Aperture Illumination Research Articles

Foreground Detection via Background Subtraction and Improved Three-Frame Differencing

Moving object detection is a widely used and important research topic in computer vision and video processing. Foreground aperture, ghosting and sudden illumination changes are the main problems in moving object detection. To consider the above problems, this work proposes two approaches: (i) improved three-frame difference method and (ii) combining background subtraction and improved three-frame difference method for the detection of multiple moving objects from indoor and outdoor real video dataset. This work accurately detects the moving objects with varying object size and number in different complex environments. We compute the detection error and processing time of two proposed as well as previously existing approaches. Experimental results and error rate analysis show that our methods detect the moving targets efficiently and effectively as compared to the traditional approaches.

Super-resolution microscopy with very large working distance by means of distributed aperture illumination

The limits of conventional light microscopy (“Abbe-Limit“) depend critically on the numerical aperture (NA) of the objective lens. Imaging at large working distances or a large field-of-view typically requires low NA objectives, thereby reducing the optical resolution to the multi micrometer range. Based on numerical simulations of the intensity field distribution, we present an illumination concept for a super-resolution microscope which allows a three dimensional (3D) optical resolution around 150 nm for working distances up to the centimeter regime. In principle, the system allows great flexibility, because the illumination concept can be used to approximate the point-spread-function of conventional microscope optics, with the additional benefit of a customizable pupil function. Compared with the Abbe-limit using an objective lens with such a large working distance, a volume resolution enhancement potential in the order of 104 is estimated.

Digital micromirror device based ophthalmoscope with concentric circle scanning.

Retinal imaging is demonstrated using a novel scanning light ophthalmoscope based on a digital micromirror device with 810 nm illumination. Concentric circles were used as scan patterns, which facilitated fixation by a human subject for imaging. An annular illumination was implemented in the system to reduce the background caused by corneal reflections and thereby to enhance the signal-to-noise ratio. A 1.9-fold increase in the signal-to-noise ratio was found by using an annular illumination aperture compared to a circular illumination aperture, resulting in a 5-fold increase in imaging speed and a better signal-to-noise ratio compared to our previous system. We tested the imaging performance of our system by performing non-mydriatic imaging on two subjects at a speed of 7 Hz with a maximum 20° (diameter) field of view. The images were shot noise limited and clearly show various anatomical features of the retina with high contrast.

Compact beam‐reconfigurable feed for large aperture antennas

This study presents a novel compact beam-reconfigurable horn antenna using a fully operational MEMS-tuned partially reflective surface (PRS). Two symmetric MEMS capacitive switches across a slot on a single layer patch PRS tune its reflection and enable the dynamic reconfiguration of the horn pattern. The authors explore the potential of this PRS in two different applications: beamwidth and beam-shape reconfiguration. Furthermore, a novel in-house process for monolithic fabrication of the MEMS-integrated full PRS and a dedicated DC bias network is developed and analysed. The proposed structure can be used as the feed of a reflector, reflectarray or transmittarray to control the aperture illumination and allow beam-reconfiguration.

Cavity-excited Huygens' metasurface antennas for near-unity aperture illumination efficiency from arbitrarily large apertures.

One of the long-standing problems in antenna engineering is the realization of highly directive beams using low-profile devices. In this paper, we provide a solution to this problem by means of Huygens' metasurfaces (HMSs), based on the equivalence principle. This principle states that a given excitation can be transformed to a desirable aperture field by inducing suitable electric and (equivalent) magnetic surface currents. Building on this concept, we propose and demonstrate cavity-excited HMS antennas, where the single-source-fed cavity is designed to optimize aperture illumination, while the HMS facilitates the current distribution that ensures phase purity of aperture fields. The HMS breaks the coupling between the excitation and radiation spectra typical to standard partially reflecting surfaces, allowing tailoring of the aperture properties to produce a desirable radiation pattern, without incurring edge-taper losses. The proposed low-profile design yields near-unity aperture illumination efficiencies from arbitrarily large apertures, offering new capabilities for microwave, terahertz and optical radiators.

Assessing the imaging performance of light sheet microscopies in highly scattering tissues.

