Showing papers on "Spatial filter published in 2005"

10-10 temporal contrast for femtosecond ultraintense lasers by cross-polarized wave generation

Abstract: We take advantage of nonlinear properties associated with ?(3) tensor elements in BaF2 cubic crystal to improve the temporal contrast of femtosecond laser pulses. The technique presented is based on cross-polarized wave (XPW) generation. We have obtained a transmission efficiency of 10% and 10?10 contrast with an input pulse in the millijoule range. This filter does not affect the spectral shape or the phase of the cleaned pulse. It also acts as an efficient spatial filter. In this method the contrast enhancement is limited only by the extinction ratio of the polarization discrimination device.

Shadow effects in spiral phase contrast microscopy.

Abstract: Recently it has been demonstrated that spatial filtering of images in microscopy with a spiral phase element in a Fourier plane of the optical path results in a strong edge enhancement of object structures. In principle the operation is isotropic, i.e., all phase edges of a sample object are highlighted simultaneously, independent of their local direction. However, here we demonstrate that the symmetry can be broken intentionally by controlling the phase of the central area of a spiral phase hologram, which is displayed at a computer controlled spatial light modulator. This produces an apparent shadow effect which can be rotated at video rate. The resulting relieflike impression of the sample topography with a longitudinal resolution in the subwavelength regime is demonstrated by imaging a standard low contrast test sample consisting of a human cheek cell.

Method and apparatus for detecting defects

Abstract: A method and apparatus for detecting defects are provided for detecting harmful defects or foreign matter with high sensitivity on an object to be inspected with a transparent film, such as an oxide film, by reducing noise due to a circuit pattern. The apparatus for detecting defects includes a stage part on which a substrate specimen is put and which is arbitrarily movable in each of the X-Y-Z-θ directions, an illumination system for irradiating the circuit pattern with light from an inclined direction, and an image-forming optical system for forming an image of an irradiated detection area on a detector from the upward and oblique directions. With this arrangement, diffracted light and scattered light caused on the circuit pattern through the illumination by the illumination system is collected. Furthermore, a spatial filter is provided on a Fourier transform surface for blocking the diffracted light from a linear part of the circuit pattern. The scattered and reflected light received by the detector from the specimen is converted into an electrical signal. The converted electrical signal of one chip is compared with that of the other adjacent chip. If these signals are not identical to each other, the foreign matter is determined to exist on the specimen in detection.

Symmetric phase only filtering: a new paradigm for DPIV data processing

Abstract: The standard approach in digital particle image velocimetry (DPIV) data processing is to use fast Fourier transforms to obtain the cross-correlation of two single exposure subregions, where the location of the cross-correlation peak is representative of the most probable particle displacement across the subregion. This standard DPIV processing technique is analogous to matched spatial filtering, a technique commonly used in optical correlators to perform the cross-correlation operation. Phase only filtering is a well-known variation of matched spatial filtering, which when used to process DPIV image data yields correlation peaks which are narrower and up to an order of magnitude larger than those obtained using traditional DPIV processing. In addition to possessing desirable correlation plane features, phase only filters also provide superior performance in the presence of dc noise in the correlation subregion. When DPIV image subregions contaminated with surface flare light or high background noise levels are processed using phase only filters, the correlation peak pertaining only to the particle displacement is readily detected above any signal stemming from the dc objects. Tedious image masking or background image subtraction is not required. Both theoretical and experimental analyses of the signal-to-noise ratio performance of the filter functions are presented. In addition, a new symmetric phase only filtering (SPOF) technique, which is a variation on the traditional phase only filtering technique, is described and demonstrated. The SPOF technique exceeds the performance of the traditionally accepted phase only filtering techniques and is easily implemented in standard DPIV FFT-based correlation processing with no significant computational performance penalty. The SPOF-based optical correlation processing approach is presented as a new paradigm for more robust cross-correlation processing of low signal-to-noise ratio DPIV image data.

Thin-film spatial filters

Abstract: A thin-film optical filter used as a one-dimensional spatial filter is presented, and its design is briefly examined. The filter consists of a stack of quarter-wave dielectric layers upon a right-angle prism that selectively cancel a reflected or transmitted plane-wave front for various angles of incidence. Transmittance and reflectance are low-pass functions or high-pass functions of the angle of incidence with a high degree of steepness. In combination, these filters exhibit bandpass transmittance with a variable bandwidth. Applications to detection of extrasolar planets are briefly discussed.

