In this paper the basic principles of LCSI are briefly described. Furthermore, we investigate the interference signal coming from a semitransparent, multilayer object theoretically and experimentally. The interference signal is modelled using a one-dimensional transmission line approach. The theory for this model is well known from the exact matrix theory of multilayered systems. We have extended the model for phase investigations and simulated the effect of a delamination when measuring through a semitransparent layer. The model is verified by experimental results from LCSI measurements.
Electronic Speckle Pattern Interferometry with a light source of short coherence length allows depth-resolved deformation analysis below the surface of light-scattering objects (Low Coherence Speckle Interferometry - LCSI). Interference is tuned to a thin layer - called the coherence layer - by appropriate adjustment of the length of the reference path. The quality of the results is degraded by background light from outside the coherence layer and by de-correlations due to the passage of the useful light through regions that have been altered by the overall deformation field. Basic experimental studies are conducted on a simple two-interface object (two roughened surfaces of glass slides) to determine the effects that one interface (and its deformation) exerts on the quality of the deformation measurement in the other interface. Analytical theoretical calculations of speckle de-correlation on the basis of Fresnel diffraction provide comparative data.
TV-holography or electronic speckle pattern interferometry (ESPI) is used for the detection of micro-deformations on a variety of optically rough surfaces. These deformations are often a result of processes in the whole volume. If the investigated object is, for example, composed of different layers the interaction between these layers can result in a deformation of the surface. Such effects are often seen on antique paintings and result in a net-like structure of cracks, the so-called craquelee. To understand these processes it is necessary to measure the movement of each layer separately. ESPI can be used for sub-surface deformation measurements on layered objects, when it is operated with a low-coherence light source like a superluminescense diode as used in optical coherence tomography. Interferometric information is only registered if the difference between the path length of object and reference beam is less than the coherence length. By changing the path length of one of the beams it is possible to select the region where deformations will be measured even if it is located below the surface, provided the light penetrates the material sufficiently. A serious problem is the separation of the coherent light and the incoherent background. This is performed by a combination of spatial phase shift (SPS) and Fourier filtering.
Parametric acoustic arrays are built to generate highly directional audio sound by nonlinear interaction of ultrasound. Arrays especially built for applications at high audio sound pressure use the most effective ultrasonic transducer for airborne sound, i.e., a piezoelectric (PZT) bimorph. Since the individual transducer elements are very small (<16 mm in diameter) several hundred of them have to be combined to reach the desired audio sound pressure level. For high performance it is a prerequisite that all transducers radiate in phase. However, fluctuations in their properties result in according fluctuations in their phases. The construction of such a device therefore requires a non-intrusive technique for monitoring amplitude and phase of a three-dimensional sound field without creating any nonlinearity. TV-holography or Electronic Speckle Pattern Interferometry (ESPI) in its time-averaging mode combined with reference wave modulation has been applied for this purpose. The recordings represent a two-dimensional projection of the sound field integrated along the viewing direction. The three-dimensional field is obtained from many such projections through the sound field at different angles in a tomographic setup. Inversion by filtered backprojection yields the three-dimensional sound amplitude and phase that can be utilized to optimize transducer operation. The performance of such a system is demonstrated in the development of an ultrasonic array at 38.5 kHz. It is shown how the generation of highly directive audio sound has been improved by guidance from the optical results. The highly directional source of audio sound finally produced is needed for an application in monument research where loose areas in historical murals have to be identified.
In this paper the basic principles of Low Coherence Speckle Interferometry (LCSI) are described. Theoretical background and experimental results for the systematic investigation of LCSI are presented. To understand and quantify the measurement results of adhesive bonded joints a modelling of the interference signal is required. For this purpose, a one-dimensional transmission line model is developed, including changes in the refractive index in a stressed adhesive layer and delamination of the glued interface. A new method for the detection of zero path length difference is introduced. Investigations of the probing depth in semi-transparent adhesive and recent experimental results of the characterisation of adhesive-bonded aluminium joints are presented.