Light sheet microscopy (LSM) has emerged as an optical-imaging method for high spatiotemporal volumetric imaging of relatively transparent samples. While this capability has allowed the technique to be highly impactful in fields such as developmental biology, applications involving highly scattering thick tissues have been largely unexplored. Herein, we employ Monte Carlo simulations to explore the use of LSM for imaging turbid media. In particular, due to its similarity to dual-axis confocal (DAC) microscopy, we compare LSM performance to point-scanned (PS-DAC) and line-scanned (LS-DAC) dual-axis confocal microscopy techniques that have been previously shown to produce high-quality images at round-trip optical lengths of ~9 - 10 and ~3 - 4 respectively. The results of this study indicate that LSM using widefield collection (WF-LSM) provides comparable performance to LS-DAC in thick tissues, due to the fact that they both utilize an illumination beam focused in one dimension (i.e. a line or sheet). On the other hand, LSM using confocal line detection (CL-LSM) is more analogous to PS-DAC microscopy, in which the illumination beam is focused in two dimensions to a point. The imaging depth of LSM is only slightly inferior to DAC (~2 - 3 and ~6 - 7 optical lengths for WF-LSM and CL-LSM respectively) due to the use of a lower numerical aperture (NA) illumination beam for extended imaging along the illumination axis. Therefore, we conclude that the ability to image deeply is dictated most by the confocality of the microscope technique. In addition, we find that imaging resolution is mostly dependent on the collection NA, and is relatively invariant to imaging depth in a homogeneous scattering medium. Our results indicate that superficial imaging of highly scattering tissues using light sheet microscopy is possible.

Active limited-angle tomographic phase microscope

We demonstrate an active, holographic tomography system, working with limited angle of projections, realized by optical-only, diffraction-based beam steering. The system created for this purpose is a Mach–Zehnder interferometer modified to serve as a digital holographic microscope with a high numerical aperture illumination module and a spatial light modulator (SLM). Such a solution is fast and robust. Apart from providing an elegant solution to viewing angle shifting, it also adds new capabilities of the holographic microscope system. SLM, being an active optical element, allows wavefront correction in order to improve measurement accuracy. Integrated phase data captured with different illumination scenarios within a highly limited angular range are processed by a new tomographic reconstruction algorithm based on the compressed sensing technique: total variation minimization, which is applied here to reconstruct nonpiecewise constant samples. Finally, the accuracy of full measurement and the proposed processing path is tested for a calibrated three-dimensional micro-object as well as a biological object--C2C12 myoblast cell.

Wavefront engineering: selective aperture illumination for increased depth of focus and super-resolution for photolithography

Photolithography has been and is the driving force behind enhancement and miniaturization of computers and their peripherals. Advances in photolithographic processes have enabled modern integrated circuits to have millions of electrical devices on a single, small circuit chip. Diffraction optics plays a central role in the photolithographic process, as during etching, the incoming light passes through a moderate-aperture focusing lens and converges on every point on the chip. Thus, the structure of the electromagnetic field in the focal region is of considerable interest. While techniques such as implantation and deposition processes control the vertical architecture of the integrated circuit chip, the lateral dimensions are defined solely by diffraction optics. The conventional imaging approach has a severe restriction because depth of focus shortens with an increase in resolution. Even though using shorter wavelength light such as deep ultraviolet is helpful, it too is reaching a point where further increase in resolution is necessary while, at the same time, preserving or increasing the depth of focus. Shortening of depth of focus with increase resolution is nonetheless the most severe limitation of photolithography. Thus, reduction in wavelength and increase in numerical aperture are viable options, but this approach has tremendous difficulties. In this study, we present a novel approach to achieve both super-resolution and increased depth of focus through a technique called selective aperture illumination (SAI). In SAI, the incoming light is redistributed by means of an optical element, phase, and attenuation masks. It is shown that both the increased depth focus and super-resolution are achieved by selectively choosing appropriate phase and amplitude at the entrance pupil.

Influence of the illumination coherency and illumination aperture on the ptychographic iterative microscopy**Project supported by the Funds from the Engineering and Physical Sciences Research Council (Grant No. EP/E034055/1) and the One Hundred Talent Project of the Chinese Academy of Sciences and the Financial Support (Grant No. GFZX0205010502.12).

While ptychography is an algorithm based on coherent illumination, satisfactory reconstructions can still be generated in most experiments, even though the radiation sources that are used are not ideally coherent. The underlying physics of this phenomenon is that the diffraction patterns of partially coherent illumination can be treated as those of purely coherent illumination by altering the intensities of the diffracted beams relative to their real values. On the other hand, due to the inconsistency in the altering interference among all the diffraction beams, noise/distortion is always involved in the reconstructed images. Furthermore, for a weak object, the noise/distortion in the reconstruction can be mostly reduced by using a highly curved beam for illumination in the data recording and forcing the dark field diffraction to be zero in the reconstruction.

