After more than 30 years since the first experiment we may say today that laser techniques are being widely employed by conservators and restorers as the best choice for the most important tasks: 1) diagnostics about the state of conservation; 2) restoration intervention in the crucial cleaning phase; 3) monitoring of the deterioration processes. Many important experiences have been carried out in Europe along these topics, with significant validation cases and sustainable technology transfer. In this paper the monitoring carried out by the EC Action COST G7 "Artworks Conservation by Laser" about the most advanced activities will be reported, showing how laser techniques in conservation of artworks are developing in Europe, providing a real change in the professional methods.
The state of the art of laser techniques employed in conservation of cultural heritage is continuously growing in Europe.
Many research projects organised at the European level have contributed to this achievement, being complementary to the development carried out at national level. The COST Action G7 is playing its unique role since the year 2000 in promoting the experimentation, comparing the experiences and disseminating best practices. This role has been particularly effective for monitoring of the results of many short-term research projects completed along the G7 Action lifetime. After that several laser cleaning techniques have been followed and evaluated it appears now clear an evolution of the systems, a specialization of the cleaning task, the achievement of side-effect free procedures. The validation of these advanced cleaning techniques has been extensive and diffused in many European countries, especially for stone and metals. Laser-based diagnostics have also specialised their tasks toward material analysis, defects detection and multidimensional documentation. Laser and optical methods successfully monitor deterioration effects. In many European countries interdisciplinary networks are managing the experimentation of these techniques giving them a sound scientific approach, but also a technology transfer to end-users. So doing the appreciation for these techniques is growing in all the conservation institutions involved at national level, disseminating a positive evaluation
about the benefits provided by laser techniques in conservation. Several laser systems became products for the activity of professional restorers and their increasing sales demonstrate a growing utilisation throughout all Europe.
Laser techniques have demonstrated very promising applications for diagnostic and restoration purposes in art conservation. Nevertheless only in the last decade a growing interest in Europe has brought this innovative approach to be tested and validated on various important tasks: laser cleaning of stone, metals, paintings, paper etc; structural laser diagnostics of frescoes and art objects; compositional laser diagnostics of materials; environmental laser monitoring etc. Many programs funded by the European Commission have contributed to the development of new laser instruments and techniques. Presently the COST Action G7 is pursuing the main task of monitoring the advancements achieved in the development of new instrumentation, accumulating validation of laser based techniques with case studies, extending the use of laser for conservation in Europe and other countries, selecting best practices and preparing safety guidelines.
After 30 years since the first experiment we may say that laser techniques are being widely employed today by conservators and restorers as the best choice for the most important tasks: (1) diagnostics about the state of conservation; (2) restoration intervention in the crucial cleaning phase; (3) monitoring of the deterioration processes. Many important experiences have been carried out in Europe along these topics, with significant validation cases and sustainable technology transfer. In this paper the activity carried out in the Tuscany Region will be reported with the monitoring of the most advanced activities carried out by the EC Action COST G7 "Artworks Conservation by Laser."
Optoelectronics and Information Science may usefully employed in conservation of artworks, providing a mutual benefit for the technology innovation in this field. In the Tuscany Region the Optocantieri project is pursuing this task, involving in a network a number of research centers, conservation institutions, restoration laboratories, high-technology producers and professional end-users. This article describes objectives, structure and preliminary results.
In this work we investigated for the first time the laser cleaning process of encrusted stones by employing a high power diode laser system. The test have been carried out using a Rofin-Sinar mod. DL025S emitting up to 2.5 kW CW power to clean various samples representing natural encrustation by pollution exposition and graffiti, typically encountered on historical monuments and buildings in urban environment.
In the present work we investigated in a general way the laser cleaning of coated bronzes by Q-switched Nd:YAG lasers. IN particular, the cleaning problems of tinned, slivered and gilded bronzes were theoretically and experimentally analyzed. Besides the laser fluence, the most critical irradiation parameter resulted to be the pulse duration. We demonstrated the advantage to employ specific Q-switching laser sources providing pulse duration in the range of tens of nanoseconds. The optimized laser ablation conditions were successfully experimented in laser cleaning test on original ancient objects.
