Photorefractive multichannel correlator based on stoichiometric LiTaO3

We demonstrate a Vander Lugt type correlator based on photorefractive stoichiometric LiTaO₃ as the volume holographic material. Accurate correlator output was obtained for binary phase-modulated input images. The correlator is tested with binary amplitude- and phase-modulated input images. Phase-modulated images show a high-quality correlation, whereas amplitude-modulated images suffer in part from a photorefractive self-focusing effect. Using 100 previously stored holograms as filters, a rate of 10000 correlations/s are demonstrated. A paraxial theoretical analysis of the correlator, giving analytic expressions for the shift invariance, is also described in detail. Received: 22 November 2000 / Revised version: 9 February 2001 / Published online: 20 April 2001     

Rotation-invariant pattern recognition using morphological phase-only correlation

We present a rotation-invariant nonlinear correlator based on the circular harmonic filter (CHF) and the previously proposed morphological phase-only correlator (MPC) [Q. Wang, S. Liu, Opt. Commun. 244 (2005) 93]. We refer to this correlator as a rotation-invariant MPC (RIMPC). Through computer simulation, we compare the output results of RIMPC with those of rotation-invariant MC (RIMC) and CHF when input scene is corrupted by salt-and-pepper noise, white additive Gaussian noise and cluttered background. Our results show that RIMPC yields higher discriminability, sharper and higher correlation peaks and displays better stability against the above three kinds of noise than do the RIMC and common CHF.     

基于光学相关的空间相机像移测量方法研究

为了测量由卫星姿态不稳定或振动等原因引起的空间相机的亚像元像移,使用了光学联合变换相关的方法对安装在相机焦面上的辅助面阵CCD采集到的相邻两帧图像进行相关运算.给出了使用该方法测量像移的原理,提出对联合变换相关器的输入面采用拉普拉斯卷积核进行边缘锐化,对功率谱相减并以0值进行二值化的处理方法提高像移测量准确度,并使用该方法对不同信噪比下的不同景物进行了像移测量仿真.结果表明:对于信噪比SNR=1的输入图像,当像移范围在 0~55个像元内时,该方法的像移测量误差小于0.2个像元.     

Conjugate Image Plane Correlator with Holographic Disk Memory

A conjugate image plane correlator with holographic disk memory is proposed. Optical correlation between conjugate images reconstructed from a holographic disk and an input image on liquid-crystal television is executed with the rotation of the disk. Regardless of Fourier hologram recording with the pseudorandom diffuser, it is found possible to take out the diffuser from the original hologram recording scheme using an image reconstruction process and to get correlation signals between input and reconstructed conjugate images in the output plane of a two-lens imaging system. Generation of conjugate replicas with high contrast causes exact matching with an input image which results in high recognition performance for autocorrelation signals. The transfer function of an optical system can be controlled by adjustment of either hologram size or hologram area illuminated with a laser beam. Hence, the output intensity distribution can be adjusted by selecting a proper pupil function and the size of an output pupil defined by the input pupil size and the optical system magnification factor. The real-time character recognition by optical parallel high-speed processing for two dimensional images with position normalization is demonstrated.     

修正振幅调制的光电混合圆谐联合变换相关器作旋转不变的目标检测

提出一种修正振幅调制的光电混合圆谐联合变换相关器作旋转不变的目标检测的实验系统。参考图像（即圆谐展开分量的实部和虚部）、圆盘状的局部偏置函数以及目标图像同时显示于输入面。此方法对联合功率谱作了修正,先将联合功率谱减去纯输入景物（含局部偏置函数）的功率谱和参考图像（含局部偏置函数）的功率谱,再加上局部偏置函数的功率谱,然后将所得修正的联合功率谱乘以振幅调制滤波函数。这种方法能产生比普通的圆谐联合变换     

Target recognition in clutter scene based on wavelet transform

Edge extraction based on wavelet for optical correlation detection is presented. Optical experiments with joint transform correlator (JTC) show that there is a bright application prospect in the field of optical correlation detection by extracting the edge features of input image with the method of wavelet transform. In the course of processing, the multi-scale character of wavelet is used sufficiently. The energy of correlation peaks and the detection ratio of various targets are greatly enhanced by the approach. To demonstrate the feasibility of edges extraction based on WT, small targets and targets in clutter scene are successfully detected.     

Phase-coded aperture for optical imaging

For the optical spectrum region, we describe a novel phase-coded aperture imaging system that can be used in a computational imaging camera. The optical design includes a phase-only screen followed by a detector array. A specific diffraction pattern forms at the detector array when the wavefront from a point source object passes through the phase screen. Since diffraction effects cannot be ignored in the optical regime, an iterative phase retrieval method is used to calculate the phase coded screen. Correlation type processing can be applied for the image recovery. Computer simulation results are presented to illustrate the excellent imaging performance of this camera.     

