Hidden detail in "The greatest painting in the world"

A new non-destructive IR imaging technique has been developed to reveal submerged details on a fresco and a painting that were not detected by current imaging methods.

The technique, named thermal quasi-reflectography (TQR) relies on measuring reflected radiation from objects in the mid-wave IR (MWIR) region at 3-5 µm rather than the thermal radiation measured in conventional IR imaging. "By working in the MWIR band and using a suitable MWIR source, the recorded IR radiation is dominated by the reflected energy, which, conversely, is strongly related to the surface properties," say the researchers.

The method was described in the open-access journal Optics Express and illustrated by the analysis of a fresco situated in the Chapel of Theodelinda, Duomo of Monza, in Italy, which is currently under restoration. TQR revealed hidden gold and silver decoration and also old restorations which had been repainted.

In a second illustration, TQR was applied to "The Resurrection” by Piero della Francesca, which was described by Aldous Huxley as "the greatest painting in the world." Pigments that were visibly similar and only slightly different in the near-IR spectrum were clearly differentiated by TQR and highly reflective retouches were revealed.

Although TQR has relatively low spatial resolution compared with colour photography and near-IR imaging, it shows great promise for analysing artworks, with "good pigment differentiations, selective mapping of gold and silver decorations and a clearer identification (with respect to near-IR 0.9-1.1 μm reflectography) of retouches and painting integrations."