共查询到20条相似文献,搜索用时 109 毫秒
1.
Resink S Jose J Willemink RG Slump CH Steenbergen W van Leeuwen TG Manohar S 《Optics letters》2011,36(15):2809-2811
Recently, we presented a method using laser-induced ultrasound from an external absorber (passive element) to image the ultrasound transmission parameters of an object under photoacoustic tomographic investigation. The method suffers from long measurement times due to the requirement for a large number of views and consequently physical projections around the object. Here we propose and validate an approach that permits a multitude of views to be obtained within a limited projection scenario. The approach uses a plurality of spatially distributed external absorbers in the path of the light, that results in multiple laser-induced ultrasound sources to interrogate the object from a number of angles. This reduces the required number of rotation angles or physical projections around the object, permitting a considerable reduction in imaging time without significant degradation in image quality. The approach brings the concept of hybrid imaging of ultrasound transmission parameters together with photoacoustic imaging, into the realm of practical application. 相似文献
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Philipp Berssenbrügge Markus Dekiff Björn Kemper Dieter Dirksen 《Optics and Lasers in Engineering》2012,50(3):465-472
The increasing importance of optical 3D measurement techniques and the growing number of available methods and systems require a fast and simple method to characterize the measurement accuracy. However, the conventional approach of comparing measured coordinates to known reference coordinates of a test target faces two major challenges: the precise fabrication of the target and - in case of pattern projecting systems - finding the position of the reference points in the obtained point cloud. The modulation transfer function (MTF) on the other hand is an established instrument to describe the resolution characteristics of 2D imaging systems. Here, the MTF concept is applied to two different topometric systems based on fringe and speckle pattern projection to obtain a 3D transfer function. We demonstrate that in the present case fringe projection provides typically 3.5 times the 3D resolution achieved with speckle pattern projection. By combining measurements of the 3D transfer function with 2D MTF measurements the dependency of 2D and 3D resolutions are characterized. We show that the method allows for a simple comparison of the 3D resolution of two 3D sensors using a low cost test target, which is easy to manufacture. 相似文献
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Realization of Absolute‐Phase Unwrapping and Speckle Suppression in Laser Digital Holography 下载免费PDF全文
Mingqing Wang Fang Li Ming Zheng Wei Lu Yan Jia Qingnan Yu Sufen Zhao Jian Wu 《Annalen der Physik》2017,529(8)
In this paper, an absolute‐phase unwrapping and speckle suppression approach to reconstruct a three‐dimensional (3‐D) image of an object with laser digital holography is described. This method offers three advantages to enhance the performance of the phase reconstruction technique. First, both speckle suppression and phase unwrapping are processed in the complex amplitude domain rather than in the single phase or amplitude domain. With this approach, the phase details of the object are better preserved upon phase reconstruction. Second, the proposed algorithm requires no threshold determination and thus achieves self‐adaptive speckle suppression and robust phase unwrapping, in contrast to other methods. Finally, an improved dual‐domain image denoising method is applied to further remove speckle‐remnant‐induced phase distortion. Ideal 3‐D phase reconstruction results are obtained both theoretically and experimentally for the first time. 相似文献
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If a laser beam illuminates a continual deformation object surface, it will lead to a temporal speckle pattern on the observation plane. Recording this time-dependent speckle pattern the deformation of the surface of an object can be obtained. Two methods, scanning phase method (SPM) and time sequence phase method (TSPM), have been introduced for measuring the displacement caused by the deformation in temporal speckle pattern interferometry (TSPI). Their principle is that by capturing a series of speckle interference patterns related to the object deformations, the fluctuations in the intensity of the interference patterns can be obtained. Through scanning these fluctuations and estimating both the average intensity and modulation of the temporal speckle interference patterns, the phase maps for whole-field displacements are calculated. In this way one is capable of quantitatively measuring continual displacements simply using a conventional electronic speckle pattern interferometry (ESPI) system without phase shifting or a carrier. The elaboration on the new methods is given in this paper and experiments are performed to demonstrate their performance with a conventional ESPI system. 相似文献
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The bispectrum, which is an ensemble average of a product of three spectral components, is shown to be a very useful diagnostic tool in experimental studies of nonlinear wave interactions in random media. In particular, it is shown that the bicoherence spectrum may be used to discriminate between nonlinearly coupled waves and spontaneously excited waves and to measure the fraction of wave power due to the quadratic wave coupling in a self-excited fluctuation spectrum. Practical aspects of digital bispectral analysis techniques, such as estimation and statistical variability of the estimator, are also discussed. Finally, applications of bispectral analysis techniques in the analysis and interpretation of plasma fluctuation data are described. 相似文献