Compact and self-aligned all-optical image correlator based on third-harmonic generation

We demonstrate a compact optical correlator using a diffractive optical element (DOE) beam splitter for 2D optical image processing. Image frequency conversion and correlation are demonstrated using third-harmonic generation (THG) in an organic film with a 1550 nm femtosecond laser. Spatial and temporal alignment of the femtosecond pulses are obtained by imaging the DOE onto the organic film.     

基于多变量鉴别分析的光学畸变不变性图像识别

提出一种基于鉴别分析的光学畸变不变性图像识别方法：对包含各种畸变的训练图像集采用主成分分析得到若干本征图像,作为参考模式与测试图像做光学相关,利用本民输入图像的相关结果作为识别特征,采用最佳鉴别分析做了训练和识别,即可实现对输入图像的畸变为性快速识别。采用非相干光相关器为光学实现硬件,给出了实验结果。     

Methods of Fourier optics in digital holographic microscopy

In this paper, processing methods of Fourier optics implemented in a digital holographic microscopy system are presented. The proposed methodology is based on the possibility of the digital holography in carrying out the whole reconstruction of the recorded wave front and consequently, the determination of the phase and intensity distribution in any arbitrary plane located between the object and the recording plane. In this way, in digital holographic microscopy the field produced by the objective lens can be reconstructed along its propagation, allowing the reconstruction of the back focal plane of the lens, so that the complex amplitudes of the Fraunhofer diffraction, or equivalently the Fourier transform, of the light distribution across the object can be known. The manipulation of Fourier transform plane makes possible the design of digital methods of optical processing and image analysis. The proposed method has a great practical utility and represents a powerful tool in image analysis and data processing. The theoretical aspects of the method are presented, and its validity has been demonstrated using computer generated holograms and images simulations of microscopic objects.     

Multitarget Detection Using a Hybrid Optical Joint Transform Correlator with Power Spectrum Subtraction

Multitarget detection using a hybrid optical joint transform correlator with power spectrum subtraction is proposed. This architecture is a programmable single spatial light modulator joint transform correlator. It consists of a liquid crystal display panel, an imaging lens, and a liquid crystal light valve to enhance resolution and space-bandwidth production. In this technique, it has two joint images in the input plane. In the second joint image, the reference image is contrast-reversal. With this technique, the second joint power spectrum is subtracted from the first joint power spectrum and the subtracted joint power spectrum is then taken the inverse Fourier transform. This technique is found to yield better correlation output performance than that of the classical joint transform correlator. Computer simulation results and the optical experimental results for input scene of the multitarget are given.The results have verified the correctness of the system design and performance analysis.     

Binary phase only filters for rotation and scale invariant pattern recognition with the joint transform correlator

The joint transform correlator (JTC) is one of the two main optical image processing architecture which provides a highly effective way of comparing images in a wide range of applications. Traditionally, an optical correlator is used to compare an unknown input scene with a pre-captured reference image library, to detect if the reference occurs within the input. Strength of the correlation signal decreases rapidly as the input object rotates or varies in scale relative to the reference object. The aim of this paper is to overcome the intolerance of the JTC to rotation and scale changes in the target image. Many JTC systems are constructed with the use of ferroelectric liquid crystal (FLC) spatial light modulators (SLMs) as they provide fast two-dimensional binary modulation of coherent light. Due to the binary nature of the FLC SLMs used in the JTC systems, any image addressed to the device need to have some form of thresholding. Carefully thresholding the grey scale input plane and the joint power spectrum (JPS) has significant effect on the quality of correlation peaks and zero order (DC) noise. A new thresholding technique to binarise the JPS has been developed and implemented optically. This algorithm selectively enhances the desirable fringes in the JPS which provide correlation peaks of higher intensity. Zero order noise is further reduced when compared to existing thresholding techniques.Keeping in mind the architecture of the JTC and limitations of FLC SLMs, a new technique to design rotation and scale invariant binary phase only filters for the JTC architecture is presented. Filers design with this technique have limited dynamic range, higher discriminability among target and non-target objects, and convenience for implementation on FLC SLMs. Simulation and experiments shows excellent results of various rotation and scale invariant filters designed with this technique. A rotation invariant filter is needed for various machine vision applications of the JTC. By fixing the distance between camera and input object, the scale sensitivity of the correlator can be avoided. In contrast to the industrial machine vision applications, scale factor is very important factor for the applications of the JTC systems in defence and security. A security system using a scale invariant JTC will be able to detect a target object well in advance and will provide more time to take a decision.     

