Showing papers on "Spatial filter published in 2002"

Large-Eddy Simulation on Curvilinear Grids Using Compact Differencing and Filtering Schemes

Abstract: This work investigates the application of a high-order finite difference method for compressible large-eddy simulations on stretched, curvilinear and dynamic meshes. The solver utilizes 4th and 6th-order compact-differencing schemes for the spatial discretization, coupled with both explicit and implicit time-marching methods. Up to 10th order, Pade-type low-pass spatial filter operators are also incorporated to eliminate the spurious high-frequency modes which inevitably arise due to the lack of inherent dissipation in the spatial scheme. The solution procedure is evaluated for the case of decaying compressible isotropic turbulence and turbulent channel flow. The compact/filtering approach is found to be superior to standard second and fourth-order centered, as well as third-order upwind-biased approximations. For the case of isotropic turbulence, better results are obtained with the compact/filtering method (without an added subgrid-scale model) than with the constant-coefficient and dynamic Smagorinsky models. This is attributed to the fact that the SGS models, unlike the optimized low-pass filter, exert dissipation over a wide range of wave numbers including on some of the resolved scales

A phase-based approach to the estimation of the optical flow field using spatial filtering

Abstract: We introduce a new technique for estimating the optical flow field, starting from image sequences. As suggested by Fleet and Jepson (1990), we track contours of constant phase over time, since these are more robust to variations in lighting conditions and deviations from pure translation than contours of constant amplitude. Our phase-based approach proceeds in three stages. First, the image sequence is spatially filtered using a bank of quadrature pairs of Gabor filters, and the temporal phase gradient is computed, yielding estimates of the velocity component in directions orthogonal to the filter pairs' orientations. Second, a component velocity is rejected if the corresponding filter pair's phase information is not linear over a given time span. Third, the remaining component velocities at a single spatial location are combined and a recurrent neural network is used to derive the full velocity. We test our approach on several image sequences, both synthetic and realistic.

Fully dynamic multiple-beam optical tweezers

Abstract: We demonstrate a technique for obtaining fully dynamic multiple-beam optical tweezers using the generalized phase contrast (GPC) method and a phase-only spatial light modulator (SLM). The GPC method facilitates the direct transformation of an input phase pattern to an array of high-intensity beams, which can function as efficient multiple optical traps. This straightforward process enables an adjustable number of traps and realtime control of the position, size, shape and intensity of each individual tweezer-beam in arbitrary arrays by encoding the appropriate phase pattern on the SLM. Experimental results show trapping and dynamic manipulation of multiple micro-spheres in a liquid solution.

Use of single-mode waveguides to correct the optical defects of a nulling interferometer.

Abstract: In the area of long-baseline nulling interferometry, high rejection ratios are needed to cancel out the light of a bright central source and look for nearby faint structures or companions. These rejection requirements directly translate into drastic optical constraints. We discuss the possibility of using single-mode waveguides for such applications. Conversely to simple pinholes, single-mode waveguides efficiently correct wave-front defects of both high- and low-order spatial frequencies, ensure a perfect matching of the amplitude profiles coming from the various beams, and can be used with almost optimum coupling efficiency over a broad optical bandpass (typically an octave). They then appear to greatly enhance the feasibility of high-dynamic interferometric coronagraphs.

Phase sensitive demodulation in multiphoton microscopy.

Abstract: Multiphoton laser scanning microscopy offers advantages in depth of penetration into intact samples over other optical sectioning techniques. To achieve these advantages it is necessary to detect the emitted light without spatial filtering. In this nondescanned (nonconfocal) approach, ambient room light can easily contaminate the signal, forcing experiments to be performed in absolute darkness. For multiphoton microscope systems employing mode-locked lasers, signal processing can be used to reduce such problems by taking advantage of the pulsed characteristics of such lasers. Specifically, by recovering fluorescence generated at the mode-locked frequency, interference from stray light and other ambient noise sources can be significantly reduced. This technology can be adapted to existing microscopes by inserting demodulation circuitry between the detector and data collection system. The improvement in signal-to-noise ratio afforded by this approach yields a more robust microscope system and opens the possibility of moving multiphoton microscopy from the research lab to more demanding settings, such as the clinic.

