Showing papers on "Spatial filter published in 2001"

Adaptive sensitivity encoding incorporating temporal filtering (TSENSE).

Abstract: A number of different methods have been demonstrated which increase the speed of MR acquisition by decreasing the number of sequential phase encodes. The UNFOLD technique is based on time interleaving of k-space lines in sequential images and exploits the property that the outer portion of the field-of-view is relatively static. The differences in spatial sensitivity of multiple receiver coils may be exploited using SENSE or SMASH techniques to eliminate the aliased component that results from undersampling k-space. In this article, an adaptive method of sensitivity encoding is presented which incorporates both spatial and temporal filtering. Temporal filtering and spatial encoding may be combined by acquiring phase encodes in an interleaved manner. In this way the aliased components are alternating phase. The SENSE formulation is not altered by the phase of the alias artifact; however, for imperfect estimates of coil sensitivities the residual artifact will have alternating phase using this approach. This is the essence of combining temporal filtering (UNFOLD) with spatial sensitivity encoding (SENSE). Any residual artifact will be temporally frequency-shifted to the band edge and thus may be further suppressed by temporal low-pass filtering. By combining both temporal and spatial filtering a high degree of alias artifact rejection may be achieved with less stringent requirements on accuracy of coil sensitivity estimates and temporal low-pass filter selectivity than would be required using each method individually. Experimental results that demonstrate the adaptive spatiotemporal filtering method (adaptive TSENSE) with acceleration factor R = 2, for real-time nonbreath-held cardiac MR imaging during exercise induced stress are presented.

A novel approach for precise simulation of the EMG signal detected by surface electrodes

Abstract: The authors propose a new electromyogram generation and detection model. The volume conductor is described as a nonhomogeneous (layered) and anisotropic medium constituted by muscle, fat and skin tissues. The surface potential detected in space domain is obtained from the application of a two-dimensional spatial filter to the input current density source. The effects of electrode configuration, electrode size and inclination of the fibers with respect to the detection system are included in the transfer function of the filter. Computation of the signal in space domain is performed by applying the Radon transform; this permits to draw considerations about spectral dips and clear misunderstandings in previous theoretical derivations. The effects of generation and extinction of the action potentials at the fiber end plate and at the tendons are included by modeling the source current, without any approximation of its shape, as a function of space and time and by using again the Radon transform. The approach, based on the separation of the temporal and spatial properties of the muscle fiber action potential and of the volume conductor, includes the capacitive tissue properties.

Theory of synthetic aperture radar imaging of a moving target

Abstract: Two novel image processing techniques have been developed to refocus a moving target image from its smeared response in the synthetic aperture radar (SAR) image which is focused on the stationary ground. Both approaches may be implemented with efficient fast Fourier transform (FFT) routines to process the Fourier spatial spectrum of the image data. The first approach utilizes a matched target filter that is derived from the signal history along the range-Doppler migration path mapped onto the SAR image from the moving target trajectory in real space. The coherent spatial filter is specified by the apparent target range in the image and the magnitude of the relative target-to-radar velocity. The second approach eliminates the range-dependence by reconstructing the moving target image from a spectral function that is obtained from the SAR image data spectrum via a spatial frequency coordinate transformation.

Peering into darkness with a vortex spatial filter

Abstract: I propose to use as a window the dark core of an optical vortex to examine a weak background signal hidden in the glare of a bright coherent source. Applications such as the detection of an astronomical object, forward-scattered radiation, and incoherent light are described whereby signal enhancements of at least 7 orders of magnitude may be achieved.

Guided wave spatial filter

Abstract: The invention concerns a guided wave spatial filter (300) for receiving input radiation and outputting corresponding filtered output radiation, the filter comprising first and second waveguide sections (30, L3, L5) connected in series, the sections: (a) mutually matched for transmitting fundamental mode radiation components present in the input radiation therethrough to provide the output radiation; and (b) mutually mismatched for hindering higher-order mode radiation components present in the input radiation from propagating therethrough and contributing to the output radiation. The spatial filter (300) is implemented using rib waveguides (30) for the sections with associated relatively deeply and relatively shallowly etched structures (20, 310, 320) for imparting to the sections their radiation mode filtration characteristics. The spatial filter according to the invention can be incorporated into optical splitters and optical modulators to enhance their performance and desensitise them to higher-order mode radiation injected thereinto.

