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

Cucu, R.

Culshaw B.

Dobre, George

Jackson, David A.

Liang, H.

Lopez-Higuera J.M.

Pannell, C.

Pedro, J.

Podoleanu, Adrian G.H.

Saunders, D.

English

Crossref

Application of OCT to examination of easel paintings

Kent Academic Repository

Liang, H

Cucu, R

Dobre, GM

Jackson, DA

Pedro, J

Pannell, C

Saunders, D

Podoleanu, AG

Nottingham Trent Institutional Repository (IRep)

Application of OCT to Examination of Easel PaintingsHaida Liang*a, Rada Cucu', George M. Dobre' David A. Jacksonb, Justin Pedroc,Chris Panneild, David Saundersa, Adrian Gh. Podoleanuba Scientific Dept., The National Gallery, Trafalgar Square, London WC2N 5DN, UK;bApplied Optics Group, School of Physical Sciences, University of Kent, Canterbury, CT2 7NR, UKCOphthalic Technologies Inc., Toronto, CanadadGh & Housego Group, Optronic Laboratories Inc., 4632 Street, Orlando, Florida 32811,USAABSTRACTWe present results of applying low coherence interferometry to gallery paintings. Infrared low coherence interferometryis capable of non-destructive examination of paintings in 3D, which shows not only the structure of the varnish layer butalso the paint layers.Keywords: Low coherence interferometry, optical coherence tomography, antiquities, art conservation1. INTRODUCTIONScientific examinations of easel paintings are routinely carried out in major galleries and museums to assist inconservation treatment and as part of technical or art historical examinations. Care is taken to examine the paintings non-destructively as far as possible, though it is sometimes still necessary to take tiny samples from paintings for analysis.Examples of non-destructive examination include X-radiography and infrared reflectography. X-radiography is routinelyemployed to examine the structure of the support of a painting, as well as details of areas painted with pigmentscontaining heavy elements. Infrared reflectography is one of the most useful techniques for art historians studying thepreparatory drawings or underdrawings underneath the painted layers, which would be otherwise invisible to the eye.Infrared reflectography normally operates in the wavelength range of 0.8 to 2 tim, where most of the paint layers arerelatively transparent, but the carbon-based materials often used in drawings absorb strongly. Both techniques collapsethe 3D information of a painting into 2D, thus losing the detailed information perpendicular to the painting plane.Hence, in order to study the paint and varnish layers, it is still currently necessary to take tiny samples of a painting toexamine the cross section of a small area of the painting under a microscope.Recently, efforts have been made to find alternative methods for pigment identification using mutli-spectral imagingtechniques (e.g. Berns and Imai', Liang et al.2), which are non-destructive and non-contact. However, in order to retrievethe true spectral reflectance of the pigments, it is necessary to 'subtract' the optical effects of the varnish layer that isnormally applied above the paint layers. Varnish layers are usually applied to paintings for both protective and aestheticpurposes. Application of a varnish layer to an oil painting not only gives it a uniform gloss, but also increases its coloursaturation, contrast and clarity. However, as the varnish layers on oil paintings age they become yellow and form cracks,which result in darkening or increased haziness. Consequently, it is necessary to clean the aged varnish off to reveal thetrue colours of paintings. However, the removal of a varnish layer can be very time consuming, and there is also a slightdanger of the solvents used in cleaning the varnish layer affecting the paint layers. In a recent study of the relativeimportance of surface roughness and refractive index in the appearance of varnished paintings, Berns and de la Rie3found that the dominant optical parameter was a varnish's ability to level a roughened paint surface. In general, the lowerthe molecular weight, the better the varnish is able to level a rough surface. Varnishes with a high refractive index andlow molecular weight are thought to be more desirable, hence the preference for natural resins such as dammar andmastic.