Experimental study of the angular correlation function in the double passage of waves through a one-dimensional random phase screen

Abstract

Experimental results are presented for the angular correlation function of far-field speckle patterns scattered in the double passage of waves through a one-dimensional random phase screen. The experiment for the correlation measurement was set up to use a CCD camera to obtain the image of the speckle patterns in the scattering directions for each given angle of incidence; the cross-correlation function is then calculated from the digitized images. The theoretical analysis of the motion of the speckle as the source is moved, as given by Escamilla, is verified experimentally. It is found that in contrast with the memory effect line of speckle motion, the speckle pattern produced in the region of observation tracks the backscattering direction.     

Contrast of time-averaged image speckle pattern for a vibrating object

In vibration analysis by speckle photography it is qualitatively known that the contrast of speckle patterns averaged in the exposure time varies, depending on the object motion. In this paper, the contrast of time-averaged image speckle patterns is theoretically evaluated and found to be determined by the ratio of the sinusoidally oscillating amplitude of the object motion to the average size of static speckles. An effect of the film response on the contrast of time-averaged speckle patterns is also studied.     

Evaluation of the quality of a speckle pattern in the digital image correlation method by mean subset fluctuation

The quality of the speckle pattern used in digital image correlation is studied in this paper using a parameter called mean subset fluctuation. A numerical translation test is performed on four speckle patterns captured from actual experiments. The translated images are analyzed and the results show that the mean bias error of the calculated displacement is linear with the value of the mean subset fluctuation. The results from the numerical calculation illustrate that speckle size and density have an influence on the quality of speckle pattern.     

Use of a joint transform correlator architecture for study of speckle displacements

The optical speckle–displacement correlation technique (OSDCT) is proposed for study of in-plane speckle displacements. The joint transform correlator architecture is used to evaluate the displacements of speckle subimages of strained surfaces. The procedure of the correlation response producing as a result of cross-correlation of strainless and strained surfaces in a conventional joint transform correlator (JTC) is considered. The convenient techniques for a joint power spectrum transformation are selected. The systematic and random errors of a mean speckle pattern displacement of a rigid body motion calculated by using the OSDCT and the digital speckle–displacement measurement technique are compared. The robustness of some JTC versions (a conventional JTC, a JTC with median thresholding, a JTC with subset median thresholding, and a fringe adjusted filter JTC) to input noise of speckle patterns and output noise of a correlation response is studied.     

空间散斑的运动规律(续)——物表面位移微分量的影响

本文讨论了由物表面变形所引起的空间散斑的运动。从Fresnel-Kirchhoff积分出发,导出了受物表面变形影响的散斑运动必须遵从的基本方程组,并对它们的物理意义进行了讨论。把物表面刚体运动和变形影响下的基本方程组综合起来,导出了物表面发生一般运动和变形时,空间散斑运动的公式。实验结果与公式一致。 关键词：     

一种抑制合成孔径雷达图像相干斑的各向异性扩散滤波方法

本文提出了一种抑制合成孔径雷达图像乘性相干斑噪声的各向异性扩散滤波新方法.该方法将受自适应耦合函数控制的平均曲率运动嵌入到传统相干斑抑制的各向异性扩散方程中,形成了一种可有效抑制边缘区域相干斑与同质区块效应现象的各向异性扩散新方程,同时在新建的扩散方程中,引入了由改进Frost滤波与局部方向比率联合构建的一种带方向约束的新扩散函数,进一步削弱了块效应现象且明显改善了抑斑图像的边缘抖动扭曲问题.实验结果表明该方法在有效保护图像边缘的同时,能充分平滑同质区与边缘区域的相干斑,明显削弱块效应现象,有效改善抑斑图像边缘抖动扭曲问题,而抑斑图像无论视觉效果还是参数指标均比多种传统抑斑方法更具优势.     

