Showing papers on "Spatial filter published in 2009"

Spatial Filtering for Wall-Clutter Mitigation in Through-the-Wall Radar Imaging

Abstract: Radio-frequency imaging of targets behind walls is of value in several civilian and defense applications. Wall reflections are often stronger than target reflections, and they tend to persist over a long duration of time. Therefore, weak and close by targets behind walls become obscured and invisible in the image. In this paper, we apply spatial filters across the antenna array to remove, or at least significantly mitigate, the spatial zero-frequency and low-frequency components which correspond to wall reflections. Unmasking the behind-the-wall targets via the application of spatial filters recognizes the fact that the wall electromagnetic (EM) responses do not significantly differ when viewed by the different antennas along the axis of a real or synthesized array aperture which is parallel to the wall. The proposed approach is tested with experimental data using solid wall, multilayered wall, and cinder block wall. It is shown that the wall reflections can be effectively reduced by spatial preprocessing prior to beamforming, producing similar imaging results to those achieved when a background scene without the target is available.

Super-resolution and reconstruction of sparse sub-wavelength images.

Abstract: We show that, in contrast to popular belief, sub-wavelength information can be recovered from the far-field of an optical image, thereby overcoming the loss of information embedded in decaying evanescent waves. The only requirement is that the image is known to be sparse, a specific but very general and wide-spread property of signals which occur almost everywhere in nature. The reconstruction method relies on newly-developed compressed sensing techniques, which we adapt to optical super-resolution and sub-wavelength imaging. Our approach exhibits robustness to noise and imperfections. We provide an experimental proof-of-principle by demonstrating image recovery at a spatial resolution 5-times higher than the finest resolution defined by a spatial filter. The technique is general, and can be extended beyond optical microscopy, for example, to atomic force microscopes, scanning-tunneling microscopes, and other imaging systems.

Laser light source device and laser irradiation apparatus using the same

Abstract: A laser light source device includes a pump light source which emits transverse-multimode light; a plurality of resonator mirrors which define a resonator, at least part of the resonator mirrors outputting light to the outside, where the output light having plural wavelengths; a laser medium arranged in the resonator, the laser medium being pumped with the transverse-multimode light emitted from the pump light source; and a wavelength conversion element arranged in the resonator, the wavelength conversion element being irradiated with a transverse-multimode line beam of fundamental wave obtained by oscillation at the laser medium and outputting a line beam of converted wave.

Measuring the Modal Content of Large-Mode-Area Fibers

Abstract: Spatially and spectrally resolved imaging of mode content in fibers, or more simply, S2 imaging, is a new measurement technique for analyzing the mode content of large-mode-area (LMA) fibers. It works by spatially resolving the spectral interference that occurs when light in a few-mode fibers scatters into different modes that then propagate with different group delays. A scanning spatial filter in the form of a single-mode fiber probe coupled to an optical spectrum analyzer is utilized to provide both spatial and spectral resolution, and the data are analyzed via the Fourier transform of the optical spectrum. The wealth of data allows for imaging multiple modes simultaneously propagating in the fiber under test as well as quantifying their relative power levels. In addition, the ability to analyze mode images as a function of modal group delay allows distinguishing between discrete scattering at fiber surfaces and distributed scattering that occurs along the length of the LMA fiber. The all-fiber nature of the setup makes the measurement sufficiently stable to measure phase images of the higher order modes (HOMs). Because the method is interferometrically based, even very weak HOMs can be detected.

