具有平面结构物体的有限角衍射断层成象

本文介绍一个新方法来处理具有平面结构物体的断层成象问题,这个方法称为线性代数法。对于一个具有两层结构的物体,用两个投影就可得到它的断层象,对入射角受限的情况,文中进一步用伪反滤波方法解决病态问题。模拟计算结果表明,对一具有十层平面结构的物体,入射波的角度变化范围被限制在-8°-8°之间,仅用19个投影,就可得到该物体内各层结构的重建象。     

反射式超声计算机层析成像算法研究

本文通过数学推导证明,对于单发、单收反射式超声计算机层析成像,当声换能器离被成像物体较远时,可采用X-CT中的滤波-逆投影方法进行重建;当距离较近时,上述方法得到的重建像会大大失真。本文提出一种迭代修正方法,通过计算机模拟说明,与X-CT中常用的滤波-逆投影法相比,此方法可使重建像得到明显改善。     

具有颜色恒常性的光谱反射率重建

给定物体三刺激值,重建物体反射率在跨媒体颜色复制领域有着重要应用.常见的重建反射率算法包括基向量法、维纳估计法、加权伪逆方法等,这些方法大多都是为了建立由低维度三刺激值或者RGB向高维度光谱反射率的映射关系,以重建的反射率与原始反射率的接近程度为评价指标,并且需要光谱反射率数据进行训练.但是很多工业领域在产品设计时都需...     

Automated phase retrieval of a single-material object using a single out-of-focus image

Phase retrieval is widely used in phase contrast microscopy. Here we present an autofocus algorithm that allows the phase of the exit wave function, from a single-material object, to be reconstructed at medium resolution from a single phase contrast image without any a priori knowledge of the imaging system or object. The algorithm is demonstrated on coherent out-of-focus electron micrographs of 30 nm latex sphere calibration standards, giving <10% RMS error over a large defocus range.     

Off-axis phase-only holograms of 3D objects using accelerated point-based Fresnel diffraction algorithm

A method for calculating off-axis phase-only holograms of three-dimensional (3D) object using accelerated point-based Fresnel diffraction algorithm (PB-FDA) is proposed. The complex amplitude of the object points on the z-axis in hologram plane is calculated using Fresnel diffraction formula, called principal complex amplitudes (PCAs). The complex amplitudes of those off-axis object points of the same depth can be obtained by 2D shifting of PCAs. In order to improve the calculating speed of the PB-FDA, the convolution operation based on fast Fourier transform (FFT) is used to calculate the holograms rather than using the point-by-point spatial 2D shifting of the PCAs. The shortest recording distance of the PB-FDA is analyzed in order to remove the influence of multiple-order images in reconstructed images. The optimal recording distance of the PB-FDA is also analyzed to improve the quality of reconstructed images. Numerical reconstructions and optical reconstructions with a phase-only spatial light modulator (SLM) show that holographic 3D display is feasible with the proposed algorithm. The proposed PB-FDA can also avoid the influence of the zero-order image introduced by SLM in optical reconstructed images.     

Reconstruction of protein structures from a vectorial representation

We show that the contact map of the native structure of globular proteins can be reconstructed starting from the sole knowledge of the contact map's principal eigenvector, and present an exact algorithm for this purpose. Our algorithm yields a unique contact map for all 221 globular structures of PDBselect25 of length N相似文献   

Study of the algorithm in the phase retrieval using the logarithmic Hilbert transform

The phase retrieval from a modulus of the scattered wave field at the Fourier transform plane of an object is considered in this paper by using the logarithmic Hilbert transform. When the phase is evaluated by using the logarithmic Hilbert transform from the modulus at the Fourier transform plane of the object with the direct component (for example, the phase object), the distortion appears in the reconstructed object obtained from an inverse Fourier transform of the complex wave consisting of the evaluated phase and the modulus. Here, the cause for the appearance of the distortion is made clear and a simple algorithm is presented to reduce the distortion of the reconstructed object.     

