Showing papers on "Speckle imaging published in 2004"
TL;DR: In this paper, the state-of-the-art of digital particle holography is reviewed to allow estimates of its future in experimental flow analysis, and the practical advantage of video and digital techniques over photographic recording is obvious.
Abstract: Holography is truly the key to three dimensions in particle image velocimetry, i.e.?the measurement of all spatial components of the velocity vector - and this over a deep measuring field. Sophisticated instruments have been designed that successfully tackle practical problems such as the low scattering efficiency of particles, the inferior depth resolution or the aberrations and distortions in the reconstruction. Furthermore, efficient strategies are introduced to interrogate the holographic storage and process the huge amount of data towards a final flow field representation. Recently, phase-sensitive metrology, familiar in many fields of experimental mechanics, has been examined for use in particle velocimetry. Suitable methods are holographic and speckle interferometry or the optical processing of data for three-dimensional correlation. While in these techniques the power of optics is unrivalled, the practical advantage of video and digital techniques over photographic recording is obvious. The electronic version of speckle interferometry (ESPI/DSPI) is a well-established method used in laser metrology and has received further exploitation for applications in flow analysis recently. Finally, the state-of-the-art of digital particle holography is reviewed to allow estimates of its future in experimental flow analysis.
183 citations
TL;DR: A new high-speed method for implementing angular compounding by path length encoding (ACPE) for reducing speckle in OCT images is described, by averaging images obtained at different incident angles, with each image encoded by pathlength.
Abstract: Angular compounding by path length encoding (ACPE), a new method for reducing speckle in OCT images is presented. ACPE images demonstrate a qualitative improvement over traditional OCT and an increased SNR.
110 citations
TL;DR: In this article, the authors proposed synchronous interferometric speckle suppression to improve the performance of ground-based telescopes with adaptive optics systems for direct imaging of faint companions.
Abstract: The performance of high-contrast imaging systems is very often limited by the presence of speckles in the point-spread function (PSF) of the central source. Since this unwanted light is coherent with the central source, it is possible to make it interfere with light extracted from the center of the PSF. The light of a faint companion, however, will not interfere with the central source. By taking advantage of this fundamental difference, it is possible to detect faint companions superposed on speckles more than 100 times brighter, even if the speckles are rapidly "boiling." Possible optical designs to use this technique on interferometers or imaging telescopes are shown, and a data analysis algorithm is proposed. Synchronous interferometric speckle suppression greatly improves the performance of ground-based telescopes with adaptive optics systems for direct imaging of faint companions. On a space telescope, where the speckle lifetime is expected to be longer, companions 104 times fainter than the speckle halo can be detected in a few minutes.
69 citations
TL;DR: An optical setup that can be switched to produce in-plane and out-of-plane sensitivity interferometers was designed for three-dimensional deformation measuring by electronic speckle pattern interferometry, and results with an elastic target that is exposed to a mechanical load are reported.
Abstract: An optical setup that can be switched to produce in-plane and out-of-plane sensitivity interferometers was designed for three-dimensional deformation measuring by electronic speckle pattern interferometry. Divergent illumination is considered in the evaluation of sensitivity vectors to measure both in-plane and out-of-plane displacement components. The combination of these interferometers presents the advantage of greater sensitivity in directions u, v, and w than a typical interferometer with three illumination beams provides. The system and its basic operation are described, and results with an elastic target that is exposed to a mechanical load are reported.
54 citations
TL;DR: The results of 3047 speckle interferometric observations of double stars, made with the 26 inch (66 cm) refractor of the US Naval Observatory, are presented in this article.
Abstract: The results of 3047 speckle interferometric observations of double stars, made with the 26 inch (66 cm) refractor of the US Naval Observatory, are presented. Each speckle interferometric observation of a system represents a combination of over a thousand short-exposure images. These observations are averaged into 1572 mean relative positions and range in separation from 020 to 6286, with a median separation of 419. This is the 10th in a series of papers presenting measures obtained with this system and covers the period 2003 January 13 through 2003 December 1. Included in these data are nine older measures whose positions were previously deemed possibly aberrant but are no longer classified this way following a confirming observation. Four of these systems have new orbital elements, which are presented here as well.
53 citations
TL;DR: A mathematical model that relates the first- and second-order statistical properties of the intrinsic speckle noise to relevant holographic system parameters is constructed and shows that the noise-limited information capacity can be expressed as the product of particle number density and the extent of the particle field along the optical axis.
