Digital holographic interferometry for cultural heritage structural diagnostics: A coherent and a low‐coherence optical set‐up for the study of a marquetry sample

Cultural heritage conservation is an active field of research, where there is an ever‐growing demand for nondestructive and noninvasive diagnostic techniques, for performing remote analysis and diagnosis of the condition of historical structures and pieces of art, often of very high cultural and historical value. In this context, holographic interferometry is a very well‐established optical technique for research in cultural heritage, which brings together some very basic and critical properties such as contactless examination and nondestructivity, accuracy, repeatability, and a wide range of applicability. In this paper, the optical technique of digital holographic interferometry is tested on mock‐up, art‐related targets, with 2 different light sources, in an attempt to expand the technique towards a new approach that will profit from an easy‐to‐operate, inexpensive, and tunable source, offering a broad spectrum and wavelength selectivity, according to the needs of the experiments. Examples are presented, and the results demonstrate the effectiveness of the proposed modified experimental scheme for defect mapping, to be used in structural documentation reports, and for its exploitation in future hybrid optical diagnostic systems and data processing.     

Full‐Field Vibration Measurement by Time‐Average Speckle Interferometry and by Doppler Vibrometry – A Comparison

Abstract: Identification of dynamic material properties, non‐destructive testing and study of vibroacoustic behaviour of different structures require the use of complex, pointwise and full‐field measurements, which are capable of providing data for experimental modal analysis or model updating. Nowadays, among other techniques, optical non‐contact techniques represent the favourite choice as they do not add mass, stiffness or damping to the structure under test. When the range of vibration amplitudes allows it, most of these techniques are based on interferometric principles. Development of laser sources and detectors leads to a continuous improvement of vibration measurement techniques. However, a hard choice still has to be made between spatial resolution and temporal resolution. Another difficult choice is between space bandwidth product and energetic sensitivity of the detector. While the number of pixels of a camera is continuously increasing, the pixel size seems limited at its lower end. The paper presents a comparative study of the vibration amplitude fields as measured by two full‐field non‐contact techniques, speckle interferometry and laser Doppler vibrometry, and predicted by finite‐element model. The measurements concern the free and the forced vibrations of a thick, composite plate with free boundaries.     

Art on the Nanoscale and Beyond

Methods of forming and patterning materials at the nano‐ and microscales are finding increased use as a medium of artistic expression, and as a vehicle for communicating scientific advances to a broader audience. While sharing many attributes of other art forms, miniaturized art enables the direct engagement of sensory aspects such as sight and touch for materials and structures that are otherwise invisible to the eye. The historical uses of nano‐/microscale materials and imaging techniques in arts and sciences are presented. The motivations to create artwork at small scales are discussed, and representations in scientific literature and exhibitions are explored. Examples are presented using semiconductors, microfluidics, and nanomaterials as the artistic media; these utilized techniques including micromachining, focused ion beam milling, two‐photon polymerization, and bottom‐up nanostructure growth. Finally, the technological factors that limit the implementation of artwork at miniature scales are identified, and potential future directions are discussed. As research marches toward even smaller length scales, innovative and engaging visualizations and artistic endeavors will have growing implications on education, communication, policy making, media activism, and public perception of science and technology.     

An electro-optic holography system with real-time arithmetic processing

This paper reports the use of a real-time arithmetic image processor in an electro-optic holography system. A speckle interferometer is used to combine an image of an object, lit by laser light, with a mutually coherent reference beam. A CCD TV camera detects the interference pattern, and the phase of the reference beam is advanced by 90° between frames. An image is generated from each set of four sequential TV frames by subtracting alternate frames, squaring, and adding the two results. The result is improved picture quality compared with the use of binary pixels and compared with electronic speckle pattern interferometry. Experimental results are shown.     

Electronic speckle pattern interferometry on a microscopic scale

An electronic speckle pattern interferometer that incorporates a commercial, long-working-distance microscope is described which provides new opportunities to perform nondestructive inspection for applications in fields such as microelectronics. The long-working-distance microscrope was attached directly to the CCD camera to form a compact, portable system with a field of view that was variable over several mm in width. Alignment was greatly simplified because the skewed Michelson interferometer configuration used a speckled reference beam imaged from a diffusely reflecting reference surface that was positioned adjacent to the test object. To demonstrate the performance of the micro-ESPI instrument, fringe patterns were recorded for the quasi-static cantilever deflection of a 1 mm-wide interconnect clip from an electronics socket.     