Simultaneous dual-band optical coherence tomography in the spectral domain for high resolution in vivo imaging

Abstract: Optical coherence tomography (OCT) in the spectral domain is demonstrated simultaneously at two wavelength bands centered at 800 nm and 1250 nm. A novel commercial supercontinuum laser is applied as a single low coherence broadband light source. The emission spectrum of the source is shaped by optical and spatial filtering in order to achieve an adequate double peak spectrum containing the wavelength bands 700 - 900 nm and 1100 - 1400 nm for dual-band OCT imaging and thus reducing the radiation exposure of the sample. Each wavelength band is analyzed with an individual spectrometer at an A-scan rate of about 12 kHz which enables real-time imaging for the examination of moving samples. A common path optical setup optimized for both spectral regions with a separate single fiber-based scanning unit was realized which facilitates flexible handling and easy access to the measurement area. The free-space axial resolutions were measured to be less than 4.5 microm and 7 microm at 800 nm and 1250 nm, respectively. Three-dimensional imaging ten times faster than previously reported with a signal-to-noise-ratio of above 90 dB is achieved simultaneously in both wavelength bands. Spectral domain dual-band OCT combines real-time imaging with high resolution at 800 nm and enhanced penetration depth at 1250 nm and therefore provides a well suited tool for in vivo vasodynamic measurements. Further, spatially resolved spectral features of the sample are obtained by means of comparing the backscattering properties at two different wavelength bands. The ability of dual-band OCT to enhance tissue contrast and the sensitivity of this imaging modality to wavelength-dependent sample birefringence is demonstrated.

Direct generation of accelerating Airy beams using a 3/2 phase-only pattern

Abstract: Accelerated Airy beams have previously been generated using a cubic phase pattern that represents the Fourier transform of the Airy beam. The Fourier transform of this pattern is formed using a system length of 2f, where f is the focal length of the Fourier transform lens. In this work, we directly generate the Airy beam using a 3/2 phase pattern encoded onto an LCD. Experimental results show a deflection that depends on the square of the distance from the LCD and match theoretical predictions. However the system length is much shorter.

Spiral phase filtering and orientation-selective edge detection/enhancement

Abstract: A spiral phase plate with an azimuthal structure exp[iϕ](0⩽ϕ<2π) has been used as a filter in a 4f system to achieve edge enhancement. Generally such edge-enhanced effect is isotropic, i.e., each edge of an input pattern is enhanced to the same degree regardless of its orientation. We found that one can achieve anisotropic edge enhancement by breaking down the symmetry of the filtering process. This can be done in two ways: first, by use of a fractional spiral phase filter (SPF) with a fractional topological charge and a controllable orientation of the edge discontinuity, and second, by the lateral shifting of the SPF. We interpret this process as a vortex formation due to the diffraction of the Fourier spectrum of the input pattern by a SPF with an integer and fractional topological charge. Optical experiments using a spatial light modulator were carried out to verify our proposal.

Simultaneous multi-spot inspection and imaging

Abstract: A compact and versatile multi-spot inspection imaging system employs an objective for focusing an array of radiation beams to a surface and a second reflective or refractive objective having a large numerical aperture for collecting scattered radiation from the array of illuminated spots. The scattered radiation from each illuminated spot is focused to a corresponding optical fiber channel so that information about a scattering may be conveyed to a corresponding detector in a remote detector array for processing. For patterned surface inspection, a cross-shaped filter is rotated along with the surface to reduce the effects of diffraction by Manhattan geometry. A spatial filter in the shape of an annular aperture may also be employed to reduce scattering from patterns such as arrays on the surface. In another embodiment, different portions of the same objective may be used for focusing the illumination beams onto the surface and for collecting the scattered radiation from the illuminated spots simultaneously. In another embodiment, a one-dimensional array of illumination beams are directed at an oblique angle to the surface to illuminate a line of illuminated spots at an angle to the plane of incidence. Radiation scattered from the spots are collected along directions perpendicular to the line of spots or in a double dark field configuration.

Spatial filtering of light by chirped photonic crystals

Abstract: We propose an efficient method for spatial filtering of light beams by propagating them through two-dimensional (also three dimensional) chirped photonic crystals, i.e., through the photonic structures with fixed transverse lattice period and with the longitudinal lattice period varying along the direction of the beam propagation. We prove the proposed idea by numerically solving the paraxial propagation equation in refraction-index--modulated media and we evaluate the efficiency of the process by harmonic-expansion analysis. The technique can be also applied for filtering (for cleaning) of the packages of atomic waves (Bose condensates), also to improve the directionality of acoustic and mechanical waves.