In the conservation of historical murals it is important to identify loose plaster sections that threaten to fall off. Electronic speckle interferometry in combination with acoustic excitation of the object has been employed to monitor loose areas. To avoid disadvantages of high sound irradiation of the complete building a novel directional audio-sound source based on nonlinear mixing of ultrasound has been introduced. The optical system was revised for optimum performance in the new environment. Emphasis is placed on noise suppression to increase sensitivity. Furthermore, amplitude and phase data of object response over the frequency-range inspected are employed to gain additional information on the state of the plaster or paint. Laboratory studies on sample specimen supplement field campaigns at historical sites.
Low Coherence Speckle Interferometry (LCSI) combines the depth-resolved measurement of Low Coherence Interferometry (LCI) with the high-accuracy deformation measurement of Electronic Speckle Pattern Interferometry (ESPI). Depth-resolved deformation measurement enables the characterization of the behavior of interfaces in multi-layer materials or structures while changing the ambient conditions. In this paper LCSI is introduced as a new tool for characterization of adhesion. The experimental set-up and the principle of work are described. A FEM-model of an adhesive bonded aluminum joint is developed to analyze the behavior of the Al-adhesive interface during mechanical testing. Some recent results are shown. This application demand measurements on a microscopic scale (camera field of view down to 500 x 500 μm).
Electronic speckle pattern interferometry (ESPI) is discussed for the detection of out-of-plane deformations in small objects. For increasing the resolution in object space a laser source of small wavelength is combined with a microscope with a high numerical aperture. Fringe quality is increased by using spatial phase-shifting and the Fourier transform method to allow deformation detection also under non-optimum conditions. The efficacy will be shown in some measurements on different specimens where deformations are successfully recorded in areas down to a few micrometers in size.
In the conservation of historical murals an important issue is the detection of plaster or paint layers that detach from the supporting material and thus threaten to fall off. Commonly, walls are inspected by the acoustic response to a gentle finger-tapping (percussion method). Since this is a costly and cumbersome technique there is need for a metrological instrument serving the same purpose. In the last few years we have shown, that a time-average version of electronic speckle pattern interferometry (ESPI) with increased sensitivity in combination with acoustic excitation of the object can be a powerful tool for monitoring of loose areas. It offers full-field, video real time capability and has the advantage of non-contact and remote operation which, for example, is extremely useful in large buildings. Recently, a fully computer-based evaluation and control system was added to the system to assist in the introduction of the method as a generally approved tool in artwork monitoring. Principles of the method and instrumental features of the equipment are presented and some results and their interpretation obtained with the computerized system in the church and chapel at St. John's convent at Mnstair, Switzerland are demonstrated.
The decorrelation of the characteristic speckle pattern, produced by the scattering of coherent light from a random rough surface, is used to monitor changes in the microtopography of the surface. In a basic optical setup minute alterations down to a few nanometers can be measured. To enlighten the basic relations between the speckle field decorrelation and the surface changes a theoretical model is developed, numerical calculations are carried out and comparisons with high resolution AFM measurements on two frozen states of a surface process are undertaken. Whereas the reproducibility of the AFM measurement turns out to be not precise enough to measure nanometer changes on a micrometer scale rough surface, the theoretical model is consistent with results gained from experimental variation of wavelength and angle of illumination and a-priori knowledge about the surface. It is then used to obtain information about magnitude, time constants and statistical parameters of a corrosion process.
A sensitized TV-holography system combined with acoustical object excitation is shown to be a new powerful tool for remote monitoring of detached plaster areas in historical murals. Some experimental results obtained in the laboratory and at historical sties will be presented and compared with results received with traditional percussion method.
Two remote and nondestructive testing techniques for the study of the decay mechanism of stone and plaster surfaces are presented. While electronic speckle pattern interferometer (ESPI) measures the 3-D-microdeformations of the surface, transient thermography shows at the same time invisible defects and structural faults as a reaction to the impact of short heat pulses. The combination of these two different techniques demonstrates its feasibility to detect restorative work and shows the compatibility of classical and modern substitutes and restoration techniques to historical materials.
The design and performance of an actively phase-compensated fiber-optic speckle pattern interferometer (ESPI)
is presented. Due to its portability, illuminating beam flexibility and lightweight design it provides an alternative
to standard bulk-optics FSPI systems for many applications. Phase stabilization allows for long-term
measurements up to several days. In addition, the provided phase stepping uses the four-step algorithm, is self
calibrating, and can be performed at quasi video rate. The use of all highly birefringent fiber-optics components
ensures linearly polarized light for the illuminating beams under all operating conditions.