Application of Double‐Focus Speckle Interferometry for Non‐Destructive Testing

Abstract: This paper deals with the application of a speckle interferometer, which works on the new operating principle of double‐focusing presented by the authors in recent studies, to the field of non‐destructive testing (NDT). Using this interferometer, the components of displacement given by holographic interferometry can be measured, but with no need for an external reference beam. The implementation of this interferometry can be indifferently carried out by adopting a Michelson or a Mach–Zender configuration. In the paper a double‐focus interferometer based on the Michelson design and sensitive to out‐of‐plane displacements was implemented and applied to a metallic specimen which simulates the deformations of a typical debonding.     

Construction and characterization of a continuous atom beam interferometer

In this article, we investigate both theoretically and experimentally the method of Ramsey interference applied to cold atoms being emitted from a 2-dimensional magneto-optical trap (2D-MOT) crossing continuous wave (cw) laser fields that drive Raman transitions between ground-state hyperfine sublevels. In our apparatus, the atomic beam divergence is large (56 mrad half-angle), necessitating the inclusion in our model of both the atomic divergence and the laser beam divergence as well as averaging over the transit time of the atoms through the laser beam. We discuss the effects of the divergence of the atomic and laser beams, and then we describe our apparatus and qualitatively compare our experimental results to the theoretical results. We find good agreement between our model and our experimental results.     

Werkstoffmechanische Bewertung von Laserstrahl‐Schweißnähten in Feinkornbaustählen

Materials and Mechanics Effects in the Assessment of Laser Beam Weld Joints in Structural Steels For fine‐grained structural steels of different strength levels the consequences of mechanical inhomogeneity (mismatch) in laser‐beam welds on their deformation and failure behaviour without and with defects are quantified; requirements for fitness‐for‐service are derived.     

浅析表现主义对动画艺术的影响

在绘画艺术悠久的历史长河中,表现主义作为现代重要的流派于十九世纪出现在艺术界,随后不断影响着音乐、戏曲、文学、电影等众多艺术领域。背弃了欧洲传统绘画流派对人物、事物的客观描摹,在印象主义发展的基础上,提出“要表现而不是再现”、“艺术不是现实,而是精神”等思想。     

Plotting holographic interferograms for visualization of dynamic results from finite‐element calculations

Procedures for plotting computer generated interferograms from the results of finite‐element analysis based on the principles of optical holography can provide a realistic view of dynamic processes taking place in the analysed structures. Such visualization is based on a solid physical background, does not require animation for representing dynamic processes, and is also important from the point of view of interpretation of experimental holograms. The numerical method for obtaining time‐averaged digital interferograms for structural dynamics applications is presented. Intensity mapping as well as the methods of digital stroboscopic analysis are used for plotting clearer images due to the fact that the intensity of higher interference bands decreases rapidly with the growth of vibration amplitudes. The digital time averaging of intermediate states of the moving surface together with varying direction of incident laser beam and estimation of diffuse and specular reflection phenomena enables the generation of realistic interferograms. Such a procedure is scalable in parallel computations and applicable to a wide variety of problems. Copyright © 2003 John Wiley & Sons, Ltd.     

Dual holographic interferometry for measuring the three velocity components in a fluid plane

A technique that allows one to measure simultaneously the three velocity components in a fluid plane is presented. One obtains the quantitative information from only one holographic recording by combining two different reconstruction processes. As both processes use an interferometric comparison of two waves, we refer to this technique as dual holographic interferometry. The far-field fringe pattern that is obtained when reconstruction is made with an expanded laser beam allows one to determine the in-plane velocity components. The image-field fringe pattern that is obtained when a pointwise laser beam is used for reconstruction contains information about an out-of-plane velocity component. As the two reconstruction processes have different sensitivities, two different ways to combine them are proposed. The system has been demonstrated in a fluidlike solid object and in a convective flow.     

