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.     

New fourth-order partial differential equations for filtering in electronic speckle pattern interferometry fringes

Partial differential equations (PDEs) based methods have been demonstrated to be a powerful tool for filtering in electronic speckle pattern interferometry fringe or wrapped phase patterns. In this paper, we derive the new fourth-order partial differential equations (NFOPDE) with a better performance for filtering in electronic speckle pattern interferometry fringe patterns based on the variational methods. We test the proposed models on two computer-simulated speckle fringe patterns and an experimentally obtained fringe pattern, respectively, and compare our models with the widely used, well-known the second-order selective degenerate diffusion PDE model (SOSDPDE) and the published fourth-order PDE model (PFOPDE). The proposed NFOPDE can overcome the shortages that both the SOSDPDE and PFOPDE encounter. In all cases, our NFOPDE outperforms SOSDPDE and PFOPDE.     

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.     

Continual deformation analysis with scanning phase method and time sequence phase method in temporal speckle pattern interferometry

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.     

Noise reduction in electronic speckle pattern interferometry fringes by merging orthogonally polarised speckle fields

The noise in sawtooth fringes generated by electronic speckle pattern interferometry (ESPI) is investigated. When deformations of depolarising objects are studied, the scattered object light can be decomposed into two orthogonal linearly polarised speckle patterns which are partially decorrelated. Their correlation coefficient decreases with increasing depolarisation coefficient of the object. By suitable merging of the phase distributions of these two speckle fields on the basis of a modulation depth analysis, the rms phase error in the ESPI sawtooth fringes can be reduced significantly.     

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.     

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.     

Measurement of the near-tip displacement field in a fatigue damaged steel plate by digital speckle pattern interferometry

A digital speckle pattern interferometry system with automatic data analysis is used to measure the near-tip in-plane displacement field in a stainless steel specimen previously cracked by a fatigue test. In-plane displacements over a zone of 3.15 mm×3.15 mm are determined through the evaluation of the optical phase distribution. A theoretical solution developed for elasto-plastic materials is used to evaluate the obtained experimental results.     

Measurement of out-of-plane static and dynamic deformations by processing digital speckle pattern interferometry fringes using wavelet transform

In this paper, we are presenting measurement of static and dynamic deformations in rectangular plate fixed at one end using digital speckle pattern interferometry (DSPI). To improve the measurement accuracy, we have developed a new filtering scheme based on combination of average/median filtering and Symlet wavelet filtering which enhances the signal-to-noise ratio (SNR) in the speckle interferogram obtained from DSPI. Experimental results show that our filtering scheme is quite effective in improving SNR of the speckle interferogram. The measurements by DSPI and calculations from beam theory in the case of out-of-plane static deformation and the measurements by DSPI and accelerometer in the case of dynamic deformation are in good agreement.     

Volume-grating phase-shifting digital speckle pattern interferometry used for measurement of out-of-plane displacement field

A new digital speckle pattern interferometry, called volume-grating phase-shifting digital speckle pattern interferometry, is discussed in this paper. The out-of-plane displacement field of a bent plate can be quantitatively measured using volume-grating phase-shifting digital speckle pattern interferometry proposed in this paper. Theoretical and experimental results, as well as absolute errors, are given.     

Feasibility study of a digital speckle pattern interferometry and bending test combined technique to investigate coating adhesion

This paper presents a feasibility study to assess whether digital speckle pattern interferometry could be used as a possible technique to investigate the adhesive performance of coatings. The approach is based on the measurement of the deflections produced by a pre-notched coated specimen subjected to a four-point bending test. When the bending load is increased, a delamination between the coating and the substrate is propagated with its length depending on the adhesion strength. Experiments carried out with specimens having simulated delaminations confirm that digital speckle pattern interferometry can be used to estimate the delamination length.     

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。     

Comparison of phase recovery methods in spiral speckle pattern interferometry correlation fringes

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.     

Accuracy and sensitivity of a hole drilling and digital speckle pattern interferometry combined technique to measure residual stresses

This paper reports on the accuracy and sensitivity of digital speckle pattern interferometry (DSPI) when it is combined with the hole drilling technique for measuring residual stresses. The in-plane displacement field generated by the introduction of a small hole is determined using an automated data analysis approach. This method is based on the calculation of the optical phase distribution through a phase-shifting method and the application of a robust iterative phase unwrapping algorithm. It is experimentally demonstrated that residual stresses can be measured with a relative uncertainty of 7.5%. It is also shown that the minimum value of residual stress that can be determined with the DSPI and hole drilling combined technique is about 10% of the yield stress of the material.     

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.     

Measuring thickness change of transparent plate by electronic speckle pattern interferometry and digital image correlation

The thickness change of transparent plates was measured by electronic speckle pattern interferometry (ESPI) method and digital image correlation (DIC) method. An out-of-plane ESPI system was developed based on the Michelson interferometer, and a new thickness measurement method was designed, which is on the basis of Snell's law of refraction and DIC. The main principles and experimental procedures of these two methods were presented. The thickness change of polymethyl specimens under uniaxial tensile loading were measured by the optical techniques and compared with each other. The results reveal that the data obtained with DIC method achieve better linearity than ESPI.     

Measurement of transverse vibrations/visualization of mode shapes in square plate by using digital speckle pattern interferometry and wavelet transform

New filtering scheme is investigated and implemented on digital speckle pattern interferometric fringes to enhance the signal-to-noise ratio (SNR) in the speckle interferograms. To establish the potential of new filtering scheme the experiment was conducted on the vibrating cantilever beam. Experimental results revealed that the new filtering scheme is more powerful than other known filtering schemes (Kumar et al. Opt Laser Eng 2004;41:81–93. Kumar et al. Opt Laser Technol 2001;33:567–571. Shakher et al. Opt Eng 2002;41:176–180. Shakher et al. Proceedings ICICS, Singapore, 9–12 September 1997, p. 953–956. Bowler et al. Proceedings of IEE second international conference on image processing, London, 1986, p. 24–26. Lim. Opt Eng 1981;20:670–678. Devila et al. J Mod Opt 1995;42:1795–1804; Kaufmann et al. Opt Eng 1996;35:9–14). The new scheme reduced the speckle noise and improved SNR in speckle interferograms. Further, the new investigated filtering scheme is implemented to study the mode shapes of square plates under two different boundary conditions. In the first condition all the edges of the square plate were fixed while in the second condition two adjacent edges were fixed and the remaining two edges were free. From the experimental results it is clear that the mode shapes are more distinctly visible with the implementation of the new filtering scheme as compared to other known schemes. Under both the boundary conditions the recorded resonance frequencies were compared with the calculated values of resonance frequencies based on classical theory. The results obtained form DSPI show good agreement with classical theory.     

Heisenberg principle applied to the analysis of speckle interferometry fringes

Optical techniques that are used to measure displacements utilize a carrier. When a load is applied the displacement field modulates the carrier. The accuracy of the information that can be recovered from the modulated carrier is limited by a number of factors. In this paper, these factors are analyzed and conclusions concerning the limitations in information recovery are illustrated with examples taken from experimental data.