Opt. Express 20, 14746–14753 (2012)

Credit: © 2012 OSA

Light is a powerful, versatile and attractive tool for art conservation, as measurements are non-invasive and can be highly sensitive. 'Enhanced' visual techniques such as infrared reflectography involve broadband imaging in spectral regions other than the visible, and can reveal underlying features in a painting. By reversing the basic concept of thermography, in which infrared radiation emitted from a surface provides a measure of temperature, Dario Ambrosini and co-workers in Italy have developed a new non-contact imaging tool called thermal quasi-reflectography. Their technique involves measuring the mid-infrared light (3–5 μm) reflected from a painting's surface, and requires only a mid-infrared camera and a light source, such as a low-cost halogen lamp. The recorded infrared radiation is dominated by the energy reflected from the material's surface, which is strongly related to its surface properties. The researchers tested their technique on two famous artworks: the Zavattari's frescoes in the Chapel of Theodelinda; and Piero della Francesca's 'The Resurrection'. In both cases, thermal quasi-reflectography provided good pigment differentiation, selective mapping of gold and silver decorations and clearer retouch identification than near-infrared reflectography.