Angular Filtering Research Articles

Study of Spacecraft Attitude Tracking Using Variable Speed Control Moment Gyros

研究了以变速控制力矩陀螺(VSCMG)作为执行机构的航天器姿态跟踪问题.建立了以VSCMG为执行机构的航天器姿态动力学模型, 引入一阶稳定的线性角速度滤波方程, 同时, 根据Lyapunov稳定性定理, 设计了闭环系统的控制律. 利用加权的最小范数解得到VSCMG的姿态控制输入矢量. 提出了表征VSCMG构型的新奇异度量, 在其基础上利用梯度法构建了VSCMG的零运动, 以回避VSCMG的构型奇异, 并使转子转速趋于期望值. 以四陀螺金字塔构型为例进行仿真,仿真结果验证了该算法的可行性和有效性.

The effect of angular dose distribution on the detection of microcalcifications in digital breast tomosynthesis

Substantial effort has been devoted to the clinical development of digital breast tomosynthesis (DBT). DBT is a three-dimensional (3D) x-ray imaging modality that reconstructs a number of thin image slices parallel to a stationary detector plane. Preliminary clinical studies have shown that the removal of overlapping breast tissue reduces image clutter and increases detectability of large, low contrast lesions. However, some studies, as well as anecdotal evidence, suggested decreased conspicuity of small, high contrast objects such as microcalcifications. Several investigators have proposed alternative imaging methods for improving microcalcification detection by delivering half of the total dose to the central view in addition to a separate DBT scan. Preliminary observer studies found possible improvement by either viewing the central projection alone or combining all views with a reconstruction algorithm. In this paper, we developed a generalized imaging theory based on a cascaded linear-system model for DBT to calculate the effect of variable angular dose distribution on the 3D modulation transfer function (MTF) and noise power spectrum (NPS). Using the ideal observer signal-to-noise ratio (SNR), d', as a figure-of-merit (FOM) for a signal embedded in a uniform background, we compared the detectability of objects with different sizes under different imaging conditions (e.g., angular dose distribution and reconstruction filters). Experimental investigation was conducted for three different angular dose schemes (ADS) using a Siemens Novation(TOMO) prototype unit. Our results show excellent agreement between modeled and experimental measurements of 3D NPS with different angular dose distribution. The ideal observer detectability index for the detection of Gaussian objects with different angular dose distributions depends strongly on the applied reconstruction filter as well as the imaging task. For detection tasks of small calcifications with reconstruction filters used typically in a clinical setting, variable angular dose distribution with more dose delivered to the central views may lead to higher d' than a uniform angular dose distribution. The conspicuity of the detection of small calcifications may be improved, under certain imaging conditions, by delivering higher dose toward the central views of a tomosynthesis scan, while also reducing the dose at peripheral angles to keep total administered radiation dose equivalent. The degree of improvement depends on the choice of reconstruction filters as well as the imaging task. The improvement is more substantial for high-frequency imaging tasks and when an aggressive slice-thickness (ST) filter is applied to reduced the high-frequency noise at peripheral angles.

Bringing some photonic structures for solar cells to the fore

A review on the use of photonic structures enabling a better absorption of solar radiation within solar cells is proposed. Specific geometric configurations, such as folded solar cells or fiber-based architectures, are shown to be promising solutions to reach better light absorption. Electromagnetic optimization of thin-film solar cells and the use of angular thin-film filters, proposed by several research groups, also provide solutions to better concentrate solar radiation within the active layers of solar cells. Finally, results on "photonized" solar cells comprising gratings or more advanced photonic components, such as photonic crystals or plasmonic structures, and their effects on light-matter interaction in solar cells are highlighted.