Functional imaging in photorefractive tissue speckle holography

Cellular motion produces highly dynamic speckle in wide-field depth-gated holographic optical coherence imaging. The speckle provides cell scale information such as cell membrane and organelle motion, even though the spatial resolution of the optics is above the cell scale. The statistical properties of dynamic speckle can be used for functional imaging of motile activity at depth within tissue. In this paper we develop a motility metric using time autocorrelation to create a three-dimensional motility map of tissue. The effect of noise and the shimmering showerglass effect in functional optical coherence imaging are evaluated quantitatively, and we demonstrate that meaningful motility information can be extracted from functional imaging using dynamic speckle.     

Digital ultrasonic speckle phase-shifting method

The digital ultrasonic speckle phase-shifting method (USPM), which is introduced in this paper, can be applied to the measurement of small displacement that is smaller than speckle size at the test point compared to traditional ultrasonic speckle correlation method (USCM). Using USPM, a digital ultrasonic reference signal is introduced to interfere with the ultrasonic speckle signal, which is picked up at the test point on an object surface and is referred to as the object signal. As the phase of the reference signal is shifted several times using the software and then they superimpose with the object signal respectively, the phase of the object signal can be calculated according to the intensities of the superimposed signals. If the object surface moves a small distance, the phase variation of the object speckle can be detected by the same process. As a result, the displacement of the object surface can be measured. Based on the feature of ultrasonic speckles, inner surface displacement of an object can be measured using this proposed method. In this case, the effect of outer surface roughness to the measurement accuracy of USPM is examined experimentally. The experimental results show that the measurement is successful when the displacement is smaller than half of the speckle size at the test point and the roughness parameter R_a of the outer surface of the specimen is less than about 5.47 μm.     

Analysis of motion tracking in echocardiographic image sequences: Influence of system geometry and point-spread function

This paper focuses on motion tracking in echocardiographic ultrasound images. The difficulty of this task is related to the fact that echographic image formation induces decorrelation between the underlying motion of tissue and the observed speckle motion. Since Meunier’s seminal work, this phenomenon has been investigated in many simulation studies as part of speckle tracking or optical flow-based motion estimation techniques. Most of these studies modeled image formation using a linear convolution approach, where the system point-spread function (PSF) was spatially invariant and the probe geometry was linear. While these assumptions are valid over a small spatial area, they constitute an oversimplification when a complete image is considered. Indeed, echocardiographic acquisition geometry relies on sectorial probes and the system PSF is not perfectly invariant, even if dynamic focusing is performed.This study investigated the influence of sectorial geometry and spatially varying PSF on speckle tracking. This was done by simulating a typical 64 elements, cardiac probe operating at 3.5 MHz frequency, using the simulation software Field II. This simulation first allowed quantification of the decorrelation induced by the system between two images when simple motion such as translation or incompressible deformation was applied. We then quantified the influence of decorrelation on speckle tracking accuracy using a conventional block matching (BM) algorithm and a bilinear deformable block matching (BDBM) algorithm. In echocardiography, motion estimation is usually performed on reconstructed images where the initial sectorial (i.e., polar) data are interpolated on a cartesian grid. We therefore studied the influence of sectorial acquisition geometry, by performing block matching on cartesian and polar data.Simulation results show that decorrelation is spatially variant and depends on the position of the region where motion takes place relative to the probe. Previous studies did not consider translation in their experiments, since their simulation model (spatially invariant PSF and linear probe) yields by definition no decorrelation. On the opposite, our realistic simulation settings (i.e., sectorial probe and realistic beamforming) show that translation yields decorrelation, particularly when translation is large (above 6 mm) and when the moving regions is located close to the probe (distance to probe less than 50 mm).The tracking accuracy study shows that tracking errors are larger for the usual cartesian data, whatever the estimation algorithm, indicating that speckle tracking is more reliable when based on the unconverted polar data: for axial translations in the range 0-10 mm, the maximum error associated to conventional block matching (BM) is 4.2 mm when using cartesian data and 1.8 mm for polar data. The corresponding errors are 1.8 mm (cartesian data) and 0.4 mm (polar data) for an applied deformation in the range 0-10%. We also show that accuracy is improved by using the bilinear deformable block matching (BDBM) algorithm. For translation, the maximum error associated to the bilinear deformable block matching is indeed 3.6 mm (cartesian data) and 1.2 mm (polar data). Regarding deformation, the error is 0.7 mm (cartesian data) and 0.3 mm (polar data). These figures also indicates that the larger improvement brought by the bilinear deformable block matching over standard block matching logically takes place when deformation on cartesian data is considered (the error drops from 1.8 to 0.7 mm is this case).We give a preliminary evaluation of this framework on a cardiac sequence acquired with a Toshiba Powervision 6000 imaging system using a probe operating at 3.25 MHz. As ground truth reference motion is not available in this case, motion estimation performance was evaluated by comparing a reference image (i.e., the first image of the sequence) and the subsequent images after motion compensation has been applied. The comparison was quantified by computing the normalized correlation between the reference and the motion-compensated images. The obtained results are consistent with the simulation data: correlation is smaller for cartesian data, whatever the estimation algorithm. The correlation associated to the conventional block matching (BM) is in the range 0.45-0.02 when using cartesian data and in the range 0.65-0.2 for polar data. The corresponding correlation ranges for the bilinear deformable block matching are 0.98-0.2 and 0.98-0.55. In the same way these figures indicate that the bilinear deformable block matching yield a larger improvement when cartesian data are considered (correlation range increases from 0.45-0.02 to 0.98-0.2 in this case).     

