Phase retrieval in digital speckle pattern interferometry by use of a smoothed space-frequency distribution

We evaluate the use of a smoothed space-frequency distribution (SSFD) to retrieve optical phase maps in digital speckle pattern interferometry (DSPI). The performance of this method is tested by use of computer-simulated DSPI fringes. Phase gradients are found along a pixel path from a single DSPI image, and the phase map is finally determined by integration. This technique does not need the application of a phase unwrapping algorithm or the introduction of carrier fringes in the interferometer. It is shown that a Wigner-Ville distribution with a smoothing Gaussian kernel gives more-accurate results than methods based on the continuous wavelet transform. We also discuss the influence of filtering on smoothing of the DSPI fringes and some additional limitations that emerge when this technique is applied. The performance of the SSFD method for processing experimental data is then illustrated.     

Measurement of nanometric displacements by correlating two speckle interferograms

This paper presents a method to measure nanometric displacement fields using digital speckle pattern interferometry, which can be applied when the generated correlation fringes show less than one complete fringe. The method is based on the evaluation of the correlation between the two speckle interferograms generated by both deformation states of the object. The performance of the proposed method is analyzed using computer-simulated speckle interferograms. A comparison with the performance given by a phase-shifting technique is also presented, and the advantages and limitations of the proposed method are discussed. Finally, the performance of the proposed method to process real data is illustrated.     

Adaptive correction to the speckle correlation fringes by using a twisted-nematic liquid-crystal display

An innovative technique for reducing speckle noise and improving the intensity profile of the speckle correlation fringes is presented. The method is based on reducing the range of the modulation intensity values of the speckle interference pattern. After the fringe pattern is corrected adaptively at each pixel, a simple morphological filtering of the fringes is sufficient to obtain smoothed fringes. The concept is presented both analytically and by simulation by using computer-generated speckle patterns. The experimental verification is performed by using an amplitude-only spatial light modulator (SLM) in a conventional electronic speckle pattern interferometry setup. The optical arrangement for tuning a commercially available LCD array for amplitude-only behavior is described. The method of feedback to the LCD SLM to modulate the intensity of the reference beam in order to reduce the modulation intensity values is explained, and the resulting fringe pattern and increase in the signal-to-noise ratio are discussed.     

Two improved algorithms with which to obtain contoured windows for fringe patterns generated by electronic speckle-pattern interferometry

Fringe patterns generated by electronic speckle-pattern interferometry are full of high-spatial-frequency and high-contrast speckle noise. Filtering with contoured windows has proved to be an efficient approach to filtering out speckle noise while retaining the fringe patterns. Furthermore, with contoured windows the contoured correlation fringe pattern method can be used to derive smooth, normalized, consistent fringes. Contoured windows previously were determined by fringe orientation only, and this process generated accumulated errors. We propose two new algorithms with which to obtain the contoured windows according to the fringe intensity slope and the distance ratio to neighboring skeletons. These new techniques can determine contoured windows more precisely.     

Phase retrieval in digital speckle pattern interferometry by application of two-dimensional active contours called snakes

We propose a novel approach to retrieving the phase map coded by a single closed-fringe pattern in digital speckle pattern interferometry, which is based on the estimation of the local sign of the quadrature component. We obtain the estimate by calculating the local orientation of the fringes that have previously been denoised by a weighted smoothing spline method. We carry out the procedure of sign estimation by determining the local abrupt jumps of size pi in the orientation field of the fringes and by segmenting the regions defined by these jumps. The segmentation method is based on the application of two-dimensional active contours (snakes), with which one can also estimate absent jumps, i.e., those that cannot be detected from the local orientation of the fringes. The performance of the proposed phase-retrieval technique is evaluated for synthetic and experimental fringes and compared with the results obtained with the spiral-phase- and Fourier-transform methods.     

