Mario Ferri De Collibus

Underwater three-dimensional imaging with an amplitude-modulated laser radar at a 405 nm wavelength

We report the results of underwater imaging with an amplitude-modulated single-mode laser beam and miniaturized piezoactuator-based scanning system. The basic elements of the device are a diode laser source at 405 nm with digital amplitude modulation and a microscanning system realized with a small-aperture aspheric lens mounted on a pair of piezoelectric translators driven by sawtooth waveforms. The system has been designed to be a low-weight and rugged imaging device suitable to operate at medium range (approximately 10 m) in clear seawater as also demonstrated by computer simulation of layout performance. In the controlled laboratory conditions a submillimeter range accuracy has been obtained at a laser amplitude modulation frequency of 36.7 MHz.

Remote Colorimetric and Structural Diagnosis by RGB-ITR Color Laser Scanner Prototype

Since several years ENEAs Artificial Vision laboratory is involved in electrooptics systems development. In the last period the efforts are concentrated on cultural heritage remote diagnosis, trying to develop instruments suitable for multiple purposes concerning restoration, cataloguing, and education. Since last five years a new 3D (three-dimensional) laser scanner prototype (RGB-ITR) based on three amplitude-modulated monochromatic laser sources mixed together by dichroic filters is under development. Five pieces of information per each sampled point (pixel) are collected by three avalanche photodiodes and dedicated electronics: two distances and three target reflectivity signals for each channel, red, green, and blue. The combination of these pieces of information opens new scenarios for remote colorimetry allowing diagnoses without the use of scaffolds. Results concerning the use of RGB-ITR as colorimeter are presented.

Laser in vessel-viewing system for nuclear fusion reactors

An amplitude modulated laser radar has been developed by ENEA (Italian Agency for New Technologies, Energy and Environment) for periodic in-vessel inspection in large fusion machines. Its overall optical design has been developed taking into account the extremely high radiation levels and operating temperatures foreseen in large European fusion machines such as JET (Joint European Torus) and ITER (International Thermo- nuclear Experimental Reactor). The viewing system is based on a transceiving optical radar using a RF modulated single mode 840 nm wavelength laser beam. The sounding beam is transmitted through a coherent optical fiber and a focusing optic to the inner part of the nuclear reactor vessel by a stainless steel probe on the tip of which a suitable scanning silica prism steers the laser beam along a linear raster spanning a -90 degree(s) to +60 degree(s) in elevation and 360 degree(s) in azimuth for a complete mapping of the vessel itself. All the electronics, including the laser source, avalanche photodiode and all the active components are located outside the bioshield, while passive components (receiving optics, transmitting collimator, fiber optics), located in the torus hall, are made of fused silica so that the overall laser radar is radiation resistant. The signal is acquired, the raster lines being synchronized with the aid of optical encoders linked to the scanning prism, thus yielding a TV like image. Preliminary results have been obtained scanning large sceneries including several real targets having different backscattering properties, colors and surface reflectivity ranging over several decades to simulate the expected dynamic range of the video signals incoming from the vessel.

High-resolution laser radar for 3D imaging in artwork cataloging, reproduction, and restoration

A high resolution Amplitude Modulated Laser Radar (AM-LR) sensor has recently been developed, aimed at accurately reconstructing 3D digital models of real targets, either single objects or complex scenes. The sensor sounding beam can be swept linearly across the object or circularly around it, by placing the object on a controlled rotating platform, enabling to obtain respectively linear and cylindrical range maps. Both amplitude and phase shift of the modulating wave of back-scattered light are collected and processed, providing respectively a shade-free, high resolution, photographic-like picture and accurate range data in the form of a range image. The resolution of range measurements depends mainly on the laser modulation frequency, provided that the power of the backscattered light reaching the detector is at least a few nW (current best performances are ~100 µm). The complete object surface can be reconstructed from the sampled points by using specifically developed software tools. The system has been successfully applied to scan different types of real surfaces (stone, wood, alloys, bones), with relevant applications in different fields, ranging from industrial machining to medical diagnostics, to vision in hostile environments. Examples of artwork reconstructed models (pottery, marble statues) are presented and the relevance of this technology for reverse engineering applied to cultural heritage conservation and restoration are discussed. Final 3D models can be passed to numeric control machines for rapid-prototyping, exported in standard formats for CAD/CAM purposes and made available on the Internet by adopting a virtual museum paradigm, thus possibly enabling specialists to perform remote inspections on high resolution digital reproductions of hardly accessible masterpieces.

A Quadratic Model with Nonpolynomial Terms for Remote Colorimetric Calibration of 3D Laser Scanner Data Based on Piecewise Cubic Hermite Polynomials

The processing of intensity data from terrestrial laser scanners has attracted considerable attention in recent years. Accurate calibrated intensity could give added value for laser scanning campaigns, for example, in producing faithful 3D colour models of real targets and classifying easier and more reliable automatic tools. In cultural heritage area, the purely geometric information provided by the vast majority of currently available scanners is not enough for most applications, where indeed accurate colorimetric data is needed. This paper presents a remote calibration method for self-registered RGB colour data provided by a 3D tristimulus laser scanner prototype. Such distinguishing colour information opens new scenarios and problems for remote colorimetry. Using piecewise cubic Hermite polynomials, a quadratic model with nonpolynomial terms for reducing inaccuracies occurring in remote colour measurement is implemented. Colorimetric data recorded by the prototype on certified diffusive targets is processed for generating a remote Lambertian model used for assessing the accuracy of the proposed algorithm. Results concerning laser scanner digitizations of artworks are reported to confirm the effectiveness of the method.