Optimal phase contrast in common-path interferometry

Abstract: We have developed an analytical model for the design and optimization of common-path interferometers (CPI's) based on spatial filtering. We describe the mathematical analysis in detail and show how its application to the optimization of a range of different CPI's results in the development of a graphical framework to characterize quantitatively CPI performance. A detailed analytical treatment of the effect of curvature in the synthetic reference wave is undertaken. We show that it is possible to improve the linearity and fringe accuracy of certain standard interferometers by a modification of the Fourier filter, and we propose and analyze a dual CPI system for the unambiguous mapping of phase to intensity over the complete input phase range.

Interference management for CDMA systems through power control, multiuser detection, and beamforming

Abstract: Among the ambitious challenges to be met by the third-generation systems is to provide high-capacity flexible services. Code-division multiple access (CDMA) emerges as a promising candidate to meet these challenges. It is well known that CDMA systems are interference-limited, and interference management is needed to maximally utilize the potential gains of this access scheme. Several methods of controlling and/or suppressing the interference through power control, multiuser detection (temporal filtering), and receiver beamforming (spatial filtering) have been proposed to increase the capacity of CDMA systems up to date. We investigate the capacity increase that is possible by combining power control with intelligent temporal and spatial receiver filter design. The signal-to-interference ratio maximizing joint temporal-spatial receiver filters in unconstrained and constrained filter spaces are derived. Two-step iterative power control algorithms that converge to the optimum powers and the joint temporal and spatial receiver filters in the corresponding filter domains are given. A power control algorithm with a less complex filter update procedure is also given. We observe that significant savings in total transmit power are possible if filtering in both domains is utilized compared with conventional power control and joint optimal power control and filtering in only one domain.

Concentric-ring electrode systems for noninvasive detection of single motor unit activity

Abstract: New recording techniques for detecting surface electromyographic (EMG) signals based on concentric-ring electrodes are proposed in this paper. A theoretical study of the two-dimensional (2-D) spatial transfer function of these recording systems is developed both in case of rings with a physical dimension and in case of line rings. Design criteria for the proposed systems are presented in relation to spatial selectivity. It is shown that, given the radii of the rings, the weights of the spatial filter can be selected in order to improve the rejection of low spatial frequencies, thus increasing spatial selectivity. The theoretical transfer functions of concentric systems are obtained and compared with those of other detection systems. Signals detected with the ring electrodes and with traditional one-dimensional and 2-D systems are compared. The concentric-ring systems show higher spatial selectivity with respect to the traditional detection systems and reduce the problem of electrode location since they are invariant to rotations. The results shown are very promising for the noninvasive detection of single motor unit (MU) activities and decomposition of the surface EMG signal into the constituent MU action potential trains.

Coupling of large telescopes and single-mode waveguides: application to stellar interferometry

Abstract: The coupling between a turbulence-distorted optical beam and a single-mode waveguide is addressed. The coupling efficiency and the coupled phase are derived, both without aberrations and with small aberrations. These analytical expressions are validated by numerical simulations. Correction with adaptive optics is investigated. In the general case, the Strehl ratio is a pessimistic estimator, and the coupled phase is different and has a smaller variance than the classical phase averaged over the pupil. Application fields are heterodyne detection and stellar interferometry, for which spatial and modal filtering are distinguished.

Noise reduction pre-processor for digital video using previously generated motion vectors and adaptive spatial filtering

Abstract: Noise is reduced in a video system by applying motion compensated temporal filtering using previously generated motion vectors and adaptive spatial filtering at scene change frames. Various types of noise can be introduced into video prior to compression and transmission. Artifacts arise from recording and signal manipulation, terrestrial or orbital communications, or during decoding. Noise introduced prior to image compression interferes with performance and subsequently impairs system performance. While filtering generally reduces noise in a video image, it can also reduce edge definition leading to loss of focus. Filtering can also tax system throughput, since increased computational complexity often results from filtering schemes. Furthermore, the movement of objects within frames, as defined by groups of pixels, complicates the noise reduction process by adding additional complexity. In addition to improvements made to FIR spatial filtering, the present invention improves on previous filtering techniques by using Infinite Impulse Response (IIR) temporal filtering to reduce noise while maintaining edge definition. It also uses motion vectors previously calculated as part of the first-pass image encoding or alternatively by transcoding to reduce computational complexity for P-frame and B-frame image preprocessing. Single stage P-frame temporal noise filtering and double stage B-frame temporal noise filtering are presented.

