The development of portable sensors for fast screening of crime scenes is required to reduce the number of evidences useful to be collected, optimizing time and resources. Laser based spectroscopic techniques are good candidates to this scope due to their capability to operate in field, in remote and rapid way.
In this work, the prototype of a multispectral imaging LIF (Laser Induced Fluorescence) system able to detect evidence of different materials on large very crowded and confusing areas at distances up to some tens of meters will be presented. Data collected as both 2D fluorescence images and LIF spectra are suitable to the identification and the localization of the materials of interest. A reduced scan time, preserving at the same time the accuracy of the results, has been taken into account as a main requirement in the system design. An excimer laser with high energy and repetition rate coupled to a gated high sensitivity ICCD assures very good performances for this purpose. Effort has been devoted to speed up the data processing. The system has been tested in outdoor and indoor real scenarios and some results will be reported. Evidence of the plastics polypropylene (PP) and polyethilene (PE) and polyester have been identified and their localization on the examined scenes has been highlighted through the data processing. By suitable emission bands, the instrument can be used for the rapid detection of other material classes (i.e. textiles, woods, varnishes).
The activities of this work have been supported by the EU-FP7 FORLAB project (Forensic Laboratory for in-situ evidence analysis in a post blast scenario).
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.
LIBS is one of the most promising techniques for rapid, in-situ elemental analyses of artworks. It does not require sample preparation, it is almost non destructive (micro sampling) and information both about major and trace elements could be obtained simultaneously. LIBS has been used to recognize the elements present in different archaeological materials and has been also proposed for on-line monitoring during the object cleaning by lasers. Quantitative determination of the material composition can supply useful information to restorers and help the object cataloguing. However, the analytical LIBS measurements on the archaeological materials were rarely reported, mainly due to difficulties to obtain the corresponding matrix-matched standards, required for the initial calibration. Alternatively, Calibration-Free (CF) approach could be used on some class of materials if all the major sample elements are detected and if the laser plasma preserves the material stochiometry. The latter condition is sometimes missing, as in the case of bronzes under nanosecond pulse laser ablation. We have developed a theoretical model for laser ablation of quaternary copper alloys, which allows for correction of the missing plasma stochiometry in CF approach. The model also predicts the optimal calibration for this type of material. In our recent work, we also obtained quantitative LIBS results on marbles by realizing the calibration standards starting from doped CaCO3 powders and by applying the corrections on the plasma parameters, different for the laboratory standards and marbles. Semi-quantitative LIBS results have been also obtained on multi-layered renaissance ceramics by subtraction of the contribution to plasma of each ceramic layer.
The effects of energy and pulse duration in laser ablation experiments have been investigated in the case of copper based alloys. Experiments were carried out near 530nm by using two laser sources with different widths (8 ns and 250 fs). The craters generated by lasers were examined and their characteristics were related to different mechanisms involved in the ablation by laser pulses acting on different timescales. The optical emission of plasma produced was analyzed by LIPS, acquiring time resolved line intensities of the major elements contained in the samples, and determining plasma characteristics (temperature, electron density). Experiments demonstrated that fs and ns laser pulse must be modeled by different ablation regimes, the onset of which can be used to decrease the effect of fractionation on brasses and bronzes.
Diamond like carbon (DLC) can be an ideal candidate as coating material for optical components owing to its high hardness value, chemical inertness and high transparency in the IR spectral region. DLC thin films have been deposited by RF magnetron sputtering from a graphite target in argon atmosphere without hydrogen on glass and silicon substrates in order to study the influence of some parameters on the mechanical and optical properties. Increasing the power input values, from 100W to 400W, the DLC films compactness increases too as it can be deduced from the raise in this range of power of both hardness and refractive index values, calculated by microindentations size and reflectance and transmittance spectrophotometer measurements respectively. Moreover, a set of sample was deposited at different substrate temperatures and at the highest used value a lower value of hardness and refractive index as well are obtained. The possibility to correlate the films compactness to their absorption humidity has been investigated by means of the IR transmittance spectra analysis. Densifying effect of RF sputtered DLC thin films examined in this work seems to be related, in conclusion, to the power input increasing rather than to the substrate temperature.
One of the main responsibilities for adhesion failure of antireflection coatings on plastic substrates is the stress developing at the film/substrate interface during film deposition and subsequent system cooling. Coating a substrate by a different material always induces stress at the interface, because it enforces the film to copy the substrate structure. Further, non-equilibrium conditions during film growth and thermal deformations due to post- deposition cooling, also contributes to the total stress of the system. In particular, thermal stress plays an important role in adhesion failure of coated plastic,s because the expansion coefficient of polymers is about an order of magnitude greater than that of hard oxides. In order to prevent delamination of the film, this strain must be balanced by bond strength between the film and the substrate. In this work, we investigate the possibility of improving the film/substrate adhesion by reducing the total stress at that interface. In fact, the intrinsic stress of ion assisted hafnium oxide films is influenced by the assistance conditions, and consequently the total stress can be changed by varying the experimental parameters. Hafnium oxide layers were deposited on plastic substrate by dual ion beam sputtering, at different growth conditions. Stress values were obtained by measuring the curvature of the substrate before an dafter deposition. Practical adhesion properties were evaluated by mechanical tests.
In this work, ZnSe thin films were deposited by radio frequency magnetron sputtering onto oriented silicon substrates. Three sets of samples were produced by varying the argon working gas pressure, and changing the sputtering power supply. The effect of the different growth conditions on the structural and optical properties of the ZnSe films was investigated by using XRD and FTIR spectroscopy. In particular, x-ray diffraction was used in order to study the residual strains and texture. The ZnSe coatings were grown in the cubic polymorph with the grains preferentially oriented for all the investigated pressures with the exception of 0.5 Pa. An in-plane residual stress reversal, which changed from compressive to tensile by going from the low to the high-pressure sample set, was observed. At the 0.5 Pa pressure, the ZnSe coatings were both in compressive and tensile state, according to the power supply value. These result have been correlated with the momentum of reflected neutral in order to find an exhaustive description of the deposition process. At low momentum values, the ZnSe films show tensile state, low refractive index, grains preferentially oriented, while at high momentum they were in compressive state, refractive index very alike to the bulk and grains randomly oriented.
Evaporated coatings need generally high substrate temperature to obtain a more dense structure of the layers avoiding the humidity adsorption when the coating itself is exposed to the atmosphere. Moreover, the employment of fluorides as low refractive index layers can increase the stress level of the coating because of their frequent hygroscopicity. Under severe operational conditions, the adsorbed water causes an expansion of the layer with a consequent destruction of the coating if the adhesion strengths is not strong enough to counter balance the increased stress. The ion beam assistance can be a useful method to increase the density of the coating produced by the evaporation technique. In fact, the ion bombardment during the thin film growth improves the packing density and enhances thin film resistance to moisture penetration. Magnesium fluoride (MgF2) and barium fluoride (BaF2) are two low refractive index materials employed in the optical coatings production whereas yttria material (Y2O3) can be used as high refractive index layer. In this work the effect of the ion beam assistance on the optical coating capability to offer resistance to atmosphere humidity penetration was investigated. Since the presence of an absorption band at a wavelength of 3 micrometers reveals the presence of the water in the layers, some of the investigated coatings were deposited on silicon substrates to measure the transmittance in the infrared region.
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