Application of OCT to Examination of Easel Paintings

Haida Liang*a, Rada Cucu', George M. Dobre' David A. Jacksonb, Justin Pedroc,
Chris Panneild, David Saundersa, Adrian Gh. Podoleanub
a
Scientific Dept., The National Gallery, Trafalgar Square, London WC2N 5DN, UK;
bApplied Optics Group, School of Physical Sciences, University of Kent, Canterbury, CT2 7NR, UK
COphthalic Technologies Inc., Toronto, Canada
dGh & Housego Group, Optronic Laboratories Inc., 4632 Street, Orlando, Florida 32811,
USA

ABSTRACT
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.

Keywords: Low coherence interferometry, optical coherence tomography, antiquities, art conservation

1. INTRODUCTION
Scientific examinations of easel 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, 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 routinely
employed to examine the structure of the support of a painting, as well as details of areas painted with pigments
containing heavy elements. Infrared reflectography is one of the most useful techniques for art historians studying the
preparatory 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 are
relatively transparent, but the carbon-based materials often used in drawings absorb strongly. Both techniques collapse
the 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 to
examine 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 imaging
techniques (e.g. Berns and Imai', Liang et al.2), which are non-destructive and non-contact. However, in order to retrieve
the true spectral reflectance of the pigments, it is necessary to 'subtract' the optical effects of the varnish layer that is
normally applied above the paint layers. Varnish layers are usually applied to paintings for both protective and aesthetic
purposes. Application of a varnish layer to an oil painting not only gives it a uniform gloss, but also increases its colour
saturation, 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 the
true colours of paintings. However, the removal of a varnish layer can be very time consuming, and there is also a slight
danger of the solvents used in cleaning the varnish layer affecting the paint layers. In a recent study of the relative
importance of surface roughness and refractive index in the appearance of varnished paintings, Berns and de la Rie3
found that the dominant optical parameter was a varnish's ability to level a roughened paint surface. In general, the lower
the molecular weight, the better the varnish is able to level a rough surface. Varnishes with a high refractive index and
low molecular weight are thought to be more desirable, hence the preference for natural resins such as dammar and
mastic.

*
haida.liang@ng-london.org.uk; phone 44 207 747 2570; fax 44 207 839 3897; www.nationalgallery.org.uk

378 Second European Workshop on Optical Fibre Sensors, edited by

José Miguel López-Higuera, Brian Culshaw, Proceedings of SPIE Vol. 5502
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Given that the optical thickness of the varnish and its success in reducing underlying surface roughness will affect the
optical properties of the painting, including its colour saturation, and that the optical depth will also affect the degree of
yellowing imparted by a discoloured varnish, it would be extremely useful for museums and galleries to have at their
disposal 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 of
layers 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. These
systems are similar to those used to image medical structures such as the retina reported in Podoleanu eta15, using single
mode fibre couplers6.

Laser interferometry such as holography and speckle interferometry have been applied to works of art to detect structural
defects. However, since these methods employ a laser source which has high temporal coherence, they do not offer the
ability to distinguish between successive reflective layers. A wide band source with high spatial coherence used in OCT
can 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 TARGETS
3.1 TEST SAMPLES
A series of sample boards to which oil paint layers had been applied was prepared. Films of dammar or mastic varnish
were applied to either side of the sample leaving the centre region unvarnished (see Fig. 1). In some of the samples the
paint patches were applied with a brush, simulating a real painting, where the depth varies from point to point, while in
others a film-spreader was used to give an ideal situation where the depth is uniform. Different pigments covering a
broad colour range were used to test whether the measurements would be independent of the absorption characteristics of
the 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 Teflon
board was sanded so that the ground layer would be able to bind to it. A traditional painting ground was made by the
addition of chalk to warmed rabbit skin glue. Teflon boards were either covered by spreading the ground to a thickness
of 200 jim, or by brushing it on. Once dry, the ground was sealed by the application of two size layers so that the oil
from the oil paint was not absorbed into the porous chalk ground. Diluted rabbit skin glue was used as the size. Two
pigments were then painted onto each board.

3.2 TEST PAINTING

A roughly 50-year-old test painting, which has half the aged yellow varnish removed, was used to test the technique on a
real painting (see Fig. 2). Two small patches of new mastic and dammar varnish were applied to the top left hand corner
of the painting where the old varnish had been removed.

Proc. of SPIE Vol. 5502 379

 Figure 1: Sample board of yellow ochre and smalt (dark Figure 2: A 50-year-old test painting: Points 1, 2,
blue in colour) on a ground layer of chalk and rabbit skin 3, 6, and A were covered with the original varnish,
glue. One third of the sample has three coats of dammar which is now yellowed; point 6 has a rough paint
varnish applied, and another one third of the painting has 3 surface; point 4 is covered in new mastic varnish;
coats of mastic varnish applied, leaving the central part of point 5 is unvarnished.
the sample unvarnished (marked 'none').

4. RESULTS
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 and
viewing this under a microscope. This measurement found a varnish thickness of 80—l00tm, which is consistent with the
thickness measured by OCT (see Fig. 6). Varnish thickness measurements at point A on the test painting (Fig. 2) using
OCT systems working at 800nm and l300nm were consistent with each other and found to be — 80 tm in the thicker
regions, which is also consistent with the measurements made by cross-sectional analysis.

I
C
C 4I
I layer
Chalk
ground
layer

5mm

Figure 3 An 800 nm OCT image at point A on the test Figure 4: An 800nm OCT image of the sample board shown in
painting showing a well delineated varnish layer on Fig. 2, at the edge between the smalt and yellow ochre paint
top of the paint layer and also showing the microscopic areas. Smalt is transparent to 800nm radiation, which allows
variation in the structure of both the paint and varnish the ground layer of chalk to be detected underneath the small
layers. The vertical scale represents depth measured in layer. Yellow ochre is less transparent at 800nm, and the
air. ground layer cannot be detected beneath the yellow ochre
paint layer. The vertical scale represents depth measured in air.

1cm ÷ 1cm

380 Proc. of SPIE Vol. 5502

 Figure 5: A l300nm OCT image at point 5 on the test Figure 6: A l300nm OCT image of a mastic varnish layer
painting, showing a bare paint layer (which was expected, on a piece of glass dating from 1952; the varnishlair and
since the varnish had been cleaned off that area). The varnish/glass interfaces are clearly delineated. The vertical
vertical 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. 4
shows the potential of an infrared OCT in obtaining cross-section information on paintings. The 800 nm OCT instrument
was used to scan across the boundary between the yellow ochre and smalt paints on the sample board (Fig. 1); the smalt
area is to the right and the yellow ochre area is to the left. The transparency of smalt to radiation at 800 nm enables the
OCT to see through it and measure the tomography of the ground layer beneath.

5. CONCLUSIONS
The OCT images present better microscopic tomography of the surface of the varnish and paint layers than any other
system currently employed in the examination of easel paintings. OCT also provides an accurate measure of the
thickness of the varnish layer on a painting; its advantage over normal cross-section examination under a microscope
being that it would be possible to make a thickness measurement across the surface of the paintings rather than only at
those discrete points selected for sampling. Infrared OCT has the potential to provide a 3D infrared reflectogram during
the examination of a painting which would not only show the underdrawings underneath the paint layers, but also
provide information on the structure of the paint and varnish layers. It might therefore be possible to see, without taking
samples, the extent to which a particular layer overlapped another and to make sense of the layer sequence in cases
where the painting displayed a complex stratigraphy.

ACKNOWLEDGEMENTS
H.L. would like to thank Marika Spring for help with cross-section measurements, and Emily Gore for preparing the
painted samples.

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