Measurement of poisson''s ratio using real-time digital speckle pattern interferometry

This paper describes an application of Digital Speckle Pattern Interferometry (DSPI) to the measurement of Poisson's ratio. This real-time technique provides several advantages over conventional holographic methods. A suitable operator interactive algorithm has been developed to evaluate the fringe pattern and then calculate the Poisson's ratio. Experimental results have been presented for a few specimens.     

Measuring velocity of speckle fields with digital speckle pattern interferometry

We investigate a digital method for detecting the velocity of a diffusing object. The technique is based on Digital Speckle Pattern Interferometry (DSPI). A set of reference fringes is generated externally through the reference beam in a digital interferometer. As the object moves, subsequent frames are acquired and subtracted according to the normal DSPI procedure and stored. By means of the theory of first order speckle statistics applied to speckle intensity correlation, we relate the visibility variations in the reference fringes with the object velocity. Thus, by measuring the fringe visibility variation in the resulting DSPI stored frames the mean object velocity can be obtained. The theoretical results are experimentally verified.     

Accurate measurement of composite laminates deflection using digital speckle pattern interferometry

The modern digital speckle pattern interferometry (DSPI) technique is applied for the accurate measurement of the full-field deflection distribution of a bent composite laminates. Two kinds of powerful phase-shifting methods, phase of differences method (PDM) and difference of phases method (DPM), are described briefly and employed in DSPI to quantitatively extract the phase information, respectively. A comparison of the deflection distributions measured by DPM and PDM indicates that the former has a better measurement accuracy than the latter in laminate's static test experiment.     

数字散斑干涉三维变形测量技术研究进展

三维变形可以转换为应力/应变分布,是材料性能测试和结构可靠性分析的关键参数。在众多三维变形测量技术中,数字散斑干涉技术可以高精度地测量三维变形信息,在航空航天、汽车、先进制造、土木工程和生物医学等行业发挥着十分重要的作用。从散斑干涉基本原理出发,详细介绍了几类三维变形散斑干涉测量技术,并分析比较各类方法的优缺点;同时介绍了散斑干涉三维变形测量技术的国内外研究进展和最新应用;最后展望了散斑干涉三维变形测量技术在动态同步测量、测量系统简化以及应用范围扩宽等方面的发展趋势。     

Development of Matlab filtering techniques in digital speckle pattern interferometry

In interferometric fringe pattern analysis, specular and speckle fringe patterns are the two main divisions. While specular fringes are characterized by quality fringes, speckle (that obtains due to the diffuse scattering of the coherent radiation from an optically rough surface) fringe patterns are characterized by noisy fringes. This paper concentrates on this aspect and the Matlab based filtering methods to improve the quality of speckle fringe patterns by developing the appropriate software. Further, the newly developed software “Macurv” will be presented which can give the second order derivative (curvature) fringe information. A software with several functions is written using Matlab. The objective of the software is to provide a more effective way for the post-processing of speckle interferometric fringes. The algorithm and functions of the developed software “Macurv” will be explained.     

Electronic speckle pattern interferometry using optical fibers

Initial experiments using electronic speckle pattern interferometry with fiberoptic imaging and illumination are described.     

Electronic speckle pattern interferometry using vortex beams

We show that it is possible to perform electronic speckle pattern interferometry (ESPI) using, for the first time to our knowledge, vortex beams as the reference beam. The technique we propose is easy to implement, and the advantages obtained are, among others, environmental stability, lower processing time, and the possibility to switch between traditional ESPI and spiral ESPI. The experimental results clearly show the advantages of using the proposed technique for deformation studies of complex structures.     

Investigation of in-plane stresses on bolted flange joints using digital speckle pattern interferometry

Digital Speckle Pattern Interferometry (DSPI) is a promising field for a wide range of applications such as measurement of displacements/deformations, contouring, nondestructive testing, etc. It has holographic sensitivity and circumvents delays in photographic processing/filtering. Speckle correlation interferometry can be made sensitive to only one of the components of in-plane displacement by illuminating the object with two beams symmetric about the viewing axis and imaging along this axis using a viction. This configuration allows measurement of in-plane displacements independent of out-of-plane displacements.