*haida.liang@ng-london.org.uk; phone 44 207 747 2570; fax 44 207 839 3897; www.nationalgallery.org.ukSecond European Workshop on Optical Fibre Sensors, edited byJosé Miguel López-Higuera, Brian Culshaw, Proceedings of SPIE Vol. 5502(SPIE, Bellingham, WA, 2004) 0277-786X/04/$15 · doi: 10.1117/12.566780378Given that the optical thickness of the varnish and its success in reducing underlying surface roughness will affect theoptical properties of the painting, including its colour saturation, and that the optical depth will also affect the degree ofyellowing imparted by a discoloured varnish, it would be extremely useful for museums and galleries to have at theirdisposal an efficient means of measuring the varnish optical thickness and surface roughness non-destructively.In an attempt to solve the problem we have used an low coherence interferometry to resolve the spatial distribution oflayers in depth. This is similar to the principle of Optical Coherence Tomography (OCT)4 widely applied in the non-invasive imaging of tissue. We have used two OCT systems capable of producing depth resolved 3D images. Thesesystems are similar to those used to image medical structures such as the retina reported in Podoleanu eta15, using singlemode fibre couplers6.Laser interferometry such as holography and speckle interferometry have been applied to works of art to detect structuraldefects. However, since these methods employ a laser source which has high temporal coherence, they do not offer theability to distinguish between successive reflective layers. A wide band source with high spatial coherence used in OCTcan overcome most of these difficulties offering the opportunity to study the cross-section of the paint and varnish layers.Recently Targowski et at7 reported the use of OCT for a similar application.3. TEST TARGETS3.1 TEST SAMPLESA series of sample boards to which oil paint layers had been applied was prepared. Films of dammar or mastic varnishwere applied to either side of the sample leaving the centre region unvarnished (see Fig. 1). In some of the samples thepaint patches were applied with a brush, simulating a real painting, where the depth varies from point to point, while inothers a film-spreader was used to give an ideal situation where the depth is uniform. Different pigments covering abroad colour range were used to test whether the measurements would be independent of the absorption characteristics ofthe underlying paint layers. The pigment mixtures and layer combinations reflect those found in real oil paintings.Ground, paint and varnish layers were painted onto six 7 x 8.5 cm Teflon (PTFE) boards. The surface of each Teflonboard was sanded so that the ground layer would be able to bind to it. A traditional painting ground was made by theaddition of chalk to warmed rabbit skin glue. Teflon boards were either covered by spreading the ground to a thicknessof 200 jim, or by brushing it on. Once dry, the ground was sealed by the application of two size layers so that the oilfrom the oil paint was not absorbed into the porous chalk ground. Diluted rabbit skin glue was used as the size. Twopigments were then painted onto each board.3.2 TEST PAINTINGA roughly 50-year-old test painting, which has half the aged yellow varnish removed, was used to test the technique on areal painting (see Fig. 2). Two small patches of new mastic and dammar varnish were applied to the top left hand cornerof the painting where the old varnish had been removed.Proc. of SPIE Vol. 5502     379Figure 1: Sample board of yellow ochre and smalt (darkblue in colour) on a ground layer of chalk and rabbit skinglue. One third of the sample has three coats of dammarvarnish applied, and another one third of the painting has 3coats of mastic varnish applied, leaving the central part ofthe sample unvarnished (marked 'none').4. RESULTSFigure 2: A 50-year-old test painting: Points 1, 2,3, 6, and A were covered with the original varnish,which is now yellowed; point 6 has a rough paintsurface; point 4 is covered in new mastic varnish;point 5 is unvarnished.Selected points on the sample board shown in Fig. 1 and on the test painting (Fig. 2) were imaged using OCT.The thickness of a varnish sample applied to glass in 1952 was measured independently by making a cross-section andviewing this under a microscope. This measurement found a varnish thickness of 80—l00tm, which is consistent with thethickness measured by OCT (see Fig. 6). Varnish thickness measurements at point A on the test painting (Fig. 2) usingOCT systems working at 800nm and l300nm were consistent with each other and found to be — 80 tm in the thickerregions, which is also consistent with the measurements made by cross-sectional analysis.4II layerChalkgroundlayerFigure 4: An 800nm OCT image of the sample board shown inFig. 