Near infrared spectroscopic reflectance imaging: a new tool in art conservation

doi:10.1016/S0924-2031(01)00145-X

Vibrational Spectroscopy

Volume 28, Issue 1, 28 February 2002, Pages 59-66

https://doi.org/10.1016/S0924-2031(01)00145-X Get rights and content

Abstract

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.

Introduction

Infrared (IR) photography has been used for decades in the analysis of works of art [1], [2]. The ability of infrared radiation to penetrate through some pigments has allowed researchers to visualize under-drawings not seen with visible light. Infrared images made using black-and-white photographic film have provided much useful technical information to conservators. However, a lack of chemical specificity, problems with the graininess of images, and a need to bracket exposure times to ensure good quality photographs have limited the usefulness of IR photography. Some of these limitations have been addressed by the related technique of infrared reflectography, which uses infrared-sensitive Vidicon cameras [3], [4] or charge-coupled device (CCD) cameras [5] to detect the infrared radiation reflected from the art work. However, all these cameras have a broad wavelength response, and without some form of wavelength filter, the images obtained have reduced discriminating power [6]. Using a selection of filters, multispectral imaging is possible [7]. Nevertheless, obtaining chemical information requires making measurements at a larger number of distinct wavelengths, in order to generate a hyperspectral image.

Advances in scientific CCD cameras and liquid crystal tunable filter (LCTF) technology have enabled the development of infrared spectroscopic imaging systems [8]. We have developed such a system and have used it for the analysis of several works of art from The Winnipeg Art Gallery [9], [10]. The system allows the automated collection of wavelength-specific digital images at any wavelength in the near-infrared region between 650 nm and approximately 1100 nm. This can provide useful information on the reflectivity of pigments at specific wavelengths. More important, the specificity is sufficient to enable the differentiation of pigments based on their infrared spectra. Infrared spectroscopy is commonly used in conservation science to identify pigments and other materials [11]. When combined with imaging, it becomes an even more powerful analytical technique. By acquiring an image at regular intervals through an extended wavelength range, we can build up what is known as a spectroscopic imaging data cube (Fig. 1). The slices through the data cube defined by the X and Y dimensions represent the images taken at individual wavelengths, and the Z dimension represents the wavelength of each image. By following any given pixel through the Z dimension of the data cube, it is possible to extract the spectral information corresponding to that particular point on the sample.

The other advantages of infrared spectroscopic imaging result from its digital nature. Because a computer is used both to collect images from the digital CCD camera and to tune the LCTF, this method is from the beginning a digital imaging technique. It therefore allows images to be viewed immediately, enables the easy duplication and printing of copies, and allows the power of both digital image processing methods and multivariate methods of spectral analysis to be applied to art conservation problems. In particular, because we collect digital images, standard mathematical methods for processing spectra can be used. By ratioing against a background image taken of a whiteboard, the reflectance images can be converted into optical density (OD) images, which are better suited to spectroscopic analysis. These have been used for the subsequent image processing, including the subtractions and pseudo-colour imaging described below.

The instrumentation used for this project was developed at the Institute for Biodiagnostics for use in non-invasive investigation of tissue health [12]. Since it is small and portable, it could be taken to The Winnipeg Art Gallery for imaging works of art on location. A custom user interface has been created to allow easy data collection and operation by non-specialists. One application of this instrumentation to an art conservation problem has previously been published [10]. Here, we present the results of a related study, including measurements made on individual pigments and spectroscopic images of a drawing and a painting.

Section snippets

Experimental

Near-IR spectroscopic images were collected using a Princeton Instruments CCD camera consisting of a 512×512 pixel back-illuminated CCD sensor and a 14/16-bit ST-138 A/D converter run in 16-bit mode (Princeton Instruments, Trenton, NJ). The camera was fitted with a Nikon Micro AF60 lens with the f-stop set to 8. The art objects being examined were placed on an easel 1–2 m in front of the camera, and were evenly illuminated using quartz halogen floodlights. A liquid crystal tunable filter (LCTF)

Results and discussion

An investigation was first conducted of brown and black materials that could have been used for drawing. Nine pigments were chosen to be representative of materials commonly used in 15th and 16th century drawing. The pigments were applied on mattboard and were imaged in the same way as the works of art, with the data processed to generate spectra. The spectra in the near-IR region between 650 and 1040 nm generally contained broad peaks and were rather featureless (Fig. 2). Nevertheless, the

Conclusion

Near infrared spectroscopic imaging is non-invasive, non-destructive, and utilizes relatively inexpensive equipment. Provided that UV-filtered lighting is used, and that the lights do not overheat the artwork, the technique is safe for most works of art. It retains many of the advantages of visible imaging and photography, since it can be used with most standard photographic accessories, including zoom lenses and optical microscopes. This means that spectroscopic images can be acquired from a

Acknowledgements

We are grateful for the enthusiastic cooperation of staff at The Winnipeg Art Gallery, as well as for useful comments from two anonymous reviewers.

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Near infrared spectroscopic reflectance imaging: a new tool in art conservation

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgements

J. Cultural Heritage

Vibrat. Spectrosc.

Stud. Conserv.

Mater. Evaluation

Stud. Conserv.

Stud. Conserv.

Stud. Conserv.