Reflected infrared light unveils never-before-seen details of Renaissance paintings

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When restoring damaged and faded works of art, artists often employ lasers and other sophisticated imaging techniques to study intricate details, analyze pigments, and search for subtle defects not visible to the naked eye. To refine what can be seen during the restoration process even further, a team of Italian researchers has developed a new imaging tool that can capture features not otherwise detectable with the naked eye or current imaging techniques. The system, known as Thermal Quasi-Reflectography (TQR), is able to create revealing images using reflected light from the mid-infrared part of the spectrum (3-5 micrometers in wavelength). Researchers from the University of L'Aquila, the University of Verona, and Italy's National Institute of Optics in Florence successfully demonstrated the TQR system on two famous works of art: the Zavattari frescos in the Chapel of Theodelinda and "The Resurrection" by the Italian Renaissance artist Piero della Francesca. The researchers detail their work in a paper published June 18 in the Optical Society's (OSA) open-access journal Optics Express.

Thermography, the traditional infrared imaging technique in this part of the spectrum (greater than 3 micrometers) detects subtle temperature differences due to the pigmentation on the surface of paintings. These thermal maps can be used during art restoration to reveal internal defects that are not evident in visible light.

In contrast, the TQR imaging system uses a very different tactic and doesn't detect heat emitted from paintings at all; in fact it tries to minimize it: The TQR system shines a faint mid-infrared light source onto the surface of the painting and records the light that is reflected back to a camera.

"This is, to the best of our knowledge, the first time that this technique has been applied on artworks," said Dario Ambrosini of the University of L'Aquila in Italy, one of the paper's authors. "This novel method represents a powerful yet safe tool for artwork diagnostics." All objects emit some infrared radiation. Depending on their temperature, certain materials shine more brightly in one wavelength than in others. At normal room temperature (20° C or 68° F), paintings typically emit more energy in the longer infrared wavelengths (42 percent) than they do in the mid-infrared (1.1 percent).

To take advantage of this weak mid-IR emission, the researchers applied the basic tools of thermography and ran them in reverse. Since the painting would not normally shine brightly in the mid-IR, the researchers used under-powered halogen lamps as very simple yet effective sources of mid-IR radiation. To measure only the reflected light, special care had to be taken to prevent the lamp from heating the surface of the painting and to exclude all other potential sources of mid-IR radiation.

The researchers developed the TQR system to find a thermal imaging tool capable of better differentiating materials in a painted surface. The mid-infrared has advantages over other wavelengths in this regard. It also has better contrast and produces sharper images than studies in the far-infrared, and can detect features not seen in the near-infrared (NIR) with wavelength less than approximately 2 micrometers.

In its first test on a small section of the Zavattari frescos in the Chapel of Theodelinda, the TQR system revealed details that were missed by earlier optical and near-infrared studies. "Our system easily identified old restorations in which missed gold decorations were simply repainted," said lead author Claudia Daffara of the University of Verona. "The TQR system was also much better at visualizing armor on some of the subjects in the fresco."

To further evaluate the potential, the TQR system also studied a painting known as "The Resurrection" by Piero della Francesca. The TQR system identified interesting features, such as highly reflective retouches from previous restorations, all while operating during normal museum hours without interruption. The most surprising feature was an area around a soldier's sword that was painted by using two different fresco techniques. This subtle distinction was not detected by NIR photography.

"For mural paintings the use of the mid-infrared regions reveals crucial details," said Daffara. "This makes TQR a promising tool for the investigation of these artworks."

The researchers are currently conducting tests to determine if the TQR system can also provide infrared spectra of the surface of paintings, which may be able to identify the pigments used. "Determining the chemical makeup of the pigments is important in determining how best to protect and restore the artwork," said Ambrosini. And they note that TQR may have applications beyond art preservation. "In principle, it should work whenever we desire to differentiate surface materials," said Ambrosini.