A novel technique has revealed never-before-seen details of Renaissance artworks in Italy.
The method images the faint reflections of low-power infrared light - the invisible light waves typically associated with heat.
However, in contrast to existing infrared imaging, the technique deposits little heat in precious works.
The approach, called Thermal Quasi-Reflectography or TQR, is described in Optics Express.
It joins a host of light-based techniques that restoration experts have at their disposal to analyse and care for artworks.
At the high-energy end of the electromagnetic spectrum, X-rays can be used to not only see through layers of pigments but also to identify the very atoms used in them - a crucial step in determining the age or authenticity of some works.
But at the other end of the useful spectrum lies the much lower-energy infrared light, with wavelengths longer than those we can see.
Established techniques called near-infrared spectroscopy and thermography use chunks of this part of the spectrum, but TQR reveals details that these other techniques miss.
It makes use of a comparatively low-power halogen light, and a camera which can see some of the "mid-infrared" wavelengths of light - from three to five millionths of a metre.
A TQR analysis of a 15th-Century fresco called The Resurrection by Piero della Francesca showed retouches (bright spots marked A above), unevenness in the painting of a shield (marked B) and even changes in the painting technique (marked C) that do not show up in a near-infrared image (left-hand panel).
A similar study of frescoes in the Theodelinda's Chapel in the Duomo of Monza first showed that TQR adds unique new details to complement existing methods.
"Our system easily identified old restorations in which missed gold decorations were simply repainted," said Claudia Daffara of the University of Verona, lead author of the study.
"The TQR system was also much better at visualising armour on some of the subjects in the fresco."
The team, made up also of members from the University of L'Aquila and the National Institute of Optics in Florence, analysed an 1930s copy of a fresco by Ghirlandaio, finding that a TQR image (right panel) showed the use of cinnabar that did not appear in visible light (left panel) and two near-infrared (middle panels) bands.
However, Dario Ambrosini of the University of L'Aquila said that further studies are needed to turn TQR into a method that can actually identify pigments, rather than just showing that different pigments or techniques were employed.
"Determining the chemical makeup of the pigments is important in determining how best to protect and restore the artwork," he said.
He added that the approach could also find use in other analytical applications beyond those in art restoration.
"In principle, it should work whenever we desire to differentiate surface materials."
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