Noise performance of multimode VCSELs

Abstract: Relative intensity noise (RIN) of selective oxidized multimode VCSELs has been investigated. The VCSELs show an excellent noise performance with a RIN/spl les/-140 dB/Hz at 500 MHz and only one resonance peak in the RIN spectrum, as long as the total light power is detected. The situation deteriorates if the transmission link contains mode-selective losses. Introducing external filtering of the emitted light by an aperture or a polarizer, one may observe an increase of RIN by several orders of magnitude and multiple-resonance peaks in the RIN spectrum. However, even in spite of a very restrictive external filtering a RIN of -120 dB/Hz can still be achieved, which enables a 1.25 Gb/s transmission. The multiple-resonance peaks in the filtered RIN spectra do not describe the different relaxation resonance frequencies of transverse modes. We believe there is only one relaxation resonance frequency, which is proportional to the square root of the photon number in the active layer, irrespective of the number of existing modes. The other peaks in the RIN spectrum ran be considered as "mode partition frequencies," which result from the carrier interchanges between the modes. Using a simplified numerical model, which takes two modes into account, analytical expressions describing the RIN of each mode, as well as of the total power have been derived. It has been found that the larger the overlap between the modes, the smaller the "mode partition frequency" and the larger is the maximum mode partition noise (MPN).

Double-resonance-absorption microscope

Abstract: The present invention provides various techniques useful for a double-resonance-absorption microscope which realizes a super-resolution by using a double resonance absorption. As one technique, there is provided a double-resonance-absorption microscope comprising a light source for a pump light of a wavelength λ1 which excites a sample molecule to a first electronic excited state from a ground state, a light source for an erase light of a wavelength λ2 which excites the sample molecule to a second electronic excited state or a higher excited state from the first electronic excited state and overlap means for partially overlapping irradiating areas of the pump light and the erase light with each other, wherein an emission area upon deexcitation of the sample molecule to the ground state from the first electronic excited state is partially inhibited by irradiating the pump light and the erase light through the overlap means, said double-resonance-absorption microscope characterized in that there is provided, on an optical path of the erase light, a spatial filter which has a condenser lens and a collimate lens and a pinhole therebetween and performs condensing of the erase light onto the pinhole by the condenser lens and collimating of the erase light passed through the pinhole into a parallel beam by the collimate lens.

Method and apparatus for measuring refractive errors of an eye

Abstract: Method and apparatus for measuring refractive errors of an eye that improve upon wavefront type refractors using a conventional Hartmann-Shack sensor. Specifically, one embodiment of the present invention is an apparatus (100) for measuring refractive errors of an eye (30) which includes: (a) a source (12) of a probe beam; (b) a first Badal lens system (15) adapted to project the probe beam into a subject's eye (30) to form an illumination spot (32) on a retina (31); (c) a second Badal lens system (41) adapted to image the illumination spot (32) onto an image plane (R') substantially conjugate to the retina (31); and (d) a spatial filter (44) disposed in the image plane (R') adapted to transmit at least a portion of the image.

Influence of tip modulation on image formation in scanning near-field optical microscopy

Abstract: Modulation of the probe height in a scanning near-field optical microscope (SNOM) is a technique that is commonly used for both distance control and separation of the near-field signal from a background. Detection of higher harmonic modulated signals has also been used to obtain an improvement in resolution, the elimination of background, or artifacts in the signal. This article presents a theoretical model for the effects induced in SNOM images by modulation of the probe. It is shown that probe modulation introduces a spatial filter into the image, generally suppressing propagating field components and enhancing the strength of evanescent field components. A simple example of detection of a single evanescent field above a prism is studied in some detail, and a complicated dependence on modulation parameters and waveform is shown. Some aspects of the application of this theory in a general experimental situation are discussed. Simulated images are displayed to explicitly show the effects of varying modulation amplitude with first and second harmonic detection. Finally, we discuss the suppression of background artifacts due to propagating fields through the use of higher harmonic detection.