On Spatial Aliasing in Microphone Arrays

Abstract: Microphone arrays sample the sound field in both space and time with the major objective being the extraction of the signal propagating from a desired direction-of-arrival (DOA). In order to reconstruct a spatial sinusoid from a set of discrete samples, the spatial sampling must occur at a rate greater than a half of the wavelength of the sinusoid. This principle has long been adapted to the microphone array context: in order to form an unambiguous beampattern, the spacing between elements in a microphone array needs to conform to this spatial Nyquist criterion. The implicit assumption behind the narrowband beampattern is that one may use linearity and Fourier analysis to describe the response of the array to an arbitrary wideband plane wave. In this paper, this assumption is analyzed. A formula for the broadband beampattern is derived. It is shown that in order to quantify the spatial filtering abilities of a broadband array, the incoming signal's bifrequency spectrum must be taken into account, particularly for nonstationary signals such as speech. Multi-dimensional Fourier analysis is then employed to derive the broadband spatial transform, which is shown to be the limiting case of the broadband beampattern as the number of sensors tends to infinity. The conditions for aliasing in broadband arrays are then determined by analyzing the effect of computing the broadband spatial transform with a discrete spatial aperture. It is revealed that the spatial Nyquist criterion has little importance for microphone arrays. Finally, simulation results show that the well-known steered response power (SRP) method is formulated with respect to stationary signals, and that modifications are necessary to properly form steered beams in nonstationary signal environments.

Adaptive Fuzzy Filtering for Artifact Reduction in Compressed Images and Videos

Abstract: A fuzzy filter adaptive to both sample's activity and the relative position between samples is proposed to reduce the artifacts in compressed multidimensional signals. For JPEG images, the fuzzy spatial filter is based on the directional characteristics of ringing artifacts along the strong edges. For compressed video sequences, the motion compensated spatiotemporal filter (MCSTF) is applied to intraframe and interframe pixels to deal with both spatial and temporal artifacts. A new metric which considers the tracking characteristic of human eyes is proposed to evaluate the flickering artifacts. Simulations on compressed images and videos show improvement in artifact reduction of the proposed adaptive fuzzy filter over other conventional spatial or temporal filtering approaches.

Median-modified Wiener filter provides efficient denoising, preserving spot edge and morphology in 2-DE image processing

Abstract: Denoising is a fundamental early stage in 2-DE image analysis strongly influencing spot detection or pixel-based methods. A novel nonlinear adaptive spatial filter (median-modified Wiener filter, MMWF), is here compared with five well-established denoising techniques (Median, Wiener, Gaussian, and Polynomial-Savitzky-Golay filters; wavelet denoising) to suggest, by means of fuzzy sets evaluation, the best denoising approach to use in practice. Although median filter and wavelet achieved the best performance in spike and Gaussian denoising respectively, they are unsuitable for contemporary removal of different types of noise, because their best setting is noise-dependent. Vice versa, MMWF that arrived second in each single denoising category, was evaluated as the best filter for global denoising, being its best setting invariant of the type of noise. In addition, median filter eroded the edge of isolated spots and filled the space between close-set spots, whereas MMWF because of a novel filter effect (drop-off-effect) does not suffer from erosion problem, preserves the morphology of close-set spots, and avoids spot and spike fuzzyfication, an aberration encountered for Wiener filter. In our tests, MMWF was assessed as the best choice when the goal is to minimize spot edge aberrations while removing spike and Gaussian noise.

