Ewa A. Kwiatkowska

Structural Examination of Easel Paintings with Optical Coherence Tomography

Identification of the order, thickness, composition, and possibly the origin of the paint layers forming the structure of a painting, that is, its stratigraphy, is important in confirming its attribution and history as well as planning conservation treatments. The most common method of examination is analysis of a sample collected from the art object, both visually with a microscope and instrumentally through a variety of sophisticated, modern analytical tools. Because of its invasiveness, however, sampling is less than ideally compatible with conservation ethics; it is severely restricted with respect to the amount of material extirpated from the artwork. Sampling is also rather limited in that it provides only very local information. There is, therefore, a great need for a noninvasive method with sufficient in-depth resolution for resolving the stratigraphy of works of art. Optical coherence tomography (OCT) is a noninvasive, noncontact method of optical sectioning of partially transparent objects, with micrometer-level axial resolution. The method utilizes near-infrared light of low intensity (a few milliwatts) to obtain cross-sectional images of various objects; it has been mostly used in medical diagnostics. Through the serial collection of many such images, volume information may be extracted. The application of OCT to the examination of art objects has been in development since 2003. In this Account, we present a short introduction to the technique, briefly discuss the apparatus we use, and provide a paradigm for reading OCT tomograms. Unlike the majority of papers published previously, this Account focuses on one, very specific, use of OCT. We then consider two examples of successful, practical application of the technique. At the request of a conservation studio, the characteristics of inscriptions on two oil paintings, originating from the 18th and 19th centuries, were analyzed. In the first case, it was possible to resolve some questions concerning the history of the work. From an analysis of the positions of the paint layers involved in three inscriptions in relation to other strata of the painting, the order of events in its history was resolved. It was evident that the original text had been overpainted and that the other inscriptions were added later, thus providing convincing evidence as to the paintings true date of creation. In the second example, a painting was analyzed with the aim of confirming the possibility of forgery of the artists signature, and evidence strongly supporting this supposition is presented. These two specific examples of successful use of the technique on paintings further demonstrate how OCT may be readily adaptable to other similar tasks, such as in the fields of forensic or materials science. In a synergistic approach, in which information is obtained with a variety of noninvasive techniques, OCT is demonstrably effective and offers great potential for further development.

Absolute LIBS stratigraphy with optical coherence tomography

In this contribution preliminary studies on the application of Optical Coherence Tomography (OCT) to absolute depth calibration of Laser Induced Breakdown Spectroscopy (LIBS) data in application to revealing stratigraphy of easel paintings are presented. The procedure of in-situ monitoring of LIBS by means of OCT is described. Numerical method developed for precise extraction of the depth of the LIBS ablation crater is explained. Results obtained with model paintings are discussed.

Application of digital image correlation (DIC) for tracking deformations of paintings on canvas

A non-invasive and non-contact optical method for tracking overall and local deformations of canvas painting is presented. The technique was tested on a model painting on canvas with inhomogeneities introduced by infilling gaps, mending tears, and applying patches on the reverse of the canvas. The deformation of the sample was induced by changes of relative humidity. The feasibility of 3D Digital Image Correlation technique for evaluation of conservation methods is discussed.

Picosecond laser ablation system with process control by optical coherence tomography

In this contribution we describe an apparatus for precise laser ablation of delicate layers, like varnish on pictures. This specific case is very demanding. First of all any changes in colour of remaining varnish layer as well as underneath paint layers are unacceptable. This effect may be induced photochemically or thermically. In the first case strong absorption of the radiation used will eliminate its influence on underlying strata. The thermal effect is limited to so called heat affected zone (HAZ). In addition to colour change, a mechanical damage caused by overheating of the structure adjacent to ablated region should be considered also. All kinds of treads must be carefully eliminated in order to make laser ablation of varnish commonly accepted alternative to chemical and/or mechanical treatments [1]. Since the varnish ablation process is obviously irreversible its effective monitoring is very important to make it safe and trusted. As we showed previously [2-6] optical coherence tomography (OCT) originated from medicine diagnostic method for examination and imaging of cross-sections of weakly absorbing objects can be used for this task. OCT utilises infrared light for non-invasive structure examination and has been under consideration for the examining of objects of art since 2004 [7-10]. In this case the in-depth (axial) resolution is obtained by means of interference of light of high spatial (to ensure sensitivity) and very low temporal coherence (to ensure high axial resolution). In practice, IR sources of bandwidths from 25 to 150 nm are utilised. Resolutions obtained range from 15 down to 2 μm in the media of refracting index equal 1.5. In this contribution we expand application of OCT to space resolved determination of ablation rates, separately for every point of examined area. Such data help in better understanding of the ablation process, fine tuning the laser and finally permit increase of the safety of the ablation process.

Application of optical coherence tomography (OCT) for real time monitoring of consolidation of the paint layer in Hinterglasmalerei objects

Optical coherence tomography (OCT) is a fast non-contact and non-invasive technique for examination of objects consisting of transparent or semitransparent layers. Since it is a useful tool for inspection of Hinterglasmalerei paintings, the aim of the experiment was to explore its feasibility for monitoring of the consolidation process, which plays the most important role in the conservation treatment of such artefacts.

Optical Coherence Tomography for non-destructive investigations of structure of easel paintings

In this contribution the application of Optical Coherence Tomography (OCT) for non-invasive structural imaging of easel paintings will be presented. Since the technique permits imaging semi-transparent layers accessible for infrared light, the varnish and glaze layers are usually under investigation. The major emphasis will be laid on application of OCT to resolving specific conservation problems, arising during the restoration process. The examples of imaging multilayer varnishes and subsequent alterations will be given and the application of these images for authentication of inscriptions will be discussed. Since the thickness of imaged layers may be directly measured with OCT in completely non-destructive, quick and convenient way as many times as necessary, the application of the technique to generation of varnish thickness maps will be presented.

Twofold improvement in axial resolution of optical coherence tomography by four-pass sample probing

In this contribution a proof of concept for the alternate way of twofold increasing the axial resolution of Optical Coherence Tomography systems is shown. On the contrary to expanding the bandwidth of the light source, the number of passes of light between sample and the Michelson interferometer is increased. In two simplified novel configurations of Spectral OCT devices designed for this research, the interferometer is equipped with polarization controlling elements in order to force light to pass the distance from the beam splitter to the sample four times: during the first pass the initial linear polarization of the probing beam is converted to the perpendicular one and on return to the interferometer deflected by the polarization sensitive beam splitter towards the additional mirror reflecting it back to the sample. After the second pass the state of polarization is changed again and restored to the initial one in order to interfere with the reference beam. As a result in both set-ups optical paths difference between both arms of the Michelson interferometer is twofold longer comparing to the standard system. This results in two times smaller axial calibration coefficient and finally twofold increase of an effective axial resolution for the same coherence length of the light source. In the paper the experimental evidences are given and limitations of the method discussed.