Abstract: In holographic imaging of particle fields, the interference among coherent wave fronts associated with particle scattering gives rise to intrinsic speckle noise, which sets a fundamental limit on the amount of information that particle holography can deliver. It has been established that the intrinsic speckle noise is especially severe in in-line holography because of superposition of virtual image waves, the direct transmitted wave, and the real image. However, at sufficiently high particle number densities, such as those typical in holographic particle image velocimetry (HPIV) applications, intrinsic speckle noise also arises in off-axis particle holography from self-interference among wave fronts that form the real image of particles. To overcome the latter problem we have constructed a mathematical model that relates the first- and second-order statistical properties of the intrinsic speckle noise to relevant holographic system parameters. Consistent with our experimental data, the model provides a direct estimate of the information capacity of particle holography. We show that the noise-limited information capacity can be expressed as the product of particle number density and the extent of the particle field along the optical axis. A large angular aperture of the hologram contributes directly to achievement of high information capacity. We also show that filtering in either digital or optical form is generally ineffective in removing the intrinsic speckle noise from the particle image as a result of the similar spectral properties of the two. These findings emphasize the importance of angular aperture in designing holographic particle imaging systems.
47 citations
TL;DR: The results of 3056 speckle interferometric observations of double stars, made with the 26 inch (66 cm) refractor of the US Naval Observatory, are presented in this article.
Abstract: The results of 3056 speckle interferometric observations of double stars, made with the 26 inch (66 cm) refractor of the US Naval Observatory, are presented. Each speckle interferometric observation of a system represents a combination of over a thousand short-exposure images. These observations are averaged into 1675 mean relative positions and range in separation from 019 to 4521, with a median separation of 299. This is the ninth in a series of papers presenting measures obtained with this system and covers the period 2002 January 1 through 2002 December 29. Included in these data are 28 older measures whose positions were previously deemed possibly aberrant but are no longer classified this way following a confirming observation. Nine of these systems have new orbital elements, which are presented here as well.
46 citations
TL;DR: In this article, an in-plane displacement is measured by the optical measurement arrangement using the two collimated beams using the spatial fringe analysis method under an optimal measurement condition, and the optimum condition is discussed as the measurement parameters concerning the speckle's size and the passband of the bandpass filter.
Abstract: Speckle interferometry is used to measure the deformation of an object with a rough surface. In particular, the precise measurement can be easily performed with electronic-speckle-pattern interferometry (ESPI) using fringe scanning technology. Then, the measurement accuracy is influenced by the ratio between the speckle size and the pixel size of a CCD. Sometimes, this causes a problem concerning optical dislocations. An in-plane displacement is measured by the optical measurement arrangement using the two collimated beams. The measurement is performed by ESPI technology with the spatial fringe analysis method under an optimal measurement condition. Then, the optimum condition is discussed as the measurement parameters concerning the speckle's size and the passband of the bandpass filter. In the experimental measurement of in-plane displacement, it is shown that the optimal measurement condition can evade the problem of optical dislocations. At the same time, it is confirmed that a precise measurement can be performed.
43 citations
TL;DR: In this article, the temporal intensity variation of each pixel on the recorded images is analyzed by a robust mathematical tool, which is called Morlet wavelet transform instead of conventional Fourier transform.
Abstract: When a continuously deforming object is measured by electronic speckle pattern interferometry (ESPI), the speckle pattern recorded on a camera sensor changes constantly. These time-dependent speckle patterns would provide the deformation history of the object. Various objects are applied with both linearly and nonlinearly varying loads and speckle patterns are captured using a high-speed CCD camera. The temporal intensity variation of each pixel on the recorded images is analyzed by a robust mathematical tool—Morlet wavelet transform instead of conventional Fourier transform. The transient velocity and displacement of each point can be retrieved without the necessity of the temporal or spatial phase unwrapping process. The displacements obtained are compared with those from a temporal Fourier transform, and the results show that the wavelet transform minimizes the influence of noise and provides better results for a linearly varying load. System error in the wavelet analysis for nonlinear load is also discussed.
42 citations
22 Jun 2004
TL;DR: In this article, a new numerical simulation of speckle behavior for prediction of noise level in a real measurement is presented, which provides real insight into the phase and amplitude modulation of the Doppler signal as a result of specckle changes.
Abstract: This paper begins with a review of the fundamental mechanism by which speckle noise is generated in Laser Vibrometry before describing a new numerical simulation of speckle behavior for prediction of noise level in a real measurement. The simulation data provides real insight into the phase and amplitude modulation of the Doppler signal as a result of speckle changes. The paper also includes experimental data looking at the influence of speckle noise in measurements on rotors with a selection of surface treatments and in scanning and tracking configurations.