Deterioration of historical murals often leads to partial detachments of the paint carrying plaster layers. To identify such regions an acousto-optical method is described. Loose portions of the plaster are excited by sound waves and the resulting vibrations are detected by sensitized analog TV-holography.
Sophisticated optical metrology is conquering more and more dimensions of complex flow fields. A survey of more-dimensional techniques from particle image velocimetry to stereo- photography and particle image holography is presented including some special emphasis on stereoscopic back-projection and multiple light sheet holography.
Electronic speckle pattern interferometry is elaborated in such a way that also spatially resolved image decorrelation can be measured. While retaining the typical ESPI setup for deformation measurements, a speckle correlation formalism is implemented based on the phase-shift method. In many practical situations decorrelation is directly related to surface microstructure changes of a test specimen. Feasibility and restrictions of the method are illustrated by measurements of water-induced changes at the surfaces of natural stones and by monitoring microbiological activity on stones.
An extension of a common electronic speckle pattern interferometry system is presented, which in addition to deformation measurements enables the spatially resolved quantitative determination of image decorrelation. In the absence of other decorrelation effects this quantity is directly related to surface microstructure changes of test specimen. Without principal variations of a typical ESPI setup a speckle correlation formalism is implemented based on the phase-shift method. Feasibility and restrictions are illustrated in measurements of water-induced changes of natural stone surfaces.
Major environmental problems all over the world require immediate action. Often a quantitative observation of the environmental effects on natural or man-made systems is asked for. Useful information about the object is collected in the speckle fields produced by scattered coherent light. Object changes are detected by correlation of corresponding speckle fields. Optical methods are employed for this task: holography, holographic interferometry, double- exposure speckle photography, speckle correlation, or electronic speckle pattern interferometry. Modifications of these techniques are adapted to on-site and real-time applications in environmental diagnostics. Practical results are presented in examples from the study of plants and the monitoring of climatic effects on cultural stone monuments.
Substitution of common lasers by small laser diodes gains increasing popularity since it serves for compactness of measurement systems. The possibility of tuning the emitted wavelength simply by changing the injection current is very attractive for systems applying phase shift methods. Usage of optomechanical elements to produce the required phase shifts is superseded. Phase shifting by laser diode tuning was implemented into the presented electronic speckle pattern interferometry (ESPI) setup. Automatic evaluation of fringe systems is done using the Carre phase shift method. Influences of variations of laser power due to changed injection current and decorrelation of speckle patterns due to the altered wavelength are found to be neglectable.
Subject of the investigation are time dependent processes that cause small particles in rough surfaces to displace or deform. These small particles must be identified in a region several cm2 in size. The method proposed is a simultaneous application of electronic speckle pattern interferometry (ESPI) and a holographic double exposure technique using suited carrier fringe systems. The time dependence of the process is observed by ESPI indicating suitable states of the process to be compared by the double exposure technique. Detection of displacement and identification of the particles takes place in the reconstruction real image of the double exposure hologram. The real image was chosen because of the better accessibility of the finely spaced carrier fringes. The resolution obtainable with this method is subject of discussion. The power of the method is illustrated experimentally.
Electronic speckle pattern interferometry (ESPI) is applied to 3-D deformation mapping in practical problems from constructional engineering. The shrinking behavior and bonding ability of repair mortars applied in the restoration of historical stone buildings is monitored. Strategies are illustrated to process the huge amount of deformation data toward parameters characteristic of the relevant processes. Thus, ESPI is extended to routine applications in industrial inspection.
A videoholographic system was developed which allows three-dimensional deformation measurements. The system was applied in inyestigations of deformations occurring in the hardening process of a special restoration mortar. 1 .
Particle image velocimetry (PIV) is extended to 30 velocity measurements by stereoscopic imaging. Optical processing yields all velocity components in a single output.
Holograms of a particle field illuminated by a set of light sheets are made simultaneously on one recording material differing each in reference beam direction and coherence properties. Each sheet can thus be evaluated separately by ordinary PlY techniques. 2.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.