Synthetic aperture microscopy using off-axis illumination and polarization coding

A new method to improve the resolution of optical imaging systems beyond the classical Rayleigh resolution limit is presented. The technique relies on synthetic aperture generation in three stages. The first one (encoding stage) uses an illumination procedure that combines both on-axis and off-axis illumination beams with different polarization states onto the object. After the imaging system, a second stage (decoding stage) allows the recovering of the encoded spatial-frequency object information by means of an interferometric configuration based on the polarization coding carried out in the previous stage. Finally, a third stage (digital post-processing stage) is used to generate a synthetic aperture that is three times larger than the conventional aperture of the imaging system. The whole process allows us to obtain a superresolved image of the object. Experimental implementation of the approach for a commercial microscope objective is presented.     

Lensless imaging of an arbitrary object

A new class of imaging systems that do not require the use of lenses or similar optical devices is introduced and theoretically investigated. In particular, it is demonstrated that, if an arbitrary plane object is illuminated by an appropriate spherical wave front (generated from a monochromatic point source), then a magnified image of the object intensity distribution can be observed in any transversal plane along the light-propagation direction within the far-field (Fraunhofer) diffraction region. The phenomenon is based on the fact that, under certain conditions, the spherical wave front can modify the energy's angular spectrum of the field distribution in the object plane such that this spectrum replicates the spatial intensity distribution of the object.     

利用维纳滤波改善声透镜光声成像系统的分辨率

为了克服衍射效应对光声成像系统分辨率的限制,需要采用逆卷积方法进行图像反演.从理论上分析了声透镜成像原理,模拟仿真了声透镜的点扩展函数对声透镜成像系统分辨率的影响和维纳滤波解卷积方法复原光声成像的过程,并利用自搭建的声透镜光声成像系统进行了深入的实验研究,得到了物平面上相距4 mm和3 mm的两个黑胶带点的直接成像光声...     

光学稀疏孔径系统的成像及其图像复原

由多个小口径成像系统通过特殊排列综合而成的光学稀疏孔径系统是实现高分辨率天文目标成像观测的一种新方法.建立了光学稀疏孔径系统的衍射受限非相干成像模型,并针对某一具体的光学稀疏孔径系统,运用图像复原维纳滤波器完成了系统成像和图像恢复处理的计算机仿真实验.结果表明,光学稀疏孔径系统可以等效实现单个大口径成像系统的成像观测效果.     

Precise speed measurement using an interlaced scan image

In this paper, we propose a novel scheme for speed measurement of a moving object with translational motion. First, this scheme uses one interlaced scan CCD camera to obtain only one interlaced scan image of a moving object. The odd and even field images are extracted and resized. Second, image matte is applied in these two field images to extract the moving object’s silhouettes. The distance between two centroids in the two silhouettes is then computed. Finally, the object’s speed is calculated using the above distance and the camera imaging parameters. Simulation and real experiments prove that our scheme can fulfill the speed measurement for translational motion accurately.     

Reduction of speckle in an image by a moving aperture - second order statistics

In an earlier paper the second-order statistics of time-averaged speckle were investigated for the case of speckle formed in the Fraunhofer plane when an aperture was moved over the surface of a uniformly illuminated diffuse object. In this paper it is the intention to substantiate a claim that was made in that paper that the same treatment will also apply to time-averaged speckle in an image plane when an aperture is moved in the pupil plane of the imaging lens. The optical system is allowed to have aberrations of any degree of severity, and the object is allowed to have non-uniform brightness, subject to the condition that the brightness variations are slow in comparison to the size of the (aberrated) point spread function. The treatment makes use of the usual assumption that a very large number of independent scattering points in the object contributes to any one point in the image: for this assumption to be realistic, however, it will be shown that the point spread function of the lens must be of a size that restricts us either to systems where the numerical aperture in object space is low or, alternatively, to systems where the aberrations are severe.     

Reconstruction of an acoustical image using an ‘acousto-electronic’ lens device

Present acoustic imaging apparatuses give only a virtual image of the insonified object. Several studies have been conducted in order to pick up the information from such an image, such as holography and computer data processing. These techniques are very involved, and so simpler processing systems have been analysed in our laboratory in order to reconstruct a true image. Our first system used optical processing of the acoustic echo. In this paper, a second, acousto-electronic, system is described, which enables like the first the attainment of very high image rates in the B-scan mode.     