The PRIMA fringe sensor unit

Abstract: Context. The fringe sensor unit (FSU) is the central element of the phase referenced imaging and micro-arcsecond astrometry (PRIMA) dual-feed facility and provides fringe sensing for all observation modes, comprising off-axis fringe tracking, phase referenced imaging, and high-accuracy narrow-angle astrometry. It is installed at the Very Large Telescope Interferometer (VLTI) and successfully served the fringe-tracking loop during the initial commissioning phase.Aims. To maximise sensitivity, speed, and robustness, the FSU is designed to operate in the infrared K -band and to include spatial filtering after beam combination and a very-low-resolution spectrometer without photometric channels. It consists of two identical fringe sensors for dual-star operation in PRIMA astrometric mode.Methods. Unique among interferometric beam combiners, the FSU uses spatial phase modulation in bulk optics to retrieve real-time estimates of fringe phase after spatial filtering. The beam combination design accommodates a laser metrology for pathlength monitoring. An spectrometer across the K -band makes the retrieval of the group delay signal possible. The calibration procedure uses the artificial light source of the VLTI laboratory and is based on Fourier transform spectroscopy to remove instrumental effects.Results. The FSU was integrated and aligned at the VLTI in July and August 2008. It yields phase and group delay measurements at sampling rates up to 2 kHz, which are used to drive the fringe-tracking control loop. During the first commissioning runs, the FSU was used to track the fringes of stars with K -band magnitudes as faint as mK = 9.0, using two VLTI auxiliary telescopes (AT) and baselines of up to 96 m. Fringe tracking using two Very Large Telescope (VLT) unit telescopes was demonstrated.Conclusions. The concept of spatial phase-modulation for fringe sensing and tracking in stellar interferometry is demonstrated for the first time with the FSU. During initial commissioning and combining stellar light with two ATs, the FSU showed its ability to improve the VLTI sensitivity in K -band by more than one magnitude towards fainter objects, which is fundamental for achieving the scientific objectives of PRIMA.

Analysis of a Spatial-Filtering Passive Fiber Laser Beam Combining System

Abstract: In this paper, a formulation of the cavity mode structure, loss, and beam quality of a ring-cavity fiber laser beam combining apparatus is developed, which shows that the resonator is rigorously single mode. The passive phasing property is clearly understood on the basis of the theory, and conditions are specified for a cavity that permits the attainment of optimum phasing conditions. An important result is that reduction of over-all cavity loss must be achieved at the price of reduction of maximum array size, although mitigated by the property that this loss occurs in the low-power feedback leg. Application of the analysis to specific examples gives the dependence of cavity loss and Strehl ratio on fiber and cavity properties.

Narrowband tunable photonic notch filter.

Abstract: We propose a technique for realization of a high-contrast, tunable, low-insertion-loss notch filter using polarization selectivity of whispering-gallery-mode resonators. We demonstrate a 10 MHz filter with 5.5 dB insertion loss and 45.5 dB of in-band rejection. The measured rejection value is limited by the finite (3 kHz) linewidth of our laser. We show that the filter can potentially have tunable bandwidth without significant rejection modification.

Subdiffraction and spatial filtering due to periodic spatial modulation of the gain-loss profile

Abstract: We investigate light propagation in the materials with periodically modulated gain-loss profile in both transverse and longitudinal directions with respect to the direction of light propagation. We predict the effects of self-collimation (diffraction-free propagation) of the beams, as well as the superdiffusion (spatial frequency filtering) of the beams depending on the geometry of the gain-loss lattice, and justify the predictions by numerical simulations of the paraxial wave propagation equations.

Beamforming and imaging with the BYU/NRAO L-band 19-element phased array feed

Abstract: An experimental 19-element L-band phased array feed was installed on the Green Bank 20-Meter Telescope in October 2007 and July 2008 to measure sensitivity and effciency and demonstrate signal processing algorithms for array calibration, multiple beam formation, imaging, and adaptive spatial filtering methods for interference mitigation. System noise performance was characterized using a warm absorber/cold sky Y-factor setup. The peak beam aperture effciency was 69% and the minimum beam equivalent system temperature was 66K. With a single reflector pointing, a high sensitivity image of a field of view approximately two half-power beamwidths in diameter can be produced. Measured figures of merit compare well to numerical simulations, indicating that complicating effects such as mutual coupling are understood well enough to enable the next phase of array feed development to proceed on firm grounds.

