Electronic Speckle Pattern Interferometry Setup Research Articles

Development of a small-scale testing machine for use with interferometric monitoring methods

Material testing procedures are increasingly using image-based methods for strain field measurements, with extensive usage of Digital Image Correlation in particular. Nonetheless, there is interest in extending this use to other image-based interferometric methods with increased resolution, such as Electronic Speckle Pattern Interferometry (ESPI) and Shearography. In particular, ESPI requires very stable conditions for measurement, usually achieved by assembly on an optical table.Therefore, this work reports the development of a portable testing machine that can be assembled on INEGI’s optical table and is compatible with our previously developed 3D ESPI setup. The machine is an inexpensive custom design for static or quasi-static loading, with the main objective of withstanding a maximum load and velocity of 20 kN and 240 mm/min, respectively. The machine was built around a brushless dc motor and controller, both from Trinamic, and is currently being successfully used with ESPI methodology.

Deformation Wave Theory and Application to Optical Interferometry.

A method to diagnose the deformation status of solid objects under loading is discussed. The present method is based on a recent field theory of deformation and fracture and optical interferometry known as the Electronic Speckle-Pattern Interferometry (ESPI). Using one of the most fundamental principles of physics referred to as symmetry in physics, this field theory formulates all stages of deformation and fracture on the same theoretical basis. In accordance with the formalism, the theory has defined the criteria for different stages of deformation (linear elastic, plastic and fracturing stages) expressed by certain spatiotemporal features of the differential displacement (the displacement occurring during a small time interval). The ESPI is used to visualize the differential displacement field of a specimen as two-dimensional, full-field interferometric fringe patterns. This paper reports experimental evidence that demonstrates the usefulness of the present method. A tensile load is applied to an aluminum-alloy plate specimen at a constant pulling rate and the resultant in-plane displacement field is visualized with a two-dimensional ESPI setup. The differential displacement field is obtained at each time step and the interferometric fringe patterns are interpreted based on the criterion for each stage of deformation. It has been found that the criteria of linear elastic deformation, plastic deformation and fracturing stage are clearly observed in the corresponding fringe patterns and that the observations are consistent with the loading characteristics.

A Simple Method for Detecting a Plastic Deformation Region Formed by Local Loading Using Electronic Speckle Pattern Interferometry

A simple method for detecting a plastic deformation region formed by local loading using electronic speckle pattern interferometry (ESPI) is proposed. The plastic deformation region is recognized as the decorrelation region of interference fringes by adjusting the beam angle and the iris in the ESPI setup. Not only the relationship between the decorrelation limit and the strain but allowable rigid body displacement for positioning the specimen are investigated. Then, the proposed method is validated by detecting the plastic deformation region of an aluminum specimen deformed by local impact. As a result, the plastic deformation region is easily identified. As the proposed method is simple and easy, it can be widely applied to damage evaluation.

Portable electronic speckle interferometry device for the damages measurements in veneered wood artworks

Deterioration of wood artwork is often connected to mechanical material degradation that starts on microscopic scales. Insight into decay mechanisms can be obtained by monitoring surface deformation and displacement fields. This paper presents the application of Electronic Speckle Pattern Interferometry (ESPI) to detect damages of wood samples invisible into the surface. Two different damages were created on the model simulating cabinet wood panels: delaminating and worm galleries, all defects are invisible by naked eye. All optical arrangements of speckle interferometry are divided in two types of fundamentally different speckle patterns: photographic speckle patterns, which contain information only about the light wave amplitude, and holographic speckle patterns, which contain both phase and amplitude information for comparison potentialities of two approaches we present the results obtained by thermography and ESPI investigations of the wood panel under test. The fiber-optic ESPI set-up based on a He–Ne CW laser has been developed and used for studying the possibility to reveal the invisible damages and determine their locations, sizes and shapes. For this end the two digital holograms of the test object, corresponding to the non-heated and heated states of object, are captured at two video frames of the CCD camera, and then processed in a PC. The resulted fringe pattern has the information about the damages. The purpose of the work was determining the possibilities of ESPI method in revealing of different kinds of invisible damages. Our results indicate that developed variant ESPI system is well adapted to reveal the under surface damages in veneered wood samples. The set-up may be used in out-of-laboratory conditions and without severe anti-vibration preoccupations. The comparison of results obtained by developed ESPI system and thermo camera shows the higher sensitivity the ESPI system. The thermo camera does not indicate the presence of under surface damages like delaminating and worm galleries in contrast with ESPI system which is well adapted to extract information about these defects. Applying ESPI set-up we have determined the presence of different kind of damages located under surface: big delaminating and little worm galleries. The developed ESPI set-up is capable of predicting the position, shape and size of revealed damages. The results presented in this paper show that the ESPI technique is a promising tool for testing the wood artworks.