Laser diagnostics developed for conservation and restoration of cultural inheritance

Different laser induced diagnostics, originally developed for different purposes including material characterization and environmental monitoring, have been applied in the field of Cultural Inheritance preservation with the aim to facilitate successive conservation and restoration actions. In this paper results relevant to three different techniques are reviewed. The use of topologic laser and 3D sensor in checking small artifacts and large surfaces is discussed, the application of Speckle interferometry to defect analysis of ceramic artwork is represented, a demonstration of the capabilities of a time resolved LIF system in the characterization of surface composition of ancient ceramics and frescoes is finally given.

Imaging topological radar technology as a general purpose instrument for remote colorimetric assessment, structural security, cataloguing, and dissemination

Abstract While today 3D digitisation techniques are commonly applied in several areas of cultural heritage, introducing new ways for monitoring, cataloguing, and studying masterpieces, the use of these technologies is not always worthwhile in terms of costs/benefits. The ENEA UTAPRAD-DIM laboratory has developed opto-electronic devices for cultural heritage applications. Two different 3D laser scanners for terrestrial and underwater inspection have been the subject of laboratory research for the last decade and a new technique known as imaging topological radar (ITR) has been developed and patented. The ITR system is based on the superimposition of three amplitude-modulated laser sources for the simultaneous acquisition of data related to colour and structure. This approach opens new scenarios for colour measurement and remote/non-invasive analysis, reducing the gap between costs and benefits from the technology. Several factors affect the quality of data collected by ITR, such as the precision of the scanner mechanism, the material and shape of the work studied, and the geometry of scanning (i.e. distance and angular dependencies). This paper explores the effect of the geometry of scanning on point cloud quality, focussing attention on data correction algorithms and their practical application. The data collected during the digitisation of the Sistine Chapel using the RGB-ITR scanner serve as a case study for validating the theoretical assumptions, models, and algorithms.

High-resolution laser radar: a powerful tool for 3D imaging with potential applications in artwork restoration and medical prosthesis

A high-resolution laser radar has been developed for laboratory applications at an accurate 3D reconstruction of real objects. The laser scanner can be used to produce single cylindrical range image when the object is placed on a controlled rotating platform or, alternatively, 3 or more linear range images, in order to fully characterize the surface of the object as seen from different points of view. From the sample points, characterized by an uncertainty as small as 100 μm, the complete object surface can be reconstructed by using specifically developed software tools. The system has been successfully applied to scan different types of real surfaces (stone, wood, bones) with relevant applications in industrial machining, artwork classification and medical diagnostics. Significant examples of 3D reconstructions are shown and discussed in view of a specific utilization for reverse engineering applied to artwork restoration and medical prosthesis.

Laser vision sensor for in-vessel inspection of fusion reactors

An optical amplitude modulated laser radar has been developed for periodic in-vessel inspection in large fusion machines and its overall optical aiming is developed taking into account the extremely high radiation levels and operating temperatures foreseen in the large European fusion machines (JET and ITER). In this paper an in vessel viewing system based on a transceiving optical radar using an RF modulated single mode 840 nm wavelength laser beam is illustrated. The sounding beam is transmitted through a coherent optical fiber and a focusing collimator to the inner part of the vessel by a stainless steel probe on the tip of which a suitable scanning silica prism steers the laser beam along a linear raster spanning a -90 degree to +90 degree in elevation and 360 degrees in azimuth for a complete mapping of the vessel itself. All the electronics, including laser source, avalanche photodiode and all the active components are located outside the bioshield, while passive components (receiving optics, transmitting collimator, fiber optics), located in the torus hall, are in fused silica so that the overall vision system is radiation resistant. The Active and passive components are contained in separated stainless steel boxes connected through two silica fiber optics. The laser radiation backscattered by the resolved surface element of the vessel is received by a collecting silica optics and remotely transmitted through a multimode fiber on the surface of an avalanche photodiode detector located in the active module at 120 m distance. The received signal is then acquired, the raster lines being synchronized with the aid of optical encoders linked to the scanning prism, to give a TV like image. The scanning accuracy expected in scanning process is less than 1 mm at 10 m of distance: this is a suitable resolution to yield a high quality image showing all the damages due to plasma disruptions. Preliminary results have been obtained scanning large sceneries including several real targets having different light backscattering properties, colors and surfaces reflectivity ranging over several decades to simulate the expected dynamic range of the video signals incoming from the vessel.

Development of a high-resolution laser radar for 3D imaging in artwork cataloging

A high resolution Amplitude Modulation Laser Radar (AM-LR) sensor has recently been developed, aimed at accurately reconstructing 3D digital models of real targets -- either single objects or complex scenes. The sensor sounding beam can be swept linearly across the object or circularly around it, by placing the object on a controlled rotation platform. Both intensity and phase shift of the back-scattered light are then collected and processed, providing respectively a shade-free photographic-like picture and accurate range data in the form of a range or depth image, with resolution depending mainly on the laser modulation frequency. Starting from the sample points, with an uncertainty that can be made as small as 100 μm, the complete object surface can be reconstructed by using specifically developed software tools. The system has been successfully applied to scan different types of real surfaces (stone, wood, bones) and is expected to have significant applications in industrial machining, artwork cataloguing and medical diagnostics. Examples of 3D reconstructions are presented and the relevance of this technology for reverse engineering applied to artwork restoration and conservation is briefly discussed.