Spatial filtering of RF interference in radio astronomy

Abstract: We investigate spatial filtering techniques for interference removal in multichannel radio astronomical observations. The techniques are based on the estimation of the spatial signature vector of the interferer from short-term spatial covariance matrices followed by a subspace projection to remove that dimension from the covariance matrix, and by further averaging. The projections will also modify the astronomical data, and hence a correction has to be applied to the long-term average to compensate for this. As shown by experimental results, the proposed technique leads to significantly improved estimates of the interference-free covariance matrix.

Piecewise linear spatial phase modulator using dual-mode micromirror arrays for temporal and diffractive fourier optics

Abstract: One embodiment of the present invention provides a spatial light phase modulator, which can perform piecewise linear phase modulation of a light beam. This spatial light phase modulator includes an array of movable micromirrors and an array of actuators. Each actuator of the array of actuators is movably coupled to one micromirror of the array of movable micromirrors and can move the micromirror both vertically and rotationally. Additionally, the present invention provides an optical function generator that is a femtosecond pulse shaper. This optical function generator includes a diffraction grating that disperses an input pulse into a dispersed spectrum, a lens assembly to focus the dispersed spectrum onto a micromirror array, and the micromirror array to provide spatial filtering to the dispersed spectrum to provide the filtered spectrum.

Simultaneous Measurement of Particle Size and Particle Velocity by the Spatial Filtering Technique

Abstract: The objective of this study was to compare the measuring results of a fiber-optical probe based on a modified spatial filtering technique with given size distributions of different test powders and also with particle velocity values of laser Doppler measurements. Fiber-optical spatial filtering velocimetry was modified by fiber-optical spot scanning in order to determine simultaneously the size and the velocity of particles. The fiber-optical probe system can be used as an in-line measuring device for sizing of particles in different technical applications. Spherical test particles were narrow-sized glass beads in the range 30–100 μm and irregularly shaped test particles were limestone particles in the range 10–600 μm. Particles were dispersed by a brush disperser and the measurements were carried out at a fixed position in a free particle-laden air stream. Owing to the measurement of chord lengths and to the influence of diffraction and divergent angle, the probe results show differences from the given test particle sizes. Owing to the particle-probe collisions, the mean velocity determined by the probe is smaller than the laser Doppler mean velocity.

A theoretical and experimental investigation of low-frequency acoustic vector sensors

Abstract: An acoustic vector sensor is a compact device which simultaneously measures the scalar acoustic pressure p(t, r/spl I.oarr//sub 0/) and the gradient of the acoustic pressure, /spl nabla/p(t, r/spl I.oarr//sub 0/), at some measurement point r/spl I.oarr//sub 0/. Thus, a three-dimensional (3D) acoustic vector sensor produces four time series outputs, which can be processed to provide some degree of spatial filtering and a direction of arrival (DoA) estimate to an acoustic target. Historically, two-dimensional (2D) acoustic vector sensors have been used in several Navy systems, like the DIFAR sonobuoy (AN/SQQ-53 series) and the AN/WLR-9 acoustic intercept receiver (which uses the multimode hydrophone). These devices have made important contributions to the Navy sonar community, where it is desirable to obtain an accurate azimuthal DoA estimate for bearing to a low-frequency target from a single point in space. Other researchers have used the outputs of acoustic vector sensor to estimate intensity vector (watts/m/sup 2/). In the literature, these intensity-related vector sensors have been referred to as intensity probes or "acoustic watt-meters". In this paper, we consider 3D acoustic vector sensors that uses three orthogonal underwater acoustic accelerometers with a scalar acoustic pressure sensor of hydrophone packaged in a single housing. The sensors will be referred to as low-frequency vector sensors since they were optimized for the 50 Hz to 2 kHz band. We will show that these vector sensors can produce a frequency-independent spatial response over the aforementioned band. Specifically, a 3-dB beamwidth of 105 degrees across the stated frequency band and an array gain against isotropic noise of 6 dB can be achieved. We will also show that the four time series vector sensor outputs (x, y and z accelerations and 1 pressure) can be processed to produce multiple nulls in the spatial response or beam pattern, which can be used to null out interfering noise sources. Experimental results, recorded at the US Navy Seneca Lake Test Facility, Dresden, NY using two different designs of the aforementioned 3D vector sensor, show how vector sensors perform in the presence of multiple low-frequency sources and interferers.