Raman microprobe: a diagnostic tool for processed silicon. Analysis of microindented silicon

Laser-assisted processes are currently used in silicon technology. The response of the material to the laser beam depends strongly on its own physical properties and on the laser power density. The use of a microRaman system, allows the structural characteristics of the material to be analysed by varying the excitation laser power density on the sample over a large power range with a submicrometre lateral resolution. Results are reported on microindented crystalline silicon, showing that changes in the physical properties of the material, introducing grain boundaries, dislocations and cracking, result in a strong modification of the Raman spectrum. These spectral changes are enhanced for increasing laser power densities. Several mechanisms are pointed out as possible sources of the observed spectral modifications. These results show that Raman microprobe is a very promising technique for the diagnosis of technologically processed semiconductors and devices.     

The Sensitivity of Acoustic-Laser Technique for Detecting the Defects in CFRP-Bonded Concrete Systems

This paper presents an experimental study to evaluate the sensitivity of acoustic-laser technique in defect detection. The technique is particularly useful towards the detection of near-surface defects in fiber reinforced polymer-bonded concrete by vibrating the material with an acoustic excitation and measuring the vibration signals with a laser beam. However, relatively little is known about the sensitivity of acoustic-laser technique. More research work should be conducted to evaluate the effectiveness of the technique when adopted for defect detection. It is also important to investigate the limits of the technique performance with respect to varying operational conditions so as to determine ways of improving the detectability. For this purpose, operational conditions in terms of acoustic excitation and laser beam incidence are investigated for their effectiveness in detecting near-surface defects and a reliable defect detection scheme using our portable equipment is therefore recommended. This work provides a basis for further improving such technique which can be used in other engineering applications including quality control of materials and product development process.     

Null detection of ballistic photons for locating objects buried in turbid media

We describe an optical system in which a vibration induced to a laser probe beam combined with a half-blocked photodiode allows determination of the position and width of objects buried in turbid media. Our system is based on the detection of an AC signal which drastically decreases under the presence of an obstructing buried object. We describe the technique and include experimental results showing that the system is capable of detecting 2?mm wide objects buried at depths up to 3?cm from the front surface of a sample simulating scattering properties of soft tissue.     

Fluorescence lifetime imaging and spectroscopy as tools for nondestructive analysis of works of art

A system for advanced fluorescence investigation of works of art has been assembled and integrated in a characterization procedure that allows one to localize and identify organic compounds that are present in artworks. At the beginning of the investigation, fluorescence lifetime imaging and spectroscopy address a selective microsampling of the artwork. Then analytical measurements of microsamples identify the chemical composition of the materials under investigation. Finally, on the basis of fluorescence lifetime and amplitude maps, analytical data are extended to the whole artwork. In such a way, information on the spatial distribution of organic materials can be inferred. These concepts have been successfully applied in an extensive campaign for analysis of Renaissance fresco paintings in Castiglione Olona, Italy. Residue of various types of glue and stucco left from a restoration carried out in the early 1970s was localized and classified. Insight into the technique used by the painter to make gilded reliefs was also obtained.     

On the Industrial Applications of Moiré and Fringe Projection Techniques

Abstract: This paper describes a set of tools and procedures based on the moiré and fringe projection techniques, which were developed to suit the requirements of real industrial situations. Particular attention was paid to flexibility, robustness and reliability, ease of use and calibration, and cost efficiency. Three case studies are being discussed in the paper. The first case describes a prototype shadow moiré instrument, which was developed for the detection of surface defects such as barely visible impact damage or corrosion inside lap-joints, during maintenance inspections of aeronautical structures. The second case presents a portable instrument based on the fringe projection technique for the automatic, full-field measurement of the shape and deformation of an arbitrary object. The system has applications ranging from deformation measurement of bioprostheses to correction of shape effects in the photoelastic analysis of complex aerospace components. Finally, a portable instrument was developed for the measurement of the location, orientation, shape and deformation of an arbitrary object, by combining the fringe projection technique with the laser spot technique for distance measurement. The system was used to assist in the integration of non-destructive evaluation techniques throughout the life cycle of an aircraft.     

