Autofocus laser system for multi-NIR scanning imaging of painting surfaces

A variety of optical investigation methods applied to paintings are, by now, an integral part of the repair process, both to plan the restoration intervention and to monitor its various phases. Among them infrared reflectography in wide-band modality is traditionally employed in non-invasive diagnostics of ancient paintings to reveal features underlying the pictorial layer thanks to transparency characteristics to NIR radiation of most of the materials composing the paints. This technique was improved with the introduction of the multi-spectral modality that consists in acquiring the radiation back scattered from the painting into narrow spectral bands. The technology, widely used in remote sensing applications such as satellite or radar imaging, has only recently gained importance in the field of artwork conservation thanks to the varied reflectance and transmittance of pigments over this spectral region. In this work we present a scanning device for multi-NIR spectral imaging of paintings, based on contact-less and single-point measurement of the reflectance of painted surfaces. The back-scattered radiation is focused on square-shaped fiber bundle that carries the light to an array of 16 photodiodes equipped with pass-band filters so to cover the NIR spectral range from 900 to 2500 nm. In particular, we describe the last instrument upgrade that consists in the addition of an autofocus system that keeps the optical head perfectly focused during the scanning. The output of the autofocus system can be used as a raw map of the painting shape.