High-resolution imaging ellipsometer.

Abstract: We report on a novel imaging ellipsometer using a high-numerical-aperture (NA) objective lens capable of measuring a two-dimensional ellipsometric signal with high resolution. Two-dimensional ellipsometric imaging is made possible by spatial filtering at the pupil plane of the objective. A Richards-Wolf vectorial diffraction model and geometrical optics model are developed to simulate the system. The thickness profile of patterned polymethyl methacrylate is measured for calibration purposes. Our instrument has a sensitivity of 5 A and provides spatial resolution of approximately 0.5 µm with 632.8-nm illumination. Its capability of measuring refractive-index variations with high spatial resolution is also demonstrated.

Squeezing in a LiNbO3 integrated optical waveguide circuit

Abstract: We report on traveling-wave quadrature squeezing generated in an integrated optical circuit fabricated with periodicallypoled lithium niobate waveguides. The device integrates a secondharmonic-generation stage, waveguide couplers, spatial mode filters, and a degenerate-optical-parametric-amplification stage. The integrationpromises to create a low power, compact, and simple source of wideband squeezed light. -1 dB of squeezing is directly measured using 20 ps pulses with peak powers near 6W.

Image processing method and apparatus and image forming apparatus for reducing moire fringes in output image

Abstract: A spatial filter applies a modulation transfer function correction process so as to increase a number of bits of input image data. A high-resolution converter converts the image data into output image data having a density higher than a sampling density of the input image data. One of the spatial filter and the resolution converter processes the image data after the other of the spatial filter and the resolution converter has processed the image data A possibility of saturation of an output of the spatial filter is decreased, thereby decreasing generation of moire in a saturation calculation.

Evaluation of rosette infrasonic noise-reducing spatial filters

Abstract: This paper presents results from recent tests of rosette infrasonic noise-reducing spatial filters at the Pinon Flat Observatory in southern California Data from 18- and 70-m aperture rosette filters and a reference port are used to gauge the reduction in atmospheric wind-generated noise levels provided by the filters and to examine the effect of these spatial filters on spatially coherent acoustic signals in the 002- to 10-Hz band At wind speeds up to 55 m/s, the 18-m rosette filter reduces wind noise levels above 02 Hz by 15 to 20 dB Under the same conditions, the 70-m rosette filter provides noise reduction of up to 15 to 20 dB between 002 and 07 Hz Standing wave resonance inside the 70-m filter degrades the reception of acoustic signals above 07 Hz The fundamental mode of the resonance, 15 dB above background, is centered at 265-Hz and the first odd harmonic is observed at 795 Hz in data from the large filter Analytical simulations accurately reproduce the noise reduction and resonance observed in the 70-m filter at all wind speeds above 125 m/s Resonance theory indicates that internal reflections that give rise to the resonance observed in the passband are occurring at the summing manifolds, and not at the inlets Rosette filters are designed for acoustic arrivals with infinite phase velocity The plane-wave response of the 70-m rosette filter has a strong dependence on frequency above 35 Hz at grazing angles of less than 15° from the horizontal At grazing angles, complete cancellation of the signal occurs at 5 Hz Theoretical predictions of the phase and amplitude response of 18- and 70-m rosette filters, that take into account internal resonance and time delays between the inlets, compare favorably with observations derived from a cross-spectral analysis of signals from the explosion of a large bolide

Minimum length of a single-mode fiber spatial filter.