等离子体中散斑光场的传输特性

为了深入了解激光驱动惯性约束核聚变系统中连续位相板所产生的散斑在抑制等离子体非线性效应时起到的作用,采用统计光学理论及矩阵光学方法,分析了散斑在等离子体中的传输特性,并通过数值模拟计算了散斑的自相关函数值.在此基础上,解释了散斑抑制等离子体非线性效应的机制,通过定量计算揭示了散斑光场的自相关长度在传输过程中的变化.结果表明,高密度等离子体中的散斑自相关长度更短,这有助于对等离子体中各类非线性效应的抑制.     

三温区溶质传输助熔剂法生长BaTiO_3晶体

ＢａＴｉＯ３晶体新的生长方法，即感应加热三温区溶质传输熔剂法。其要点使高温熔体分为三个温区，坩埚顶部为生长区（Ａ区），温度最低，坩埚底部为溶解区（Ｃ区），温度高于生长区，营养料置于此区，坩埚中部为高温区（Ｂ区），在此区将杂晶熔解。还阐述了此法的机理，调试创造了稳定适宜的三温区温场。用此法生长出了优质ＳＴｉＯ３大单晶，尺寸达３０×３０×１５ｍｍ，并初步观测了其畴结构形貌，实验测定了晶格常数和居里点。     

数字剪切散班干涉术中的刚体位移补偿的新方法

给出一个数字剪切散斑干涉术中的刚体位移补偿的新方法。当刚体位移大于一个像素时，加载前后的散斑图将会由于位置的变化导致失相关。重新安排每一个像素将会克服此失相关。散斑平均、条纹重构以及迭代方法用来改善条纹质量，消除散斑噪声，最后得到可取结果。     

Spacetime correlation functions of dynamic speckles produced by a series of moving phase screens

Abstract

Spacetime correlation functions of the time-varying speckle intensity have been evaluated for the case where speckle patterns are produced in the Fresnel diffraction region by light propagating through a series of moving random-phase screens. The correlation functions are given by a set of recursion formulae which are useful for numerical computations. Three statistical quantities relating to the motion of speckle patterns are derived to examine the relationship between the velocity distribution of the phase screens and the temporal behaviour of resultant speckle patterns. It is found that the velocity differences between the phase screens have a strong effect on both the decorrelation of speckle patterns and the fluctuating speed of the speckle intensity.     

经成像系统二次调制的动态散斑的空时相关性质

高斯光束经位于成像系统共轭面的两个散射体(其中一个是恒速运动)后,在距像面为菲涅尔区的观测面上形成了串级动态散斑场.理论分析和实验测量结果均表明,这种动态散斑的空-时相关函数与一次散射形成的动态散斑相比,对散射体的运动十分敏感,但其时间相关长度与速度的倒数仍然保持线性关系.     

