Phase-conjugate holographic system for high-resolution particle-image velocimetry

A novel holographic particle-image velocimeter system has been developed for the study of threedimensional (3-D) fluid velocity fields. The recording system produces 3-D particle images with a resolution, a signal-to-noise ratio, an accuracy, and derived velocity fields that are comparable to high-quality two-dimensional photographic particle-image velocimetry (PIV). The high image resolution is accomplished through the use of low f-number optics, a fringe-stabilized processing chemistry, and a phase conjugate play-back geometry that compensates for aberrations in the imaging system. In addition, the system employs a reference multiplexed, off-axis geometry for the determination of velocity directions with the cross-correlation technique, and a stereo camera geometry for the determination of the three velocity components. The combination of the imaging and reconstruction subsystems makes the analysis of volumetric PIV domains feasible.     

基于双目相机的光栅立体印刷图像合成

目的研究一种基于双目相机的光栅立体图像合成方法。方法首先用双目相机采集场景中2个观察角度的二维平面图像数据。基于双目立体视觉理论,采用一种鲁棒性较强的基于区域分割的图像匹配方法,得到精度较高的深度图。然后分析序列视差图像的成像模型,建立一种基于双目图像对生成序列视差图像的方法,得到连续角度等间隔的序列图像。最后基于柱镜光栅的光学特性形成的立体印刷图像编码规则,对序列视差图像进行纵向条纹抽样分割,等间隔的抽取每幅序列视差图像中的对应列实现光栅立体图像的合成。结果验证了该光栅立体图像合成方法的有效性。结论基于双目相机的光栅立体合成方法,可以使立体印刷产品实现个性化、便捷化的即时输出。     

用于毫米波焦面阵成像系统的扩展半球介质透镜

毫米波成像是近年来毫米波领域的一个研究热点,而焦面阵成像因其具有实时成像的优点更加受到重视。中分析可用为面阵成像的扩展半球介质透镜。这种焦面阵成像结构将集成天线阵贴附在透镜背面接收透镜聚焦的功率,消除了集成天线工作在毫米波频段时存在的表面波对天线性的影响,具有尺寸紧凑、损耗小的特点。采用Stratton-Chu公式和射线追迹分析了电磁波入射到扩展半球透镜上时在其背面的场分布,即透镜的焦区场分布,以获得透镜用于焦面阵成像时的性能。为验证分析方法的正确性,对平面波垂直入射和会聚高斯束入射两种情形进行了实验验证,实验结果与理论分析吻合较好。该透镜天线还可用来消除常规集成毫米波系统中抛物面天线与集成前端之间的过渡,以降低损耗,改善系统性能,也可用于与准光系统的连接或耦合。该结果将对上述应用提供理论指导。     

Volume holographic imaging in transmission geometry

We address the performance of transmission geometry volume holograms as depth-selective imaging elements. We consider two simple implementations using holograms recorded with spherical and plane beams. We derive the point-spread function (PSF) of these systems using volume diffraction theory and use the PSF to estimate depth resolution. Furthermore, we show that appropriately designed objective optics can significantly improve the depth resolution or the working distance of plane-wave reference holographic imaging systems. These results are confirmed experimentally and demonstrated for objects with millimeter axial features, imaged from the 5- to 50-cm range.     

Incorrect depth sense due to focused object distance

In stereoscopy, stereoscopic depth distortion is a serious problem in terms of determining the correct depth sense. There have been few studies pertaining to the problem in terms of the focused object distance (FOD). In this investigation, we discuss the FOD as one of the factors inducing an incorrect depth sense in using common stereo camera systems and propose a method for compensating the incorrect depth sense, which is strongly related to the process of demagnifying the size of displayed stereo image on the screen. The incorrect depth sense increases when the FOD becomes shortened. Our method illustrates that the depth sense difference between a correct depth sense and an induced depth error is compensated completely. We verified the validation of our concerns by both a theoretical simulation and a practical experiment.     

立体视觉曲面重建与系统误差分析

采用立体视觉空间曲面重建技术对三维曲面表面成像进行边缘提取、图像匹配、匹配点空间位置计算等步骤,得到三维曲面表面点的空间位置,利用空间点信息对三维曲面形状进行重建,恢复曲面三维形状;并讨论了立体视觉系统的摄像机分辨率、测量范围和摄像机间距等参数之间的关系.利用该方法对堆积物表面形状及体积进行测量实验结果表明,该方法能准确、快速、方便地给出三维曲面的形状.     

Performance Analysis and Evaluation of Geometric Parameters in Stereo Deflectometry

This paper presents a novel geometric parameters analysis to improve the measurement accuracy of stereo deflectometry. Stereo deflectometry can be used to obtain form information for freeform specular surfaces. A measurement system based on stereo deflectometry typically consists of a fringe-displaying screen, a main camera, and a reference camera. The arrangement of the components of a stereo deflectometry system is important for achieving high-accuracy measurements. In this paper, four geometric parameters of a stereo deflectometry system are analyzed and evaluated: the distance between the main camera and the measured object surface, the angle between the main camera ray and the surface normal, the distance between the fringe-displaying screen and the object, and the angle between the main camera and the reference camera. The influence of the geometric parameters on the measurement accuracy is evaluated. Experiments are performed using simulated and experimental data. The experimental results confirm the impact of these parameters on the measurement accuracy. A measurement system based on the proposed analysis has been set up to measure a stock concave mirror. Through a comparison of the given surface parameters of the concave mirror, a global measurement accuracy of 154.2 nm was achieved.     