39 citations
TL;DR: In this paper, a new filtering scheme based on combination of average/median filtering and Symlet wavelet filtering was proposed to enhance the signal-to-noise ratio (SNR) in the speckle interferogram obtained from DSPI.
TL;DR: In this article, the in-plane displacement field generated by the introduction of a small hole is determined using an automated data analysis approach based on the calculation of the optical phase distribution through a phase-shifting method and the application of a robust iterative phase unwrapping algorithm.
TL;DR: In this article, the authors describe the use of shearography for surface slope and shape measurement, which is a full field non-contact optical technique usually used to measure the gradient of the displacement of a surface subjected to thermal or mechanical loading.
TL;DR: In this paper, a combined system based on digital speckle pattern interferometry and laser annealing is proposed to determine residual stresses in ductile materials, which allows the measurement of the radial in-plane displacement field generated when a specimen subjected to residual stresses is locally heated.
TL;DR: In this article, a hybrid procedure for the in-plane mechanical characterization of orthotropic materials is presented, where the material identification reverse engineering problem is solved by combining speckle interferometry and numerical optimization.
Abstract: In this paper we present a novel hybrid procedure for the in-plane mechanical characterization of orthotropic materials. The material identification reverse engineering problem is solved by combining speckle interferometry and numerical optimization. The rationale behind the entire process is the following: for any specimen to be characterized and which has been subjected to some loading condition, it is possible to express the difference between experimental data and analytical/numerical predictions by means of an error function ψ, which depends on the elastic constants of the material. The ψ error will decrease as the elastic constants come close to their target values. Here, we build the ψ function as the difference between the displacement field measured with speckle interferometry and its counterpart computed by means of finite element analysis. Since the ψ function is highly non-linear, it has to be optimized with a global optimization algorithm, which perform a random search in the elastic constants design space. The hybrid material identification process finally allows us to determine values of the elastic constants. In order to prove the feasibility of the present approach, we have determined the in-plane elastic properties of an eight-ply composite laminate (woven fiberglass-epoxy) used as a substrate for printed circuit boards. The results indicate that the procedure proposed in this paper was able to accurately characterize the material under investigation. Remarkably, the elastic constants found by the identification procedure were less than 0.7% different from their target values, while the residual error between the displacements measured by speckle interferometry and those computed at the end of the optimization process was less than 3%.
TL;DR: The author proposes a novel method, the contoured correlation-fringe-pattern (CCFP) method, by which speckle-noise-free fringe patterns can be generated for ESPI, and eliminates the two main disadvantages of the phase-shifting (stepping) methods of ESPI.
Abstract: Speckle fringe patterns of electronic speckle pattern interferometry (ESPI) are full of high-level speckle noise and are mainly processed by phase-shifting methods that normally require three speckle fringe patterns or more. The author proposes a novel method, the contoured correlation-fringe-pattern (CCFP) method, by which speckle-noise-free fringe patterns can be generated for ESPI. The application of this novel method is extended to the phase-shifting or phase-stepping for ESPI after its improvement. It generates speckle-noise-free phase fringes and remains valid for single phase-step condition, thus eliminates the two main disadvantages of the phase-shifting (stepping) methods of ESPI.
TL;DR: A phase-shifting electronic speckle pattern shearing interferometer with a very simple shearing device is proposed, which is simple, flexible, and low cost.
Abstract: A phase-shifting electronic speckle pattern shearing interferometer with a very simple shearing device is proposed. Two partially reflective glass plates are used to introduce the shear in this new interferometer. The reflection coefficients of the coatings on the two plates are 0.3 and 0.7. The distance between the two glass plates controls the size of the shear. The proposed new interferometric system is simple, flexible, and low cost.
TL;DR: In this article, the displacement uncertainty depends strongly on the incidence angles of the illuminating beams of the interferometer, and faults in the alignment of the incident beams have more influence on the uncertainty than errors in their collimation.
TL;DR: In this article, the performance of ground-based single-mode and multimode interferometers in the presence of partial adaptive optics correction of atmospheric turbulence is compared, and it is shown that the fiber estimator is much more robust than the speckle estimator with respect to Strehl ratio variations during the calibration procedure.
Abstract: We compare performances of ground-based single-mode and multimode (speckle) interferometers in the presence of partial Adaptive Optics correction of atmospheric turbulence. It is first shown that for compact sources (i.e. sources smaller than the Airy disk of a single telescope) not entirely resolved by the interferometer, the remarkable property of spatial filtering of single-mode waveguides coupled with AO correction significantly reduces the speckle noise which arises from residual wavefront corrugations. Focusing on those sources, and in the light of the AMBER experiment (the near infrared instrument of the VLTI), we show that single-mode interferometry produces a better Signal-to-Noise Ratio on the visibility than speckle interferometry. This is true for bright sources (K < 5), and in any case as soon as Strehl ratio of 0.2 is achieved. Finally, the fiber estimator is much more robust - by two orders of magnitude - than the speckle estimator with respect to Strehl ratio variations during the calibration procedure. The present analysis theoretically explains why interferometry with fibers can produce visibility measurements with a very high precision, 1% or less.
