Image processing techniques for 3D colour data from remote laser scanner prototype with Cultural Heritage applications

Danielis A.

3D color laser scanning  Colorimetry  Remote color calibration  Cultural Heritage  Near infrared imaging  Remote structural monitoring  Remote diagnosis  Image enhancement  Feature extraction 

This PhD project is a collaboration between ENEA Research Center of Frascati and the Department of Electronic Engineering of the University of Rome "Tor Vergata" focused on the development of image processing techniques for 3D colour images acquired by a terrestrial laser scanner for Cultural Heritage (CH) purposes. The imaging system is a patented 3D remote tristimulus laser scanner prototype completely designed and realized at ENEA laboratories, called RGB-ITR (Red Green Blue - Imaging Topological Radar). This opto-electronic device is based on the amplitude modulation of three monochro- matic sources (660 nm, 514 nm, and 440 nm), and able to simultaneously collect colour and structure information for any investigated sampled surface point in a working range of about 3-30 meters, without being affected by ambient illumination. This capabil- ity makes colour information as important as range data, opening new scenarios and problems for 3D remote colorimetric processing. To authors' knowledge, this is the first amplitude modulated 3D scanner ever realised that can natively acquire colour infor- mation. In CH area the purely geometric information provided by the vast majority of currently available scanners is not enough for most applications, where indeed accu- rate colorimetric and range data are both needed. Furthermore, the modularity of the ITR system permits to replace the RGB colour sources with a near-infrared (800 nm) laser, making also possible multisensiorial data processing and integration. Multisenso- rial data integration is particularly important for CH conservation applications, since it enables restorers to map diagnostic data directly onto the 3D model of the work of art being restored, so as to carefully plan potentially invasive operations and prevent possible damages. Within this framework, the scientific challenge of this research was to enhance and highlight the potentialities that the 3D, colorimetric and infrared technique that the laser scanner prototype is able to deliver on works of art of medium and large dimensions at short and large distances, and to provide benefits for dissemination, catologuing, education, remote diagnosis and restoration purposes. This study involves various issues and tasks including remote colorimetric calibration, colour, near-infrared and range image enhancement and segmentation, and image fu- sion. These issues and tasks have been addressed, analysed and improved during the PhD period, and are here described emphasizing the mathematical methods used for implementing the algorithms. In experts opinion this is a really solid piece of work that will be very useful within the field of colour and structural monitoring in various heritage conservation fields. The techniques here presented will show that the laser scanner prototype is a tool that could help to improve accurate diagnosis about conservation issues based on images and colour change, that is, without any kind of affection on the heritage. The algorithms used meet the requirements of identify areas of alteration - on ITR digitisations of artworks - based just on colour and range information. These could be very useful tools for the future as well. To the best of the authors knowledge this is the first time that a similar approach has been carried out, so that this study can be considered a significant contribution to heritage preservation studies.