Additive-subtractive phase-modulated speckle interferometry: Fringe visibility under partial decorrelation

Additive-subtractive phase modulated speckle interferometry (ASPMSI) is a technique that minimizes the susceptibility of speckle methods to environmental noise while providing fringes of good visibility. The method requires the acquisition of two consecutive video frames of additive-speckle images of the same two deformed states of an object at a rapid enough rate such that ambient noise is not a problem. The additive-speckle images as expected are of very poor visibility due to the presence of the self-interference term. An interframe phase-modulation is introduced and the two additive-speckle images are digitally subtracted to improve the fringe visibility by removing the self-interference term. The ASPM-SI method works with in-plane and out-of-plane deformation sensitive ESPI as well as with displacement-gradient sensitive speckle-shearing interferometry. It is shown that the ASPM-SI scheme has higher visibility than conventional additive-SI and performs consistently better than subtractive-SI schemes in the presence of partial interframe speckle decorrelating optical noise. Furthermore, it is shown that the fringe visibility of the out-of-plane displacement sensitive interferometer which uses a protected reference beam separate from the object beam can be made to be essentially unity even at complete interframe decorrelation.     

一种新的数字散斑振动定量分析系统

在数字散斑干涉(DSPI)测量方法中，数字散斑时间平均法测量振动时得到的是贝塞尔条纹，其定量分析比较困难。该文采用贝塞尔条纹准相移技术进行数字散斑振动定量分析，即在参考光路中引入和物体同频率的偏置参考振动,当参考振动的相位改变时,振动条纹会产生移动,这相当于静态余弦条纹的相移,可以采用相移算法对数字散斑振动测量的贝塞尔条纹进行定量分析。文中开发了一种新的数字散斑振动定量分析系统，研制了一种低压光学相移器和测量控制电路，给出了圆周固定圆片和四周固定方铜板的振动测量结果。实验结果表明：采用本系统可以方便地定量测出物体的振动模态。     

Effect of image fiber on the speckle fringe pattern in image fiber-guided DSPI endoscopy

The effect of using an image fiber on the speckle fringe visibility in an endoscopic DSPI is analyzed here. An endoscope system was designed and developed, using image fiber as the speckle pattern image conduit, to work in the out-of-plane speckle interferometric configuration and experiments were carried out using a curved phantom tissue specimen as the test target. Experimental results obtained using the developed system are compared with that obtained using conventional DSPI system. To obtain a relative and quantitative comparison of the quality of the fringes obtained employing the two methods, an evaluation method that can give a quantified ‘score’ is proposed here.     

Measurement of curvature distribution using digital speckle three-shearing aperture interferometry

Digital speckle three-shearing-aperture interferometry used for the measurement of curvature distribution fields of a deformation object is proposed for the first time. In this method, pure curvature distribution fringes without containing slope distribution fringes can be obtained by using digital speckle three-shearing-aperture interferometry. Two specklegrams, one before and the other after object deformation, are recorded by a CCD camera and stored in a computer. A 2-D fast Fourier transform (FFT) and a 2-D inverse FFT (IFFT) are performed on the two specklegrams. The subtraction of the transformed specklegrams will produce curvature distribution fringes, and the speckle noises in curvature distribution fringes can be removed after these fringes pass through low-pass filtering. Results from theory and experiments are in good agreement.     

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.     

Two wavelength simultaneous DSPI and DSP for 3D displacement field measurements

A novel optical system is proposed that implements digital speckle pattern interferometry (DSPI) and digital speckle photography (DSP) simultaneously using two wavelength illumination of an object for simultaneous measurement of all three components of the displacement vector field. A collimated red light illuminates both the object and a reference surface in a DSPI configuration which is sensitive to out-of-plane displacement field while a blue light illuminates the object in a DSP configuration which is sensitive to in-plane displacement fields. A color 3-CCD camera records the red and blue lights individually through its red and blue channels, respectively. Two reference images and one image in the deformed state of the object are required for quantitative measurement. Experimental results are presented to validate the system.     

Contrast enhanced and phase controlled stroboscopic additive fibre optic TV-holography for whole field out-of-plane vibration analysis

A new technique for real-time contrast enhancement and phase control of fringes in additive Stroboscopic TV-holography applied to out-of-plane vibration analysis and its implementation on a fibre-optic electronic speckle pattern interferometer (FOESPI) are presented. Synchronous Stroboscopic illumination, firing two pulses per object vibration period, is combined with simultaneous inter-pulse (high frequency) and inter-frame (low frequency) phase modulation in the reference arm of the ESPI yielding a sequence of frames (interferograms) that are grabbed and processed in real-time. With this technique both speckle and fringe phases are independently controlled by means of the parameters of modulation enabling speckle contrast inversion, as required to enhance the visibility of fringes by sequential subtraction, as well as dynamic fringe phase shifting to solve peakvalley ambiguities.     

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.     

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.     