TL;DR: In this paper, a stable and compact speckle interferometer for doing out-of-plane displacement measurements on reflective as well as diffusely scattering object surfaces is demonstrated, which is based on a nearly path length compensated interferometers of the Fizeau type and uses diffuse illumination of the object combined with a speckled reference wave.
TL;DR: An extended theory for speckle fringe formation on curved surfaces is proposed, which can be applied to the study of curved engineering and technical specimens under various loading conditions such as in-plane, out-of- plane, and out- of-plane shear configurations.
Abstract: Deformation study of curved engineering and technical surfaces, such as pipes and pressure vessels, has gained much importance in the recent past. Speckle interferometric techniques and their electronic and digital analogs, which are whole field techniques, have been effectively applied for practical nondestructive testing applications over the years. However, little work has been done that discusses the speckle fringe formation with a fruitful theoretical formulation to study deformation analysis of curved surfaces. We propose an extended theory for speckle fringe formation on curved surfaces, which can be applied to the study of curved engineering and technical specimens under various loading conditions such as in-plane, out-of-plane, and out-of-plane shear configurations. Simulated contours are generated by use of finite element models with similar loading conditions, and the data are analyzed and compared with the obtained experimental results.
TL;DR: A good agreement between theoretically predicted and experimental correlation data can be observed and the results allow a quantitative whole-field monitoring of surface processes by remote optical means.
Abstract: We present a detailed investigation of digital speckle correlation to measure small changes in the microstructure of random rough surfaces. The corresponding alterations in the scattered-light field are recorded by an electronic camera with subsequent numerical correlation. Among the classical theoretical approaches to describe the scattering at random rough surfaces, the composite-roughness model is advanced to calculate the speckle correlation in terms of parameters of the changes in surface microstructure. For an experimental verification, surfaces with similar microstructure are fabricated with a photolithographic technique. They are employed for comparative measurements with high-resolution scanning force microscopy and for correlation measurements under variation of experimental parameters. A good agreement between theoretically predicted and experimental correlation data can be observed. The results allow a quantitative whole-field monitoring of surface processes by remote optical means.
TL;DR: A novel method to generate aspeckle-noise-free fringe pattern from a single speckle fringe pattern and to extract the phase field from the new pattern and the resultant fringe pattern is normalized automatically so that the full phase field can be extracted from this single fringe pattern by the single-image phase-shifting method.
Abstract: Speckle fringe patterns of ESPI are full of high-level speckle noise and normally are processed by phase shifting methods that require multi speckle fringe patterns. We propose a novel method to generate a speckle-noise-free fringe pattern from a single speckle fringe pattern and to extract the phase field from the new pattern. With the new method, the correlation between two speckle patterns is performed only within contour windows instead of rectangular windows and this contoured correlation results in a smooth, normalized fringe pattern without speckle noise. The new ESPI fringe patterns are speckle-noise-free and of comparable quality to that of moire and hologram, which is unimaginable with traditional ESPI methods. In addition to the smoothness, the resultant fringe pattern is normalized automatically so that the full phase field can be extracted from this single fringe pattern by the single-image phase-shifting method.
TL;DR: A photopolymer holographic grating is used to produce the two sheared images in an electronic speckle pattern shearing interferometer that eliminates unwanted diffraction orders and removes the requirement for the CCD camera to resolve the diffraction grating's pitch.
Abstract: A photopolymer holographic grating is used to produce the two sheared images in an electronic speckle pattern shearing interferometer. A ground glass screen following the grating eliminates unwanted diffraction orders and removes the requirement for the CCD camera to resolve the diffraction grating's pitch. The sheared images on the ground glass are further imaged onto the CCD camera. The fringe pattern contrast was estimated to be above 90%. A validation of the system was done by comparing the theoretical phase difference distribution with the experimental data from the three-point bending test.
TL;DR: It is shown that anomalously bright pinned speckles are not included in the traditional expression for speckle power in an image, (1 - S), on which sensitivity estimates of future high-performance AO systems have been based.
