Input-image homogenization as a method to improve a correlator's discrimination capability

We propose an input-image preprocessing method consisting of homogenization of the image to improve the discrimination capability of a correlation-based recognition process. This method is an approximation of the optimal filter. It offers the advantage that correlation with the preprocessed images can easily be implemented in an optical correlator working with phase-only spatial light modulators.     

Passive ranging and a three-dimensional imaging system through wavefront coding

A single-image passive ranging and three-dimensional(3 D) imaging system with chiral phase encoding was proposed in 2011 [Opt. Lett. 36, 115(2011)]. A new theoretical analysis of the system in space domain is presented in this paper. We deduce the analytic relationships between the object distance and the point spread function, and between the object distance and the encoded image, respectively. Both the point spread function and the processed spectrum of the encoded image have two spots, which will rotate with the variation of the object distance. Then the depth map is extracted from the encoded image and it can be used to set up 3 D images. The theoretical analysis is verified by a wavefront coding system with a chiral phase which is generated by a phase-only liquid-crystal spatial light modulator. The phase generated by the liquid-crystal spatial light modulator is more flexible than the fixed phase mask and can be adjusted in real time. It is especially suitable for observing the object with a large depth of field.     

Reconfigurable generation of high-repetition-rate optical pulse sequences based on time-domain phase-only filtering

We propose and demonstrate a fiber-based phase-only filtering technique for programmable optical pulse shaping, in which the filtering operation is implemented in the time domain by means of an electro-optical (EO) phase modulator. The technique has been applied for generating customized ultrahigh-repetition-rate optical pulse sequences (>40 GHz) from single input pulses by driving the EO phase modulator with a periodic electronic waveform (RF tone). The generated output pulses are replicas of the input pulse and both the repetition rate and the envelope profile of the generated sequences can be controlled and tuned electronically using this approach.     

Image processing with the radial Hilbert transform: theory and experiments

The Hilbert transform is useful for image processing because it can select which edges of an input image are enhanced and to what degree the edge enhancement occurs. However, the transform operation is one dimensional and is not applicable for arbitrarily shaped two-dimensional objects. We introduce a radially symmetric Hilbert transform that permits two-dimensional edge enhancement. We implement one-dimensional, two-dimensional, and radial Hilbert transforms with a programmable phase-only liquid-crystal spatial light modulator. Experimental results are presented.     

Subband mutiresolution by multi-object phase-only joint transform correlator

Subband multiresolution technique presents decomposition and reconstruction properties for 2D images using multichannel spatial filtering. Each subband image represents multiresolution of the input image. In this paper, optic subband multiresolution architecture is proposed based on multi-object phase-only joint transform correlator (JTC). Multichannel filtering is realized using multi-object input joint image that includes bipolar data. The bipolar data (containing positive, zero and negative values) is encoded by phase in the input and Fourier planes of the phase-only JTC. For image decomposition and reconstruction, two optoelectronic subband multiresolution architectures have been developed. Each architecture has been tested using a simulation program.     

Optimal Incoherent Filtering for Distortion-Invariant Image Recognition

An optimal incoherent filter is proposed for distortion-invariant correlation. The optical transfer function (OTF) of the correlator is specified as a realizable phase-only term which is optimized by the simulated annealing algorithm. The phase-only OTF is used in image recognition to produce a sharp peak response and can be generated by dual-filter synthesis. Image recognition is achieved by subtracting two spatially incoherent correlation outputs produced from a positive and negative filters with an input image. Computer simulations show that the proposed correlator can yield a sharp correlation peak with excellent distortion tolerance.Presented at 1996 International Topical Meeting on Optical Computing (OC ‘96), April 21–25, Sendai, Japan     

Compact optical correlator based on one phase-only spatial light modulator

An optical correlator that utilizes one phase-only spatial light modulator (SLM) combined with a mirror is proposed and demonstrated. This system is compressed by displaying the input and filter pattern on different parts of the same SLM. The background noise is separated from the correlation signal by superimposing a high-frequency carrier with the filter pattern except in the zero-frequency regions, which will improve the signal-to-noise ratio in pattern recognition. Our architecture is compact and the cost is relatively low by utilizing only one SLM.     

仅用参考物功率谱取阈的二元联合变换相关器

提出一种用特征物功率谱取阈值的新型二元联合变换相关器。与传统的联合变换相关器相比较,它有高光能效率、高鉴别率和输入空间宽带的优点;与现行的二元联合变换相关器相比较,它有鉴别率高和不需靶物功率谱测量的特点,因而是一种简便和高性能的相关器。     

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.     

Phase contrast enhancement in microscopy using spiral phase filtering

We demonstrate two methods based on Fourier plane filtering using (a) a fractional spiral phase plate (SPP) and (b) an off-axial SPP for phase contrast enhancement in optical microscopy. In comparison to previous works, a spatially incoherent LED is used in the Köhler illumination as the light source to illuminate the biological specimen. We demonstrate that both these methods can transform the phase specimen into a relief-like view even under such illumination. The degree and orientation of enhancement can be controlled by changing the phase structure of the filter. The SPP is displayed on a phase-only spatial light modulator, and can be integrated into the optical path of standard microscopes.     