3D imaging with a single-sided sensor: an open tomograph

An open tomograph to image volume regions near the surface of large objects is described. The central achievement in getting such a tomograph to work is the design of a fast two-dimensional pure phase encoding imaging method to produce a cross-sectional image in the presence of highly inhomogeneous fields. The method takes advantage of the multi-echo acquisition in a Carr-Purcell-Meiboom-Gill (CPMG)-like sequence to significantly reduce the experimental time to obtain a 2D image or to spatially resolve relaxation times across the sensitive volume in a single imaging experiment. Depending on T(2) the imaging time can be reduced by a factor of up to two orders of magnitude compared to the one needed by the single-echo imaging technique. The complete echo train decay has been also used to produce T(2) contrast in the images and to spatially resolve the T(2) distribution of an inhomogeneous object, showing that variations of structural properties like the cross-link density of rubber samples can be distinguished by this method. The sequence has been implemented on a single-sided sensor equipped with an optimized magnet geometry and a suitable gradient coil system that provides two perpendicular pulsed gradient fields. The static magnetic field defines flat planes of constant frequency parallel to the surface of the scanner that can be selected by retuning the probe frequency to achieve slice selection into the object. Combining the slice selection obtained under the presence of the static gradient of the open magnet with the two perpendicular pulsed gradient fields, 3D spatial resolution is obtained.     

Improvement of images generated reconstructed from computer- holograms using an iterative method

An iterative method is presented which can uniformly distribute the Fourier transformed amplitudes of an original object. The method is effective in making phase quantized holograms whose amplitudes have constant values, and further, it is effective for such imaging systems where the decreased spectral information is desirable for transmission and storage. The hologram is produced by iterations of the Fourier transform and inverse Fourier transform operations. The image quality depends upon the number of iterations, however, a great many iterations will not necessarily lead to a marked improvement in the image quality because a rapid improvement is often achieved by only a few iterative operations. From our experiments, the iterative operations have made remarkable improvements in reconstructed images. The proposed method is also suitable for other types of holograms and for applications in imaging systems.     

Incoherent coincidence imaging and its applicability in X-ray diffraction

Entangled-photon coincidence imaging is a method to nonlocally image an object by transmitting a pair of entangled photons through the object and a reference optical system, respectively. The image of the object can be extracted from the coincidence rate of these two photons. From a classical perspective, the image is proportional to the fourth-order correlation function of the wave field. Using classical statistical optics, we study a particular aspect of coincidence imaging with incoherent sources. As an application, we give a proposal to realize lensless Fourier-transform imaging, and discuss its applicability in x-ray diffraction.     

Object plane detection and phase-amplitude imaging based on transport of intensity equation

By using transport of intensity equation (TIE), phase distribution of an object is retrieved from through-focus intensity images. This technique allows simple and robust phase imaging compared with an interferometric approach. However, it is hard to measure phase distribution when a dynamic object moves in the direction of an optical axis. To clear this problem, autofocusing TIE which is based on local statistics is proposed. The proposed technique achieves the detection of the object plane and the retrieval of a focused object phase distribution simultaneously. In this approach, an object plane is determined by the focusing techniques based on local statistics such as variance, gradient, and Laplacian of amplitude distribution, after phase distribution in an image sensor plane is retrieved by the TIE. The performance of these three statistics is evaluated in numerical and optical experiments, and a suitable focus value is determined for precise phase imaging.     

光学扫描全息术研究进展

光学成像技术极大地拓展了人类的视觉极限,提高了人们观察和理解现实世界的能力。越多地获得目标的光学信息,对其的认识越充分。数字全息术是一种可以将样本的三维信息以二维全息图的形式编码记录下来的一种成像技术。通过获得由携带物体信息的物光波和参考光波叠加产生的干涉图案,可以以数字化的方式实现多种重建模态,例如图像恢复、相位成像和切片成像等。光学扫描全息术是一种独特的数字全息成像技术,通过主动式二维化扫描对三维物体进行成像,其完整的波前信息可以被单像素探测器记录,并基于光外差检测进行信号解调,从而恢复出复数全息图。对光学扫描全息术的最新进展进行介绍。首先,基于双光瞳成像系统,通过特殊的硬件和算法设计,提高光学成像系统的性能,如提高空间分辨率、缩短扫描时间。其次,基于计算成像原理,通过改进和优化全息像重建算法,实现高质量的图像恢复,主要涉及切片成像和三维成像等重建模态。第三,介绍光学扫描全息术的其他研究方向,并讨论该领域未来可能的发展方向。     

X-ray phase contrast imaging with optical magnification using three-block interferometer with bi-level Fresnel zone plates

The possibility of X-ray phase contrast imaging using already suggested three-block interferometer consisting of bi-level Fresnel zone plates is considered. The interferometer operates in the amplitude-division mode and does not impose strong requirements to spatial and temporal coherences of an initial radiation. The use of the Fresnel zone plates as the interferometer blocks allows one to obtain an optically magnified image of an object and to condense the radiation incident on the tested object.     