In this paper, the in-plane deformation/stress distribution has been studied for a typical bolted flange joint of a pressure vessel using two beam illumination DSPI. The information obtained can be very useful in predicting the leakage behaviour of such joints employed in pressure vessels. Experimental investigations along with theoretical predictions are presented.     

Optical distance sensing using digital speckle shearing interferometry

A novel distance sensing technique using digital speckle shearing interferometry is outlined in this paper. The primary incentive for the development of this technique is of one that can be: (i) incorporated into any TV imaging system; (ii) used as a focusing mechanism; (iii) suited to measure from diffuse targets; and (iv) relatively insensitive towards the environment. Results obtained reveal the feasibility of further research and development of this technique for practical applications.     

Stroboscopic digital speckle pattern interferometry for vibration analysis of microsystem

In this paper, vibration measurement and analysis of microsystems, such as micro-electro-mechanical systems (MEMS) by using stroboscopic digital speckle pattern interferometry (DSPI) is presented. Because of the speckle interferometry, the technique is suited for samples which have a rough surface or whose surfaces can be sprayed into a scattering surface. A laser speckle microinterferometer incorporated with optoelectronic devices for a stroboscopic illumination and a synchronization of the signals between excitation and stroboscopic illumination is described and demonstrated. The system can measure both out-of-plane and in-plane displacement under either a static or dynamic loading. The fundamental is explained and some applications are demonstrated.     

Quantitative vibration amplitude measurement with time-averaged digital speckle pattern interferometry

A new method of using time-averaged digital speckle pattern interferometry for the quantitative measurement of vibration amplitude was developed. Signal processing techniques especially the Hilbert transformation for quantitative evaluation of the Bessel fringes obtained in time-averaged digital speckle pattern interferometry were explored. The quadrature signal after Hilbert transformation is equivalent to a 90° phase-shifted interferogram for a monotonically increasing or decreasing phase function. An algorithm was developed for Bessel fringe contrast enhancement and phase extraction. The techniques were tested numerically and experimentally. Sub-fringe quantification of the time-averaged vibration fringes is realised with the proposed method. Compared with the commonly used phase shift method which requires a minimum of two images for image processing, this method requires only one fringe pattern for data extraction.     

Wavelet filtering applied to time-average digital speckle pattern interferometry fringes

In this paper we are presenting a filtering scheme using Symlet wavelet to remove the speckle noise from the time-averaged digital speckle pattern interferometry fringes. To demonstrate the potential of Symlet wavelet filtering, experiments are conducted to remove the speckle noise from the fringes recorded for the surface of computer hard disk. Experimental results demonstrate that this filtering removes the speckle noise to the large extent.     

A comparative study of phase-shifting algorithms in digital speckle pattern interferometry

Digital speckle pattern interferometry (DSPI) is a tool for making qualitative as well as quantitative measurements of deformation of objects. Phase-shifting algorithms in DSPI are useful for extracting quantitative deformation data from the system. Comparative studies of the different phase-shifting algorithms in DSPI for object deformation measurement are presented. Static and quasi-dynamic deformation of the object can be measured using these algorithms. Error compensating five-step phase-shifting method is used for the algorithms.     

Quantitative modal analysis using electronic speckle pattern interferometry

Existing modal analysis techniques based on pointwise methods such as accelerometers and laser vibrometers, suffer from the compromises required to infer whole field vibration behaviour from a predetermined number of discrete measurement data points. The measurement grid is normally chosen before the most sensitive areas of the test piece have been identified and this can lead to a requirement for further grid refinement, with the consequent spatial and temporal disadvantages. Whole-field modal analysis using electronic speckle pattern interferometry (ESPI) has been developed, and focuses on the manipulation of optical information to provide a grid of data points which is transferred to a modal analysis software package for comparison with traditional point wise modal data sets. The advantages of obtaining the initial whole-field vibration picture in real time and subsequently overlaying a chosen measurement data grid, are demonstrated. Modal analysis of a steel plate is presented, which demonstrates single displacement vector measurements. Data is transferred into modal analysis software allowing ESPI mode shape information to be directly compared with measurements using other pointwise techniques and finite element analysis (FEA), through a modal assurance criterion (MAC) calculation. Further results are presented for multiple displacement vector analysis of high-power ultrasonic components, demonstrating the ability to accurately measure the modal characteristics of complex dynamic components.     