Performance analysis of spatial filtering of RF interference in radio astronomy

Abstract: Radio astronomical observations are increasingly contaminated by man-made RF interference (RFI). If these signals are continuously present, then they cannot be removed by the usual techniques of detection and blanking. We have previously proposed a spatial filtering technique, where the impact of the interferer is projected out from the estimated covariance data. Assuming that the spatial signature of the interferer is time-varying, several such estimates can be combined to recover the missing dimensions. We give a detailed performance analysis of this algorithm. It is shown that the spatial filter introduces a small increase in variance of the estimates (because of the loss in information) and that the algorithm is unbiased in case the true spatial signatures of the interferers are known but that there may be a bias in case the signatures are estimated from the same data. Some of the bias may be removed, and moreover, the bias only affects the auto-correlations, whereas the astronomical information is mostly in the cross-correlations.

Efficient computation of spatial filter matrices for steering transmit diversity in a MIMO communication system

Abstract: Techniques for efficiently computing spatial filter matrices are described. The channel response matrices for a MIMO channel may be highly correlated if the channel is relatively static over a range of transmission spans. In this case, an initial spatial filter matrix may be derived based on one channel response matrix, and a spatial filter matrix for each transmission span may be computed based on the initial spatial filter matrix and a steering matrix used for that transmission span. The channel response matrices may be partially correlated if the MIMO channel is not static but does not change abruptly. In this case, a spatial filter matrix may be derived for one transmission span l and used to derive an initial spatial filter matrix for another transmission span m. A spatial filter matrix for transmission span m may be computed based on the initial spatial filter matrix, e.g., using an iterative procedure.

Apparatus and method for filtering digital image signal

Abstract: An apparatus and method of filtering a digital image signal. The apparatus includes: a noise reduction filter which selectively outputs one of results obtained by temporally and spatially filtering pixel values of pixels of each of frames of an image as a temporal or spatial filtering value in response to magnitudes of the results of temporal and spatial filtering; and a sharpness enhancement filter which highlights and outputs a high pass component of the temporal or spatial filtering value.

Spatial Filtering Velocimetry: Fundamentals and Applications

Abstract: Principle and Properties of the Spatial Filtering Method.- Optical System.- Signal Analysis.- Spatial Filtering Devices and Systems.- Applications.

A Spatial Two-Dimensional Discrete Filter for Limited-Area-Model Evaluation Purposes

Abstract: A two-dimensional discrete spatial filter was developed. It serves as a means to classify meteorological fields on a limited-area grid according to their spatial dimensions by filtering certain wavenumber ranges. Thereby it performs an isotropic spatial-scale separation of the atmospheric fields. A general algorithm was developed, which allows the construction of a filter that closely approximates a specific isotropic response function. The filter is simple in the construction and easy to apply while giving reasonable results. The method allows for considerable flexibility in choosing this specific response. This way, low-, band-, and high-pass filters are obtained. Examples show an effective scale separation of atmospheric fields on limited-area grids that can be used for process studies, model evaluation, or comparisons.

Mach-Zehnder interferometer for piston and tip-tilt sensing in segmented telescopes: theory and analytical treatment.

Abstract: A study is presented of a Mach-Zehnder interferometer for the measurement of phasing errors of the type found in segmented telescopes. We show that with a pinhole much larger than the Airy disk and an optical path difference between the arms equal to a quarter of the wavelength, the interferometric signal is related to the second derivative of the wave front. In this condition the signal is produced mostly by the segmentation errors and is marginally sensitive to other aberrations including atmospheric turbulence. The signal has distinguishable symmetric and antisymmetric properties that are related to segment aberrations. We suggest using the antisymmetric component of the signal to retrieve piston, tip, and tilt. The symmetric component of the signal serves as an estimate of the measurement error. In this way we proceed with a study of the errors associated with the misalignment of the interferometer, the segment edge imperfections, and the nonaveraged atmospheric perturbations. The entire study is performed on a theoretical basis, and numerical simulations are used to cross check the analytical results.

Shear Layers and Aperture Effects for Aero-Optics

Abstract: *† ‡ This paper examines the effect of a finite aperture on wavefronts, especially with regard to aero-optic distortions from a shear layer. When the net deflection of a beam is corrected in real time to bring a beam on target, a common practice in optic applications even if no other corrections are performed, this removal of the off-target tilt and the finite aperture of the beam act as a spatial filter. This restricts the tip-tilt correction and the wavefront correction to separate frequency ranges, causing the rms magnitude of the remaining distortion to be corrected to vary with aperture size. It has also been found that for a shear layer, aperture size is a key factor in scaling the severity of optical distortions, and it is likely to be a key component in scaling other types of optically-aberrating flows.