Transillumination hyperspectral imaging for histopathological examination of excised tissue

Angular domain spectroscopic imaging (ADSI) is a novel technique for the detection and characterization of optical contrast in turbid media based on spectral characteristics. The imaging system employs a silicon micromachined angular filter array to reject scattered light traversing a specimen and an imaging spectrometer to capture and discriminate the largely remaining quasiballistic light based on spatial position and wavelength. The imaging modality results in hyperspectral shadowgrams containing two-dimensional (2D) spatial maps of spectral information. An ADSI system was constructed and its performance was evaluated in the near-infrared region on tissue-mimicking phantoms. Image-based spectral correlation analysis using transmission spectra and first order derivatives revealed that embedded optical targets could be resolved. The hyperspectral images obtained with ADSI were observed to depend on target concentration, target depth, and scattering level of the background medium. A similar analysis on a muscle and tumor sample dissected from a mouse resulted in spatially dependent optical transmission spectra that were distinct, which suggested that ADSI may find utility in classifying tissues in biomedical applications.

Angular domain fluorescence lifetime imaging: a tissue-like phantom study

We describe a fluorescence lifetime imaging technique employing the collimation detection capabilities of an angular filter array (AFA). The AFA accepts minimally scattered photons emitted from fluorophores up to 2 mm deep within turbid media. The technique, referred to as Angular Domain Fluorescence Lifetime Imaging (ADFLI), is described and its performance evaluated in comparison to a conventional (lens and pinhole) system. Results from a tissue-mimicking phantom demonstrated that ADFLI provides better spatial resolution and image contrast for fluorescent probes at greater depths compared to a lens and pinhole system.

Uniaxial epsilon-near-zero metamaterial for angular filtering and polarization control

We describe a unique class of metamaterials that exhibit strong uniaxial anisotropy with epsilon-zero response along the optical axis and which optical properties depend strongly on polarization. In an example of array of silver nanowires grown in anodic alumina membrane, the proposed singular uniaxial metamaterial is shown to function as a polarizer and narrowband angular transmittance filter.

Guided-mode resonance excitation on multimode planar periodic waveguide

Planar periodic waveguides are applied to design numerous passive guided-mode resonance (GMR) elements. Key to achieving these devices with desired spectral properties is placement and manipulation of the resonance peaks. It has been shown that, the resonance locations closely track the dispersion curves of the leaky modes. In this paper, taking Bragg reflection due to periodicity and coupling between different modes into account, we investigate the dispersion relations of leaky modes in multimode planar periodic waveguides, both for s-polarized (TE mode) and p-polarized (TM mode) incident waves. Exploiting the characteristics of leaky mode dispersions in full range of the first Brillouin zone, we offer the ways to construct some novel optical elements, for example, polarization independent and angular tolerant GMR filters.

Angular Selective Light Filter Based on Photonic Crystals for Photovoltaic Applications

In this paper, we present a new angular selective filter design that is based on photonic bandgap engineering principles. The operation of the filter is based on the occurrence of angular-dependent photonic stopbands and passbands in a photonic crystal structure. Such a filter allows for the propagation of normally incident light while disallowing the propagation of obliquely incident light waves. When the filter is applied to a solar cell structure that consists of a diffraction grating structure on the back surface, a high-efficiency light trap can be formed. Hence, the light-trapping capacity of the new structure is dependent on the photonic band structure of the light filter rather than on refractive optical properties of the active photovoltaic material. This paper presents a model of such a structure and investigates the possibilities afforded by the new structure.

Angular domain transillumination imaging optimization with an ultrafast gated camera

By employing high-aspect-ratio parallel microchannels as an angular filter, quasiballistic photons sensitive to internal structures in a turbid medium can be captured. Scattered photons exiting the turbid medium typically exhibit trajectories with random angles compared to the initial trajectory and are mostly rejected by the filter. However, angular filter arrays cannot differentiate between quasiballistic photons (early arriving) and photons that happen to attain a scattered trajectory that is within the acceptance angle (late arriving). Therefore, we have two objectives: (1) to experimentally characterize the angular distribution and proportion of minimally deviated quasiballistic photons and multiply scattered photons in a turbid medium and (2) to combine time and angular gating principles so that early and late arriving photons can be distinguished. From the angular distribution data, the angular filter with angular acceptance about 0.4 deg yields the highest image contrast for transillumination images. The use of angular domain imaging(ADI) with time-gating enables visualization of submillimeter absorbing objects with approximately seven times higher image contrast compared to ADI in a turbid medium with a scattering level of six times the reduced mean free path.