Combination of FEM simulations and shearography for defect detection on artwork

Digital shearography is an interferometric technique, which is often used for defect detection on composite structures. The measurement is fast and non‐destructive and thus suitable for the application to cultural heritage. However, a major drawback of the technique is the indirect measurement, because surface strain is used to detect subsurface defects. So the determination of depth, size, and type of the defects is rather difficult. To overcome this issue, we use a combination of finite element method simulation and shearographic measurement. Due to the comparison of both data sets, the inverse problem can be solved in a more reliable way. In this paper, we investigate the application of finite element method simulation for an improved defect detection on anisotropic material. We validate the technique on a wooden sample with notches on the backside.     

Double bridge circuit for self‐validated structural health monitoring strain measurements

In structural health monitoring (SHM) applications, sensor faults and structural damage need to be assuredly discriminated. A self‐diagnosis strain sensor operating in a continuous online SHM scenario is considered. The strain sensor is based on full electric resistance strain gauge Wheatstone bridges. The state of the art shows that such a sensor has not yet been developed. The loop current step response (LCSR) is a well‐known method to detect strain gauge debonding. However, applying the LCSR method to a full strain gauge Wheatstone bridge has some limitations analysed in this paper. To enable the use of the LCSR method in an online SHM scenario, the double bridge circuit is proposed in this work. Two new strain gauge debonding fault detection methods and a new debonding fault isolation method—based on the double bridge circuit measurements—are proposed and evaluated. Two new sensor fusion weighting approaches are also proposed and evaluated—to achieve strain gauge debonding fault tolerance on the double bridge circuit. The experimental results show that the proposed methods can detect, isolate, and tolerate a strain gauge grid debonding fault and can be applied in an online SHM self‐diagnosis sensor scenario.     

Risky intensity peaks resulting from nonlinear holographic imaging

A study of the maximal intensity peaks due to nonlinear holographic images of obstacles such as obscurations or phase defects in a high-power laser system is presented. It is shown that the interference of the high-power plane wave and the converging image wave results in the formation of intensity maximums in the vicinity of the image plane, the values of which significantly exceed the intensity in the image plane itself. For round obstacles, analytical expressions that describe magnitudes and locations of the maxima depending on the radius and the type of obstacle are given. A procedure of numerical modeling that allows estimation of the influence of beam size, medium thickness, type, size, and shape of obstacles onto the properties of nonlinear images is described. It is demonstrated that for a given combination of the nonlinear medium and the high-power beam parameters, there is an intrinsic size of obstacles that is most harmful for the laser system components.     

Laser weld pool management through diffractive holographic optics

Abstract

Conduction laser welding involves initiating a melt pool by exposure to high power laser induced light and controlled thermal conduction. Existing welding techniques generally provide enough energy to join the component but have no real control over the melt pool. This process can invariably lead to overheating in adjacent areas or even the melt pool itself, often causing unavoidable effects, such as ‘burn through’. The present work presents a procedure in which a desired melt pool shape is conceived, and a bespoke beam irradiance distribution is designed to match. The beam is shaped not by conventional lenses but by a diffractive holographic optical element (DHOE). The DHOE utilises holography to wholly create highly complex three-dimensional energy distributions through constructive and destructive interference. This technique allows novel beam irradiance distributions to be applied to conduction mode laser welding, with the melt pool transverse profile being shaped to a specific design. Holographic conduction laser welding has been shown to be successful and represents a significant step forward in the industry, as demonstrated in this case in both mild and stainless steels. The fusion zone is shown to be particularly influenced by the shape of the illuminating laser beam profile, and many of the welds demonstrate a highly novel weld profile because of this. The use of a bespoke beam irradiance distribution allows control of the heat flow to the workpiece, and this allows greater control over material migration due to surface tension effects. Many of the welds demonstrate unique surface solidification patterns directly linked to the beam profile used. The DHOE also presents a number of additional advantages, such as an increased usable depth of field, allowing for less stringent set-up tolerances. Comprehensive metallography has been performed on samples of these welds through the use of optical microscopy, electron microscopy, electron backscatter diffraction and energy dispersive (X-ray) spectroscopy. These techniques offer in depth analysis of crystal size, shape, orientation and phase. By incorporating DHOEs into a laser welding process, not only does the melt pool shape become controllable, but also the crystal growth is highly influenced. Many of the undesirable attributes of a conventional laser weld are reduced by using a beam distribution created by a DHOE, bringing the microstructure of the weld pool closer to that of the parent material.