Abstract: Using the concept of leaky modes, we derive the minimum length of a single-mode fiber required to act as a spatial-mode filter of given quality. The degree of filter action is defined by the ratio of power carried by the fundamental mode to that carried by the leaky modes.

Method and system for 3D imaging of target regions

Abstract: A system including confocal and triangulation-based scanners or subsystems provides data which is both acquired and processed under the control of a control algorithm to obtain information such as dimensional information about microscopic targets which may be “non-cooperative.” The “non-cooperative” targets are illuminated with a scanning beam of electromagnetic radiation such as laser light incident from a first direction. A confocal detector of the electromagnetic radiation is placed at a first location for receiving reflected radiation which is substantially optically collinear with the incident beam of electromagnetic radiation. The system includes a spatial filter for attenuating background energy. The triangulation-based subsystem also includes a detector of electromagnetic radiation which is placed at a second location which is non-collinear with respect to the incident beam. This detector has a position sensitive axis. Digital data is derived from signals produced by the detectors. In this way, data from at least one triangulation-based channel is acquired in parallel or sequentially with at least one slice of confocal image data having substantially perfect temporal and spatial registration with the triangulation-based sensor data. This allows for fusion or further processing of the data for use with a predetermined measurement algorithm to thereby obtain information about the targets.

Image processing apparatus and method that avoid generation of moire

Abstract: An image processing apparatus includes a spatial filtering unit which applies spatial filtering to input image data to generate filtered image data such that the spatial filtering provides a broader dynamic range for outputs than for inputs, and a high-resolution conversion unit which interpolates the filtered image data in a first direction by an average of values of two pixels adjacent in the first direction, in a second direction perpendicular to the first direction by an average of values of two pixels adjacent in the second direction, and in a direction diagonal to the first and second directions by an average of four pixels surrounding a pixel of interest, thereby converting the filtered image data into high-resolution image data.

Method of spatially filtering a digital image using chrominance information

Abstract: A method of processing a color digital image having a luminance channel and one or more chrominance channels to improve the tone scale characteristics of the image, includes the steps of: producing a control signal from the one or more chrominance channels; generating a pedestal signal containing mainly low frequency modulation by filtering the luminance channel with a spatial filter, whereby the operation of the spatial filter is modulated by the control signal; and producing a texture signal by subtracting the pedestal signal from the luminance channel.

Combined multiuser detection and beamforming for CDMA systems: filter structures

Abstract: Code-division multiple-access (CDMA) systems are interference limited, and therefore efficient interference management is necessary to enhance the capacity of a CDMA system. In this paper, we consider combining two effective receiver-based interference management strategies: multiuser detection (temporal filtering) and receiver beamforming (spatial filtering). We formulate and examine the performance of several two-dimensional linear filter structures, which are all based on minimum mean squared error (MMSE) criterion but differ in how the MMSE problems are defined in the temporal and spatial domains, i.e., jointly or in cascade. It is shown that while the joint optimum MMSE filter achieves the maximum signal-to-interference ratio (SIR) among all possible matrix filters, the constrained optimum MMSE filter which results in a single temporal and single spatial filter, outperforms all combined single-user/multiple-user approaches and cascaded optimization approaches either uniformly or asymptotically. The constrained optimum MMSE filter is near-far resistant in all but very highly loaded systems and enjoys low complexity.

Optical system for improving the brightness of a stack of lensed diode lasers

Abstract: The optical system includes a stack of lensed, AR-coated laser diode bars and optics to brighten the stack's output. A spatial filter forces each bar to lase in one or a few high-order modes having two strong emission lobes. Radiation in the first lobe is passed to a collimating optic to form a filtered image of each of the lensed diode bars on an associated grating, whose angle then determines the lasing wavelength of that bar. Radiation in the second lobe is directed into an output path where another collimating optic produces an array of spatially separated, collimated beams. The wavelengths set for each beam allow them to be spectrally combined into a single, multi-wavelength, collimated output beam possessing substantially the same cross section and divergence as an individual input beam.