60-GHz Thin Broadband High-Gain LTCC Metamaterial-Mushroom Antenna Array

A low-profile broadband metamaterial-mushroom antenna array is proposed for high-gain 60-GHz band applications. The antenna array consists of a single-layer mushroom radiating structure and a simplified single-layer substrate integrated waveguide (SIW) feeding network. A new transmission-line based model is presented to estimate the resonant frequencies of the operating TM10 and antiphase TM20 modes. With the metamaterial-mushroom structure, the closely-placed antenna elements realize less mutual couplings, higher aperture illumination efficiency compared to conventional patch antennas. An 8 × 8 mushroom antenna array is designed and prototyped using low-temperature cofired ceramic (LTCC), the measurement shows the antenna array with the feeding transition proposed an impedance bandwidth of 56.3-65.7 GHz with a boresight gain greater than 21.2 dBi and up to 24.2 dBi at 62.3 GHz. The proposed antenna array features the merits of high gain, broadband, compact size, and low cross-polarization levels.

Why do green rods of frog and toad retinas look green?

Amphibian “green” rods express a blue-sensitive cone visual pigment, and should look yellow. However,when observing them axially under microscope one sees them as green. We used single-cell microspectrophotometry (MSP) to reveal the basis of the perceived color of these photoreceptors. Conventional side-on MSP recording of the proximal cell segments reveals no selective longwave absorbing pigment explaining the green color. End-on MSP recording shows, in addition to the green rod visual pigment, an extra 2- to 4-fold attenuation being almost flat throughout the visible spectrum. This attenuation is absent in red (rhodopsin) rods, and vanishes in green rods when the retina is bathed in high-refractive media, and at wide illumination aperture. The same treatments change the color from green to yellow. It seems that the non-visual pigment attenuation is a result of slender green rod myoids operating as non-selective light guides. We hypothesize that narrow myoids, combined with photomechanical movements of melanin granules, allow a wide range of sensitivity regulation supporting the operation of green rods as blue receptors at mesopic-to low-photopic illumination levels.End-on transmittance spectrum of green rods looks similar to the reflectance spectrum of khaki military uniforms. So their greenness is the combined result of optics and human color vision.

The possibilities of compressed-sensing-based Kirchhoff prestack migration

An approximate subsurface reflectivity distribution of the earth is usually obtained through the migration process. However, conventional migration algorithms, including those based on the least-squares approach, yield structure descriptions that are slightly smeared and of low resolution caused by the common migration artifacts due to limited aperture, coarse sampling, band-limited source, and low subsurface illumination. To alleviate this problem, we use the fact that minimizing the L1-norm of a signal promotes its sparsity. Thus, we formulated the Kirchhoff migration problem as a compressed-sensing (CS) basis pursuit denoise problem to solve for highly focused migrated images compared with those obtained by standard and least-squares migration algorithms. The results of various subsurface reflectivity models revealed that solutions computed using the CS based migration provide a more accurate subsurface reflectivity location and amplitude. We applied the CS algorithm to image synthetic data from a fault model using dense and sparse acquisition geometries. Our results suggest that the proposed approach may still provide highly resolved images with a relatively small number of measurements. We also evaluated the robustness of the basis pursuit denoise algorithm in the presence of Gaussian random observational noise and in the case of imaging the recorded data with inaccurate migration velocities.

Quantitative Optical Microscopy: Measurement of Cellular Biophysical Features with a Standard Optical Microscope