Adaptive-neighborhood speckle removal in multitemporal synthetic aperture radar images.

Abstract: We present a new method for multitemporal synthetic aperture radar image filtering using three-dimensional (3D) adaptive neighborhoods. The method takes both spatial and temporal information into account to derive the speckle-free value of a pixel. For each pixel individually, a 3D adaptive neighborhood is determined that contains only pixels belonging to the same distribution as the current pixel. Then statistics computed inside the established neighborhood are used to derive the filter output. It is shown that the method provides good results by drastically reducing speckle over homogeneous areas while retaining edges and thin structures. The performances of the proposed method are compared in terms of subjective and objective measures with those given by several classical speckle-filtering methods.

Image filters and source of illumination for optical navigation upon arbitrary surfaces are selected according to analysis of correlation during navigation

Abstract: Optical navigation upon grainy surfaces whose orientation is inclined at about 45° to the X and Y axes of the navigation mechanism is enhanced by: First, detect that a spatial filter in use is inappropriate for the orientation presently occurring, and; Second, employ a different and more appropriate spatial filter subsequent to such detection. Two spatial filters have been developed that are respectively effective about the 45° and 135° inclinations of the Standard filter. The shape of a correlation surface used in the navigation process is tested for the presence of a transverse ridge in the correlation surface. This generates control metrics whose filtered excursions are tracked by a control system that changes the spatial filter in use. The control system incorporates a time constant to prevent thrashing and excessive sensitivity to isolated random variations. The direction from which illumination arrives relative to the X and Y axes affects the range of angles (relative to, say, the Y axis) that a filter is effective, by changing the apparent size and proportions of the highlights and shadows that are the perceived features ultimately navigated upon. Stable operation of the control system can be enhanced by ensuring overlap of the filters' ranges, and can be promoted by dynamically altering the direction from which illumination reaches the navigation surface, so that it corresponds to, or varies in relation with, the filter in use.

Microscopic imaging with high energy x-rays by Fourier transform holography

Abstract: Experimental results obtained with an x-ray microscope based on Fourier transform holography are discussed. The experiments were done with monochromatic x rays at an energy of 14 keV. The x-ray source was a high brilliance undulator with a small vertical source size providing a spatial coherent beam. The necessity of the increase of the spatial coherence by spatial filtering is discussed together with the generation of a smallest possible reference point source. A Fresnel zone plate was used as a beam splitter. It formed the reference point source by the first order diffracted beam. The sample to be imaged is placed close to the first order focal spot and in parallel the sample is illuminated by the zero order transmitted beam. The hologram is generated by the interference of the reference wave and the object wave at a large distance from the object. A characteristic of these holograms is that a small distance in the object plane gives rise to large fringe spacing in the hologram plane and vice versa. In ...

Dynamic complex wave-front modulation with an analog spatial light modulator.

Abstract: A method of producing an arbitrary complex field modulation by use of two pixels of an analog ferroelectric spatial light modulator (SLM) is demonstrated The method uses the gray-scale modulation capabilities of a SLM to spatially encode the complex data on two pixels A spatial filter is used to remove the carrier signal This technique gives fast gray-level amplitude and phase modulation

A method and an apparatus for generating a phase-modulated wave front of electromagnetic radiation

Abstract: The present invention provides a method and a system for generating a phase-modulated wave front. According to the present invention, the spatial phase-modulation is not performed on the different parts of the wave front individually as in known POSLMs. Rather, the spatial phase-modulation of the present invention is performed by generating an amplitude modulation in the wave front, Fourier or Fresnel transforming the amplitude modulated wave front, filtering Fourier or Fresnel components of the Fourier or Fresnel distribution with a spatial filter such as a phase contrast filter, and regenerating the wave front whereby the initial amplitude modulation has transformed into a phase-modulation.