时域反卷积算法用于提高超声反射CT成像分辨率

超声反射CT成像通常采用反射回波的包络作为投影,按一定算法进行重建,以实现对物体内某一截面的成像。由于超声换能器的发射脉冲具有一定宽度,这样,被检测物体内相距较近的缺陷回波信号将产生混叠,使得重建像中难以区分各相距较近的缺陷。针对这一问题,本文应用时域反卷积（TDD）算法对缺陷回波信号进行预处理,再进行图像重建,理论和实验结果表明,成像分辨率得到提高,图像质量明显改善。     

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.     

Phase deviation analysis and phase retrieval for partial intensity saturation in phase-shifting projected fringe profilometry

Intensity saturation may take place as a sinusoidal fringe pattern is projected onto an object which has a relative high reflective index on some regions of the surface. If a phase-shifting method is used, the illuming light intensities at the same point of the object may have different values for each projected phase-shifting fringe pattern. Therefore, when the intensity at a point of the object exceeds the saturation level for a certain phase step of projected fringe pattern, other intensities obtained at the same point with different phase steps of projected fringe patterns do not exceed the saturation level. This kind of saturation is defined as partial intensity saturation. In the case of partial intensity saturation, a relative larger phase error is introduced when a conventional phase reconstruction algorithm is used in the phase-shifting projected fringe profilometry (PSPFP). To get rid of such disadvantage, an improved algorithm for phase reconstruction is proposed in this paper. By using the new algorithm, the effect of partial intensity saturation for the phase reconstruction can be decreased greatly and a good quality of reconstructed phase map can be obtained. Phase deviation of the reconstructed phase is also analyzed. Finally, an experimental result with PSPFP is presented to validate the feasibility of the proposed algorithm.     

非相干照明条件下的ptychographic iterative engine成像技术

在传统多波长相干衍射成像理论的基础上提出适用于 X-射线和电子束等非相干光源照明成像的改进多波长ptychographic iterative engine 方法, 同时将小孔形状和照明光谱信息用于叠代计算, 可以在非相干照明条件下精确重建出物体的强度透射像和相位透射像, 并对光源带宽对重建精度的影响进行了分析, 对于解决如何在非相干照明条件下对大尺寸物体进行精确相位成像的问题具有较好的科研和实用价值.     

数字全息中的一些基本问题分析

利用全息理论、傅里叶频谱分析和采样定理，在模拟和数字全息光栅实验基础上，分析了数字全息记录和再现中的一些基本问题。结果表明：在物体和CCD尺寸确定的情况下，记录光路结构参量只取决于对图像采样的要求及CCD的像素尺寸，只要物体到CCD的距离满足采样要求，数字全息图再现光场的三个部分就可以分离；用准直平行光作为记录参考光和模拟再现光，可以得到与物体大小和形状完全一致的再现像；采样条件对再现像的影响大于分离条件，减小参物光的夹角记录适当过采样的数字全息图，有利于提高再现像的质量；另外，在获得高质量再现像方面，根据物体的具体特征，尽可能记录高质量的数字全息图，与满足采样条件和分离条件具有同样重要的意义。     

A new pixel matching method using the entire modulation of the measured object in online PMP

A method using entire modulation information of the measured object itself for pixel matching is proposed in online phase measuring profilometry (PMP). Only one fixed sinusoidal grating is projected onto the moving measured object and then five deformed patterns are captured by the imaging system (CCD). Because the contrast of the sinusoidal grating is a constant, the modulation distribution of each deformed pattern reflecting the gray of the measured region can be extracted. The entire modulation of the object cut from one of them is regarded as a template, and then the distance of the measured object's movement between each two adjacent deformed pattern can be calculated by the correlation operation so that we can cut the corresponding parts containing the measured object's information from five deformed patterns. Then the discrete phase can be calculated with Stoilov algorithm. By the phase unwrapping and the mapping algorithm, the object can be reconstructed exactly. A series of simulations and experiments confirm its feasibility and validity.     