Contrast enhancement for electronic speckle pattern interferometry fringes by the differential equation enhancement method

Electronic speckle pattern interferometry fringe patterns usually have poor contrast so it is important to enhance fringe contrast for the extraction of phase from a single fringe pattern. We present new enhancement methods based on differential equations (called DE enhancement methods) to electronic speckle pattern interferometry fringes. The DE enhancement methods transform the image processing to solve differential equations. With the proposed methods, the visibility of the correlation speckle fringe patterns can be improved significantly. We tested the proposed methods on computer-simulated speckle correlation fringes and experimentally obtained fringes, and we compared the new method with other contrast enhancement techniques. The experimental results illustrate the performance of this approach.     

Phase extraction from electronic speckle pattern interferometry addition fringes

Addition fringes are obtained in real time from electronic speckle pattern interferometry (ESPI) by use of a twin-pulsed laser when two pulses are fired during a single field of a CCD camera. This enables object deformations to be studied in harsh environmental conditions. However, the fringe patterns have poor visibility because optical noise is additive. To our knowledge automatic phase extraction from addition fringes has not previously been achieved: Low-pass filtering to suppress random speckle noise also eliminates the fringes because of their low visibility. Two phase-stepping algorithms that calculate phase from ESPI fringes without the need for a preprocessing filter are presented. In the first ESPI subtraction fringes are considered, for which an improvement in accuracy is seen, and in the second ESPI addition fringes are considered, which, we believe, has enabled the phase to be extracted for the first time. The algorithms are demonstrated with theoretical data and with experimental ESPI fringepatterns recorded with a cw laser. As presented, they form the first step toward a procedure that can beused with twin-pulsed ESPI.     

Recording of In-plane Surface Displacement by Double-exposure Speckle Photography

Local displacements in the plane of an object surface illuminated by coherent light may be measured by recording double-exposure photographs of it. When the surface is illuminated symmetrically by two oblique beams, ‘speckle correlation fringes’ appear in the doubly exposed negative, due to the non-linear nature of the photographic recording. Factors affecting the visibility of these fringes and the range of displacement which can be measured are discussed. The fringe visibility falls to zero for displacements larger than a speckle width, but measurements can then be performed upon the optical transform of the negative image. With uni-directional object illumination, the in-plane displacement can be measured on a point-by-point basis in magnitude and in direction from Young's fringes observed. With two symmetrical oblique illuminating beams the effect of a small surface strain is displayed, even if large lateral displacement has occurred, by using spatial filtering when viewing the photographic image.     

Denoising in digital speckle pattern interferometry using fast discrete curvelet transform

In digital speckle pattern interferometry, the denoising of speckle fringe patterns is of vital importance for quantitative extraction of phase distribution. A filtering method of fast discrete curvelet transform based on weighted average thresholding technique is proposed in this paper for noise removal in speckle fringe patterns. Both computer-simulated and experimental digital speckle pattern interferometry fringe patterns are adopted to evaluate the performance of the proposed filtering method. In addition, a widely used and representative filtering method, windowed Fourier filter, is introduced for making a comparison and validation in the image processing effect, and the parameter of peak signal noise ratio is also used for assessment of denoising effect. It is shown from the filtered results that the filtering method of fast discrete curvelet transform is effecitve to remove speckle noises and simultaneously preserve fringe structure information.     

Noise reduction in digital speckle pattern interferometry using bidimensional empirical mode decomposition

We propose a bidimensional empirical mode decomposition (BEMD) method to reduce speckle noise in digital speckle pattern interferometry (DSPI) fringes. The BEMD method is based on a sifting process that decomposes the DSPI fringes in a finite set of subimages represented by high and low frequency oscillations, which are named modes. The sifting process assigns the high frequency information to the first modes, so that it is possible to discriminate speckle noise from fringe information, which is contained in the remaining modes. The proposed method is a fully data-driven technique, therefore neither fixed basis functions nor operator intervention are required. The performance of the BEMD method to denoise DSPI fringes is analyzed using computer-simulated data, and the results are also compared with those obtained by means of a previously developed one-dimensional empirical mode decomposition approach. An application of the proposed BEMD method to denoise experimental fringes is also presented.     

