Three-dimensional deformation field measurement with digital speckle correlation

Digital speckle correlation is based on a detailed analysis of changes in speckle images that are recorded from laser-illuminated rough surfaces. The two in-plane components are obtained by cross-correlation of corresponding subimages, a method also known as digital speckle photography. The local gradient of the hitherto inaccessible out-of-plane component is determined from the characteristic dependence of the speckle correlation on the spatial frequency. A detailed experimental study is carried out to analyze the new technique for systematic and random measuring errors. For moderate decorrelation the accuracy of the out-of-plane measurement is better than lambda/10 and thus comparable with interferometric techniques. Yet the extremely simple and robust optical setup is suited to nondestructive-testing applications in harsh environments. The quality of the deformation maps is demonstrated in a practical application.     

Defocus measurement using speckle correlation

Abstract

A technique using correlation fringes is presented to estimate the out-of-plane displacement of the image plane. The width of the fringe is estimated using autocorrelation of the Young's fringes and the visibility of the autocorrelation of these fringes are used as an estimator of the focusing error. This technique allows us to estimate focusing errors of the order of few tens of microns.     

A digital correlation speckle shift interferometer

The optical circuit of a speckle shift interferometer with two arms of different lengths is presented and its operating principle described. The device includes a semiconductor laser that serves as the radiation source and a digital camera interfaced to a computer.     

Measurement of random processes at rough surfaces with digital speckle correlation

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.     

Distance measurements by speckle correlation of objective speckle patterns, structured by the illumination

Objective speckles produced by two beams overlapping and interfering on a rough object surface contain information about the angle of incidence of the two beams, and how well they overlap. We obtain the autocovariance function for such a speckle pattern, and demonstrate how the information carried by the objective speckles can be used to probe the distance between the object and the observation plane. From a distance of 75 mm to a distance of 150 mm, and using an angle of 0.3 deg between the two incident beams, we can measure the actual distance with an uncertainty of better than ±0.1% of the full range. As long as the beams overlap at the object surface, the proposed method can measure distance with an uncertainty inversely proportional to the spot size at the object.     

Clipped correlation functions of aperture integrated laser speckle

The doubly and singly clipped correlation functions of aperture integrated laser speckle are evaluated and compared.     

Whole-field determination of surface roughness by speckle correlation

A whole-field method of double-exposure speckle photography is employed to determine metal surface roughness by correlation between two speckle patterns. A movable rectangular aperture that is mounted before an image lens is shifted between the exposures, which results in a decrease in the contrast of the reconstructed Young's fringes with increasing roughness. The technique permits evaluation of the roughness of particular points on a surface as well as the average roughness of an entire surface. Four sets of random surfaces that were prepared by different machine-finishing processes and with roughnesses ranging from 0.6 to 13 μm have been tested. Different methods have been carried out to process the test data, and a practical method for the evaluation of surface roughness is proposed.     

Laser monitoring of the flow velocity in lymphatic microvessels based on a spatiotemporal correlation of the dynamic speckle fields

A method and the corresponding experimental setup for the in vivo laser monitoring of temporal variations in the velocity and direction of flow in lymphatic microvessels are described. Experimental results on the laser monitoring of flow in the mesenteric microvessels of rat are presented. The method is based on an analysis of the statistical properties of the dynamic speckle fields and provides for a high spatial resolution. The results of determination of the lymph flow velocity by the proposed method agree well with analogous data of functional transmission microscopy.     

In-plane dynamic speckle interferometry: comparison between a combined speckle interferometry/speckle correlation and an update of the reference image

A common problem during study of, for instance, tensile tests with interferometers is that the sample moves too much so that the speckles decorrelate and no phase information is obtained. Two ways to overcome this problem are compared: a combination of speckle interferometry and speckle correlation and a method in which the reference image is updated during the experiment. The comparison shows that both techniques can be used to measure the deformation of an object even if it is exposed to rigid body motions. Both techniques are applied to measurements of microscale deformation fields of an adhesive joint in a carbon-fiber epoxy composite.     

