a series of digital cameras that have combined the high color accuracy of the original 'scientific' imaging system with the familiarity and portability of a medium format camera. This has culminated in the program of digitization now in progress at the National Gallery. By the middle of 2001 we will have digitized all the major paintings in the collection at a resolution of 10,000 pixels along their longest dimension and with calibrated color; we are on target to digitize the whole collection by the end of 2002. The images are available on-line within the museum for consultation and so that Gallery departments can use the images in printed publications and on the Gallery's web- site. We describe the development of the imaging systems used at National Gallery and how the research we have conducted into high-resolution accurate color imaging has developed from being a peripheral, if harmless, research activity to becoming a central part of the Gallery's information and publication strategy. Finally, we discuss some outstanding issues, such as interfacing our color management procedures with the systems used by external organizations.
The European Commission-funded MARC project ended in April 1996, with the publication of Flemish Baroque Painting, Masterpieces of the Alte Pinakothek, Muenchen (Hirmer 1996). To the best of our knowledge, this is the world's first all- digital colorimetric art catalogue. This paper will briefly introduce the MARC camera and the MARC printing technology, and then present a critical evaluation of the final book. The application of MARC results since the end of the project will be covered, and related EC imaging projects surveyed.
With the aim of providing a digital electronic replacement for conventional photography of paintings, a scanner has been constructed based on a 3000 X 2300 pel resolution camera which is moved precisely over a 1 meter square area. Successive patches are assembled to form a mosaic which covers the whole area at c. 20 pels/mm resolution, which is sufficient to resolve the surface textures, particularly craquelure. To provide high color accuracy, a set of seven broad-band interference filters are used to cover the visible spectrum. A calibration procedure based upon a least-mean-squares fit to the color of patches from a Macbeth Colorchecker chart yields an average color accuracy of better than 3 units in the CMC uniform color space. This work was mainly carried out as part of the VASARI project funded by the European Commission's ESPRIT program, involving companies and galleries from around Europe. The system is being used to record images for conservation research, for archival purposes and to assist in computer-aided learning in the field of art history. The paper will describe the overall system design, including the selection of the various hardware components and the design of controlling software. The theoretical basis for the color calibration methodology is described as well as the software for its practical implementation. The mosaic assembly procedure and some of the associated image processing routines developed are described. Preliminary results from the research will be presented.
As more research is conducted into the storage of colour images by digital image processing, the quality of colour has come under greater scrutiny; this has in turn led to a reassessment of the methods for coding and analyzing colour.