Eigenvector based spatial filtering of fetal biomagnetic signals

Abstract: In this paper we demonstrate the usefulness of an eigenvector based spatial filtering method for signal processing of multi-channel fetal magnetocardiogram (fMCG) and fetal magnetoencephalogram (fMEG) recordings. This method of filtering can separate signal and interference by exploiting the considerable spatial information contained in multi-channel recordings. Typically, fMCGs and fMEGs suffer from large cardiac interference and low signal-to-noise ratio. To isolate the signal from the interference, we identify their respective subspaces using one portion of the record dominated by the signal and another dominated by the interference. Each subspace is approximately rank two since the sources of the signal and interference can be modeled as current dipoles. The spatial filter consists of a linear transformation that preserves the signal subspace and annihilates the interference subspace. It is easier to implement than a matched filter, preserves the morphology and topography of the fetal signal, and effectively removes maternal cardiac interference, even when the maternal and fetal complexes overlap strongly in time or when small maternal movements or maternal arrhythmias alter the temporal character of the interference.

Quantitative image quality analysis of a nonlinear spatio-temporal filter

Abstract: Digital temporal and spatial filtering of fluoroscopic image sequences can be used to improve the quality of images acquired at low X-ray exposure. In this study, we characterized a nonlinear edge preserving, spatio-temporal noise reduction filter, the bidirectional multistage (BMS) median filter of Arce (1991). To assess image quality, signal detection and discrimination experiments were performed on stationary targets using a four-alternative forced-choice paradigm. A measure of detectability, d', was obtained for filtered and unfiltered noisy image sequences at different signal amplitudes. Filtering gave statistically significant, average d' improvements of 20% (detection) and 31% (discrimination). A nonprewhitening detection model modified to include the human spatio-temporal visual system contrast-sensitivity underestimated enhancement, predicting an improvement of 6%. Pixel noise standard deviation, a commonly applied image quality measure, greatly overestimated effectiveness giving 67% improvement in d'. We conclude that human testing is required to evaluate the filter effectiveness and that human perception models must be improved to account for the spatio-temporal filtering of image sequences.

Spatial filter for enhancing hartmann-shack images and associated methods

Abstract: A system and method improves a quality of sensor image data from a wavefront emanating from an eye The sensor image data include a pixel array of intensities A filter array is populated with a coefficient at each position of the array and the filter array is applied to each position in the pixel array The system includes a processor and a software package adapted to perform the above method steps

Method of coding artefacts reduction

Abstract: A method to reduce coding artefacts within a discrete decoded picture includes a spatial and/or temporal filtering with respective filter characteristics dependent on an image quality value (Q). Preferrably, said spatial filtering includes a deblocking filtering, wherein the deblocking filter operation decreases with an increasing image quality value (Q) and/or said deblocking filtering chooses a image quality value (Q) dependent weighting function for a pixel to be filtered according to an activity of both blocks to which common border said pixel belongs.

Spatial filter approach for comparison of the forward and inverse problems of electroencephalography and magnetoencephalography.

Abstract: We present an analysis of the relative information content of cortical current source reconstructions from electroencephalogram (EEG) and magnetoencephalogram (MEG) forward calculations by examining the spatial filters that relate the internal sources with the externally measured electric potentials and magnetic fields. The forward spatial filters are seen to be low-pass functions of spatial frequency and spatial resolution degrades in external measurements. Inverse spatial filters may be used to reconstruct cortical sources from external data, but since they are high-pass functions of spatial frequency, they must be regularized to avoid instabilities caused by noise at higher spatial frequencies. The regularization process limits the spatial resolution of source reconstructions. EEG forward spatial filters fall off at lower spatial frequencies than MEG filters; hence, there is less information available in higher spatial frequencies resulting in lower spatial resolution in inverse reconstructions. The tangential component of the magnetic field provides even higher spatial resolution than can be obtained using the radial component. An accompanying article examines the surface Laplacian for both the EEG and the MEG. © 2001 Biomedical Engineering Society.

Experimental observation of traveling waves in the transverse section of a laser

Abstract: The selection of a transverse traveling wave by an inhomogeneous pump profile has been experimentally observed in a class B laser structure. The laser structure consisted of a wide-aperture edge-emitting laser diode operating in pulsed mode to avoid thermal guiding effects. The injection current’s profile was modified from the usual top-hat configuration to a Lorentzian-like profile by the inclusion of a 10‐μmp-type expitaxial spreading layer. Spatial dependance of the far field on the near field was observed. The same behavior is also demonstrated numerically by use of Maxwell–Bloch equations for semiconductor lasers.