Spatial filtering using directional audio coding parameters

Abstract: Directional audio coding (DirAC) is a recent method for spatial audio processing, based on a perceptually motivated representation of spatial sound. Due to its efficiency, DirAC has already been proposed for spatial audio teleconferencing scenarios. Modern hands-free communication systems usually include beamforming techniques to improve speech intelligibility by suppressing diffuse background noise and interfering sources. In this paper, we propose a novel spatial filtering method which can be integrated into the DirAC spatial codec. It uses a spectral weighting of the recorded audio signal, where the design of the corresponding spatial filter transfer function is based on the DirAC parameters, i. e., direction-of-arrival and diffuseness of the sound field. Simulation results show that compared to a standard beamformer the novel technique offers significantly higher interference attenuation, while introducing similar distortion of the desired signal.

Optical Scatter Imaging with a digital micromirror device

Abstract: We had developed Optical Scatter Imaging (OSI) as a method which combines light scattering spectroscopy with microscopic imaging to probe local particle size in situ. Using a variable diameter iris as a Fourier spatial filter, the technique consisted of collecting images that encoded the intensity ratio of wide-to-narrow angle scatter at each pixel in the full field of view. In this paper, we replace the variable diameter Fourier filter with a digital micromirror device (DMD) to extend our assessment of morphology to the characterization of particle shape and orientation. We describe our setup in detail and demonstrate how to eliminate aberrations associated with the placement of the DMD in a conjugate Fourier plane of our microscopic imaging system. Using bacteria and polystyrene spheres, we show how this system can be used to assess particle aspect ratio even when imaged at low resolution. We also show the feasibility of detecting alterations in organelle aspect ratio in situ within living cells. This improved OSI system could be further developed to automate morphological quantification and sorting of non-spherical particles in situ.

Guidelines for Adaptive-Optic Correction Based on Aperture Filtration

Abstract: Aperture filtration refers to the effects of viewing optical wavefront distortions of infinite extent through a finite aperture If the length-scale of the aberration is larger than this aperture, then the portion of the aberration visible in the aperture at any moment in time will not reach the full magnitude of the aberration seen in its entirety The aperture acts as a spatial filter, mitigating the effects of large-scale wavefront distortions while having little effect on smaller-scale aberrations, with the dividing line between large-scale and small-scale being the size of the aperture itself This dissertation presents and charts the development of a set of analytic formulas for judging and predicting the effectiveness of adaptive-optic corrective systems applied over finite apertures This includes some simplified formulas and benchmarks as guides for the minimum requirements a system will need to meet to be effective, and the maximum degree of effectiveness such systems can reasonably achieve

Multi-frame approach for image upscaling

Abstract: Video upscaling may be performed by effectively combining several different image processing techniques. The video is processed using temporal based motion estimation and subsequently filtered using a spatial filter. The spatially filtered image is sharpened and noise is suppressed. Also, the images in the video are upscaled.

Aberration-free laser beam in the soft x-ray range

Abstract: By seeding an optical-field-ionized population-inverted plasma amplifier with the 25th harmonic of an IR laser, we have achieved what we believe to be the first aberration-free laser beam in the soft x-ray spectral range. This laser emits within a cone of 1.34 mrad(1/e(2)) at a repetition rate of 10 Hz at a central wavelength of 32.8 nm. The beam exhibits a circular profile and wavefront distortions as low as lambda/17. A theoretical analysis of these results shows that this high beam quality is due to spatial filtering of the seed beam by the plasma amplifier aperture.

Time gate, optical layout, and wavelength effects on ballistic imaging

Abstract: A method to distinguish a hidden object from a perturbing environment is to use an ultrashort femtosecond pulse of light and a time-resolved detection. To separate ballistic light containing information on a hidden object from multiscattered light coming from the surrounding environment that scrambles the signal, an optical Kerr gate can be used. It consists of a carbon disulfide (CS2) cell in which birefringence is optically induced. An imaging beam passes through the studied medium while a pump pulse is used to open the gate. The time-delayed scattered light is excluded from measurements by the gate, and the multiple-scattering scrambling effect is reduced. In previous works, the two beams had the same wavelength. We propose a new two-color experimental setup for ballistic imaging in which a second harmonic is generated and used for the image, while the fundamental is used for gate switching. This setup allows one to obtain better resolution by using a spectral filtering to eliminate noise from the pump pulse, instead of a spatial filtering. This new setup is suitable for use in ballistic imaging of dense sprays, multidiffusive, and large enough to show scattered light time delays greater than the gate duration (τ=1.3 ps).