Abstract: Ground-based optical searches for faint stellar or planetary companions about other stars may be limited by speckle noise, which is the rapid intensity fluctuations that are due to motions of remnant atmospheric speckles. Adaptive optics (AO) can reduce residual wave-front phase errors to low values, substantially reducing the unwanted power in the speckle halo. At high correction, however, the noise in the halo will be dominated by anomalously bright “pinned” speckles that have a number of unusual properties. They can have negative intensities and will appear in spatially antisymmetric patterns; they are spatially pinned to Airy rings and have zero mean in a sufficiently long integration. Some of these properties may be used to reduce the unanticipated effect of pinned speckles on companion searches, depending on details of the AO system. But, in short exposures, pinned speckles dominate speckle noise over much of the inner halo for Strehl ratios S as low as 0.6 and over much of the outer halo too as Strehl and deformable-mirror actuator densities increase. I show that these anomalously bright pinned speckles are not included in the traditional expression for speckle power in an image, 1-S, on which sensitivity estimates of future high-performance AO systems have been based.
TL;DR: A two-wavelength method for endoscopic topography reconstruction is introduced that can be applied to out-of-plane sensitive electronic-speckle-pattern interferometry systems based on rigid endoscope imaging systems and the applicability is demonstrated with results from the topographic reconstruction of a free-form surface.
Abstract: A two-wavelength method for endoscopic topography reconstruction is introduced that can be applied to out-of-plane sensitive electronic-speckle-pattern interferometry systems based on rigid endoscope imaging systems. The surface measurement is performed by detection of the phase-difference distribution affected by a change in the applied laser wavelength. Furthermore, the off-axis endoscopic illumination geometry is taken into account by an approximation. Experimental results of the characterization of the endoscopic surface reconstruction technique and the measurement accuracy obtained are described and discussed. Finally, the applicability of the method is demonstrated with results from the topographic reconstruction of a free-form surface.
01 Mar 2004
TL;DR: In this paper, the principles of and new developments in three-dimensional electronic speckle pattern interferometry (3D-ESPI) for strain/stress analysis are presented.
Abstract: The principles of and new developments in three-dimensional electronic speckle pattern interferometry (3D-ESPI) for strain/stress analysis are presented in this paper. As ESPI is a relative measuring technique, a major problem in 3D analysis of ESPI measurement has been the determination of the absolute phase. In this paper, we describe a method for determining the absolute phase without additional measurements or assumptions and making accurate 3D measurements of displacement and strains. With the use of four illumination directions we generate an overdetermined equation system, which enables us to calculate the absolute phase in addition to the required 3D displacement vectors. This principle has been implemented in a compact 3D strain measurement device, which can also be used under harsh environmental conditions.
TL;DR: A real-time system for analysing data from Speckle interferometers, and speckle shearing interferometer, has been developed and has been applied to the problem of detecting sub-surface delamination cracks in carbon fibre composite panels.
TL;DR: In this article, a simple experiment for detecting small deformations by speckle interferometry is discussed, which uses a laser diode for the source together with a lens to expand the laser beam and a beam splitter dividing the beam and illuminating two scattering surfaces.
Abstract: We discuss a simple experiment for detecting small deformations by speckle interferometry. The optical setup uses a laser diode for the source together with a lens to expand the laser beam and a beam splitter dividing the beam and illuminating two scattering surfaces. A low-cost, commercial charge coupled device (CCD) photocamera provides images of the two superimposed speckle fields through the beam splitter. We first take a picture with the system at rest, and then take a second one after a deformation is made in a surface. By simple subtraction of the digital pictures, we obtain a fringe pattern that gives us information about the deformation.
TL;DR: In this article, an integrated deformation and load measurement system was developed, which consists of an electronic speckle interferometry unit, a uniaxial tensile unit and a spring beam deflection unit.
Abstract: It is very difficult to accurately measure the mechanical properties of low-dimensional film whose thickness is at the level of a micrometre or below. In this paper, we present an experimental method to measure the mechanical properties of thin films. An integrated deformation and load measurement system is also developed, which consists of an electronic speckle interferometry unit, a uniaxial tensile unit and a spring beam deflection unit. The electronic speckle interferometry unit is a double-field-of-view electronic speckle pattern interferometry system, one sensitive to the in-plane displacement to measure the film deformations and the other to the out-of-plane displacement and to measure the spring beam deflections (converted to microloads). The new system allows in situ and real-time measurements of the deformations of the thin films and microforces under uniaxial tensile testing. The tested thin films can be metals, oxide ceramics, and multi-layer composites of thickness from several tens of micrometres to less than a micrometre, and the tensile load from 88 µN to 15 N. We present the underlying principle of the method and the experimental set-up. Experiments were carried out by testing the deformations of pure Ni, Au, and Au/Cr multi-layer films. The measured performance of the method and testing system is also discussed.