Multi-scale characterization of laser speckle patterns

The interaction between coherent monochromatic radiation and scattering medium results in a speckle phenomenon. The purpose of this paper is to present a stochastic approach to characterizing speckle patterns, using the differential equation that formalizes Brownian motion. This stochastic approach is based on a differential method for the approximation of diffusion. This method is validated by the characterization of solutions of latex balls of various concentrations.     

Introducing speckle noise maps for Laser Vibrometry

When coherent light scatters from a surface, which is rough on the scale of the wavelength of the light, a speckle pattern is produced. The Laser Vibrometer measures target vibration velocity in the direction of the incident laser beam and typically samples a region of a speckle pattern on its photodetector. Target motions can cause the speckle pattern to change on the photodetector surface, particularly when target motions are non-normal to the direction of the laser beam. This speckle motion modulates the Doppler signal and adds noise to the demodulated output signal. Periodic target motions can cause the speckle noise to become pseudo-random and produce harmonic peaks, with the same fundamental frequency as the genuine target vibrations, which can be indistinguishable from the genuine target vibrations. Typical speckle noise levels are generally considered to be low-level, but they have not so far been adequately quantified. This paper reports preliminary results quantifying speckle noise levels using controlled experimental configurations incorporating periodic in-plane and tilt target motions. Working with commercial Laser Vibrometers, various target surface finishes and treatments are considered and speckle noise maps are produced for each configuration. For a tilting surface, speckle noise has been quantified at approximately 1 μm s^?1/deg s^?1 while, for surfaces with in-plane motion, the sensitivity to speckle noise has been estimated pessimistically at 0.1% of the in-plane velocity. Ultimately, these speckle noise maps will form a valuable practical resource for the Laser Vibrometer user.     

Technique for compensating excessive rigid body motion in nondestructive testing of large structures using shearography

A major limitation of speckle interferometry techniques including shearography is that the occurrence of rigid body motion produces a decorrelation of speckle patterns which destroys the fringe visibility. This paper demonstrates that digital shearography can tolerate much greater rigid body translation than a speckle size as predicted by the conventional speckle decorrelation theory. Excessive rigid body translation in digital shearography can be compensated by either translating the sensor/camera or digitally shifting one of the speckle patterns.     

Sandwich-speckle hologram: A combined method for local displacement analysis

Sandwich hologram interferometry and sandwich speckle photography tend to complement each other as displacement measuring methods; a sandwich hologram is best suited for out-of-plane movements, while speckle photography is used to determine in-plane components. The two techniques can be combined and both holographic and speckle photographic data can be stored on the same sandwich pair. Fringes caused by in-plane rigid body motion can be compensated for and local displacement evaluated.     

Fractal structure of digital speckle patterns produced by rough surfaces

We report on the fractal analysis of digital speckle patterns experimentally generated using an optical setup to record the light scattered from metallic rough surfaces in the normal direction. Using the differential box counting technique, we have calculated the fractal dimension of digital speckle patterns for six samples with different roughness. Our results show a quadratic dependence between the surface roughness and the fractal dimension of the corresponding digital speckle pattern. As an application a method to determine the surface roughness of metallic surfaces is proposed.     

Numerical study on trapping and guiding of nanoparticles in a flow using scattering field of laser light

Optical trapping and guiding using laser have been proven to be useful for non-contact and non-invasive manipulation of small objects such as biological cells, organelles within cells, and dielectric particles. We have numerically investigated so far the motion of a Brownian particle suspended in still water under the illumination of a speckle pattern generated by the interference of coherent light scattered by a rough object. In the present study, we investigate numerically the motion of a particle in a water flow under the illumination of a speckle pattern that is at rest or in motion. Trajectory of the particle is simulated in relation with its size, flow velocity, maximum irradiance, and moving velocity of the speckle pattern to confirm the feasibility of the present method for performing optical trapping and guiding of the particle in the flow.