On the holographic reconstruction of vibroacoustic fields using equivalent sources and inverse boundary element method

Abstract: In using the near-field acoustical holography based on the inverse boundary element method (BEM) for the reconstruction of vibroacoustic source parameters, an enormous number of measurements required in practice have limited the extended application of this method. To obtain the sufficient field data with a small number of actual measured data, the regeneration method of the radiated field using the multipoint equivalent sources is attempted in this paper. In using the equivalent source method, a vibrating source can be represented by distributed spherical sources inside the actual source surface. In this paper, the radiated field is expressed with a series of spherical Hankel functions and spherical harmonics. For suppressing the adverse effect of high-order spherical functions, spatial filtering of coefficients and wave-vector components by a regularization scheme is adopted. Restored field data appended with actual measured data can be used as input for the inverse BEM to reconstruct the source field. Numerical tests for spherical sources were performed for investigating the characteristics of the proposed technique. In order to validate the usefulness of the proposed method to actual irregular sources, a vacuum cleaner was taken as a demonstration example and good agreement between measured and reconstructed results could be observed.

Coherence characteristics of electrically excited tandem organic light-emitting diodes

Abstract: A tandem organic light-emitting diode structure, excited electrically in the pulsed domain and confined within a double spatial filter configuration, is observed to emit a low-divergence beam (Δθ≈2.53 mrad, or ∼1.1 times the diffraction limit) with a near-Gaussian spatial distribution. The emission originates from the laser dye Coumarin 545 T, which is used as a dopant. Spectral coherence was determined by use of a double-slit interferometer. The interferometric distribution from our device approximates the interferometric pattern obtained from well-known lasers emitting at λ≈540 nm.

Scene-based nonuniformity correction technique that exploits knowledge of the focal-plane array readout architecture

Abstract: Spatial fixed-pattern noise is a common and major problem in modern infrared imagers owing to the nonuniform response of the photodiodes in the focal plane array of the imaging system. In addition, the nonuniform response of the readout and digitization electronics, which are involved in multiplexing the signals from the photodiodes, causes further nonuniformity. We describe a novel scene based on a nonuniformity correction algorithm that treats the aggregate nonuniformity in separate stages. First, the nonuniformity from the readout amplifiers is corrected by use of knowledge of the readout architecture of the imaging system. Second, the nonuniformity resulting from the individual detectors is corrected with a nonlinear filter-based method. We demonstrate the performance of the proposed algorithm by applying it to simulated imagery and real infrared data. Quantitative results in terms of the mean absolute error and the signal-to-noise ratio are also presented to demonstrate the efficacy of the proposed algorithm. One advantage of the proposed algorithm is that it requires only a few frames to obtain high-quality corrections.

Dynamically reconfigurable optical lattices.

Abstract: We present a theoretical framework for two approaches of generating light-efficient optical lattices via a generic Fourier-filtering operation. We demonstrate how lattice geometry can be dynamically changed from fully discrete to interconnected optical arrays conveniently achieved with virtually zero computational resources. Both approaches apply a real-time reconfigurable phase-only spatial light modulator to set up dynamic input phase patterns for a 4-f spatial filtering system that synthesizes the optical lattices. The first method is based on lossless phase-only Fourier-filtering; the second, on amplitude-only Fourier-filtering. We show numerically generated optical lattices rendered by both schemes and quantify the strength of the light throughput that can be achieved by each filtering alternative.

Blind extraction of a dominant source signal from mixtures of many sources [audio source separation applications]

Abstract: This paper presents a method for enhancing a dominant target source that is close to sensors, and suppressing other interferences. The enhancement is performed blindly, i.e. without knowing the number of total sources or information about each source, such as position and active time. We consider a general case where the number of sources is larger than the number of sensors. We employ a two-stage processing technique where a spatial filter is first employed in each frequency bin and time-frequency masking is then used to improve the performance further. To obtain the spatial filter we employ independent component analysis and then select the component of the target source. Time-frequency masks in the second stage are obtained by calculating the angle between the basis vector corresponding to the target source and a sample vector. The experimental results for a simulated cocktail party situation were very encouraging.

Method and system for noise reduction in digital video

Abstract: Aspects of noise reduction in digital video may comprise monitoring at least one of memory usage and memory bandwidth usage of memory utilized to process video data. The aspect may further comprise adaptively adjusting filtering of the video data according to the monitoring. At least one of impulse filtering, temporal filtering, and spatial filtering may be utilized for the filtering of the video data. At least one of the impulse filtering, the temporal filtering, and the spatial filtering may be adaptively adjusted based on the monitoring. Furthermore, at least one of motion information and edge information may be estimated from the video data for utilizing in at least one of the impulse filtering, the temporal filtering, and the spatial filtering. At least one of the estimated motion information and the estimated edge information may be adaptively adjusted based on the monitoring.

