Researchers already knew that terahertz waves (electromagnetic radiation with frequencies between 0.3 and 3 × 1012 Hz) can penetrate a wide variety of materials without harming them. Engineers at the Georgia Institute of Technology are now using them, along with advanced signal processing techniques, to uncover the secrets of 17th Century artists. Using them, the researchers are able to peer through layers of pigment to see how painters prepared canvasses, applied undercoats, and built up layer upon layer of paint to produce their masterpieces. The level of detail produced by this terahertz reflectometry technique could help art conservators spot previous restorations of paintings, highlight potential damage, and assist in authenticating the old works. In non-art-related applications, it could also find use in detecting skin cancers, ensuring proper adhesion of turbine blade coatings, and measuring the thickness of automotive paints.
The researchers studied the painting “Madonna in Preghiera” from the workshop of Giovanni Battista Salvi da Sassoferrato, which was on loan from the Musée de la Cour d’Or, Metz Métropole, France. The examination began by placing the artwork face down on a gantry device designed to support the canvas without sagging.
Using a commercial terahertz scanner, the painting was zapped approximately every 200 microns by pulses of THz radiation. The scanner, an electromagnetic wave generator, emits signals that penetrated through successive layers of the painting. Some of that radiation reflected back from the paint, producing signals from each layer as the scanner moved across the painting in a raster pattern similar to the one used to create television images.
An optical micrograph near the edge of painting shows several layers verifying the interpretation of previous cross-sectional image based on deconvoluted data. (Courtesy of Alexandre Locquet)
A computer using a signal processing technique known as sparsity-based time-domain deconvolution processed the data, separating signals reflected by each layer to construct a three-dimensional map of the image. Researchers could identify he canvas support, ground, imprimatura (initial coating of paint), underpainting, pictorial, and varnish layers, along with a previously unknown restoration done on the varnish.
“Our technique is similar to the way in which seismology can be used to identify various layers of rock in the ground,” says David Citrin, a professor in Georgia Tech’s School of Electrical and Computer Engineering. “In that case, seismologists send in an acoustic pulse and measure the resulting echoes. In a similar way, we use a pulse of electromagnetic radiation at a frequency of around one terahertz and examine the reflections from various layers, a science known as stratigraphy.”
Without the signal processing, researchers might only be able to identify layers 100 to 150 microns thick. But using the advanced processing, they can distinguish layers just 20 microns thick. Paintings done before the 18th Century have been challenging to study because their paint layers tend to be thin, according to Citrin.
“This is really quite significant, because for years people have tried to use raw data, but you really can’t see much in that without processing the signals,” he explains. “It takes coupling the terahertz signals with the signal processing to really make a difference.”
Terahertz radiation, also known as submillimeter radiation, easily penetrates layers of paint, though it can be blocked by conductive pigments such as carbon black. THz imaging can supplement conventional art analysis techniques such as X-rays, nuclear magnetic resonance imaging, and optical imaging.
“Different techniques provide different information that useful to art conservators and historians,” says Cirtrin. “Terahertz gives us the combined ability to image a large object relatively quickly and inexpensively. We have shown that you don’t need a fancy system to extract useful information.”
Beyond paintings, Citrin’s research group has also imaged a Byzantine coin through a thick layer of oxidation, and is attempting to read an inscription in a medieval lead funerary cross also obscured by an oxide layer. The researchers are also collaborating with a research group in Hong Kong to use the technique for characterizing the layers of skin for skin cancer detection, and with another group for measuring damage in composite materials.
Terahertz imaging is still an emerging field that has to find its best applications, according to the team.