Researchers from Nottingham Trent University (NTU), UK, have partnered with the National Gallery in London to develop an instrument capable of looking below the surfaces of paintings without damaging them. The high resolution and sensitivity of the system means it can image thin varnish and glaze layers with ‘unprecedented contrast’, they say.
Often old masterpieces consist of many layers of paint, sometimes covered with varnish. Before they can replace degrading varnish, conservators need to understand the materials and structure of the painting beneath the surface so that they can do as little damage as possible.
Traditionally, analysing the layers of a painting involves taking a very small sample to view under a microscope, which can be imaged at high resolution and analysed. More recently, researchers have begun to use non-invasive imaging techniques, such as optical Coherence Tomography (OCT), originally developed for medical imaging. However, these techniques are limited in depth resolution – more than 4μm in paint and varnish – compared with conventional microscopic examination of samples (about 1μm).
In OCT, a beam of light is split so that half is directed towards the sample, and the other half towards a reference mirror. The light scatters off both surfaces. By measuring the combined signal, which effectively compares the returned light from the sample versus the reference, the instrument determines how far into the sample the light penetrated. By repeating this procedure many times, researchers can build up a cross-sectional map of the painting.
Haida Liang’s team at NTU modified a typical OCT system to use a broadband laser-like light source, which has only become commercially available recently. This concentrated beam contains a wide range of frequencies that allows for more precise data collection. Other modifications included using a mirror rather than a lens in the spectrometer to reduce variation in sensitivity.
When tested on a late 16th-century copy of a Raphael painting, housed at the National Gallery in London, the modified system performed as well as traditional invasive imaging techniques (Optics Express, 2015, 23(8),10145). It achieved a constant resolution of 1.2μm in varnish or paint throughout a depth range of 1.5mm with little drop off in sensitivity, an issue that has dogged other OCTs of similar resolution demonstrated recently.
‘We are able not only to match the resolution but also to see some of the layer structures with better contrast,’ remarks Liang. ‘That’s because OCT is particularly sensitive to changes in refractive index. In some places, the ultra-high resolution OCT setup identified varnish layers that were almost indistinguishable from each other under the microscope.’
The team is also working on improving the penetration depth of the system, which involves using a longer wavelength light source. However, shorter wavelength light as used in their current setup provides the best resolution. The next challenge is to combine both approaches in one instrument, as well as to extract chemical information from different layers, says Liang.
Adrian Podoleanu of Kent University, UK, agrees that using a supercontinuum optical source is a good choice for high resolution OCT. ‘It produces a high axial resolution which is essential for analysis of art work.’