Current-density imaging using ultra-low-field MRI with adiabatic pulses

Magnetic resonance imaging (MRI) allows measurement of electric current density in an object. The measurement is based on observing how the magnetic field of the current density affects the associated spins. However, as high-field MRI is sensitive to static magnetic field variations of only the field component along the main field direction, object rotations are typically needed to image three-dimensional current densities. Ultra-low-field (ULF) MRI, on the other hand, with B₀ on the order of 10–100 μT, allows novel MRI sequences. We present a rotation-free method for imaging static magnetic fields and current densities using ULF MRI. The method utilizes prepolarization pulses with adiabatic switch-off ramps. The technique is designed to reveal complete field and current-density information without the need to rotate the object. The method may find applications, e.g., in conductivity imaging. We present simulation results showing the feasibility of the sequence.     

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.     

Improvement of the optical image reconstruction based on multiplexed quantum ghost images

Ghost imaging allows one to obtain information on an object from the spatial correlation function between photons propagating through or reflected from the object and photons of the reference arm. In this case, detection in the object arm is performed over the entire aperture of the beam and, therefore, it does not give information on the object. The reference beam does not interact with the object, but is recorded with a scanning point detector or a CCD array permitting the measurement of the spatial correlation function of photons in two arms. The use of multimode entangled quantum light beams by illuminating the object by one beam and orienting other beams to reference arms makes it possible to obtain simultaneously several ghost images (GIs). Cross correlations of multiplexed GIs (MGIs) are determined by eighth-order field correlation functions. A special algorithm is developed for calculating higher-order correlations of Bose operators. The presence of GI cross correlations is used for improving the quality of the reconstructed object’s image by their processing using the measurement reduction method. An example of the computer simulation of the image reconstruction by MGIs formed in the field of four-frequency entangled quantum states is considered. It is found that in this case the reduced GI has a signal-to-noise ratio several times higher than that of GIs.     

像面滤波技术在无透镜傅里叶变换数字全息中的应用

无透镜傅里叶变换数字全息波前重建主要采用全息图的一次快速傅里叶变换方法,重建图像不能充分占有重建平面.本文基于像平面滤波技术,提出对物体局部区域光波场进行放大重建并让重建图像布满重建平面的方法,给出具有精细结构物体的数字全息波前重建实例.此外,将数字全息光波场重建视为具有方形出射光瞳的光学系统的相干光成像过程,导出了物体放大图像的分辨率与光学系统相关参量的关系,并通过实验给予证明.     

像面滤波技术在无透镜傅里叶变换数字全息中的应用

无透镜傅里叶变换数字全息波前重建主要采用全息图的一次快速傅里叶变换方法,重建图像不能充分占有重建平面.本文基于像平面滤波技术,提出对物体局部区域光波场进行放大重建并让重建图像布满重建平面的方法,给出具有精细结构物体的数字全息波前重建实例.此外,将数字全息光波场重建视为具有方形出射光瞳的光学系统的相干光成像过程,导出了物体放大图像的分辨率与光学系统相关参量的关系,并通过实验给予证明.     

Imaging of an Absorbing Object Embedded in a Dense Scattering Medium by Diffusing Light Topography

A new and simple imaging method for an absorbing object embedded in a dense scattering medium is proposed. The distinct characteristic is the positive usage of the diffusing light in the dense scattering medium to image the absorbing object. The principle is based on the equivalence between a probability distribution function of the path-length and a backscattered intensity distribution integrated spatially in the boundary plane between the medium and the air. The usefulness of the proposed method is experimentally confirmed for a modified pyramidal object painted black and thin tubes filled with black and red inks. The conditions under which the better image can be reconstructed are confirmed from the results of experiments and simulations. It is finally shown that our proposed method is capable of imaging the map of blood vessels distributed under the skin layers.     

Robust Ghost Imaging Based on Degenerate Spontaneous Parametric Down-Conversion

In traditional ghost imaging, the entangled photon pairs produced from the spontaneous parametric down conversion(SPDC) process are used. There is an intrinsic disadvantage that the utilization efficiency of the photon pairs is very low. Inasmuch as all the correlated photon pairs produced by the degenerate SPDC process can be used to record the image of an object, the ghost imaging scheme we present here has a higher utilization efficiency of the photon pairs. We also investigate the robustness of our experimental scheme. The experimental results show that, no matter whether the photon-pair source is two light cones or two beam-like spots, the clear image of the object can be obtained. The slight rotation of the nonlinear crystal has no influence on the imaging quality.Our experimental results also demonstrate that when the part of the photon-pair source in the signal path or the idler path is blocked by unwanted things, the clear ghost image of the object can still be recorded.