Real-time speckle interferometry fringe formation with an adaptive phase mask

We describe what we believe is a novel speckle-pattern interferometry method of applying a spatial light modulator (SLM) as an adaptive phase mask to obtain real-time fringes of a deformed object without using conventional correlation methods of electronic subtraction or addition. The method is to use a SLM to cancel initial phase in the speckled image before the object is deformed. The fringes from the deformed object can be visualized directly after the initial phase has been canceled. A commercial liquid-crystal television is used as a SLM. The performance of using this SLM in an out-of-plane speckle interferometer is demonstrated.     

散斑图样条纹自动分析的一种方法

本文论述散斑图样条纹的一种新的图象处理方法密度滤波和特征提取。介绍了散斑计量原理和基本的图象处理方法,以及逐点分析散斑图的自动系统。用该系统测得的面内位移和离面空间位移,其测量误差分别小于1.6%和2.3%。     

Signal-to-noise based local decorrelation compensation for speckle interferometry applications

Speckle-based interferometric techniques allow assessing the whole-field deformation induced on a specimen due to the application of load. These high sensitivity optical techniques yield fringe images generated by subtracting speckle patterns captured while the specimen undergoes deformation. The quality of the fringes, and in turn the accuracy of the deformation measurements, strongly depends on the speckle correlation. Specimen rigid body motion leads to speckle decorrelation that, in general, cannot be effectively counteracted by applying a global translation to the involved speckle patterns. In this paper, we propose a recorrelation procedure based on the application of locally evaluated translations. The proposed procedure implies dividing the field into several regions, applying a local translation, and calculating, in every region, the signal-to-noise ratio (SNR). Since the latter is a correlation indicator (the noise increases with the decorrelation) we argue that the proper translation is that which maximizes the locally evaluated SNR. The search of the proper local translations is, of course, an interactive process that can be facilitated by using a SNR optimization algorithm. The performance of the proposed recorrelation procedure was tested on two examples. First, the SNR optimization algorithm was applied to fringe images obtained by subtracting simulated speckle patterns. Next, it was applied to fringe images obtained by using a shearography optical setup from a specimen subjected to mechanical deformation. Our results show that the proposed SNR optimization method can significantly improve the reliability of measurements performed by using speckle-based techniques.     

Improvement to human-face recognition in a volume holographic correlator by use of speckle modulation

We show that a speckle-modulation technique can improve the parallelism and the recognition accuracy of volume holographic correlators. The object patterns are modulated by a speckle pattern generated by a diffuser. These modulated patterns are stored as Fourier holograms by use of angular-fractal multiplexing. With the speckle modulation the sidelobes are completely suppressed, the cross talk is negligible, and the correlation peak becomes a bright sharp spot. Thus higher recognition accuracy is achieved. The angular separation between adjacent patterns in the multiplexing could be much smaller, resulting in larger capacity and higher parallelism of the correlator. Also, this technique can be combined with other methods such as wavelet filtering to achieve a large invariant tolerance range. Theoretical analysis, numerical evaluation, and experimental results are presented to confirm that sidelobes and cross talk are sharply suppressed by the speckle modulation.     

Digital processing of electronic speckle pattern interferometry addition fringes

A digital image-processing method for analyzing double-pulsed electronic speckle pattern interferometry addition fringes is described. The procedure consists of three steps, forming a combination particularly suited to addressing some important practical limitations of the measurement system. In the first step it is shown that in certain cases fringe visibility may be enhanced by subtraction of a reference interferogram, so that a pattern with a quality similar to that of a subtraction one is obtained. In the second step noise is reduced by the application of a spectral subtraction image-restoration method. The third step concerns the calculation of the wrapped phase by means of a Fourier transform method with bandpass filtering. Preliminary experimental results that illustrate the performance of this approach are presented.     

