High-resolution 3D survey of artworks Raffaella Fontana 1 , Maria Chiara Gambino 1 , Cinzia Mazzotta 2 , Marinella Greco 1 , Enrico Pampaloni 1 , Luca Pezzati 1 1 Istituto Nazionale di Ottica Applicata (INOA), largo E. Fermi 6, 50125, Florence, Italy 2 Università degli Studi di Lecce, Dipartimento di Beni Culturali, via D. Birago 64, 73100 Lecce, Italy; ABSTRACT The aim of this work is to show how 3D techniques can be used to integrate standard diagnostic ones, adding useful and powerful tools for the restorers. A 3D model allows both to monitor restoration processes and to keep trace of any significant modification of an artwork. We present 3D measurements carried out on different kind of samples: a statue, a painting, a xylography board and two ancient coins. These surveys were carried out by means of a high-resolution laser micro-profilometer developed by the Art Diagnostic Group of the National Institute of Applied Optics. It is composed of a commercial distance meter mounted on a scanning device and allows dense data sampling with high quota resolution and accuracy. Keywords: optical diagnostic, 3D survey, micro-profilometry. 1. INTRODUCTION Measurement of the shape of an artwork is highly relevant both to its study and to its conservation. The possible scenarios involved in the utilization of 3D digital models range from the monitoring of deterioration due to pollutant, to the realization of digital archives, easy to access and long-lasting; from reverse-engineering to fast-prototyping; from the analysis of conservation state to the monitoring of restoration interventions. Besides that digital models allow the remote fruition of an artwork, its placing in a scenery that can be different from the actual one, the visualization of changes due to the proposed restoration intervention and the re-composition of lost parts of a whole. Moreover, by analyzing the differences between measurements at different times, it is possible to obtain information on the object deformation. Optical techniques for shape measurements are often derived from industrial metrology, but the peculiarity of an artwork does not allow for a straightforward application. In fact, industrial manufactured objects are generally regular in shape, with uniformly coloured surfaces. On the contrary, artworks are unique in their shape, having highly contrasted surfaces: this is the case, for instance, of a painting or a polychrome statue. To investigate a surface with very high resolution, a variety of instruments is available on the market. Those instruments generally have a stylus or needle to measure a surface. Depth resolution can be as good as some tens of nanometers at any measured point, whereas lateral resolution ranges from 10 to 50 μm depending on stylus diameter. This method suites when measuring small areas of very hard surfaces, but it is not suitable for soft or frail objects, because stylus sharpness can damage the surface causing micro-scratches. A new technique, called conoscopic holography, was developed in the last few years. Surface shape is computed from an optical measurement: a light beam is projected on the sample and scattered radiation is collected and split to originate an interference pattern. Height differences result in optical path differences which are seen as light and dark fringes on a video camera or diode array detection system. Analysis of the intensity and phase of the interference pattern yields information about the shape of the test surface. Proper instruments for artwork diagnostics fulfill the requirement of being non-invasive, therefore contact devices such as stylus micro-profilometer are often not suitable, regardless their high resolution . Conoscopic holography, despite a worse resolution than stylus technique, has the advantage of being non-invasive: during measurement the object is not even touched, and it allows measurements over wide areas. 2. INSTRUMENT The operating principle of a conoprobe is described in Fig. 1. A light beam is projected by a diode laser on the sample and it is both reflected and back scattered. An objective lens collects the scattered radiation thus impinges on a uniaxial