We describe the use of a standard optical microscope to perform quantitative measurements of mass, volume, and density on cellular specimens through a combination of bright field and differential interference contrast imagery. Two primary approaches are presented: noninterferometric quantitative phase microscopy (NIQPM), to perform measurements of total cell mass and subcellular density distribution, and Hilbert transform differential interference contrast microscopy (HTDIC) to determine volume. NIQPM is based on a simplified model of wave propagation, termed the paraxial approximation, with three underlying assumptions: low numerical aperture (NA) illumination, weak scattering, and weak absorption of light by the specimen. Fortunately, unstained cellular specimens satisfy these assumptions and low NA illumination is easily achieved on commercial microscopes. HTDIC is used to obtain volumetric information from through-focus DIC imagery under high NA illumination conditions. High NA illumination enables enhanced sectioning of the specimen along the optical axis. Hilbert transform processing on the DIC image stacks greatly enhances edge detection algorithms for localization of the specimen borders in three dimensions by separating the gray values of the specimen intensity from those of the background. The primary advantages of NIQPM and HTDIC lay in their technological accessibility using “off-the-shelf” microscopes. There are two basic limitations of these methods: slow z-stack acquisition time on commercial scopes currently abrogates the investigation of phenomena faster than 1 frame/minute, and secondly, diffraction effects restrict the utility of NIQPM and HTDIC to objects from 0.2 up to 10 (NIQPM) and 20 (HTDIC) μm in diameter, respectively. Hence, the specimen and its associated time dynamics of interest must meet certain size and temporal constraints to enable the use of these methods. Excitingly, most fixed cellular specimens are readily investigated with these methods.

Quantitative optical microscopy: measurement of cellular biophysical features with a standard optical microscope.

We describe the use of a standard optical microscope to perform quantitative measurements of mass, volume, and density on cellular specimens through a combination of bright field and differential interference contrast imagery. Two primary approaches are presented: noninterferometric quantitative phase microscopy (NIQPM), to perform measurements of total cell mass and subcellular density distribution, and Hilbert transform differential interference contrast microscopy (HTDIC)to determine volume. NIQPM is based on a simplified model of wave propagation, termed the paraxial approximation, with three underlying assumptions: low numerical aperture (NA) illumination, weak scattering, and weak absorption of light by the specimen. Fortunately, unstained cellular specimens satisfy these assumptions and low NA illumination is easily achieved on commercial microscopes. HTDIC is used to obtain volumetric information from through-focus DIC imagery under high NA illumination conditions. High NA illumination enables enhanced sectioning of the specimen along the optical axis. Hilbert transform processing on the DIC image stacks greatly enhances edge detection algorithms for localization of the specimen borders in three dimensions by separating the gray values of the specimen intensity from those of the background. The primary advantages of NIQPM and HTDIC lay in their technological accessibility using "off-the-shelf" microscopes. There are two basic limitations of these methods: slow z-stack acquisition time on commercial scopes currently abrogates the investigation of phenomena faster than 1 frame/minute, and secondly, diffraction effects restrict the utility of NIQPM and HTDIC to objects from 0.2 up to 10 (NIQPM) and 20 (HTDIC) μm in diameter, respectively. Hence, the specimen and its associated time dynamics of interest must meet certain size and temporal constraints to enable the use of these methods. Excitingly, most fixed cellular specimens are readily investigated with these methods.

An FSS-Backed 20/30 GHz Circularly Polarized Reflectarray for a Shared Aperture L- and Ka-Band Satellite Communication Antenna

A shared aperture antenna for simultaneous operation at L- (1525 to 1661 MHz) and Ka-band (19.7 to 20.2 GHz and 29.5 to 30.0 GHz) is demonstrated. This stacked antenna consists of a Ka-band reflectarray antenna with a frequency selective surface (FSS) ground-plane above an L-band patch array antenna. The reflectarray is based on the concentric dual split-loop element backed by a concentric dual-loop FSS element. The reflectarray comprises 80 × 80 elements and it is printed on a 40 ×40 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Rogers 5880 substrate, while the L-band antenna is a 2 × 2 patch array. The reflectarray antenna has been manufactured at the Technical University of Denmark (DTU) and measured at the DTU-ESA Spherical Near-Field Antenna Test Facility. The reflectarray provides a maximum directivity of 36.4 and 38.5 dBi at 20.0 and 29.8 GHz, respectively, and an aperture illumination efficiency in the two frequency bands up to 57% and 48%, respectively. There is very little degradation in the L-band patch array performance due to the reflectarray, and it provides a minimum directivity of 11.8 dBi over the L-band.

Multiband Dual-Polarization High-Efficiency Array Feed for Ku/Reverse-Band Satellite Communications

A dual-band dual-polarized planar passive phased array feed was designed and simulated, with four highly isolated ports and polarizations. The feed design provides an important feasibility demonstration for the deployment of a satellite transponder that transmits a downlink signal at 17.3-17.8 GHz, referred to as reverse band (RB), in the same orbital slot as an existing Ku-band satellite. The design utilizes a multilayer printed circuit board (PCB) to separate the complex feed distribution networks required for the four-port antenna. Simulation results indicate that the feed receives signals in dual linear polarization at Ku-band and dual circular polarization at RB with isolation between all ports better than 20 dB. With the feed illuminating a reflector antenna, the aperture illumination and radiation efficiencies are approximately 70% and 80%, respectively, for all ports, representing one of the most complex yet efficient reported planar array feed designs in the literature.