Study of adaptive elliptical ideal low-pass filters used for denoising of digital speckle interferograms obtained in phase-shifting digital speckle pattern interferometry

Ideal low-pass filtering methods were studied in detail and adaptive elliptical ideal low-pass filters proposed for the first time. The proposed adaptive elliptical ideal low-pass filters can be used effectively for denoising of digital speckle interferograms obtained in phase-shifting digital speckle pattern interferometry. The shape of adaptive elliptical ideal low-pass filters is determined according to the spectrum distribution of interference fringes by using the adaptive method. Theoretical analysis, simulation calculation, and experimental results are presented, and are in good agreement.     

用散斑干涉光谱分布检测瞬态温度

为了提高对瞬态温度检测的灵敏度,提出了基于散斑干涉条纹光谱分析的瞬态温度反演算法.系统利用散斑干涉形成干涉条纹,由于瞬态温度的变化会使材料应变,从而使散斑干涉条纹改变.被测表面形变前后获得的干涉条纹由面阵 CCD 采集,其对应的光谱密度分布函数也会发生相应的改变,即由散斑干涉条纹反演得到的中心波长振幅发生改变.通过对两次中心波长幅值的比值的检测和计算,即可获得被测的瞬态温度.在分析计算了瞬态温度变化与材料应变、材料应变与干涉条纹变化的函数关系的基础上,推导了瞬态温度变化与干涉条纹振幅及相位函数关系.实验采用660 nm 半导体激光器,SI6600型面阵 CCD 探测器,从获得的光谱分布函数中提取中心波长处幅值比值,通过计算和标定,最终温度检测精度可达到±2℃.相比传统的直接检测干涉条纹的变化量,由被测面形变量推导温度的方法精度提高了近一个数量级,其精度更高、检测均匀性更好、稳定性更好.     

Contouring of Diffused Objects by Using Lensless Fourier Transform Digital Holography and Dual‐Index Immersion Method

Abstract: A simple method for contouring of diffused objects by using lensless Fourier transform digital holography and dual‐index immersion method is presented. It is noticed that to get more accurate results speckle noise should be eliminated/reduced from the reconstructed phase map from digital holograms. Speckle noise is handled by 5 × 5 median filtering. Depth contour interval up to a maximum of 0.12 mm could be achieved without making the interference phase fringes overcrowded.     

Programmable color liquid-crystal television spatial light modulator: transmittance properties and application to speckle-correlation displacement measurement

Drive electronics developed for a color liquid-crystal television (LCTV) display enable data to be written onto individual pixels. Display transmittance characteristics obtained with the new and the original TV drive electronics are compared. The enhanced performance obtained through this development has some potential for spatial light modulator applications in color, optical information processing based on the low-cost LCTV. As an example, we describe a novel, to our knowledge, speckle metrology technique used to display fringes and to output correlation peaks resulting from in-plane object displacement. This requires only a single LC display to encode, simultaneously in three pixel colors, speckle and fringe patterns for real-time measurements. Relative merits of this technique, including displacement range and temporal resolution, are discussed.     

Three-wavelength electronic speckle pattern interferometry with the Fourier-transform method for simultaneous measurement of microstructure-scale deformations in three dimensions

We present the simultaneous measurement of three-dimensional deformations by electronic speckle pattern interferometry using five object beams and three colors. Each color, corresponding to an orthogonal direction of displacement, is separated through dichroic filtering before being recorded by a separate CCD camera. Carrier fringes are introduced by tilting the beam path in one arm of each of the three interferometers. The measured deformation modulates these carrier fringes and is extracted using the Fourier-transform method to achieve high displacement sensitivity. The field of view is on the order of a millimeter, making the system suitable for study of microstructural deformations. We compare experimental results with calculated values to validate out-of-plane and in-plane deformation measurements and demonstrate sensitivity on the order of 10 nm.