Laser cleaning of stones has collected so far the largest number of experiences, employing mainly Nd:YAG laser systems. This class of lasers has been typically used in the two standard emission regimes of Q-switching and Free Running. In our 6 years experience on laser cleaning in conservation, we have studied the interaction process of Nd:YAG laser radiation with various types of stones and surface alterations in order to optimize the laser emission parameters in terms of efficiency and safety of cleaning operations. To this aim we have carried out a critical study on the possible side-effects, which could endanger the result of the cleaning, as well as the future conservation of o artworks. Beside the standard parameters which can be easily adjusted in most of the commercial laser devices, we have considered also the role of the pulse duration, which was varied between the time limits of QS and FR regimes by using laser prototypes suitably develop by us. Such laser systems have been tested in laboratory studies on a large variety of stones and alteration, and are presently applied in the restoration of high valuable statutes and facades of historical buildings. Moreover, the definition of a correct methodology of laser cleaning applications in conservation is presently carried out in cooperation with important public conservation centers of Italy.
The achievement of high quality cleaning of encrusted stone and metals manufacts by optimised laser ablation processes was demonstrated for several deterioration typologies. Here we present some representative cases were intermediate pulse duration Nd:YAG lasers we developed allowed to achieve specific restoration goals.
The nonlinear Kerr refractive index n2 was determined in several crystal hosts, suitable for Ce3+ doping to obtain tunable laser emission in the UV (LiBaF3, LiLuF4, LiCAF, SrA1F5) and in the KMgF3, suitable for Yb2+ doping, a potential laser material in blue region. These parameters were measured by means of Z-scan measurements at 1064 nm and 532 nm, using picosecond laser pulses. The materials under test didn't exhibit two photon absorption effects, within the experimental sensitivity limits. The characterization of these new optical materials is important in view of the development of high peak power laser oscillators and amplifiers.
A 50 W CuBr laser is reported. Characteristics of the major laser parameters are presented: output power, laser pulse repetition frequency, and laser beam quality. The laser has been designed for precision material processing in industry.
After a general discussion about the optimum laser pulse duration for laser cleaning of stone artworks, we present a basic study aimed to state a methodological approach to optimize restoration interventions. As representative examples we investigated the cleaning problem of black crust removal from pliocene sandstone and marble of Siena. The trials were performed with a shot free running Nd:YAG on quarry samples, and on sample from historical buildings. Optical and thermal analyses, associated with petrological diagnostics of the irradiation effects, allowed to derive the damage thresholds and to understand the nature of any invasive phenomenon. The analysis of the experimental results provided the best irradiation conditions and the laser parameter ranges for a safe laser cleaning intervention of the lithotypes and degradation typologies here investigated.
Despite the long period of time since the first proposal of a laser approach to restoration of artworks, this technique is still under debate. The various material composition determines quite different results according to the laser interaction parameters, and their selection and optimization is the base to design laser instrumentation developed for this field of application. The principles of restoration followed by the Italian school of conversation provided quite selective criteria for the development of techniques and laser devices. In the following a description of the activity carried out by our group will be presented and discussed according to the application of new laser instrumentation for cleaning techniques of stones and metals.
The behavior of stress waves induced by Argon-Fluoride laser ablation of the cornea in the typical operative conditions of clinical laser keratectomy have been studied experimentally and analyzed in porcine eyes and in an artificial eye model. Laser-induced stresses with pressure peaks as high as 100 bar were measured in the anterior chamber of irradiated eyes. Analysis of stress wave propagation the eye evidenced diffraction effects modifying the temporal shape of the stress transient with the formation of a rarefaction phase. Besides, significant pressure enhancements caused by focusing of the stress wave front were observed to occur when the spot diameter exceeded 3 mm. For the maximum laser spot diameter of 6.5 mm, diffraction and focusing effects produced at the acoustic focus in the eye compression peaks of 250 and negative peaks of -90 bar, respectively. Implications to clinical procedures, as possible damages due to tissue stretching and cavitation formation are discussed.
In the present work we discuss in some details the development of the stress induced into the target by laser ablation in air. The analytical treatment based on the variable energy blast theory allowed to provide the scaling law of the peak and the temporal profile of the generated pressure wave. Measurements of the acoustic transient associated with excimer laser ablation of polyimide, obtained by means of polyvinyldenefloride (PVDF) sensors were successfully compared with the behaviors provided by the analysis. The description here proposed can be extended to pulsed laser ablation of soft tissues where the target material is removed in vapor or gaseous phases.
