M. Ferri de Collibus

Techniques for effective optical noise rejection in amplitude-modulated laser optical radars for underwater three-dimensional imaging

Amplitude-modulated (AM) laser imaging is a promising technology for the production of accurate three-dimensional (3D) images of submerged scenes. The main challenge is that radiation scattered off water gives rise to a disturbing signal (optical noise) that degrades more and more the quality of 3D images for increasing turbidity. In this paper, we summarize a series of theoretical findings, that provide valuable hints for the development of experimental methods enabling a partial rejection of optical noise in underwater imaging systems. In order to assess the effectiveness of these methods, which range from modulation/demodulation to polarimetry, we carried out a series of experiments by using the laboratory prototype of an AM 3D imager ( = 405 nm) for marine archaeology surveys, in course of realization at the ENEA Artificial Vision Laboratory (Frascati, Rome). The obtained results confirm the validity of the proposed methods for optical noise rejection.

Experimental evidence of signal-optical noise interferencelike effect in underwater amplitude-modulated laser optical radar systems.

We report experimental evidence that in an amplitude-modulated laser optical radar system for underwater 3D imaging the observed contrast oscillations as a function of the modulation frequency originate from an interference-like effect between target signal VT and water backscattered radiation VW. The demonstration relies on the ability to perform a direct measurement of VW in a 25 m long test tank. The proposed data processing method enables one to remove the contribution of water backscattering from the detected signal and drastically reduce signal fluctuations due to the medium. Experiments also confirm the possibility to improve the signal to optical noise ratio and contrast by increasing the modulation frequency.

Two‐wavelength infrared heterodyne transceiver with a continuous phase tracking system

We describe the performance of an optical heterodyne sensor with a two‐wavelength coherent infrared beam, which is used to detect the motion of remote targets in a real environment with an unambiguous sensing of the instantaneous direction along the line‐of‐sight propagation path. The initial phase condition of maximum sensitivity of the transceiver is stabilized and held by an active feedback system which drives the phase of the optical field of the local oscillator. The experimental results show that this feedback is particularly useful in reducing the noise generated by low‐frequency environmental disturbances, leaving the signal unaffected when the respective frequency ranges are separated. Under certain circumstances, the signal can be recovered from the noise by the linearizing properties of the phase tracking on the interferometer response. The measurements reported show that the two‐wavelength infrared heterodyne transceiver generates signals which replicate the target vibration exactly up to ampl...

Amplitude-modulated laser range-finder for 3D imaging with multi- sensor data integration capabilities

A high performance Amplitude Modulated Laser Rangefinder (AM-LR) is presented, aimed at accurately reconstructing 3D digital models of real targets, either single objects or complex scenes. The scanning system enables to sweep the sounding beam either linearly across the object or circularly around it, by placing the object on a controlled rotating platform. Both phase shift and amplitude of the modulating wave of back-scattered light are collected and processed, resulting respectively in an accurate range image and a shade-free, high resolution, photographic-like intensity image. The best performances obtained in terms of range resolution are ~100 μm. Resolution itself can be made to depend mainly on the laser modulation frequency, provided that the power of the backscattered light reaching the detector is at least a few nW. 3D models are reconstructed from sampled points by using specifically developed software tools, optimized so as to take advantage of the system peculiarities. Special procedures have also been implemented to perform precise matching of data acquired independently with different sensors (LIF laser sensors, thermographic cameras, etc.) onto the 3D models generated using the AM-LR. The system has been used to scan different types of real surfaces (stone, wood, alloys, bones) and ca be applied in various fields, ranging from industrial machining to medical diagnostics, vision in hostile environments cultural heritage conservation and restoration. The relevance of this technology in cultural heritage applications is discussed in special detail, by providing results obtained in different campaigns with an emphasis on the systems multi-sensor data integration capabilities.

Polarimetry as tool to improve phase measurement in an amplitude modulated laser for submarine archaeological sites inspection

The propagation of polarized laser beams in turbid water is a subject of relevant interest in the field of underwater quantitative visualization with active sensors like amplitude modulated laser systems. In such devices, target range determination is based on the measurement of the phase difference ΔΦ between the fraction of the amplitude modulated laser beam reflected by the target and a reference signal. As water turbidity increases, the laser radiation backscattered from the water column shined by the sounding laser beam gives rise to an optical background with detrimental effects on the accuracy of range measurement. In this paper we analyze the possibility to increase the apparatus accuracy with a polarimetric technique based on the adoption of polarized laser radiation and polarization selective detection scheme for improving the underwater imaging of real scenes (e.g. archaeological sites). The method fully takes advantages of the different polarization properties of the laser radiation backscattered by turbid water and of the Lambertian component diffusively reflected by the target as described by the associated Mueller matrices. Measurements have been performed by adopting both a co-polarized and cross-polarized detection scheme with linearly and circularly polarized laser radiation. Various degrees of turbidity were realized by adding, as diffusive element, skim milk to water in order to obtain different scattering conditions. The effect of the transition from Rayleigh to Mie scattering regime on phase accuracy determination has been investigated together with the role played by high order scatterings as the medium approaches the optical thickness condition.