2, at the edge between the smalt and yellow ochre paintareas. Smalt is transparent to 800nm radiation, which allowsthe ground layer of chalk to be detected underneath the smalllayer. Yellow ochre is less transparent at 800nm, and theground layer cannot be detected beneath the yellow ochrepaint layer. The vertical scale represents depth measured in air.ICC5mmFigure 3 An 800 nm OCT image at point A on the testpainting showing a well delineated varnish layer ontop of the paint layer and also showing the microscopicvariation in the structure of both the paint and varnishlayers. The vertical scale represents depth measured inair.1cm¶÷ 1cm380     Proc. of SPIE Vol. 5502Figure 5: A l300nm OCT image at point 5 on the test Figure 6: A l300nm OCT image of a mastic varnish layerpainting, showing a bare paint layer (which was expected, on a piece of glass dating from 1952; the varnishlair andsince the varnish had been cleaned off that area). The varnish/glass interfaces are clearly delineated. The verticalvertical scale represents depth measured in air. scale represents depth measured in air.The 1300 nm OCT measurement of point 5 on the test painting confirmed that there was no varnish in that area. Fig. 4shows the potential of an infrared OCT in obtaining cross-section information on paintings. The 800 nm OCT instrumentwas used to scan across the boundary between the yellow ochre and smalt paints on the sample board (Fig. 1); the smaltarea is to the right and the yellow ochre area is to the left. The transparency of smalt to radiation at 800 nm enables theOCT to see through it and measure the tomography of the ground layer beneath.5. CONCLUSIONSThe OCT images present better microscopic tomography of the surface of the varnish and paint layers than any othersystem currently employed in the examination of easel paintings. OCT also provides an accurate measure of thethickness of the varnish layer on a painting; its advantage over normal cross-section examination under a microscopebeing that it would be possible to make a thickness measurement across the surface of the paintings rather than only atthose discrete points selected for sampling. Infrared OCT has the potential to provide a 3D infrared reflectogram duringthe examination of a painting which would not only show the underdrawings underneath the paint layers, but alsoprovide information on the structure of the paint and varnish layers. It might therefore be possible to see, without takingsamples, the extent to which a particular layer overlapped another and to make sense of the layer sequence in caseswhere the painting displayed a complex stratigraphy.ACKNOWLEDGEMENTSH.L. would like to thank Marika Spring for help with cross-section measurements, and Emily Gore for preparing thepainted samples.REFERENCES1. R.S. Bems, and F.H. Imai, 'The use of multi-channel visible spectrum imaging for pigment identification' , in ICOMCommitteefor Conservation, 13th Triennial Meeting, Rio de Janeiro (2002) 2 17—222.2. H. Liang, D. Saunders, J. Cupitt, M. Benchouika, 'A new multi-spectral imaging system for examining paintings', toappear in IS&T's Second Conference on Color in Graphics, Imaging and Vision (CGIV 2004), Aachen, 2004.3. R. S. Bems, and E.R. de la Rie, 'Exploring the optical properties of picture varnishes using imaging techniques',Studies in Conservation, Vol. 48, No. 2, 2003, 73—83.4. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C.A. Puliafito, J. G. Fujimoto, 'Optical coherence tomography', Science, Vol. 254, 1991, 1 178-118 1.5. A. Gh. Podoleanu, J. A. Rogers, D. A. Jackson and S. Dunne, 'Three dimensional OCT images from retina and skin'Opt. Express, Vol. 7, No. 9, p. 292-298, (2000), http://www.opticsexpress.org/framestocv7n9.htm6. A. Gh. Podoleanu, M. Seeger, G. M. Dobre, D. J. Webb, D. A. Jackson, F. Fitzke, 'Transversal and longitudinalimages from the retina of the living eye using low coherence reflectometry', J. Biomed Optics, Vol. 3, 1998, 12-20.7. P. Targowski, B. Rouba, M. Wojtkowski, and A. Kowalczyk, 'The application of Optical Coherence Tomography tonon-destructive examination of museum objects', Studies in Conservation, Vol. 49, No. 1, 2004, in press.Proc. of SPIE Vol. 5502     381

A new multi-spectral imaging system for examining paintings', to appear

Application of OCT to Examination of Easel Paintings Haida Liang*a, Rada Cucu',

Exploring the optical properties of picture varnishes using imaging techniques',

Optical coherence tomography',

The application of Optical Coherence Tomography to non-destructive examination of museum objects',

The use of multi-channel visible spectrum imaging for pigment identification' ,

Three dimensional OCT images from retina and skin'

Transversal and longitudinal images from the retina of the living eye using low coherence reflectometry',

http://irep.ntu.ac.uk/id/eprint/1550/1/180372_SPIE%20OCT%20EWOFS%2004.pdf

Application of OCT to examination of easel paintings

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