Near-field radiation characteristics of shaped electrically large apertures in the spatial and angular domains

According to the equivalence principle, the aperture near-field can be expressed as the convolution of the aperture field and the spatial network response function. Therefore the aperture near-field angular spectrum can be expressed as the product of the angular-domain spectrum of the aperture field and the angular-domain spectrum of the spatial network response function. The spatial network function acts as an angular spectrum filter. Based on the aperture field convolution method, the near-field radiation of several electrically large apertures has been computed and compared in the spatial and angular domains, including a circular aperture, a square aperture and a well-designed serrated edge aperture. The following facts have been demonstrated by the numerical results: The serrated edge aperture has the most uniform near-field amplitude and phase distribution, both in the transverse plane and in the central axis. The plane-wave spectrum components received by a certain observation plane are related to its position, size and shape. The direct wave and edge diffraction waves can be observed qualitatively and quantitatively in the angular domain. The edge diffraction waves cannot be distinguished when the transverse size of the observation plane is less than a certain value. Finally, based on general rules of compact antenna test range design, the angular-domain analysis applications for the evaluation of the aperture shape design and the diagnosis of the aperture deformation are presented. All the three aperture diameters are limited to 150 wavelengths in free space. Inverse Fourier transform with a Hamming window has been adopted for the spatial–angular transform.

Angular domain fluorescence imaging for small animal research

We describe a novel macroscopic fluorescent imaging technique called angular domain fluorescence imaging (ADFI) applicable to the detection of fluorophores embedded in biological tissues. The method exploits the collimation detection capabilities of an angular filter array (AFA). The AFA uses the principle of acceptance angle filtration to extract minimally scattered photons emitted from fluorophores deep within tissue. Our goal was to develop an ADFI system for imaging near-infrared fluorescent markers for small animal imaging. According to the experimental results, the ADFI system offered higher resolution and contrast compared to a conventional lens and lens-pinhole fluorescent detection system. Furthermore, ADFI of a hairless mouse injected with a fluorescent bone marker revealed vertebral structural and morphometric data that correlated well with data derived from volumetric x-ray computed tomography images. The results suggested that ADFI is a useful technique for submillimeter mapping of the distribution of fluorescent biomarkers in small animals.

Deeply-etched two-dimensional grating in a Ta 2O 5 guiding layer for very narrow spectral filtering

Deep etching of a sub-wavelength grating in a high index material Ta 2O 5 guiding layer has been developed for high angular tolerance narrow band grating-waveguide reflective filters operating around 1550 nm. Holes with accurate size and smooth as well as vertical sidewalls have been obtained thanks to an intermediate Ge mask layer. A very narrow linewidth down to Δ λ = 0.28 nm is measured on a 3 mm sq large Ta 2O 5 grating filter close to the targeted 0.21 nm linewidth of the ideal structure.

Transmission and fluorescence angular domain optical projection tomography of turbid media