Pulsed laser ablation of corneal tissue at the typical operative fluences of Argon-Fluoride photorefractive keratectomy produces stress waves propagating into the eye. Temporal and spatial evolution of these pressure pulses during propagation were measured directly in porcine eyes with a needle hydrophone. Beside diffraction effects that give rise to a bipolar shape of the pressure pulse with a positive compression peak followed by a marked rarefaction phase, we found a significant focusing of the stress wavefront caused by the spherical shape of the cornea, that increased for larger irradiation spot diameters.
NIR spectroscopy and imaging are having an increasing interest as not invasive diagnostics, for the evolution experienced in the last years by both instrumentation technology and interpretation models. In the frequency domain, the informative content of the diffusive photons waves is primarily limited by the approximation of the reconstruction algorithm, according to the measurement precision of the whole diagnostic apparatus. In this work, the influence of optical fiber delivery and collection systems has been studied, to determine signal amplitudes and phase errors, according to refractive effects at the coupling areas, and to propagation effects as intermodal dispersion induced by bending and other mechanical instabilities of the fibers.
In this present study a pump-and-probe imaging set up was arranged to image and analyze the evolution of pressure waves induced by ArF ablation of the cornea, during their propagation into the eyeball. In vitro experiments simulating the effects of clinical PRK have been performed by using an artificial model of the human eyeball, composed of a cell filled with hyaluronic acid gel with a sample of freshly excised bovine cornea placed on the gel surface. LAser irradiation was provided at a fluence of 180 mJ/cm2. Irradiation spot diameters were varied in the range 2.0-5.0 mm. Images of the traveling acoustic waves evidenced diffraction effects, related to the diameter of laser spots on the corneal surface.
A comparison between the laser angioplasty techniques preliminary expectations and verified limits with ten years of research on the laser technology and six years experience in clinical utilization is reported. Both early enthusiasm and present criticism have been found excessive because the limits of the techniques were intuitable from the beginning and on the other hand a significant clinical usefulness is verifiable in a selected group of patients.
In this work we studied the propagation of the acoustic wave generated during laser keratectomy with a clinical ArF laser source. In vitro measurements of the temporal pressure profiles induced inside the ocular bulb were performed on porcine eyes with a fast risetime needle hydrophone introduced into the eyeball. Peak pressure values as high as 90 bars have been measured at a laser fluence of 180 mJ/cm2, depending on the diameter of the irradiation spot and on the distance from the surface of the cornea.
We devised a diagnostic technique based on a pump-and-probe scheme that provided time- resolved imaging of photofragmentation effects during laser lithotripsy. The evolution of the cavitation bubble induced on kidney stone samples by underwater irradiation with a XeCl excimer laser is presented and analyzed.
Temporal and spatial evolution of shock waves generated during drilling processes of optical materials by a high repetition rate copper vapor laser were investigated by means of a CW probe beam deflection technique. Experimental results of the cinematic behavior and of the air density distribution of the shock wave were compared to respect a theoretical model of shock wave expansion providing also quantitative information on the main physical functions associated to laser-induced acoustic phenomena.
A pump and probe laser scheme was devised to obtain time-resolved imaging of plasma formation and shock wave evolution during photoablation of solid targets by a XeCl excimer laser. The analysis of the observed illumination patterns furnished quantitative data on the interaction process.
A time-resolved imaging technique to detect shock wave evolution during photofragmentation of hard biological tissues is presented. A theoretical model, previously developed for plasma physics, was successfully applied to fit experimental results providing useful information on the main parameters involved in the acoustic process.
Efficient low voltage operation of a XeCl laser has been achieved setting a reverse-bias unit on the discharge gap of a system working in the auto-prepulse (AP) mode. Efficiencies up to 1.7% with this improved device even at a charging voltage as low as 7 kV for the main storage unit and for the bias are reported. Experimental results are presented and discussed.