Multitarget Detection Using a Hybrid Optical Joint Transform Correlator with Power Spectrum Subtraction

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基于体全息光学相关器的身份认证复合检索方法

体全息相关器具有高速、并行和多通道等特点。基于体全息相关器大幅面、高分辨率、二维数据页结构,将身份认证图文信息进行编码及格式化,组合为适用于光学相关器的图像模板。数据库的图像模板以体全息图的形式存储在铌酸锂晶体中,识别时将待识别的全部或部分身份认证信息输入,光学相关器可快速返回输入图像与所有库图像模板的相关值,从而完成对整个数据库的快速复合检索。介绍了该方法的工作原理、理论模型、编码和组页方法以及复合检索工作流程,给出了演示实验结果。     

Extended fractional wavelet joint transform correlator

An extended fractional wavelet joint transform correlator is implemented for real-time target recognition applications. The real-time input scene captured using a charge-coupled device camera along with the reference image is fractional Fourier transformed. The obtained joint power spectrum is multiplied by an appropriately scaled wavelet filter and the resultant function is differentiated. The application of wavelet filter enhances the correlation outputs and differential processing of wavelet-filtered joint power spectrum improves the detection efficiency by reducing the zero-order spectra. Targets with Gaussian and speckle noise have also been used to check the correlation output. The performance metrics: correlation peak intensity, peak-to-correlation energy, peak-to-sidelobe ratio and signal to clutter ratio have been calculated. The experimental results are presented in support of the proposed idea.     

弹道相机图像分析系统与普通扫描仪精确标定

研制该系统的目的是对弹道干板进行数字化、高精度、高自动化的判读,代替传统上低效率的人工判读方法。该系统使用普通平板扫描仪将弹道相机拍摄到的干板图像进行数字化并对扫描图像进行高精度的标定和修正,再利用数字图像处理技术对于板图像进行分析判读。着重叙述了用分布于待测图像四周的标准正交网络对包括待测图像在内的全场进行标定和修正的方法及实验结果。经过本方法标定和修正后,扫描图像几何标准偏差在2～3μm以内。该判读系统及所用到的扫描仪标定方法,实现了弹道干板的数字化、自动化的高精度判读,使普通扫描仪可用于精度要求极高的工程测量。     

Spectral band invariant wavelet-modified MACH filter

Filters synthesized with images of a specific spectral band in general fail to recognize targets in a different spectral band. In this paper, we therefore demonstrate the use of the wavelet-modified maximum average correlation height (WaveMACH) filter for automatic target recognition applications in both the visible and infrared (IR) spectral bands. As any input target appears different when imaged through two different sensors, i.e., a CCD or an IR camera, a WaveMACH filter synthesized using a CCD image shows no correlation with the image of the same target from an IR camera and vice-versa. Hence, separate filters are required to match the input targets from the two sensors. To avoid the synthesis and storage of separate filters, the images from CCD and IR camera are fused using Daubechies wavelet and then the rotation-invariant WaveMACH filter generated with the fused image. In all, 18 WaveMACH filters (each of 20° range) are required for in-plane rotation invariance in both the spectral bands for the full range of 0–360°. Computer simulation and experimental results implemented in hybrid digital–optical correlator architecture are shown for the proposed idea. The same filters have also been used to identify multiple targets in a scene. Performance measures like peak-to-sidelobe ratio (PSR), peak correlation energy (PCE) and correlation peak intensity (CPI) have been calculated as metrics of goodness.     

Performances of an all-optical subtracted joint transform correlator that uses a photorefractive crystal

An all-optical subtracted joint transform correlator (JTC) was proposed and it was experimentally shown that the correlator successfully performs the pattern recognition with the number of reference patterns. In the correlator, an optical interferometer was introduced to remove the additional correlation signals between the reference patterns themselves from the output plane and a photorefractive crystal was also introduced to record the joint power spectra as volume holograms. The processing for removing the additional correlation signals was performed by the parallel computations in the interferometer in real time. In this study it has been found that the correlator yields significantly better correlation results than the classical joint transform correlator does and the recording of the volume hologram corresponds to a nonlinear processing of the joint power spectrum in the fringe-adjusted joint transform correlator. However the volume hologram has angular selectivity for matching the Bragg condition. It would restrict the shift-invariant area of the input image plane. This restriction has been investigated experimentally.     

Laser spot location method for a laser pointer interaction application using a diffractive optical element

A diffractive optical element (DOE) is applied to effectively locate a laser pointer spot on a projection screen for laser pointer interaction applications. The DOE is placed in front of a digital web camera to blur the background image while transforming the laser spot into a large diffractive pattern, such as a circle. To calculate the diffractive pattern position on the screen, only a simple subtraction method using two successive digital images with the laser ON and OFF, respectively, is needed. This approach also improves the compressed digital image transmission latency.     

Mach-Zehnder joint transform correlator with multi-channel quantized reference functions for color pattern recognition

Based on the Mach-Zehnder interferometer, we introduce a novel construction of the optical correlator for color pattern recognition. This system could remove the zero order term, which decreases the discrimination ability of the system, in only one step. Since each color can be separated into 3 channels, we propose the multi-level quantized reference functions (MQRF) for each color channel, and utilize them in spatial domain. Consequently, a total of 3 MQRFs and 3 color channels of the test image could be implemented at the input plane of liquid crystal spatial light modulator (LCSLM) of Mach-Zehnder joint transform correlator. Each LCSLM is modulated along the real-valued axis.