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High-tech device uses rays to unveil hidden artwork

Just as X-rays let doctors see the bones beneath our skin, "T-rays" could allow art historians to see murals hidden beneath coats of plaster or paint in centuries-old buildings, engineering researchers say.
A form of radiation may help uncover murals hidden beneath coats of plaster or paint in old buildings, engineering researchers say.

T-rays, pulses of terahertz radiation, also could illuminate penciled sketches under paintings on canvas without harming the artwork, the researchers say. Current methods of imaging underdrawings can't detect certain art materials such as graphite or sanguine, a red chalk that some of the masters are believed to have used.

The team of researchers, which includes scientists at the Louvre Museum, Picometrix, LLC and U-M, used terahertz imaging to detect colored paints and a graphite drawing of a butterfly through 4 mm of plaster. They believe their technique is capable of seeing even deeper. A paper on the research is published in the February edition of Optics Communications.

In March the scientists will take their equipment to France to help archeologists examine a mural they discovered recently behind five layers of plaster in a 12th century church.

"It's ideal that the method of evaluation for historical artifacts such as frescoes and mural paintings, which are typically an inherent part of a building's infrastructure, be non-destructive, non-invasive, precise and applicable on site," says John Whitaker, an author of the paper who is a research scientist and adjunct professor in the Department of Electrical Engineering and Computer Science.

Terahertz imaging can reveal depth and detail that other techniques cannot, Whitaker says.

Other types of radiation, such as X-rays and microwaves, can penetrate plaster, but scientists can't pulse them quickly enough to achieve sufficient depth resolution. Embedded layers in the plaster are indistinguishable, Whitaker says.

Terahertz radiation has been difficult to produce in a lab because it falls between the capabilities of electronic devices and lasers.

"Terahertz is a strange range in the electromagnetic spectrum because it's quasi-optical. It is light, but it isn't," says Bianca Jackson, first author of the paper who is a doctoral student in applied physics.

The device used for this research is a hybrid between electronics and lasers. It was developed by the Ann-Arbor based company Picometrix. It's called the T-Ray™ system, and it uses pulses from an ultra-fast laser to excite a semiconductor antenna, which in turn emits pulses of terahertz radiation.

Gèrard Mourou, an electrical engineering professor emeritus, says he believes this technique especially will be useful in Europe, where historic regime changes often resulted in artworks being plastered or painted over. This was common in places of worship, some of which switched from churches to mosques and vice versa over the centuries.

"In France alone, you have 100,000 churches," Mourou said. "In many of these places, we know there is something hidden. It has already been written about. This is a quick way to find it."

And Leonardo DaVinci's "The Battle of Anghiari," for example, is believed to lurk beneath other frescos at the Palazzo Vecchio in Florence, Italy, Mourou says.

Irl Duling, director of terahertz business development at Picometrix and an author of the paper. says this method beats a hammer and a chisel.

"There are two ways to find out whether there's a hidden fresco. One way is to rip the plaster off and the other way would be to use an imaging technique to see if there's an interface and a buried piece of art underneath. There's no imaging technique currently that works sufficiently,"
Duling says.

The paper is called "Terahertz imaging for non-destructive evaluation of mural paintings."

Mourou is the A D Moore Distinguished University Professor Emeritus of Electrical Engineering and Computer Science. He currently holds a position at the Laboratoire d'Optique Appliquèe.

Other authors are: Steven Williamson of Picometrix; Marie Mourou, a U-M undergraduate student; and Michel Menu, of the Center for Research and Restoration at The Louvre Museum.

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