Curious to know what secrets Mona Lisa is hiding beneath the newest layer of varnish on her surface? Up until recently, art conservationists were required to physically remove a sliver of a painting to get an accurate cross section of its many layers. But thanks to Haida Liang and her team of researchers at Nottingham Trent University's School of Science and Technology, examining the layers beneath the surface of a painting may no longer require such invasive techniques.

Due to ethical regulations, conservationists can only remove a chip that's smaller than a millimeter in size from around an edge or a damaged part of a painting in order to examine it under a microscope. But Liang's team, working with the National Gallery in London, has developed a non-invasive technique that uses an updated form of Optical Coherence Tomography (OCT) to create a cross section of the painting. According to Science Daily,

In OCT, a beam of light is split: half is directed towards the sample, and the other half is sent to a reference mirror. The light scatters off both of these surfaces. By measuring the combined signal, which effectively compares the returned light from the sample versus the reference, the apparatus can determine how far into the sample the light penetrated. By repeating this procedure many times across an area, researchers can build up a cross-sectional map of the painting.

Commercially available OCT setups are generally used by ophthalmologists to measure the thickness of a retina's distinctive layers, but their spatial resolution (the number of pixels constructing the digital image) isn't high enough to produce a detailed map of a painting. In order to glean the same amount of information from a non-invasive procedure as from an invasive procedure, the OCT setup has to use a "broadband laser-like light source–a concentrated beam of light containing a wide range of frequencies." This produces more precise results as it can detect layers and changes that were invisible under the microscope.

As Liang and her co-authors explain in "Ultra-high resolution Fourier domain optical coherence tomography for old master paintings," OCT allows large scale sampling from any spot on the painting, and has proved useful in detecting preparatory drawings, fine layers of varnish, and changes caused by the environment. 

This type of setup isn't readily available, but once it becomes more widespread, it could also be used to examine historic manuscripts and other artifacts. 

Fig. 7. Comparison of UHR OCT and commercial OCT images of roughly the same area on an old master painting. a) UHR OCT in situ imaging of The Madonna and Child (NG929, after Raphael, probably before 1600) in the conservation studio of the National Gallery London; b) 930nm commercial OCT cross-section image of the Virgin’s cloak; c) UHR OCT cross-section image at roughly the same position as a). The red bars to the right of the image indicates the top two varnish layers 1 and 2. The OCT images in b) and c) are of the same scale (3 mm wide by 0.228 mm deep).

C. S. Cheung, M. Spring, H. Liang, Opt. Express 23, 10145-10157 (2015).

[h/t Science Daily]