Digital speckle correlation method based on wavelet-packet noise-reduction processing

Despite the advantages of being highly sensitive and nondestructive, the digital speckle correlation method (DSCM) may have difficulties in detecting tiny defects such as delaminations in multilayer ceramic capacitors. This is because the presence of background noise always complicates the data processing. We present a new algorithm, which employs the wavelet-packet noise-reduction process together with the improved DSCM, to improve data processing. Both the computational error and the noise are shown to be reduced successfully by this new algorithm. The accuracy (or precision) of the improved DSCM is increased after operation of the wavelet-packet noise-reduction process. The most important feature of this new algorithm is that it can extract a small hillock signal from a large noisy background in a DSCM deformation result. This helps to save time in the detection of tiny defects, such as delamination, in a miniaturized electronic component.     

High-resolution laser speckle correlation for displacement and strain measurement

A laser speckle correlator with high optical magnification is presented, and its performance in the measurement of strain is demonstrated experimentally. Two separated areas on a test specimen are illuminated with laser beams, and displacements of each area are measured by performance of laser speckle correlation on successive magnified images. The interplay of magnification, lens aperture, surface roughness, pixel spacing on the CCD array sensor, and the attainable precision of correlation are investigated theoretically and experimentally. Resolutions that are usually considered accessible only to interferometric techniques are achieved: displacement resolutions of less than 50 nm and strain measurements of less than 10 mustrain across distances of the order of 10 mm are demonstrated. At high magnification, speckle decorrelation due to out-of-plane displacement becomes a stringent restriction, and surface height correlation effects may limit speckle contrast and broaden speckle correlation peaks.     

散斑分数傅里叶联合变换相关测量研究

提出数字散斑联合变换分数相关测量方法,利用分数相关可以锐化相关峰的作用,在数字散斑联合变换相关运算中用分数傅里叶变换代替傅里叶变换,提高测量精度。通过对散斑图像进行相位调制,有效地解决了分数傅里叶变换的移变性带来的谱移问题。编程模拟和对拉伸试件位移场测量的结果表明,只要选择合适的分数傅里叶变换级次和相位调制函数,可以使相关峰的半宽度从4～5pixel锐化到仅1pixel,得到优于傅里叶变换相关的理想输出。     

Spatial speckle correlometry in applications to tissue structure monitoring

Asymptotic behavior of temporal autocorrelation functions of speckle intensity fluctuations induced by tissue scanning with a focused probe beam is experimentally studied for the transition from a single-scattering to a multiple-scattering mode. Such parameters as the exponential factor (or the Hurst coefficient) and the Hausdo dimension are proposed for the characterization and the visualization of the variations of the studied tissues' optical properties in generalized form. We studied reversible transition between various scattering modes stimulated by the application of certain chemical agents to the human sclera samples using speckle intensity correlation analysis; corresponding results are presented. The possibilities of the scattering structures imaging with local estimations of the exponential factor of speckle intensity fluctuations are shown in in vitro experiments with samples of human skin epidermis.     

Magnitude and direction of motion with speckle correlation and the optical fractional Fourier transform

The optical fractional Fourier transform (OFRT) in combination with speckle photography has previously been used to measure the magnitude of surface tilting and translation. Previous OFRT techniques used to determine motion have not been able to discern the direction of the tilt and translation. A simple new approach involving use of correlation is presented to overcome this limitation. Controlled variation of the minimum resolution and dynamical range of measurement is demonstrated. It is then experimentally confirmed that if a rigid body's motion is captured by two OFRT systems of different orders, the direction and magnitude of both the tilting and the in-plane translation motion of the body can be independently determined without a priori knowledge. The experimental results confirm the validity of previous theoretical predictions.     

