畸变测量中应用窗口傅里叶变换载频条纹分析

为了测量光学成像系统的径向畸变,提出了基于伸缩窗口傅里叶变换空间载频条纹相位分析的测量新方法.畸变分布测量转化为调制相位测量.首先,将纵向朗奇基准光栅作为模板,通过成像系统成为变形光栅即畸变像.接着采用伸缩窗口傅里叶变换提取畸变载频光栅条纹中心无畸变点的基频和相位信息,获得理想无畸变像的基频成分,然后对变形载频光栅条纹进行频谱分析,滤波提取基频信息、逆傅里叶变换、相位解包,提取径向调制相位分布,计算畸变图像的径向位置畸变分布.最后利用该径向位置畸变分布规律和双线性插值灰度重建对畸变图像进行校正.详细的理论分析和实验结果证明,该方法是可行的.     

基于液晶光阀微分处理的实时联合变换相关器

提出了一种用液晶光阀实现对输入图像的微分预处理和对功率谱微分处理的实时联合变换相关器。两个液晶光阀均处于非线性变换态,分别在输入面和频谱面上实现对输入图像和联合功率谱的微分处理。微分功率谱经傅里叶透镜变换后得到相关输出。实验表明,该相关器采用全光方式不仅保证了实时性,而且锐化了相关峰;经该相关器微分后的图像,其相关性能明显优于常规联合变换相关器输出的结果。     

分数傅里叶变换理论及其应用研究进展

分数傅里叶变换是傅里叶变换的广义形式,提供了介于时域和频域之间的多分数域信号表征,为非平稳信号处理和线性时变系统分析开辟了新途径,应用十分广泛。本文首先总结近年来分数傅里叶变换的理论研究成果,包括分数傅里叶变换的数值计算、衍生的离散分数变换、分数域采样、分数域滤波与参数估计、多分数域分析。然后介绍分数傅里叶变换在工程和实践中的应用,包括雷达、通信、图像加密、光学干涉测量、医学、生物、机械仪器等。最后对分数傅里叶变换理论及其应用的未来研究方向进行展望。

     

分数博里叶变换的数字仿真研究

本文介绍了一种可用作故障特征提取的新方法——分数傅里叶变换。首先,对分数傅里叶变换的定义和基本性质作了简单的说明,并分析了该方法与时频分析中Wigner分布的关系;其次给出了分数傅里叶变换的快速算法;最后,通过数字仿真对分数傅里叶变换的实现和应用特点进行了初步探讨。     

基于全相位FFT的油套环空中声速计算方法

油套环空中会产生各种噪声,使测得的接箍反射信号非常复杂,环隙中真实的声速则很难计算准确,可通过对原始接箍数据进行傅里叶变换的方法对声速进行估计,但是存在不可忽视的误差。全相位傅里叶变换是傅里叶变换的一种改进方法,能够获得更加准确的频率谱与相位谱。文章采用全相位快速傅里叶变换（all-phase Fast Fourier Transform,apFFT）得到原始接箍信号的频谱,然后通过该频谱进一步计算环隙声速,可得到更加准确的声速估计。通过对不同信噪比下的模拟接箍信号采用快速傅里叶变换（Fast Fourier Transform,FFT）和全相位快速傅里叶变换（apFFT）得到其频谱,可以验证apFFT具有很强的抑制频谱泄漏的能力,且抗噪性能比FFT更好。根据FFT谱和apFFT谱分别计算出声速并对比其精度,可以验证通过apFFT谱计算出的声速稳定性更好、精度更高。然后采用上述两种方法对不同深度井的实测接箍数据进行频谱分析与对比,验证了apFFT较之于FFT对谱峰位置的辨识能力更强,根据谱峰位置计算声速的准确性也将更高。     

分数傅里叶变换的数字仿真研究

本文介绍了一种可用故障特征提取的方法－分数值里叶变换，首称，对分数傅里叶变换的定义和基本性质作了简单的说明，并分析了该方法与时频分析听Ｗｉｇｎｅｒ分布的关系，其次给出了分数傅里变换的快速算法，最后，通过数字仿真对分数傅里叶变换的实现和应用特点进行了初步探讨。     

用功率谱复制技术实现高效联合变换相关

提出利用复制技术对高通滤波后的功率谱复制多份以获得高效的联合变换相关。平方律探测器CCD将功率谱输入计算机,经相减运算和Butter-worth高通滤波后,将其复制成N份,戍阵列形式呈现在空间光调制器上,经光学傅里叶变换得到信号相叠加的高效相关输出。计算机模拟和光学实验结果表明,这种高效相关器能够抑制相关面上的中央零级项,增强、锐化一级相关峰,信噪比提高了4倍多,N份功率谱阵列对应的输出峰值强度约是单份功率谱对应强度的N倍。     

散斑干涉照像术用于微小位移的测量

本文以Vikram等人提出的方法为基础,提出一种测量小于散斑直径的小位移的新方法,使测量范围由一维扩展至二维。 采用预加位移多次曝光的方法,将对小位移的测量转为对散斑照片傅里叶变换条纹宽度和转角的测量。文中作了理论分析并给出实验结果。     