A 2.45 GHz digital beam-forming antenna for RFID reader

Abstract: Spatial filtering using adaptive or smart antennas has emerged as a promising technique to improve the performance of cellular mobile systems. Due to the digital nature of the control of spatial filtering a quantization error is readily present. This is due to the fact that weights and phase information is provided with the aid of finite wordlength processors. This paper presents the development of a five-element digital beam-forming antenna for RFID (radio frequency identification) reader operating on the 2.4 GHz ISM-band. The paper presents a detailed description of the operation of beam-form control including the analysis of 4-bit phase shifters. In addition, the result shows that the quantization error, even if it small, can degrade the pattern so dramatically making it unacceptable. However, we show that with as low number of control bits as four the desired radiation characteristics can be achieved.

Non-diffracting beams with controlled spatial coherence

Abstract: We investigate the propagation properties of partially coherent non-diffracting beams. They are obtained as a superposition of plane waves possessing a single radial frequency and interfering with controlled angular correlation. The theoretical model is adapted to the experiments based on the use of the Fourier spatial filter illuminated by the Gaussian shell-model source. As an example, the influence of the partial coherence on the intensity distribution and phase properties of optical vortices is examined.

System and method for post-processing ultrasound color doppler imaging

Abstract: A system for ultrasonic imaging that includes a comparator for comparing at least one Doppler parameter input to a threshold value and generating a first mask. The comparator has a threshold value associated with each Doppler parameter input. A spatial filter is coupled to the comparator and produces a second mask, and a classification operator is coupled to the spatial filter and generates a third mask. The classification operator can effectively compare a value of a given pixel to the value of any nearby neighboring pixels and reclassify any given non-majority pixel to a majority value of the nearby neighboring pixels. An embodiment of the invention further includes a multi-parameter generator that outputs at least one Doppler parameter. The multi-parameter generator has a functional relationship based on at least one Doppler parameter input and at least one of any of the masks. A second spatial filter is coupled to receive the Doppler parameter input.

Optimum nonlinear composite filter for distortion-tolerant pattern recognition

Abstract: We describe a nonlinear distortion-tolerant filter for pattern recognition that is optimum in terms of tolerance to input noise and discrimination capability. This filter was derived by minimization of the output energy that is due to the overlapping additive noise and the input scene, and the output of the filter meets the design constraints obtained from the training data set. The performance of this filter was tested with an input scene containing one of the training data sets, a nontraining true target, and a false object in the presence of overlapping additive noise and nonoverlapping background noise. We carried out Monte Carlo runs to measure the statistical performance of the filter and obtained receiver operating characteristics curves to show the detection capabilities of the filter.

Spatiospectral transmission of a plane-mirror Fabry-Perot interferometer with nonuniform finite-size diffraction beam illuminations.

Abstract: The transmission of a plane-mirror Fabry–Perot (PFP) interferometer is theoretically modeled and investigated by treating the spatial and spectral features in a unified manner. A spatiospectral transfer function is formulated and utilized to describe the beam propagation and the multiple-beam interference occurring in an ideal one-dimensional strip PFP interferometer with no diffraction loss. The spatial-frequency filtration of a finite-size beam input not only determines the transmitted spatial beam profile but also plays a crucial role in affecting the overall spectral transmittance. The inherent deviations of the spectral transmittance from what we know as the standard Airy’s formula are revealed in diverse aspects, including the less-than-unity peak transmittance, the displacement of a resonance peak frequency, and the asymmetric detuning profile. Our theoretical analysis extends to the misaligned PFP interferometers, such as the cases in which non-normal-incidence beams or wedge-aligned mirrors are used that could severely degrade the effective interferometer finesse.

Measurement of an aeroacoustic dipole using a linear microphone array

Abstract: It is shown that the standard beamformer technique is inadequate for both the source location and the measurement of a simple dipole and that this is due to the assumption of monopole propagation in the calculation of the phase weights used to steer the focus of the array. A numerical simulation is used to illustrate the problem and to develop a correction to the signal processing algorithm to account for the dipole propagation characteristic. This is then applied to array measurements for an aeroacoustic dipole produced by a cylinder in a cross flow. The resulting source map and the beamformed spectrum are shown to give a true representation of the source energy and frequency content. A secondary effect of this correction is that the array becomes insensitive to other source types so that in addition to acting as a spatial filter, the array can perform as a source filter. This work also demonstrates how an array measurement can be misinterpreted if applied without consideration of the source mechanism.