基于液晶空间光调制器的全息显示

在传统的纯相位全息显示系统中, 一般基于快速傅里叶变换(FFT)算法来计算相位全息图, 在FFT的计算中需要遵循Nyquist采样定理, 因此, 重建图像的尺寸往往受限于空间光调制器的固定采样率. 这个限制可以通过卷积算法或者两步菲涅耳衍射算法来解决, 但是需要使用多个FFT的计算, 导致计算量增大. 鉴于此, 提出了一种基于透镜的纯相位全息图计算方法. 在全息图的计算中, 通过透镜的成像原理建立一个采样率可变的虚拟全息面, 通过调节相应的距离参数使得在全息图的计算中可以任意调节原始图像的采样率, 摆脱了传统方法中液晶空间光调制器带宽积对重建图像尺寸的限制, 并且这种算法只需使用一次FFT就能达到变采样率的衍射计算, 大幅提高了全息图的计算速度. 数值模拟及光学实验结果证明了此方法可以在全息显示光学系统中清晰地重建不同尺寸的图像. 同时该系统可以有效地消除由空间光调制器的像素化结构带来的零级衍射.     

Hybrid correlator employing a chirp-encoded binary phase-only filter

A novel correlator architecture is described that is similar to the VanderLugt correlator (VLC) but differs because the input Fourier transform (FT) is performed electronically rather than optically. We present results in which the FT's are reduced to binary phase-only filters (BPOF's); the required multiplication of the input and the reference FT's (performed optically in the VLC) is performed electronically simply by use of a single XOR logic gate. The problematic zero order and symmetric correlation peak are shown to be removed by use of a chirp-encoded BPOF.     

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.     

Phase-only filtering based on sliced orthogonal nonlinear generalized decomposition

The sliced orthogonal nonlinear generalized (SONG) decomposition and correlation have been demonstrated to be powerful tools in digital image processing and promising for nonlinear optical information processing. In this paper, we propose an optical phase-only filtering system based on SONG decomposition (PBS), in which the phase-only filtering of the target and the input scene binary slices are performed separately by pairs and then added together. We numerically show that the PBS has extremely high and sharp output correlation peak compared with other optical correlators. Furthermore, such a SONG decomposition based phase-only filtering naturally inherits the nature of SONG decomposition, which has strong robustness to additive Gaussian noise and substitutive noise, and also the high light efficiency of phase-only filtering. We demonstrate that the PBS may serve as an optimized optical correlation scheme, which is promising in nonlinear optical pattern recognition.     

多目标识别的联合变换相关器的研究

提出了一种可用于多目标识别的联合变换相关器。为改善相关信号的性能,对功率谱作了优化处理。为消除相关面上的零级项和目标间的相关项,可用联合功率谱减去纯目标输入的功率谱和参考图像的功率谱;为增强和锐化相关峰,将相减的功率谱作指数函数滤波处理。分析了指数滤波参数对相关结果的影响。计算机模拟结果表明,这种相关器所输出的相关信号比经典联合变换相关器和二元联合变换相关器输出的相关信号更好,互相关得到了抑制,自相关得到了增强,具有很好的抗噪能力。     

Rotation invariant Target Detection Using a Hybrid Optical Circular Harmonic Joint Transform Correlator with DC Bias

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基于液晶空间光调制器的空间滤波实验

空间滤波实验可以实现对输入物体的频谱分析及图像的边缘增强、噪声消除等各种图像处理。液晶空间光调制器可以作为衍射波面变换器件来使用。在空间滤波实验系统的滤波面上放置了液晶空间光调制器,并用CCD数码相机进行输出图像的观察和记录,分别给出了低通滤波、高通滤波、方向滤波等的仿真实验结果。该实验实现了输入数字物体的实时改变,并能够实时观察采用不同空间滤波器对输出像的影响。     

Microscope system with on axis programmable Fourier transform filtering

We propose an on-axis microscope optical system to implement programmable optical Fourier transform image processing operations, taking advantage of phase and polarization modulation of a liquid crystal on silicon (LCOS) display. We use a Hamamatsu spatial light modulator (SLM), free of flickering, which therefore can be tuned to fully eliminate the zero order component of the encoded diffractive filter. This allows the realization of filtering operation on axis (as opposed to other systems in the literature that require operating off axis), therefore making use of the full space bandwidth provided by the SLM. The system is first demonstrated by implementing different optical processing operations based on phase-only blazed gratings such as phase contrast, band-pass filtering, or additive and substractive imaging. Then, a simple Differential interference contrast (DIC) imaging is obtained changing to a polarization modulation scheme, achieved simply by selecting a different incident state of polarization on the incident beam.     

反射型LCOS器件纯相位调制特性的研究

研究了反射型的LCOS显示器件的相位调制特性.采用2×2的Jones矩阵计算仿真，从理论上分析了它的相位调制特性，建立了一套测量振幅和相位特性的测试系统.它采用He-Ne激光为光源，用干涉仪观察波前相位，将LCOS显示器件作为液晶空间光调制器，用计算机和相关的电路系统驱动控制，并用CCD采样数据，测试了1024×768 LCOS显示器件的相位调制特性.理论和实验的一致性说明了在特定的入射、出射偏振光配制下，LCOS显示器件可以做纯相位调制器. 关键词： 空间光调制器 液晶显示器 反射型LCOS     

Evaluation of a suitable threshold for binarization of power spectrum in a noise-free joint transform correlator

An analysis by digital simulation of the influence of the threshold level on the binary power spectrum of a joint transform correlator has been carried out. The correlation results are studied where binarized intensity takes the values [0, 1] and [-1, 1]. The influence of threshold level on the efficiency and discrimination are analyzed. The results obtained allow us to propose an appropriate level of binarization for noise-free scenes.