Stereoscopic imaging and computer vision of impinging fires by a single camera with a stereo adapter

In this article, the novelty of using a single camera for stereoscopic imaging and computer vision of fire dynamics has been described. Specifically, 3D surface topology of the fires can be reconstructed and visualized using stereoscopic methodology. The basic stereo apparatus used in the present study consists of a high‐resolution digital camera and a stereo adapter, which is mounted to the front filter ring of the camera. A pair of stereo images could therefore be formed through the same lens system and recorded simultaneously on the same charge coupled device (CCD) chip. The mechanism of using a stereo adapter for stereoscopic imaging and its geometry has been elaborated in detail. The digitally reconstructed 3D results have also been validated by optical stereoscopic viewing using a pair of electronic shutter glasses synchronized with a computer monitor. The results have demonstrated that the single camera technique is a very powerful and cost‐effective diagnostic tool for fire studies. © 2005 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 15, 114–122, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ima.20044     

Computer-generated holograms of a real three-dimensional object based on stereoscopic video images

A novel method of using stereoscopic video images to synthesize the computer-generated hologram (CGH) patterns of a real 3D object is proposed. Stereoscopic video images of a real 3D object are captured by a 3D camera system. Disparity maps between the captured stereo image pairs are estimated and from these estimated maps the depth data for each pixel of the object can be extracted on a frame basis. By using these depth data and original color images, hologram patterns of a real object can be computationally generated. In experiments, stereoscopic video images of a real 3D object, a wooden rhinoceros doll, are captured by using the Wasol 3D adapter system and its depth data are extracted from them. Then, CGH patterns of 1280 pixels x 1024 pixels are generated with these depth-annotated images of the wooden rhinoceros doll, and the CGH patterns are experimentally displayed via a holographic display system.     

Extended depth of field through an axicon

One of the main disadvantages of imaging systems is the limited depth of field. We present the ability of a refractive axicon for imaging with an extended depth of field. We design an imaging system with a two-step imaging approach that uses a CCD camera to capture intermediate images and uses a digital process to obtain the final images. The depth of field is analyzed based on the condition of the focal segment. In particular, the point spread functions (PSFs) are discussed in simulation in detail. The performed experiments validate the effect and feasibility of the axicon for imaging with an extending depth of field.     

Dual-Color Antenna-Coupled LEKID for Next-Generation Multi-chroic CMB Focal Planes

We present an antenna-coupled Kinetic Inductance Detector for millimeter wave astronomy. Next-generation telescopes for observing the cosmic microwave background are demanding in terms of number of detectors and focal plane area filling efficiency. Moreover, foreground reduction in B-Mode polarimetry requires sky observation with multiple frequency bands. In this context, KIDs are a promising technology because of their large multiplexing rate, while antenna coupling can provide multi-band and dual-polarization solutions in compact design. We have developed polarization-sensitive dual-band pixel at 140 and 160 GHz with a bandwidth of almost 8% for each sub-band. The design involves a microstrip-excited slot antenna and two open-stub band-pass filters to direct the signal toward two resonators. These are lumped elements capacitively coupled to the antenna and include an aluminum strip as absorber. The architecture proposed is particularly simple to fabricate, via-less and only involves two metallization levels. The transition does not require any dielectric deposition above the resonator, thus preventing limitations from any source of noise due to a non-monocrystalline substrate. Furthermore, the same coupling technique can be applied to many types of microstrip-excited antennas, which allow to accommodate band-pass filters.     

Multi-Chroic Dual-Polarization Bolometric Detectors for Studies of the Cosmic Microwave Background

We are developing multi-chroic antenna-coupled Transition Edge Sensor (TES) bolometer detectors for Cosmic Microwave Background (CMB) polarimetry. Multi-chroic detectors increase focal plane area efficiency, and thus the mapping speed per focal plane area, and provide greater discrimination against polarized galactic foregrounds with no increase in weight or cryogenic cost. In each pixel, a silicon lens-coupled dual-polarized sinuous antenna collects photons over a two-octave frequency band. The antenna couples the broadband millimeter wave signal into microstrip transmission lines, and on-chip filter banks split the broadband signal into multiple frequency bands. Separate TES bolometers detect the power in each frequency band and linear polarization state. We will describe the design and performance of these devices and present optical data taken. Our measurements of dual-polarization pixels in multiple frequency bands show beams with percent-level ellipticity, and percent-level cross-polarization leakage. We will also describe the development of large arrays of these multi-chroic pixels. Finally, we will describe kilo-pixel arrays of these detectors planned for the future CMB experiments that will achieve unprecedented mapping speed.     

