A new imaging technique for analyzing art

A new imaging technique for analyzing art. Optical methods present powerful, noninvasive tools for artwork diagnostics, a central part of art conservation and forensics. For instance, interferometry with visible laser light can detect structural defects and changes over time, due perhaps to variations in ambient conditions. Near-IR radiation can penetrate beneath the surface of a painting, so reflectance imagery—also called reflectography—in that wavelength range can uncover such details as preparatory drawings and changes made by the artist. And thermography—imaging the thermal radiation emitted from the surface in the mid and far-IR—can detect variations in temperature and emissivity due to the presence of heterogeneous materials or subsurface defects. At the other end of the spectrum, x rays are increasingly finding use in art forensics (see Physics Today, January 2012, page 58 ). Claudia Daffara, Daio Ambrosini, Luca Pezzati, and Domenica Poletti have now demonstrated another technique for the conservator’s toolkit. Termed thermal quasi-reflectography (TQR), the method maps the MIR radiation reflected by an object. The team worked in the 3- to 5-µm MIR band because room-temperature objects emit significantly less blackbody radiation at those wavelengths than in the FIR, so the background noise is reduced. For their light source, the researchers used halogen lamps that were underpowered so as to shift their spectra toward the MIR. The team showed that the reflected radiation is sensitive to surface composition and enables differentiation of surface materials. In this detail of Piero della Francesca’s The Resurrection, for example, the letters label regions where the TQR image on the right reveals subtle features—including pigment variations, retouches, and differing execution techniques—that aren’t seen in the NIR image on the left. (C. Daffara et al., Opt. Express 20, 14746, 2011.)