When imaging through turbid media, objects are often blurred by scattered light. An optical collimator (i.e., an angular filter array) improves images by accepting only photons propagating within a narrow solid angle about the direction of the incident light. These photons are expected to participate in a limited number of small-angle scattering events, maintaining their original propagation direction and, finally, contributing to the development of a faithful image of an object within a turbid medium. The collimation method, also referred to as angular domain imaging (ADI), applies to a see-through configuration where the incident collimated light beam can be aligned with the collimator in a transillumination mode of operation. In this paper, we present angular domain optical projection tomography (ADOPT), a method that can extract depth information of optical contrast in turbid media with high longitudinal resolution based on ADI technology. The resolution of the ADI system has been tested over various depths in a 5 cm optical cuvette using a resolution target suspended in a homogeneous turbid medium. The ADOPT system reconstructed images from a series of angular domain projections collected at angular intervals. The system was used to measure the attenuation of an absorbing target in transmission mode (t-ADOPT) and to measure the light emitting from a fluorescent target (f-ADOPT). Tissue-mimicking phantoms were used to validate the performance of the method. In the t-ADOPT configuration, a background scattered light estimation and subtraction methodology was introduced to improve the imaging contrast. A target consisting of two graphite rods (0.9 mm diameter) was suspended in the cuvette by a rotation stage. An Indocyanine Green-filled glass rod was used as an imaging target in the f-ADOPT arrangement. The target was placed in a manner such that the line of laser light was perpendicular to the longitudinal axis of the rods. Several projections were collected at increments of 1.8 degrees and compiled into a sinogram. A transverse image was reconstructed from the sinogram by using filtered backprojection and image contrast was improved by experimental scatter measurements using a wedge prism and an image processing algorithm. The submillimeter target embedded in a 2 cm thick scattering medium (reduced scattering coefficient < or = 2.4 cm(-1)) was discernable in both the sinograms and the reconstructed images. In the f-ADOPT system, fluorescent line targets <1 cm in diameter embedded in a 2 cm thick scattering medium (reduced scattering coefficient < or = 0.8 cm(-1)) were discernable in both the sinograms and the reconstructed images. The proposed method could be used as the basis to construct an optical tomographic scanner for simultaneous absorption and fluorescence-based imaging of biological specimens (i.e., up to 7 mm across).

Improving Recording Density of All-Optical Magnetic Storage by Using High-Pass Angular Spectrum Filters

A new design is presented to improve the magnetic recording density in all-optical magnetic storage. By using the high numerical lens with a high-pass angular spectrum filter, circularly polarized laser pulses are focused into the magneto-optic film with the perpendicular anisotropy. Magnetization of the film is induced by the inverse Faraday effect. As the obstructed angle of the filter increases the magnetic recording density increases evidently. The magnetization intensity and the sidelobe effect are also discussed.

Grating lobe reduction in transducer arrays through structural filtering of supercritical plates

The effect of placing a structural acoustic filter between water and the transducer elements of an array to help reduce undesirable grating lobes is investigated. A supercritical plate is mounted to transducer elements with a thin decoupling polyurethane layer between the transducers and the plate. The plate acts as a radiation/incidence angle filter to pass energy at angles near normal incidence, but suppress energy at large incidence angles. Grating lobe reduction is achieved at the expense of limiting the available steering of the main lobe. Within this steer angle limitation, the main lobe can be steered as normal while the grating lobe level is reduced by the plate's angular filtering. The insertion of a plate structural filter provides, in principal, an inexpensive and easily implemented approach to extend usable frequency bandwidth with reduced level grating lobes, without increasing the number of array elements. Even though the data match theory well, a practical material has yet to be found that possesses optimal material properties to make the proposed idea practical. This work represents the first attempt to advantageously utilize a plate above its critical frequency to provide angular dependent sound transmission filtering.

Sinogram bow‐tie filtering in FBP PET reconstruction

Low-pass filtering of sinograms in the radial direction is the most common practice to limit noise amplification in filtered back projection (FBP) reconstruction of positron emission tomography studies. Other filtering strategies have been proposed to prevent the loss in resolution due to low-pass radial filters, although results have been diverse. Using the well-known properties of the Fourier transform of a sinogram, the authors defined a binary mask that matches the expected shape of the support region in the Fourier domain of the sinogram ("bow tie"). This mask was smoothed by a convolution with a ten-point Gaussian kernel which not only avoids ringing but also introduces a pre-emphasis at low frequencies. A new filtering scheme for FBP is proposed, comprising this smoothed bow-tie filter combined with a standard radial filter and an axial filter. The authors compared the performance of the bow-tie filtering scheme with that of other previously reported methods: Standard radial filtering, angular filtering, and stackgram-domain filtering. All the quantitative data in the comparisons refer to a baseline reconstruction using a ramp filter only. When using the smallest size of the Gaussian kernel in the stackgram domain, the authors achieved a noise reduction of 33% at the cost of degrading radial and tangential resolutions (14.5% and 16%, respectively, for cubic interpolation). To reduce the noise by 30%, the angular filter produced a larger degradation of contrast (3%) and tangential resolution (46% at 10 mm from the center of the field of view) and showed noticeable artifacts in the form of circular blurring dependent on the distance to the center of the field of view. For a similar noise reduction (33%), the proposed bow-tie filtering scheme yielded optimum results in resolution (gain in radial resolution of 10%) and contrast (1% increase) when compared with any of the other filters alone. Experiments with rodent images showed noticeable image quality enhancement when using the proposed bow-tie filtering scheme.