二维傅里叶变换法三维曲面检测

对投影光栅方法获得的位相调制空间载波图进行二维数字图象处理,实现了散射物体的三维曲面测量,利用二维快速傅里嚅变换将空域信号转变为频域信号,对一级频谱进行傅里叶反变换,逐步解调出其位相值,重建了被测物体表面的形貌。文中给出了对人体模型的实验测量结果。     

基于MCSA的异步电机测速系统研究

在满足实时测速的前提下,直接对电机定子电流使用离散傅里叶变换,会带来频谱分辨率差、能量泄漏等问题,从而不能准确获取电机转速信息.为此,提出一种结合快速傅里叶变换、加窗函数、频谱细分和信号识别的综合算法.即将经过调理采样之后的信号加上布莱克曼窗或者凯塞窗,再对其进行复调制高分辨率的快速傅里叶变换,最后有效获取转子信号频率并求出电机速度.实验结果表明,本测速系统测量时间较短且具有较高的测量精度.     

遗传算法在分数阶Fourier变换域极值优化中的应用

分数阶Fourier变换采用线性调频基,因此,线性调频（Linear Frequency Modulation,LFM）信号在分数阶Fourier域平面能够聚焦,并形成峰值。为了克服传统步进式搜索法在LFM信号峰值搜索中效率低下的缺点,将遗传算法引入到分数阶Fourier变换极值搜索中。仿真结果表明,该方法优于传统的步进式搜索法。     

免疫算法在分数阶Fourier变换域极值优化中的应用

利用线性调频(Linear Frequency Modulation,LFM)信号在分数阶Fourier域上的聚焦性,通过搜索可实现LFM信号的检测和参数估计。通常采用步进式搜索法,效率低下。为了克服该缺点,通过对分数阶Fourier域优化问题的研究,将免疫算法引入到分数阶Fourier变换极值搜索中。仿真结果表明:该方法优于传统的步进式搜索法。     

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.     

全息激光防护薄膜的研制

介绍了全息激光防护薄膜基本理论和设计方法。研制成功的全息激光防护薄膜，其光学性能测试表明：０．５３μｍ波长激光的光密度Ｄ达４．１１，防护角达１５°，具有良好的可见光透过率，且抗激光破坏能力强，可用于人眼和光电探测系统的激光防护。     

Nonlinear joint fractional transform correlator

An important tool in optical pattern recognition, the joint fractional transform correlator (JFTC), was introduced recently. We analyze the peak properties of fractional correlation (FC) by symbolic derivation and computer simulation. We show that the FC has a maximum correlation peak when the second fractional Fourier transform is reduced to the conventional Fourier transform. We introduce nonlinear operations in a joint fractional transform power spectrum and propose a differential JFTC and a binary differential JFTC. Numerical simulations show that such nonlinear JFTCs exhibit remarkable improvement in correlation peak intensity, discrimination capability, and signal-to-noise ratio. An optoelectronic setup that can implement such nonlinear JFTCs is also proposed.     

Double-image encryption by iterative phase retrieval algorithm in fractional Fourier domain

A novel double-image encryption algorithm is proposed, which can simultaneously encrypt two images into a single one as the amplitude of a fractional Fourier transform with two different groups of fractional orders. The two original images can be retrieved independently by fractional Fourier transforms with two different groups of fractional orders, one public phase mask, and two different private phase masks. The proposed approach can enlarge the key space, achieve faster convergence in the iterative process, and avoid cross-talk between the two images in reconstruction. Numerical simulations are presented to verify its validity and efficiency.     

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.     

Flexible multiple-image encryption algorithm based on log-polar transform and double random phase encoding technique

We present a novel multiple-image encryption algorithm by combining log-polar transform with double random phase encoding in the fractional Fourier domain. In this algorithm, the original images are transformed to annular domains by inverse log-polar transform and then the annular domains are merged into one image. The composite image is encrypted by the classical double random phase encoding method. The proposed multiple-image encryption algorithm takes advantage of the data compression characteristic of log-polar transform to obtain high encryption efficiency and avoids cross-talk in the meantime. Optical implementation of the proposed algorithm is demonstrated and numerical simulation results verify the feasibility and the validity of the proposed algorithm.     

Precise and fast phase wraps reduction in fringe projection profilometry

Phase wraps in a 2D wrapped phase map can be completely eliminated or greatly reduced by frequency shifting. But it usually cannot be optimally reduced using conventional fast Fourier transform (FFT) because the spectrum can be shifted only by a integer number in the frequency domain. In order to achieve a significant phase wrap reduction, we propose a fast and precise two-step method for phase wraps reduction in this paper, which is based on the iterative local discrete Fourier transform (DFT). Firstly, initial estimate of the frequency peak is obtained by FFT. Then sub-pixel spectral peak with high resolution is determined by iteratively upsampling the local DFT around the initial peak location. Finally, frequency shifting algorithm that operates in the spatial domain is used to eliminate phase wraps. Simulations and experiments are conducted to demonstrate the superb computing efficiency and overall performance of the proposed method.