Vibration mode analysis using electronic speckle pattern interferometry

The use of electronic speckle pattern interferometry as a non-destructive testing technique has been widely reported for measuring a variety of objects. When used for vibration mode analysis, the only information presented to the operator was at the nodal area. The instrument has been developed so that, with the use of a microcomputer, the operator can now produce an isometric view of an object vibrating in a resonant mode, making the results easier to interpret. The instrument provides a real-time, non-contact alternative to other mode analysis equipment and can detect high-order modes as easily as low-order modes.     

Refractive and geometric lens characterization through multi-wavelength digital speckle pattern interferometry

Physical parameters of different types of lenses were measured through digital speckle pattern interferometry (DSPI) using a multimode diode laser as light source. When such lasers emit two or more longitudinal modes simultaneously the speckle image of an object appears covered of contour fringes. By performing the quantitative fringe evaluation the radii of curvature as well as the refractive indexes of the lenses were determined. The fringe quantitative evaluation was carried out through the four- and the eight-stepping techniques and the branch-cut method was employed for phase unwrapping. With all these parameters the focal length was calculated. This whole-field multi-wavelength method does enable the characterization of spherical and aspherical lenses and of positive and negative ones as well.     

Evaluation of the piezoelectric behaviour produced by a thick-film transducer using digital speckle pattern interferometry

This paper presents an interferometric measurement of the out-of-plane deflections produced by a piezoelectric transducer, manufactured by thick-film deposition of a ceramic paste over an alumina substrate, when is subjected to a DC electric voltage. It is shown that a digital speckle pattern interferometer with an incorporated phase-shifting facility allows the measurement of nanometer displacements generated by the piezoelectric device. These measurements are used to evaluate the effective piezoelectric charge constant along the polarization direction (d₃₃)_eff that characterizes the thick-film transducer.     

Determination of residual stresses using hole drilling and digital speckle pattern interferometry with automated data analysis

A digital speckle pattern interferometry and hole drilling combined system is developed to determine the magnitude of the residual stress in a aluminum thin plate subjected to an uniform uniaxial tensile load. Performing automated fringe analysis, the optical data contained in the speckle interferograms are quickly converted into values of residual stress. The evaluation is carried out through the measurement of the in-plane displacement field generated by the introduction of the small hole. The displacement field is determined from the calculation of the optical phase distribution by means of a phase shifting method. The magnitude of the residual stress is finally evaluated through a least-squares calculation and compared with the stress value applied to the specimen.     

散斑干涉相位条纹图的频域滤波处理

为了解决在数字散斑干涉技术测量时,散斑干涉相位条纹图像中大量噪声对相位解包裹结果和精度产生严重影响的问题,介绍了一种条纹正余弦分解和频域低通滤波结合的方法,实现了散斑干涉相位条纹图的高精度滤波。该方法的基本思路是在对相位图像进行滤波处理前,先将相位图通过正余弦函数进行映射转换成两幅图,分别经过频域滤波,然后再合成为相位图。这种分解频域滤波方法可以在滤波的同时,有效保留相位跳变信息。实验结果表明：与传统的图像降噪方法相比,该方法能够在保留图像“尖峰”信息的基础上,较好地滤除图像中的散斑噪声,方法简单有效,有效解决了传统滤波方法应用在相位条纹图中,相图灰度信息丢失10%~40%的问题。     

Measurement of out-of-plane deformation of curved objects with digital speckle pattern interferometry

Digital speckle pattern interferometry(DSPI) is a high-precision deformation measurement technique for planar objects. However, for curved objects, the three-dimensional(3 D) shape information is needed in order to obtain correct deformation measurement in DSPI. Thus, combined shape and deformation measurement techniques of DSPI have been proposed. However, the current techniques are either complex in setup or complicated in operation. Furthermore, the operations of some techniques are too slow for real-time measurement. In this work, we propose a DSPI technique for both 3 D shape and out-of-plane deformation measurement. Compared with current techniques, the proposed technique is simple in both setup and operation and is capable of fast deformation measurement. Theoretical analysis and experiments are performed. For a cylinder surface with an arch height of9 mm, the error of out-of-plane deformation measurement is less than 0.15 μm. The effectiveness of the proposed scheme is verified.