Image contrast enhancement in angular domain optical imaging of turbid media

Imaging structures within a turbid medium using Angular Domain Imaging (ADI) employs an angular filter array to separate weakly scattered photons from those that are highly scattered. At high scattering coefficients, ADI contrast declines due to the large fraction of non-uniform background scattered light still within the acceptance angle. This paper demonstrates various methods to enhance the image contrast in ADI. Experiments where a wedge prism was used to deviate the laser source so that scattered photons could be imaged and subtracted from the image obtained by standard ADI provided the greatest improvement in image contrast.

Directional selectivity and ultra-light-trapping in solar cells

Directional and energy selective optical surface structures attached to solar cells may result in both, enhancement or deterioration of photovoltaic performance of a solar cell. On the one hand, restricting the cell acceptance to the small incidence angle of direct and circumsolar irradiation enhances the maximum path length of the light in a solar cell with Lambertian surfaces even above the Yablonovitch limit (ultra-light-trapping). On the other hand, restrictions to small acceptance angles imply losses of diffuse sunlight, even for perfectly tracked cells. Using temporally resolved solar irradiation spectra, we simulate the enhancement and loss in the annual energy yield of silicon solar cells of various thicknesses. We assume an idealized angular and energy selective filter on top of the Lambertian surface of the absorber and compare the results to a Lambertian surface only. We find a maximum annual gain in the energy density of 32.5% for 1 μm, and of -10% for 10 μm and 100 μm thick perfectly tracked crystalline silicon solar cells. The simulation implies various tracking modes and two different locations. Finally, we introduce two possible realizations of such a filter; a Rugate stack and inverted opals. In experimental measurements, we could verify the absorptance enhancement by such a structure applied on top of a thin silicon wafer.

Image formation principles in coded-aperture based x-ray phase contrast imaging

A novel x-ray phase contrast imaging technique based on coded apertures was recently developed at University College London. This technique removes most limitations of previous phase contrast methods and provides image improvements comparable to those obtained with synchrotron radiation with conventional x-ray sources. Unlike other phase contrast approaches, the technique does not impose restrictive wavelength and/or angular filtering on the beam emitted by the source, meaning that the beam can be exploited in full thus minimizing exposure times. As a consequence, the method provides, for the first time, a concrete opportunity to transfer x-ray phase contrast imaging into real medical applications. This paper discusses the image formation principles, analyses the shape and nature of the phase contrast profiles obtained and draws a significant conclusion on the role of convolution integrals in the acquisition of phase contrast patterns, applicable also to other phase contrast imaging approaches.

Multi-spectral angular domain optical imaging in biological tissues using diode laser sources

Angular Domain Imaging (ADI) employs micromachined angular filter to detect non-scattered photons that pass through the micro-scale tunnels unattenuated while scattered photons are rejected. This paper describes the construction of an ADI system utilizing diode lasers at three different wavelengths in the range of the red and near infrared spectrum. Experiments are performed to verify the feasibility of ADI at multi-wavelengths. ADI results of chicken breast as a biological scattering medium are presented for different thicknesses. A spatial resolution of <0.5 mm is achieved with 5 mm thick chicken breast using a 975 nm diode laser source.