A team of Italian researchers has developed a method of imaging art using wavelengths of light in the mid-infrared (3 to 5 µm) range, which they expect to expand the toolbox for art restoration. People studying and restoring art already work with many optical methods that use the near-infrared (IR) and/or lasers to examine the subsurface, analyze pigments and look for details not visible to the naked eye—but mid-IR imaging can offer unique information.
Claudia Daffara and others at the University of Verona, University of Aquila and the National Institute of Optics (Italy) developed thermal quasi-reflectography (TQR), a noninvasive and simple in-situ imaging method (Opt. Express 20, 14746). It uses reflected light from the mid-IR to reveal details that are not visible using either the more-established near-IR or standard thermographic imaging. They demonstrated the system on two famous artworks: Zavattari's frescos in the Chapel of Theodelinda and Piero della Francesca "The Resurrection." "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."
Unlike thermography, which images the mid-IR light emitted by different pigments, the new method looks for mid-IR light reflected from the painting. The mid-IR source is an under-powered halogen lamp shining at the painting. A cooled mid-IR camera captures reflected light.
The group found that gold and silver reflect more brightly than in the near-IR. They were also able to differentiate between pigments that look similar in the near-IR, as well as distinguishing painting methods and retouched areas. The researchers are looking into mid-IR spectroscopy to obtain more useful information.
Article co-author Dario Ambrosini of the University of Aquila states: "This novel method represents a powerful yet safe tool for artwork diagnostics."