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Computer-aided tomography is a technique for providing a two-dimensional cross-sectional view of a three-dimensional object through the digital processing of many one-dimensional views (or projections) taken at different look directions. In acoustic reflection tomography, insonifying the object and then recording the backscattered signal provides the projection information for a given look direction (or aspect angle). Processing the projection information for all possible aspect angles enables an image to be reconstructed that represents the two-dimensional spatial distribution of the object's acoustic reflectivity function when projected on the imaging plane. The shape of an idealized object, which is an elliptical cylinder, is reconstructed by applying standard backprojection, Radon transform inversion (using both convolution and filtered backprojections), and direct Fourier inversion to simulated projection data. The relative merits of the various reconstruction algorithms are assessed and the resulting shape estimates compared. For bandpass sonar data, however, the wave number components of the acoustic reflectivity function that are outside the passband are absent. This leads to the consideration of image reconstruction for bandpass data. Tomographic image reconstruction is applied to real data collected with an ultra-wideband sonar transducer to form high-resolution acoustic images of various underwater objects when the sonar and object are widely separated. 相似文献
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In this paper, we investigate the diffraction tomography for quantitative imaging damages of partly through-thickness holes with various shapes in isotropic plates by using converted and non-converted scattered Lamb waves generated numerically. Finite element simulations are carried out to provide the scattered wave data. The validity of the finite element model is confirmed by the comparison of scattering directivity pattern(SDP) of circle blind hole damage between the finite element simulations and the analytical results. The imaging method is based on a theoretical relation between the one-dimensional(1D) Fourier transform of the scattered projection and two-dimensional(2D) spatial Fourier transform of the scattering object. A quantitative image of the damage is obtained by carrying out the 2D inverse Fourier transform of the scattering object. The proposed approach employs a circle transducer network containing forward and backward projections, which lead to so-called transmission mode(TMDT) and reflection mode diffraction tomography(RMDT),respectively. The reconstructed results of the two projections for a non-converted S0 scattered mode are investigated to illuminate the influence of the scattering field data. The results show that Lamb wave diffraction tomography using the combination of TMDT and RMDT improves the imaging effect compared with by using only the TMDT or RMDT. The scattered data of the converted A0 mode are also used to assess the performance of the diffraction tomography method. It is found that the circle and elliptical shaped damages can still be reasonably identified from the reconstructed images while the reconstructed results of other complex shaped damages like crisscross rectangles and racecourse are relatively poor. 相似文献
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A. A. Zelensky V. F. Kravchenko V. V. Pavlikov V. I. Pustovoit A. V. Totsky 《Doklady Physics》2014,59(7):304-308
The methods of smoothing of the bispectral density estimate when solving the problems of restoration of signals with an unknown shape in the interference environment and random signal delay are considered for the first time. The performed analysis of statistical characteristics of noise that presents in the bispectrum estimate shows that these statistical characteristics have a rather complex unsteady behavior. An ambiguous selection of a filter optimal by the criterion of the minimum of the root-mean-square error and the minimum of dynamic distortions introduced by the filter seems to be problematic because of the unsteady behavior of counts of the bispectral density estimate and the absence of a priori data on the parameters of the restored signal. Therefore, statistic investigations were performed with the use of linear and nonlinear digital filters with variations of the sliding window sizes. It is shown that the advantages of the proposed approach most pronouncedly manifest themselves with the use of the nonlinear digital filtration and small signal/noise ratios at the input and/or with a small sampling volume of observed implementations. The Kravchenko weight functions are proposed to smooth the bispectrum of the multifrequency signal with a large dynamic range of variations in amplitudes of spectral components. The presented results are of practical interest for use in applications such as radiolocation, hydrolocation, and digital communication. 相似文献
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K. D. Kihm 《Optics and Lasers in Engineering》1998,29(2-3)