Correcting for sub-grid filtering effects in particle image velocimetry data

Abstract: Particle image velocimetry methodology results in a spatial averaging of the real velocity field into a set of discrete measured velocities: one for each interrogation cell. In the absence of measurement noise, this filtering process results in a reduction of the measured turbulent kinetic energy and other second-order statistics of the velocity field. The reduction in this energy will naturally be dependent upon the amount of turbulent energy at lengthscales smaller than can be resolved by the interrogation cells that make up the measurement grid. This paper investigates the effects of sub-grid scale filtering on the second-order statistics of velocity. Several experiments are reported for which interrogation cell size to turbulent integral lengthscale ratios were varied. In addition, synthetic turbulent velocity fields with known spatial correlation functions are used to support experimental results and provide calibration for the estimation of the level of sub-grid filtering. It is suggested that to accurately capture most turbulent kinetic energy using PIV the interrogation cell should be at least of order ten times smaller than the integral lengthscale of the flow. A method is then provided to estimate the level of sub-grid filtering should the interrogation cell be larger than this limit up to around the size of the integral lengthscale. With interrogation cells larger than this lengthscale then sub-grid filtering is such that second-order statistics are reduced by over 50% and it should be considered unwise to rely on any second-order statistics from such a scenario, corrected or otherwise.

Interference mitigation using a focal plane array

Abstract: [1] We consider the use of spatial filtering algorithms for radio frequency interference (RFI) mitigation in conjunction with a focal plane feed array of electrically small elements. Numerical simulations are used to study the performance of 7 and 19 element hexagonal dipole arrays with a 25 m reflector at an operating frequency of 1612 MHz. Using the maximum SNR algorithm to generate array weights, an interfering signal was attenuated by 40 dB or more. The effective sensitivity of the system, including interferer power in the system noise temperature, was comparable to the sensitivity attained in the absence of RFI. Moving the interferer through the reflector pattern sidelobes caused fluctuations in the gain and system sensitivity. This effect was exacerbated by a reflector model with random surface distortions. These results indicate that array feeds are a promising approach for RFI mitigation, but achieving stable radiation patterns in the presence of an interferer may require a trade-off between pattern control and maximum attainable sensitivity.

Optical image processing by means of acousto-optic spatial filtration

Abstract: Results are presented of a theoretical and experimental study of a new method of optical image processing based on two-dimensional acousto-optic filtration of the spatial spectrum of images. Most attention is concentrated on the analysis of the acousto-optic cell transfer function form and its dependence on crystal cut, geometry of acousto-optic interaction and ultrasound frequency. Results of computer simulation of acousto-optic spatial filtering are illustrated by the example of an object in the form of an amplitude grating. Experiments investigating the transfer functions of acousto-optic cells with collinear and tangential geometry of acousto-optic interaction are described. Acousto-optic image processing is demonstrated experimentally using the example of the edge enhancement effect for some elementary images.

Adaptive spatial-temporal filtering applied to x-ray fluoroscopy angiography

Abstract: Adaptive filtering of temporally varying X-ray image sequences acquired during endovascular interventions can improve the visual tracking of catheters by radiologists. Existing techniques blur the important parts of image sequences, such as catheter tips, anatomical structures and organs; and they may introduce trailing artifacts. To address this concern, an adaptive filtering process is presented to apply temporal filtering in regions without motion and spatial filtering in regions with motion. The adaptive filtering process is a multi-step procedure. First a normalized motion mask that describes the differences between two successive frames is generated. Secondly each frame is spatially filtered using the specific motion mask to specify different types of filtering in each region. Third an IIR filter is then used to combine the spatially filtered image with the previous output image; the motion mask thus serves as a weighted input mask to determine how much spatial and temporal filtering should be applied. This method results in improving both the stationary and moving fields. The visibility of static anatomical structures and organs increases, while the motion of the catheter tip and motion of anatomical structures and organs remain unblurred and visible during interventional procedures.

Image production apparatus, image display apparatus, image display method and optical modulation device adjustment apparatus

Abstract: An image production apparatus, an image display apparatus and an image display method are disclosed which can reduce the ununiformity in luminance and color which appears on a display screen and can be formed compact. Also optical modulation device adjustment apparatus is disclosed which can detect and correct the ununiformity in modulation characteristic of a modulation device. The image display apparatus includes a light detection apparatus in addition to a light source section, an illumination optical system, an optical modulation section, a spatial filter, a light projection section and a screen. The light detection apparatus detects the dispersion of a modulation characteristic of each pixel element of GLV devices and the ununiformity in luminance and color displayed in accordance with an illumination condition. An optimum driving voltage for minimizing the ununiformity in color and luminance to be displayed is determined based on a signal detected by the light detection apparatus.