Real-time heterodyne speckle pattern interferometry using the correlation image sensor

A real-time method for heterodyne speckle pattern interferometry using the correlation image sensor (CIS) is proposed. The CIS demodulates the interference phase of heterodyned speckle wavefronts pixelwise at an ordinary video frame rate. The proposed method neither suffers loss of spatial resolution nor requires a high frame rate. Interferometers for out-of-plane and in-plane deformation are developed with a 200 × 200 pixel CIS camera. Experimental results confirm that the proposed method realizes real-time imaging of a rough-surfaced object under deformation. The average standard deviations of demodulated phase-difference images for the out-of-plane and in-plane interferometers are 0.33 and 0.13 rad, respectively.     

Fundamental correlation lengths of coherent speckle in medical ultrasonic images

Refinements to previous analyses of the natural correlation lengths within simple images and between images to be compounded are presented. Comparison of theoretical and experimental results show very good agreement for the case of Rayleigh scattering media: the correlation length within a simple image is comparable to the resolution cell size; the correlation length between images to be spatially compounded is comparable to, but smaller than, the transducer on array aperture; and the correlation length between images to be frequency-compounded becomes a frequency comparable to their bandwidth. Complications arising from the presence of specular scattering or due to the presence of just a few scatterers are considered. It is shown that straightforward solutions exist for either of these problems taken by itself. When they occur in combination, calibration techniques may lead to unambiguous identification of the contributions to the scattering from diffuse or incoherent scattering and from specular or coherent scattering, and to estimation of the density of diffuse scatterers.     

Data compression for speckle correlation interferometry temporal fringe pattern analysis

Temporal fringe pattern analysis is gaining prominence in speckle correlation interferometry, in particular for transient phenomena studies. This form of analysis, nevertheless, necessitates large data storage. Current compression schemes do not facilitate efficient data retrieval and may even result in important data loss. We describe a novel compression scheme that does not result in crucial data loss and allows for the efficient retrieval of data for temporal fringe analysis. In sample tests with digital speckle interferometry on fringe patterns of a plate and of a cantilever beam subjected to temporal phase and load evolution, respectively, we achieved a compression ratio of 1.6 without filtering out any data from discontinuous and low fringe modulation spatial points. By eliminating 38% of the data from discontinuous and low fringe modulation spatial points, we attained a significant compression ratio of 2.4.     

Phase rotation methods in filtering correlation coefficients for ultrasound speckle tracking

In speckle-tracking-based myocardial strain imaging, large interframe/volume peak-systolic strains cause peak hopping artifacts separating the highest correlation coefficient peak from the true peak. A correlation coefficient filter was previously designed to minimize peak hopping artifacts. For large strains, however, the correlation coefficient filter must follow the strain distribution to remove peak hopping effectively. This processing usually means interpolation and high computational load. To reduce the computational burden, a narrow band approximation using phase rotation is developed in this paper to facilitate correlation coefficient filtering. Correlation coefficients are first phase rotated to increase coherence, then filtered. Rotated phase angles are determined by the local strain and spatial position. This form of correlation coefficient filtering enhances true correlation coefficient peaks in large strain applications if decorrelation due to deformation does not completely destroy the coherence among neighboring correlation coefficients. The assumed strain used in the filter can also deviate from the true strain and still be effective. Further improvement in displacement estimation can be expected by combining correlation coefficient filtering with a new Viterbi-based displacement estimator.     

基于数字散斑相关技术的老化沥青砂浆自愈合特性

沥青砂浆作为沥青混合料结构体系的分散相,其老化后的自愈合特性是影响沥青混合料路用性能的重要部分,结合沥青路面服务环境的实际情况,将老化程度及愈合时间作为条件,采用数字散斑技术进行沥青砂浆的损伤愈合实验,分析并观测其损伤愈合过程。结果表明,利用数字散斑相关技术可以很好地观测沥青砂浆损伤愈合过程;经过短期老化后的沥青砂浆在损伤愈合过程中预切口位移最大;长期老化后两种沥青砂浆的愈合值都有所提升,并且基质沥青砂浆比SBS沥青砂浆具有更好的愈合效果;沥青砂浆在愈合过程中分为二个阶段,第一阶段愈合速率较高,愈合值较大,第二阶段愈合率下降,愈合需要更长的时间。