Correction of wave-front retrieval errors caused by the imperfect collimation of reference beam in phase-shifting interferometry

In phase-shifting interferometry (PSI) the standard wave retrieval formulae are usually derived based on the condition of a plane reference beam normal to the recording plane. In practice, however, the reference wave may be a spherical wave of large radius or even have a slight inclination at the same time due to the imperfect collimation and coaxality of the optical system, and this fact will introduce phase distortion for the reconstructed object wave-front. A simple digital processing algorithm without any additional measurements is proposed to determine the unknown parameters of the spherical reference wave and then correct the object wave errors reconstructed with standard wave retrieval formulae. The effectiveness of this method is verified by a series of computer simulations, and its limitation is also discussed.     

由傅里叶功率谱恢复物函数的研究

相位恢复工作中,针对物函数是实函数的情形,Fienup算法取得了较大的成功.本文给出一新算法,其收敛程度比Fienup算法更优.以天文学中双星情形为例,比较了两种不同算法的结果.     

Error evaluation for Zernike polynomials fitting based phase compensation of digital holographic microscopy

Combining the experimental research with the simulation calculation, the error evaluation for Zernike polynomials fitting (ZPF) based phase compensation of digital holographic microscopy (DHM) is performed. The obtained results show that the reconstructed phase with high precision can be obtained by ZPF phase compensation algorithm. Moreover, the phase error for ZPF based phase compensation algorithm increases with both the variation of object height and object transverse area, the larger variation of object height, the larger of phase error, and the larger of object transverse area, the faster increase of RMS phase error. To decrease the error of ZPF phase compensation algorithm, it is required to ensure one of the variations of object height and object transverse area to be a small value. Importantly, the proposed method supplies a useful tool for the error evaluation of phase compensation algorithm.     

基于微位移技术提高CCD分辨率的方法

利用微小相对移动和计算机合成技术提高CCD等数字图像传感器分辨率是一个行之有效的方法.本文在这一技术的基础上,将CCD图像传感器按像元尺寸和不感光间隔尺寸二者之间的关系做出了分类,提出了适应更多图像传感器类型的高分辨率图像的获取方法和算法,并给出了某些类型的CCD图像传感器通过微位移方式提高分辨率的限度.计算机模拟和实验都验证了这种新方法的可行性.     

Optical-path-length-shifting color digital holography

We propose an in-line color digital holography that can suppress a 0th-order diffraction image and a conjugate image. This technique reconstructs the image of an object from two holograms that are formed at two different distances from the object for each primary color. We numerically simulated the proposed technique and confirmed that it enabled the reconstruction of higher-quality images of an object than the Fresnel transform alone, which is the simplest reconstruction algorithm in digital holography. The effectiveness of the proposed technique was also quantitatively shown by evaluating the reconstructed images using root-mean-square errors. Thus, it was confirmed that the proposed technique was capable of recording and reconstructing both three-dimensional information and color information of an object.     

A novel three-dimensional imaging method by means of coded cameras array recording and computational reconstruction

In this paper, we propose a novel three-dimensional imaging method by which the object is captured by a coded cameras array (CCA) and computationally reconstructed as a series of longitudinal layered surface images of the object. The distribution of cameras in array, named code pattern, is crucial for reconstructed images fidelity when the correlation decoding is used. We use DIRECT global optimization algorithm to design the code patterns that possess proper imaging property. We have conducted primary experiments to verify and test the performance of the proposed method with a simple discontinuous object and a small-scale CCA including nine cameras. After certain procedures such as capturing, photograph integrating, computational reconstructing and filtering, etc., we obtain reconstructed longitudinal layered surface images of the object with higher signal-to-noise ratio. The results of experiments show that the proposed method is feasible. It is a promising method to be used in fields such as remote sensing, machine vision, etc.     

基于光栅分光法的相干衍射成像

提出一种可以通过单次曝光实现 PIE(ptychographic imaging engine)成像的方法, 该方法用正交光栅将入射细光束衍射为传播方向不同的子光束簇以对样品进行照明, 并用CCD同时记录各个子光束所形成的衍射光斑阵列. 该方法很好地克服了现有PIE方法的成像质量易受机械扫描误差影响和数据采集时间过长两个缺点, 具有很好的实用价值. 关键词： 相干衍射成像 相位恢复 显微成像 迭代算法