Evaluation of the impact of finite-resolution effects on scintillation compensation using two deformable mirrors.

Abstract: The impact of finite-resolution deformable mirrors and wave-front sensors is evaluated as it applies to full-wave conjugation using two deformable mirrors. The first deformable mirror is fixed conjugate to the pupil, while the second deformable mirror is at a finite range. The control algorithm to determine the mirror commands for the two deformable mirrors is based on a modification of the sequential generalized projection algorithm. The modification of the algorithm allows the incorporation of Gaussian spatial filters into the optimization process to limit the spatial-frequency content applied to the two deformable mirrors. Simulation results are presented for imaging and energy projection scenarios that establish that the optimal spatial filter waist to be applied is equal to the subaperture side length in strong turbulence. The effect of varying the subaperture side length is examined, and it is found that to effect a significant degree of scintillation compensation, the subapertures, and corresponding spacing between actuators, must be much smaller than the coherence length of the input field.

Method for the distribution of data marks on a medium, and method and apparatus for the holographic recording and readout of data

Abstract: The invention concerns a method for the distribution of data marks on a medium. The image of the data source (44) is recorded on an optical medium (2). The data is read out with the help of identifying marks on the detected image. The data to be recorded are coded with a coding which prohibits the immediate succession of data with identical states above a predetermined number of times. In the matrix of the data source arrays of a predetermined size and position are defined, the arrays containing identifying data marks and user data marks. There is also disclosed an apparatus having a two-dimensional data source (44), and an optical system for imaging the data source through an object beam (35) and reference beam (36) onto an optical medium (2). The optical medium is positioned in the Fourier plane associated with the image of the data source (44). Beside the Fourier plane of the hologram, the optical system comprises a further Fourier plane (51) between the data source (44) and the hologram (9), and there is a spatial filtering means in the further Fourier plane.

Photon efficient scanner

Abstract: A system for optical interrogation of a sample adaptable for multiple wavelength illumination and multiple wavelength fluorescent or luminescent light collection, wherein the illumination wavelength profile and the light collection profile may overlap. In the system, coherent light from one or more lasers is focused onto a target layer on a sample to excite fluorescent or luminescent light from the target layer. Emitted light is collected from a selected depth by a reflective light collector that transmits the collected light to detection optics. The reflective light collector directs collected light at an angle to the optical axis of the illumination light, thereby separating collected emitted light from illumination light. The light collector may collect light from a focus, whereby the focused illumination light combined with the focused light collection aid in limitation of the depth of field to a selected depth. Additionally, a spatial filter positioned between the light collector and the detection optics may be used to confine the depth of field to a selected depth. This device may be incorporated into an optical scanner by scanning of illumination light in a first direction and translation of the sample in a tangent direction. Alternatively, the illumination and detection optics may remain stationary and the detectable targets moved past a scanning location (e.g. as in electrophoretic analysis).

CMOS current/voltage mode winner-take-all circuit with spatial filtering

Abstract: A CMOS current/voltage mode winner-take-all circuit (WTA) with spatial filtering for image processing applications is presented. This WTA circuit has a unique ability of spatial filtering that allows removal of the background from the image and is suitable for integration with CMOS Active Pixel Sensors. In this circuit, image intensity has been chosen for the input saliency map. The removal process is performed by zeroing the values of the image background intensity levels and transferring the new saliency map to a standard WTA circuit, so only the potential objects of interest are compared by the WTA. Two circuits, current mode and voltage mode WTA circuits with filtering, have been implemented in 0.5 um submicron CMOS technology. Their operation is discussed and simulation results are reported.

Rotating compensator ellipsometer system with spatial filter

Abstract: Disclosed is the application of spatial filter(s) in rotating compensator ellipsometer systems prior to or after a sample system. The purpose is, for instance, to eliminate a radially outer annulus of a generally arbitrary Profile beam that presents with low intensity level irregular content, so that electromagnetic beam intensity is caused to quickly decay to zero as a function of radius.