Selective edge enhancement of images with an acousto-optic light modulator.

Abstract: We show experimental results for image processing using an acousto-optic light modulator (AOLM) where the image can be edge enhanced with respect to the input object. We can select which edges are enhanced and the degree to which they are enhanced by changing the amplitude of the acoustic wave of the AOLM. We relate this technique to the fractional Hilbert transform and the fractional derivative image-processing operations and discuss its application to phase-only input images.

Miniaturized infrared gas analyzing apparatus

Abstract: The present invention provides a miniaturized infrared gas analyzing apparatus, which is composed of various kinds of micro elements (e.g., infrared light source, tunable filter, and thermal detector) fabricated by means of silicon micromachining technology so as to meet the requirements of low power consumption and low cost and apply to qualitative and quantitative analysis of infrared absorption spectra of various kinds of gases. The miniaturized infrared gas analyzing apparatus comprises an infrared emitting unit, an infrared collimator, a bandpass and spatial filter, a tunable filter unit, a sensing unit, and a microprocessing unit. The infrared emitting unit utilizes the blackbody radiation principle of thermo-resistive filament to radiate out a wide infrared spectrum and serves as a point light source. The infrared collimator converts the infrared emitting unit into a collimated infrared light beam. The bandpass and spatial filter allows the transmission of an wide passband including at least the absorption wavelength of a specific gas to be sensed, and only let the infrared light beam passing within a specific geometric region. The Fabry-Perot tunable filter unit utilizes electric field to control the length of resonant cavity so that only the narrow-bandwidth wavelength matching the absorption spectrum of the sensed gas can pass at a time. The sensing unit determines the concentration of the sensed gas according to the light intensity. And, a microprocessing unit is used for controlling of all the components mentioned above.

Spatial filtering efficiency of monostatic biaxial lidar: analysis and applications

Abstract: Results of lidar modeling based on spatial-angular filtering efficiency criteria are presented. Their analysis shows that the low spatial-angular filtering efficiency of traditional visible and near-infrared systems is an important cause of low signal/background-radiation ratio (SBR) at the photodetector input. The low SBR may be responsible for considerable measurement errors and ensuing the low accuracy of the retrieval of atmospheric optical parameters. As shown, the most effective protection against sky background radiation for groundbased biaxial lidars is the modifying of their angular field according to a spatial-angular filtering efficiency criterion. Some effective approaches to achieve a high filtering efficiency for the receiving system optimization are discussed.

Optical filter using a linear variable filter with elliptical beam geometry

Abstract: This invention relates to a new optical geometry for using a linear variable filter as an optical spectral filter. An elliptical beam geometry with the minor beam axis oriented along the linearly varying axis of the filter is used. The beam is incident at a slight angle relative to the long axis of the beam. Ultimately this geometry produces a filter which has minimum spectral broadening from both angular and beam extent effects. Furthermore, this geometry results in low back reflectance and higher input beam power damage thresholds. The optical filter includes means for tuning a wavelength response of the filter. Further, the optical filter can afford Add/Drop functionality by providing optical circulators at the input and output ports.

Coherent micromirror arrays.

Abstract: A one-dimensional, spatial light phase modulator consists of a coherent array of programmable micromirrors. Each micromirror has rotation as well as elevation control to permit high-fidelity, piecewise linear approximations to a desired spatial phase profile. We demonstrate programmable, coherent spatial modulation by configuring a four-micromirror array to act as a blazed diffraction grating, showing five diffraction orders. In sharp contrast, adjustment of each mirror elevation converts the grating into the equivalent of a single, large tilted mirror that reflects only in a new specular direction. Higher-order diffraction is effectively suppressed.