Velocity and density field measurements by digital speckle method

Velocity and density field measurements based on image processing of laser speckle or pseudo-speckle pattern have been developed. Laser speckle velocimetry (LSV) or white-light speckle velocimetry (WSV), which corresponds to a high-image-density PIV, gives a local velocity vector map or whole field velocity contour map of a two-dimensional flow field seeded densely with fine particles. This technique has an advantage in high-speed flow measurement without limit of frame rate except for directional ambiguity. New techniques of laser speckle photography and laser speckle interferometry by means of digital image processing have been developed recently for density field measurements. In laser speckle photography, a local density gradient vector map is reconstructed by cross-correlation evaluation between the reference and the object speckle patterns. In laser speckle interferometry, an equi-density contour map is reconstructed by image subtraction between the reference and the object interferometric speckle patterns.     

基于条纹等值线相关的散斑条纹图处理新方法

在数字散斑干涉术中,通常由物体变形前后得到的两幅原始散斑场用相减方法来得到条纹图,并由相移后的多幅(三幅以上)条纹图来求取相位场。介绍了一种基于条纹等值线相关的条纹图生成新方法,并在此基础上提出了基于单幅干涉条纹图的相位场提取新方法和基于条纹等值线相关的单步相移新方法。实验结果表明,该方法具有良好的结果和发展前景。     

Holography, speckle, and computers

Surveys are presented on automatic and quantitative deformation measurements using CCDs and computers in holographic interferometry and speckle metrology for diffusely reflecting surfaces. For delivering relationships between object deformation and observed quantities, we discuss formations of fringes and signals observed in these methods in terms of correlation functions of spatially randomly varying complex amplitude of light. Dependencies of the observed patterns on object deformation and optical systems are discussed. Physical meanings of the derived relationships are explained in terms of the dynamic behaviors of speckles resulting from surface deformation. Automatic measurements are described in chronological orders. They include analysis of fringe patterns resulting from photographic recording of specklegrams, video recording and analysis of speckle patterns used in electronic speckle pattern interferometry as well as direct digital correlation techniques, and digital holography that uses both digital recording and reconstruction of holograms.     

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.     

频闪散斑照相法

本文讨论了频闪散斑照相法的基本原理,给出了简谐振动和非简谐周期振动散斑图的全场分析的平均光强分布和条纹可见度的解析式.理论和实验表明频闪散斑法较之时间平均散斑法有着明显的优点.     

Statistical searching of deformation phases on wavelet transform maps of fringe patterns

A wavelet transform (WT) analysis is presented to obtain the deformation phases from the fringes with non-uniform carrier frequency, which may appear in the pattern of varied-periodic fringes generated in displacement measurement. Based on the phase maps of the Morlet WT coefficients distributed in a space-scale spectrum, a statistical processing is carried out to search the compacted density of the phase intervals over the scale, and from that the phase modulations related to the object deformation can be determined. Numerical simulation demonstrates the validity of the pattern-processing technique, and the experimental results show the applications to the measurement of the in-plane displacement by the digital speckle pattern interferometry (DSPI) and the measurement of the out-of-plane deflection by the projecting Moiré fringes.     

Application of digital speckle pattern interferometry and wavelet transform in measurement of transverse vibrations in square plate

In this paper the study of out-of-plane or transverse vibrations in a square plate using digital speckle pattern interferometry (DSPI) is presented. To improve the measurement accuracy, we have implemented a new filtering scheme based on combination of average/median filtering and Symlet wavelet filtering which enhances the signal to noise ratio in the speckle interferogram obtained from DSPI. A large number of fringe patterns are shown for square plate with two different boundary conditions. Experimentally obtained resonance frequencies for the square plate for the boundary condition one edge fixed and other edges being free, the resonance frequencies obtained from DSPI show good agreement with that obtained from classical theory for thin plates.     

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.     

数字散斑法在局域剪切带三维变形研究中的应用

在适当的温度、应变率和预变形下,合金材料的拉伸试验中,将会出现伴随应力锯齿形跌落的雪崩式剪切变形带,即波特文-勒夏特利埃(Portevin-Le Chatelier,PLC)效应。利用高速数字摄像系统(分辨力1000 frames/s)并结合数字散斑干涉法(Digital speckle pattern interferometry,DSPI)和数字散斑相关法设计了一套光学变形测量系统,实现了拉伸试验中对试件表面三维变形的实时、精确测量。利用该光学系统对铝铜合金试件在拉伸试验中产生的跳跃传播的局域剪切带瞬态成核过程进行捕捉。通过结合数字散斑相关法得到的面内变形定量结果和数字散斑干涉法得到的表现离面变形的条纹图,再现了剪切变形带成核和传播瞬间的三维变形过程。     

Digital speckle pattern interferometry applied to thermal strain measurements of metal- ceramic compounds

An experimental set-up is described for measuring two displacement components by digital speckle pattern interferometry (DSPI). Experimental results are given for metal-ceramic components under thermal stress. Emphasizing quantitative evaluation, the effective thermal expansion coefficient was determined.     

线性相关计算法形成数字剪切散斑相关条纹图

本文提出了用线性相关计算法形成数字剪切散斑干涉条纹的新方法,从理论分析和实验研究均证明,用该方法所获得的数字剪切散斑相关条纹图在图象监视器上易于观看,条纹图象光强无直流分量影响且可见度较减模式要高,该新方法的采用为今后的条纹图象分析奠定了基础。