Long pulse excimer laser radiation can be efficiently coupled and transmitted through optical fibers allowing the achievement of both photoablative and photomechanical effects. In this work the investigation has been focussed on the urologic surgery field to demonstrate the effectiveness of an excimer laser system for very different therapeutic tasks: recanalization of urethral stenosis and lithotripsy. The choice of the suitable radiation dosimetry and the technical solutions employed provide to the surgeon a multipurpose laser system with a wide range of utility in comparison with other laser systems.
Numerical and experimental results of a study carried out to investigate the capabilities of the time gating technique to enhance images of biological tissues are reported. In particular we have investigated the confocal scanning configuration. The experimental results were obtained by using a picosecond laser and a streak camera system. The numerical results were obtained by using Monte Carlo simulations. Both elementary and semianalytic Monte Carlo methods were used. The numerical results showed that by using the time gating technique the contrast can be significantly increased when a short gating time is used. The measured time resolved spread functions were in general good agreement with numerical results. However, the experimental results showed that with our experimental setup it is difficult to measure with sufficient accuracy the small energy received during a short gating time when the slab of tissue is larger than 1 cm.
KEYWORDS: Capacitors, Electrodes, Pulsed laser operation, Computer simulations, Laser systems engineering, Prototyping, Gas lasers, Excimer lasers, Modeling, Energy transfer
The requirements for obtaining extended discharge stability in an excimer laser system are summarized. We present a system working in auto prepulse that despite an intrinsic simplicity obtains a long pulse duration and a relatively high overall efficiency. The performances of the scheme have been tested on a small scale prototype. In these conditions computer modelling of the main electrical and optical features shows a good agreement with the experimental data, offering perspectives for scalability in energy and pulse duration.
Our experience on excimer laser angioplasty in peripheral arteries is reported. During three years 34 patients were treated with improved techniques, following the evolution of the laser and of the delivery systems. Encouraging results in the laser stand alone technique allowed us to reduce the association with balloon dilatation to a limited number of cases.
A long pulse XeCl excimer laser (200 ns) was used to induce fragmentation of human urinary stones and artificial models during 'in vitro' experiments. High UV energy fluences, up to 50 J/cm2, could be delivered to the samples by means of silica optical fibers. Fragmentation thresholds of the different samples were observed in the range 5 - 20 J/cm2, well within the energy capability of the fibers. Total fragmentation was always successfully achieved, requiring a variable number of pulses (10 - 100), depending on the pulse energy and the hardness of the sample. The effects of ultraviolet laser radiation on urinary stones are also compared with those of commercial Alexandrite and dye lasers.
We report here the experience of our multidisciplinary group that has been working since 1986 on excimer laser angioplasty. After having selected the excimer laser between the available sources because of the negligible lesions left on the residual tissue, we had the purpose to develop a suitable laser and catheter system. Neglecting here all the preliminary studies, we outline only a typical phenomenon related to the energy delivery and useful for the comprehension of the recanalization process. The energy emitted by every single fiber determines, under a certain threshold, independent recanalized channels in the plaque with residual flaps. At a higher energy level the overposition of the lobes, due to the intrinsic divergence, up to the recanalization in a single large channel. In our opinion this condition is crucial in the design of the catheters to obtain an optical instead of a mechanical recanalization. The biological experimentation conducted during the preliminary tests on human hearts obtained from transplants or cadavers, convinced us that the correct goal to pursue was unique laser angioplasty without the need for further balloon dilation.
A laser angioplasty technique is presented, which utilizes a set of suitably developed catheters to enlarge in subsequent steps the recanalized neolumen.
Results and operative problems of excimer laser angioplasty in human arteries
will be presented. They pertain to a clinical experience about the coronaric
district in association with A.C. by-pass procedure and in the iliac and femoralpopliteal
districts with a percutaneous approach.
Guided propagation in silica optical fibers can overcome the relatively low emission power of a copper vapor laser to produce frequency conversion by stimulated Raman scattering. Multiple Stokes generation induced by laser radiation in the fused silica of optical fibers were obtained. Conversion efficiencies up to 75 % and pulse shortening of the Stokes waves were recorded.
The possibility to induce ablative decomposition with minimal thermal damage on organic tissues by excimer laser suggested promising medical applications of this class of lasers. Excimer laser angioplasty, which represents one of the most advanced surgical laser technique, is at present pursued by some groups at a clinical level. Original research programs in urology and dentistry, developed by our group, are also presented.
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