Efficient disparity estimation from stereo images using hybrid-guided image filter

Stereo vision process involves capturing the pictures from a camera of the same scene from at least two different locations and calculating the three-dimensional information. Conventionally, these two versions of snapshots are called left and right views which yield the depth information of an object upon relative comparison of its location in two views. Although the stereo image and its applications are becoming increasingly prevalent, there has been very limited research on disparity estimation from stereo images. Most of the existing techniques suffer from the gradient reversal artefacts issue. Therefore, to handle this issue, we have proposed a hybrid-guided image filter for improving the disparity estimation from stereo images. The hybrid filter utilizes the features of guided image filter and Bayesian non-local means with edge aware constraint. Maximum likelihood and local area homogeneity analysis are used to generate the guidance image for the proposed filter. To enhance the quality of disparity estimation from stereo images, segmentation is also done using the modified mean shift technique. Experimental results show that the proposed technique can efficiently estimate the depth maps over the available techniques. One-way ANOVA analysis on experimental results validates that the hybrid filter-based stereo matching outperforms consistently over the state-of-art approaches.     

涂层隐身目标毫米波被动探测分析与计算

毫米波系统有着较微波、红外、光学系统不同特性的优点.目前反装甲导弹和末敏弹的制导通常都是毫米波主/被动双模体制.针对涂层隐身问题,我们研究探索毫米波被动辐射计探测涂层隐身的技术.文章首先分析了毫米波被动探测隐身目标的机理、模型与实验验证.其次,根据探测到的目标天线温度数据,给出了求涂层隐身目标辐射温度方差极小最佳解的方法,并给出3 mm被动辐射计探测目标的实验数据,及涂层目标的毫米波被动探测辐射特性分析.最后给出实例仿真学习曲线.     

Development of a real-time stereo TEM

We have developed a real-time stereo transmission electron microscope (TEM) with tilting illumination. This system allows us to observe three-dimensional (3-D) images directly with a video-rate of 1/30 s. The system comprises two electrostatic deflectors. The first, included in the illumination system, is able to illuminate a specimen at two oblique stereoscopic angles (left and right of the optical axis) up to ±2.3 °. The second deflector, in the imaging system, is used to correct the image separation caused by defocusing of the tilted illumination. These deflectors are operated in synchronization with an NTSC video signal output from a CCD camera to record left projections on odd fields and right projections on even fields. The time series of stereo pairs is transferred to a 3-D display that enables viewing of the 3-D images without special glasses. Real-time observation of ZnO particles demonstrated that 3-D images were clear even while moving the specimen. We applied this system to observing dislocations in an Al thin film.     

Depth extraction by use of a rectangular lens array and one-dimensional elemental image modification

Stereo matching, a technique for acquiring depth information from many planar images obtained by several cameras, was developed several decades ago. Recently a novel depth-extraction technique that uses a lens array instead of several cameras has attracted much attention because of the advantages offered by its compact system configuration. We present a novel depth-extraction method that uses a lens array consisting of vertically long rectangular lens elements. The proposed method rearranges the horizontal positions of the pixels from the collection of perspective images while it leaves the vertical positions of the pixels unchanged. To these rearranged images we apply a correlation-based multibaseline stereo algorithm in properly modified form. The main feature of the proposed method is the inverse dependency of the disparity in depth between horizontal and vertical directions. This inverse dependency permits the extraction of exact depth from extremely periodically patterned object scenes and reduces quantization error in the depth extraction.     

Obtaining spectral stereo images with electronic spectral tuning and polarization separation

The problem of obtaining stereo images of objects in arbitrary narrow spectral intervals is considered. A method of accomplishing this task is proposed, according to which the two light beams forming a stereopair are simultaneously filtered in one acousto-optic cell and the output beams are separated with respect to polarization. An optimum geometry of diffraction is found and requirements to the acousto-optic filter are formulated. The main characteristics of the spectral stereo system are determined.     

Novel method of calibration with restrictive constraints for stereo-vision system

Regarding the calibration of a stereo vision measurement system, this paper puts forward a new bundle adjustment algorithm based on the stereo vision camera calibration method. Multiple-view geometric constraints and a bundle adjustment algorithm are used to optimize the inner and outer parameters of the camera accurately. A fixed relative constraint relationship between cameras is introduced. We have improved the normal equation construction process of the traditional bundle adjustment method, so that each iteration process occurs just outside the parameters of two images that are taken by a camera that has been optimized to better integrate two cameras bound together as one camera. The relationship between the fixed relative constraints can effectively increase the number of superfluous observations of the adjustment system and optimize higher accuracy while reducing the dimension of the normal matrix; it means that each iteration will reduce the time required. Simulation and actual experimental results show the superior performance of the proposed approach in terms of robustness and accuracy, and our approach also can be extended to stereo-vision system with more than two cameras.     

Simplified homography matrix for a large-scale camera array system

We want to use a large-scale camera array system in which each camera is placed at the desired position to photograph a subject and later render images of the subject viewed from various directions or render images for a three-dimensional display. The homography matrix for each camera should be calculated in advance to correct the captured images. In the case that each camera is physically facing toward the subject as precisely as possible but the captured image still includes geometrical distortion, if the expected error in the deviations from the ideal directions is assumed to be the zero vector, the homography matrix of each camera can be easily obtained.