Application of dynamic phase shifting with wavelet analysis to electronic speckle contouring

Dynamic phase shifting is a temporal phase unwrapping method, i.e., a method in which a sequence of speckle patterns is analyzed along the time axis. Each pixel can thus be considered as an independent detector, which is of particular interest for the study of complex surfaces. I report the application of this technique, which is based on a wavelet analysis, to contouring measurements with a dual-beam illumination electronic speckle pattern interferometry setup. I present a new, more general geometric model of the setup. I also investigate the possibility of enhancing the accuracy by using the intermediate phase values. Tests are performed on a simply described object and compared with coordinate measuring machine measurements.

Adaptive correction to the speckle correlation fringes by using a twisted-nematic liquid-crystal display

An innovative technique for reducing speckle noise and improving the intensity profile of the speckle correlation fringes is presented. The method is based on reducing the range of the modulation intensity values of the speckle interference pattern. After the fringe pattern is corrected adaptively at each pixel, a simple morphological filtering of the fringes is sufficient to obtain smoothed fringes. The concept is presented both analytically and by simulation by using computer-generated speckle patterns. The experimental verification is performed by using an amplitude-only spatial light modulator (SLM) in a conventional electronic speckle pattern interferometry setup. The optical arrangement for tuning a commercially available LCD array for amplitude-only behavior is described. The method of feedback to the LCD SLM to modulate the intensity of the reference beam in order to reduce the modulation intensity values is explained, and the resulting fringe pattern and increase in the signal-to-noise ratio are discussed.

Measurement of thermally induced vibrations of microelectronic devices by use of a heterodyne electronic speckle pattern interferometry imaging technique.

An imaging technique to measure modulated surface displacements on microelectronic devices is presented. A device is supplied by a sinusoidal current that creates a modulated variation of temperature. To measure the induced normal surface displacement, we use an electronic speckle pattern interferometry setup in which we introduce a secondary modulation using an electro-optic modulator. To extract the displacement information, we then analyze the term atthe blinking frequency, which is equal tothe difference between the frequency of the surface displacement and the frequency of the secondary modulation. As the photodetector is a visible CC D camera, weapply heterodyne detection byusing a multichannel lock-in scheme. We have experimented with this new technique on a membrane to measure the amplitude of modulated surface displacement induced by the Joule effect.

Multiplicative electronic speckle-pattern interferometry fringes

A theoretical analysis of fringe patterns and its experimental corroboration obtained by multiplication of two speckled images with electronic speckle-pattern interferometry (ESPI) are reported. A specifically designed digital filter is used to enhance the contrast and visibility of the inherently noisy multiplication fringes. Phase retrieval is achieved by a phase-stepping technique. Experimental results are presented for the in-plane-sensitive optical ESPI setup; however, out-of-plane and shearing setups may be used as well. The method represents an alternative to the subtraction and addition techniques in ESPI.

Electronic speckle pattern interferometry with a holographically generated reference wave

A new method of electronic speckle pattern interferometry (ESPI) is introduced. It is a two-stage process. In the first stage a holographic optical element (HOE) is recorded in a usual holographic arrangement. This HOE illuminated later reconstructs an object wave that serves as an ESPI reference wave. Well suited HOEs are produced on photothermoplastic or silver halide media. In the case of silver halide media, both recording and momental monobath photoprocessing of the HOE are performed in presence of an intense polychromatic illumination to meet industrial requirements. Due to the introduction of HOE the second stage, ESPI implementation can be performed with an extremely simple optical setup. The number of optical elements, apart from the video camera, can be drastically reduced to only two elements: a plane mirror and the HOE. High accuracy alignment of the optical elements is not necessary. Our method also enables HOE recording in the ESPI setup practically without modifications. Both experimental results and the ready device are presented. Our optical setup is compact and needs no sophisticated vibration insulation and no dark room, thus it is ideally suited for industrial applications.

Electronic speckle pattern interferometry using a CO 2 laser

The advantages and problems of using electronic speckle pattern interferometry (ESPI) at longer (infra-red) wavelengths are discussed. The adjustment and use of an ESPI set-up with a CO 2 laser and pyroelectric vidicon are described and illustrated by some experimental results.