Polarimetry as a valid means to reduce optical noise in underwater 3D imaging by means of amplitude-modulated laser optical radar systems

We report the results of a series of underwater imaging experiments in the visible, carried out at ENEA (Frascati, Rome) by using a bistatic, amplitude-modulated laser optical radar system. In these experiments, polarimetry is used for minimizing the water backscattering signal and improving the accuracy of phase measurements directly related to distance. The presented technique enables one to obtain 3D images of underwater real scenes characterized by high quality, space resolution, and contrast. The results are of remarkable importance for applications in the 3D imaging of submerged objects, such as submarine archaeological sites.

Color (RGB) imaging laser radar

We present a new color (RGB) imaging 3D laser scanner prototype recently developed in ENEA (Italy). The sensor is based on AM range finding technique and uses three distinct beams (650nm, 532nm and 450nm respectively) in monostatic configuration. During a scan the laser beams are simultaneously swept over the target, yielding range and three separated channels (R, G and B) of reflectance information for each sampled point. This information, organized in range and reflectance images, is then elaborated to produce very high definition color pictures and faithful, natively colored 3D models. Notable characteristics of the system are the absence of shadows in the acquired reflectance images - due to the systems monostatic setup and intrinsic self-illumination capability - and high noise rejection, achieved by using a narrow field of view and interferential filters. The system is also very accurate in range determination (accuracy better than 10-4) at distances up to several meters. These unprecedented features make the system particularly suited to applications in the domain of cultural heritage preservation, where it could be used by conservators for examining in detail the status of degradation of frescoed walls, monuments and paintings, even at several meters of distance and in hardly accessible locations. After providing some theoretical background, we describe the general architecture and operation modes of the color 3D laser scanner, by reporting and discussing first experimental results and comparing high-definition color images produced by the instrument with photographs of the same subjects taken with a Nikon D70 digital camera.

Image processing from laser scanners for remote diagnostic and virtual fruition of cultural heritage

Two types of high resolution laser scanners have been developed for remote applications with imaging capabilities for optical or spectroscopic characterization of CH surfaces: a three-chromatic imaging topologic radar (RGB-ITR) and hyperspectral LIF-scanning, collecting at each investigated point the entire fluorescence spectra relevant to the utilized UV laser excitation. Their combined use produces multi-layered images with enhanced information content, which is especially valuable in CH characterization of surface materials.

How the amplitude modulation of n-laser stimuli could change our way to observe submerged and emerged worlds

A philosophical thought experiment that raises questions regarding observation and knowledge of reality declaims: “If a tree falls in a forest and no one is around to hear it, does it make any sound?”. A similar question could be formulated in terms of color perception: “If no light illuminates an object, does the color still exist?”. A technology called RGB-ITR (Red Green Blue - Imaging Topological Radar), based on the amplitude modulation of n-lasers stimuli, has been developing in UTAPRADDIM lab of C.R. ENEA Frascati for several years now. One of the major application of this technology is remote monitoring and digitization of artistic goods in Cultural Heritage (CH) environment: the exclusive use of lasers/photo-diodes for the acquisition of both color and distance information permits to obtain not only a very accurate 3D model of the investigated target, but also hyper-photos free from external light sources influence and with calibrated whites in all the instrument working range. By performing several case studies in real environment, this work aims at demonstrating the versatility of this technique in several fields of CH sector, like remote structure and color monitoring, cataloging and fruition. Recently the significant effort has been spent on looking for novel applications of this technology in the underwater environment for archaeological and industrial purposes. The lab results, reported in this work, demonstrate the potential of this technology to bypass the barrier created by the water back-scattering, especially in case of turbid environment.

High resolution laser remote imaging innovative tools for preservation of painted surfaces: information from reflectance and fluorescence data

Two innovative laser scanning prototypes have been developed at ENEA for diagnostics of large surfaces relevant to monumental cultural heritage. The first, based on amplitude modulation technique in the visible, is a trichromatic (Red /Green /Blue) imaging topologic radar (RGB-ITR) specialized to collect high resolution 3D models. After proper color calibration, it allows for hyper-realistic rendering of colored features on painted surfaces and for precise localization of irregularities. The second is a line scanning system, working either in reflectance or laser induced fluorescence mode, capable of fast 2D monochromatic images acquisition on up to 90 different spectral channels in the visible/UV range, which was developed to investigate the presence of different substances onto the painted surface. Data collected during former field campaigns on frescos by means each scanning system will be reported and discussed extracting information of interest to conservators by means of specific data processing methodologies and respective software tools. Recent results relevant to paints of the Assumption on slate and canvas by Scipione Pulzone named “il Gaetano” collected in two churches in Rome (San Silvestro al Quirinale, Bandini chapel; Santa Caterina dei Funari, Solano della Vetera Chapel) from the late XVI century are presented in order to demonstrate the increased diagnostic capabilities coming from data integration. From combination of reflectance data from both instruments, the first true remote differential colorimetry has been implemented, giving a chance to test the color quality in the future from the archived images.