Laser speckle photography measures projected beam deflection of a phase object. The beam deflection measured at the solid surface will directly determine the wall-temperature gradient. Thus, the laser-speckle photography allows the evaluation of the convection-heat-transfer coefficients without the need to correct for conduction and radiation heat losses, which otherwise would be necessary. Selected results and brief discussions are presented for several natural convection-heat-transfer problems. Second, high-temperature applications of laser speckle photography shows potential as a nonintrusive means of measuring flame temperature. A fairly good agreement is shown in comparison of the laser speckle data with intrusive thermocouple data. Suggestive discussions are made to further improve the accuracy and sensitivity of the laser speckle photography technique in high-temperature applications. Finally, tomographic reconstruction of laser speckle photography data is discussed using both actual test fields and computer-synthesized phantom fields. The conventional algebraic reconstruction technique (ART) has been modified to apply for the laser speckle tomography. The efficacy of ART is examined particularly under a limited number of projections. 相似文献
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Synthetic aperture imaging ladar (SAIL) technique belongs fully coherent processing in both the time domain and space domain and has a rather high implement difficulty. To solve this problem, the concept of circular incoherently SAIL is introduced. A speckle version image of a two-dimensional (2D) letter ’E’ target is reconstructed from E-field projection data detected by a circular incoherently SAIL system. The experimental system is constructed by three subsystems using chirped-pulse laser as the light source and heterodyne detection to get the range information of the target. The reconstruction of the image and the noise effect are also discussed in detail. 相似文献
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Differential phase-contrast tomography, also known as beam-deflection tomography, is a method for reconstructing an object's refractive index distribution from knowledge of differential projection data that describe beam deflection angles. We describe and demonstrate an approach to determining regions of interest within an object from truncated differential projection data. 相似文献
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To test a developed reconstruction algorithm for cone beam geometry, whether it is transmission or emission tomography, one
needs projection data. Generally, mathematical phantoms are generated in three dimensions and the projection for all rotation
angles is calculated. For non-symmetric objects, the process is cumbersome and computation intensive. This paper describes
a simple methodology for the generation of projection data for cone beam geometry for both transmission and emission tomographies
by knowing the object’s attenuation and/or source spatial distribution details as input. The object details such as internal
geometrical distribution are nowhere involved in the projection data calculation. This simple approach uses the pixilated
object matrix values in terms of the matrix indices and spatial geometrical coordinates. The projection data of some typical
phantoms (generated using this approach) are reconstructed using standard FDK algorithm and Novikov’s inversion formula. Correlation
between the original and reconstructed images has been calculated to compare the image quality. 相似文献
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Spiral interferometry can be used as a solution to the problem of sign ambiguity presented in the conventional speckle pattern interferometric technique when the optical phase needs to be reconstructed from a single closed fringe system. Depressions and elevations of the topography corresponding to the object deformation are distinguished by the direction of rotation of the local spiral fringe pattern. In this work, we implement and compare several methods for optical phase reconstruction by analyzing a single image composed of spiral speckle pattern interferometry correlation fringes. The implemented methods are based on contour line demodulation, center line demodulation, Spiral Phase Quadrature Transform and the 2D Riesz transform with multivector structure. Contour line and center line demodulation approaches are exclusively dedicated to images containing a fringe system with spiral structure. The others are based on the 2D Riesz transform, these being well known approaches in conventional interferometry. We examine simulated experiments and analyze some of the emerging drawbacks for solving the phase reconstruction problem by using different mean values of speckle size and background noise levels. We also discuss several numerical procedures that may well improve the efficiency and robustness of the presented numerical implementations. The performance of the implemented demodulation methods is evaluated by using a universal image quality index and therefore a quantitative comparison is also presented. 相似文献
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This work derives the expressions for the bispectrum and bicoherence functions for multi-degree-of-freedom spring-mass systems with quadratic nonlinearities subject to inputs described by a wide class of random processes. The derivation uses a Volterra series model for the system response and yields expressions for both “homogeneous spectra”, where the output of only one degree of freedom is used, and “mixed spectra” where the bispectral density contains multiple response time series. This expression is then used to determine the optimal probability distribution of the input and the optimal bispectrum to compute for the goal of maximizing the probability of detecting the nonlinearity. 相似文献
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散斑抑制一直是激光显示技术的研究热点。对激光光束经SiO2溶液所形成散斑的特性分析,建立了多重散射与散斑颗粒大小的联系。结合动态光散射理论,提出利用斜入射引入动态多重散射机制的激光散斑抑制方法,并构建由静态散射片和装有粒径为300 nm、摩尔浓度为3.0×10-4 μm-3的SiO2悬浮液的光通管组成的散斑抑制装置,将其置入激光显示系统的光源部分。针对光束以不同入射角进入SiO2悬浮液对散斑图像对比度影响进行了系统实验分析。结果表明,光束以约8°进入SiO2悬浮液能将散斑图像的对比度从0.43降低至0.067。通过该方法实现了散斑颗粒的空间平均和散斑图像的时间平均,提高了散斑抑制的效果。散斑抑制单元无需振动装置且便于集成到激光投影系统中,不仅提高了系统的可靠性也减小了成本。 相似文献