An Adaptive Noise-Filtering Algorithm for AVIRIS Data With Implications for Classification Accuracy

Abstract: This paper describes a new algorithm used to adaptively filter a remote-sensing data set based on signal-to-noise ratios (SNRs) once the maximum noise fraction has been applied. This algorithm uses Hermite splines to calculate the approximate area underneath the SNR curve as a function of band number, and that area is used to place bands into ldquobinsrdquo with other bands having similar SNRs. A median filter with a variable-sized kernel is then applied to each band, with the same size kernel used for each band in a particular bin. The proposed adaptive filters are applied to a hyperspectral image generated by the airborne visible/infrared imaging spectrometer sensor, and results are given for the identification of three different pine species located within the study area. The adaptive-filtering scheme improves image quality as shown by estimated SNRs. Classification accuracies of three pine species improved by more than 10% in the study area as compared to that achieved by the same discriminant method without adaptive spatial filtering.

The spatial filtering method for solid particle velocity measurement based on an electrostatic sensor

Abstract: The spatial filtering method for particle velocity measurement has the advantages of simplicity of the measurement system and convenience of data processing. In this paper, the relationship between solid particles mean velocity in a pneumatic pipeline and the power spectrum of the output signal of an electrostatic sensor was mathematically modeled. The effects of the length of the sensor, the thickness of the dielectric pipe and its length on the spatial filtering characteristics of the sensor were also investigated using the finite element method. As for the roughness of and the difficult determination of the peak frequency fmax of the power spectrum characteristics of the output signal of the sensor, a wavelet analysis based filtering method was applied to smooth the curve, which can accurately determine the peak frequency fmax. Finally, experiments were performed on a pilot dense phase pneumatic conveying rig at high pressure to test the performance of the velocity measurement system. The experimental results show that the system repeatability is within ±4% over a gas superficial velocity range of 8.63–18.62 m s−1 for a particle concentration range of 0.067–0.130 m3 m−3.

Semi-active optical tracking system

Abstract: A system and method for tracking an airborne target including an illumination source (e.g., a diode laser array) is used to enhance a target signature and a detector (e.g., a passive high-speed camera) is used to detect to electromagnetic radiation (e.g., infrared radiation) reflected off the target. The received electromagnetic radiation may be processed by a digital computer and passed through a spatial filter that implements a band limited edge detection operation in the frequency domain. The filter may remove low spatial frequencies that attenuate soft edged clutter such as clouds and smoke as well as filter out artifacts and attenuated medium to high spatial frequencies to inhibit speckle noise from the detector as well as speckle from the laser return off the target.

Digital In-line Holography with femtosecond VUV radiation provided by the free-electron laser FLASH

Abstract: Femtosecond vacuum ultraviolet (VUV) radiation provided by the free-electron laser FLASH was used for digital in-line holographic microscopy and applied to image particles, diatoms and critical point dried fibroblast cells To realize the classical in-line Gabor geometry, a 1 microm pinhole was used as spatial filter to generate a divergent light cone with excellent pointing stability At a fundamental wavelength of 8 nm test objects such as particles and diatoms were imaged at a spatial resolution of 620 nm In order to demonstrate the applicability to biologically relevant systems, critical point dried rat embryonic fibroblast cells were for the first time imaged with free-electron laser radiation

Anti-aliasing spatial filter system

Abstract: A sensor module for an image capture device including an image sensor and a filter system, the image capture device having a high resolution mode of operation and one or more lower resolution modes of operation; the filter system includes an adjustable spatial filter associated with the optical path of the image capture device having an adjustable optical structure which in response to a first signal causes the adjustable spatial filter to be effective in a high resolution mode of operation and in response to a second signal causes the adjustable spatial filter to be effective in a lower resolution mode of operation.