A robust optical flow estimation technique using gradient orientations

The gradient method is a classical technique for calculating optical flows in image sequences. In spite of its serious limitations, including the extreme sensitivity to changing illuminations, it is still popular and widely used in various image processing and computer vision applications because of its efficiency in computation. We previously proposed an improved version of the gradient method based on gradient orientations instead of intensities. Since gradient orientations are known to be stable to global changes of intensities, the proposed technique can perform optical flow estimation regardless of irregular lighting. We refer to this method the gradient orientation based gradient method (GOGM). This paper is a further study of the GOGM. New simulation and experimental results show that this technique works at a video rate of 16 Hz and outperforms the traditional gradient method where the aperture problem and illumination changes are involved.

Patch Antenna Array Feed Design for a Dish Antenna

This paper presents the design and simulation of a patch antenna array as feed for a parabolic reflector antenna in C-Band. This configuration is to serve as an alternative to conventional feed horns for in-orbit satellites to solve the size and weight problems they constitute. A 4x1 C-Band circular patch antenna array configuration with a broadside radiation pattern is designed as feed for a dish antenna with aperture illumination of 140°. The result of analysis and simulations of the model using MATLAB and CST MWS-2012 software package suggests that the patch antenna array feed radiation characteristics are very competitive when compared to that achieved with conventional feeds for dish antennas of the same aperture illumination.

Full-field dual-color 100-nm super-resolution imaging reveals organization and dynamics of mitochondrial and ER networks

Most structured illumination microscopes use a physical or synthetic grating that is projected into the sample plane to generate a periodic illumination pattern. Albeit simple and cost-effective, this arrangement hampers fast or multi-color acquisition, which is a critical requirement for time-lapse imaging of cellular and sub-cellular dynamics. In this study, we designed and implemented an interferometric approach allowing large-field, fast, dual-color imaging at an isotropic 100-nm resolution based on a sub-diffraction fringe pattern generated by the interference of two colliding evanescent waves. Our all-mirror-based system generates illumination pat-terns of arbitrary orientation and period, limited only by the illumination aperture (NA = 1.45), the response time of a fast, piezo-driven tip-tilt mirror (10 ms) and the available fluorescence signal. At low µW laser powers suitable for long-period observation of life cells and with a camera exposure time of 20 ms, our system permits the acquisition of super-resolved 50 µm by 50 µm images at 3.3 Hz. The possibility it offers for rapidly adjusting the pattern between images is particularly advantageous for experiments that require multi-scale and multi-color information. We demonstrate the performance of our instrument by imaging mitochondrial dynamics in cultured cortical astrocytes. As an illustration of dual-color excitation dual-color detection, we also resolve interaction sites between near-membrane mitochondria and the endoplasmic reticulum. Our TIRF-SIM microscope provides a versatile, compact and cost-effective arrangement for super-resolution imaging, allowing the investigation of co-localization and dynamic interactions between organelles--important questions in both cell biology and neurophysiology.

Simulation of the polarization effects induced by the bilayer absorber alternating phase-shift mask in conical diffraction

Hyper numerical aperture (NA) and off-axis illumination enable extension of ArF lithography for 45 nm technology node and beyond. Also, rigorous electromagnetic field modeling is taken into account for the optical and topographical properties of the mask. A rigorous three-dimensional mask model for bilayer absorber alternating phase shift mask (AltPSM) with the rigorous coupled-wave analysis (RCWA) is established. First, the harmonic waves are expanded based on the least common multiple of the periods in order to model the diffraction of multiple grating layers with different periods. Second, Lalanne’s formulation is used to improve the convergence of RCWA for multiple grating layers in conical diffraction. Third, the enhanced transmittance matrix approach is also extended to conical diffraction to avoid the numerical instability. Given the chromium oxide/chromium AltPSM, the change of polarization state as a function of mask and incident light properties is investigated. When the linewidth is below 30 nm, the mask acts as a transverse magnetic field polarizer, which is not preferred in terms of image quality, so the mask-induced polarization effects must be considered in the hyper NA lithography.