Compact neodymium phosphate glass laser emitting 100-J, 100-GW pulses for pumping a parametric amplifier of chirped pulses

Abstract: A five-stage neodymium phosphate glass amplifier producing 1–1.5-ns radiation pulses of energy up to 110 J is described. The use of a multistage spatial filter based on an aperture line provides efficient extraction of the stored energy. The exit aperture filling factor is 0.65 and the output radiation divergence is equal to three times the diffraction-limited divergence. The energy efficiency of radiation conversion to the second harmonic is 60%. The amplifier is intended for pumping a chirped-pulse optical parametric amplifier.

Lighting device and observation device

Abstract: PROBLEM TO BE SOLVED: To provide a lighting device and an observation device which can prevent its optical member from being damaged by heat caused by the light emitted from a light source. SOLUTION: The lighting device is equipped with a xenon lamp 11 to generate illumination light, a rotatable filter plate 14 arranged on a light path of the illumination light, which has an excitation filter 28 to filter the light within a wavelength band range of an excitation beam, and a G' filter 29 and an IR' filter to filter the light within a wavelength band range of a reference beam, and a band range switching filter plate 12 provided on the light path of the illumination light and between the xenon lamp 11 and the rotatable filter plate 14, which has an infrared fluorescence observation filter 22 to filter the light within a wavelength band range including the wavelength band ranges of the excitation beam and the reference beam. In the infrared fluorescence observation filter 22, the transmittance of the reference beam within the wavelength band range is predetermined to be lower than that of the excitation beam within the wavelength band range. COPYRIGHT: (C)2007,JPO&INPIT

Autonomous Calibration for Optical Analysis System

Abstract: The present invention provides an autonomous calibration of a multivariate based spectroscopic system that is preferably implemented as a multivariate based spectrometer. The spectroscopic system is based on a multivariate optical element that provides a spectral weighting of an incident optical signal. Spectral weighting is performed on the basis of spatial separation of spectral components and subsequent spatial filtering by means of a spatial light modulator. Calibration of the spectroscopic system is based on a dedicated calibration segment of the spatial light modulator, whose position corresponds to a characteristic calibration or reference wavelength of the incident optical signal. Preferably, the calibration or reference wavelength is given by the wavelength of the excitation radiation generated by the optical source that serves to induce scattering processes in a volume of interest.

Extended object wavefront sensing based on the correlation spectrum phase

Abstract: In this paper we investigate the performance of a Fourier based algorithm for fast subpixel shift determination of two mutually shifted images subjected to noise. The algorithm will be used for Shack-Hartmann based adaptive optics correction of images of an extended object subjected to dynamical atmospheric fluctuations. The performance of the algorithm is investigated both analytically and by Monte Carlo simulations. Good agreement is achieved in relation to how the precision of the shift estimate depends on image parameters such as contrast, photon counts and readout noise, as well as the dependence on sampling format, zero-padding and field of view. Compared to the conventional method for extended object wavefront sensing, a reduction of the computational cost is gained at a marginal expense of precision.

Optical head and multiplexing methods for reflection type holographic storage using spatial filtering

Abstract: The invention describes a collinear optical head for a reflection-type holographic recording medium, which is capable of obtaining an increased capacity. The optical head presented here allows for high capacity by combining holographic multiplexing methods, based on a shift and/or phase-coded reference beam, and/or scanned reference beam and/or angle and/or confocal addressing techniques. The parallel reading method and the format of the holographic recording medium ensure high-speed data transfer. Combined multiplexing is implemented in the system by means of a confocal optical arrangement that reduces crosstalk between adjacent holograms by filtering out object beams unintentionally reconstructed from non-addressed holograms.

Appearance inspection apparatus and projection method for projecting image of sample under inspection

Abstract: An object of the present invention is to provide an appearance inspection apparatus that can change the method of illumination, the shape of illumination light, and a spatial filter for the projection of an image of a sample dynamically during inspection, and also to provide a projection method for projecting the image of the sample. Illumination optics 12 and 13 and imaging optics 21 and 22 in the appearance inspection apparatus are respectively configured to form reflecting optical systems with mirror array devices 16 and 26 disposed at points conjugate to the pupil position 8 of an objective lens 3. The mirror array devices 16 and 26 are controlled in accordance with the field-of-view position of the objective lens 3 on the sample.