Showing papers on "Spatial filter published in 1990"

Near-surface and topographic distortions in electromagnetic induction

Abstract: The most revealing description of electromagnetic (EM) distortions due to near-surface inhomogeneities and topography is in terms of galvanic and inductive effects. In either case, the distorted electric and magnetic fields can be best visualized as a vectorial sum of primary and secondary fields. Secondary electric fields due to electric charge build-up in the galvanic case persist to the longest periods. In contrast, the secondary electric and magnetic fields due to inductive, vortex currents disappear at long periods. The static shift of magnetotelluric (MT) apparent resistivity sounding curves is a classic example of the galvanic effect. Methods to correct for unwanted distortions such as the static shift can be classified into six categories: use of invariant response parameters, curve shifting, statistical averaging, spatial filtering, use of distortion tensors, and computer modeling. Although invariant impedance calculations are simple to make, they cannot, in general, recover the undistorted impedance. Short period curve shifting is best done with auxiliary soundings such as time domain EM; however, this requires multiple surveys. The shifting of long period MT sounding branches is useful if a standard curve is known and can be matched. Statistical averaging of neighboring MT soundings that are conformal but static shifted has proven very effective at removing random distortions if adaquate data are available. The new EMAP (Electromagnetic Array Profiling) method combats the inherent spatial high pass characteristics of EM distortions by low pass operations in data collection and processing. EMAP proposes the continuous, in-field measurement of electric field dipoles to avoid spatial aliasing. Distortion tensor stripping of topographic distortions is possible since terrain is deterministic but stripping the effects of uncertain subsurface inhomogeneities may be misleading. A new decomposition of the MT impedance tensor under the assumption of surficial three-dimensional (3-D) galvanic effects imposed on a one- or two-dimensional (1-D and 2-D) regional setting promises a way to recover the regional structure. There is a continual need for 3-D computer modeling to test new methods and to calculate topographic and regional effects. Computer modeling has established the value of 2-D modeling of the data identified as transverse magnetic (TM) in some 3-D environments. Ideally, EM distortion correction requires continuous, or at least many, data and the application of more than one correction-modeling scheme.

Generalized gradient schemes for the measurement of two-dimensional image motion

Abstract: This paper describes a procedure for recovering the global velocity of an image by incorporating spatial filtering, and, optionally, temporal filtering, into a scheme that employs a generalized version of the gradient algorithm of motion detection. Motion within a patch is analysed by six parallel channels, each incorporating a different spatiotemporal filter. Advantageous features of this scheme are: (a) global velocity is derived directly, without first computing local velocity at a number of image locations; (b) the filters compute first derivatives rather than second derivatives, making the scheme potentially more resistant to noise than certain other schemes; (c) two of the six filters can be chosen almost completely arbitrarily, and can therefore be tailored to maximize signal reliability, and (d) the measurement of velocity can be made as local or as global as desired by altering the size of the patch that is viewed by the filters. An analogous scheme is derived for the measurement of rotation, as well as expansion or contraction of the image.

Spatial heterodyne spectroscopy: interferometric performance at any wavelength without scanning

Abstract: Spatial heterodyne spectroscopy (SHS) employing a two-beam dispersive interferometer producing a Fizeau fringe pattern having wavelength-dependent spatial frequencies is presented. The pattern is recorded on an imaging detector and Fourier transformed to recover the input stream. It is pointed out that spectrometers operating on the SHS principle can achieve the theoretical resolution limit of the gratings without scanning, retaining at the same time the large angular input tolerance and multiplexing properties of conventional scanning Fourier-transform spectrometers. Additionally, broad spectral coverages can be achieved, and field widening can be accomplished without moving parts.

Spatial-processing techniques for the detection of small targets in IR clutter

Abstract: The detection of small targets in infrared (IR) clutter is a problem of critical importance to Infrared Search and Track (IRST) systems. This paper presents techniques for analyzing and improving the detection performance of IRST systems. Only spatial, or single-frame, processing will be addressed. For clutter with spatially slowly varying statistics, the approach is based on linear filtering. Models of target and cluttter are developed and used to analyze matched filter performance and sensitivity. This sensitivity analysis is used to improve filter bank design. A clutter classification scheme which can separate clutter of different types is presented. Finally, to improve system performance in the presence of large intensity gradients, such as cloud edges, an improved adaptive threshold scheme is presented.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Spatial amplification: an image-processing technique using the selective amplification of spatial frequencies

Abstract: A novel technique of processing optical images that we call spatial amplification is proposed and demonstrated. The basic idea is to amplify coherently a set of selected spatial frequency components of an image. Unlike in the usual spatial filtering techniques, in which the components are selectively blocked out at the Fourier plane of a coherent processor, spatial amplification does not discard any of the incident image information. The advantages of the proposed technique over spatial filtering include high energy efficiency and the possibility for cascading several stages. In the demonstrated experiments, the required amplification is provided by two-beam energy coupling in a photorefractive BaTiO3 crystal with a nonuniform pump beam.

Optical circuitry for free-space interconnections.

Abstract: A means for losslessly implementing arbitrarily space-variant optical interconnections in free space without sacrificing optical space-bandwidth product or power is described The technique uses space-variant mirrors and polarization beam splitters to separate and combine sets of device output spots, where a different interconnection operation is performed on each spatially separable set These space-variant mirrors are inexpensive and easily fabricated Example applications for Banyan interconnections and laser array combination are presented Current experimental limits are discussed

Two-dimensional current density imaging

Abstract: Imaging of electrical current by measuring the magnetic field which it produces requires the solution of the magnetic inverse problem. For a current restricted to a plane, the inverse problem can theoretically be solved by a linear spatial filtering method. Experimental results indicate that currents restricted to the surface of a printed circuit board can be imaged using this method. To measure the magnetic field, a magnetic resonance imaging technique is used. The reconstructed current density images illustrate that a tradeoff exists between the spatial resolution and the signal-to-noise ratio (S/N). The currents were restricted to the surface of a printed circuit board set in the xy plane. The circuit board was bathed in a nonconducting mineral oil to provide a magnetic resonance (MR) signal. The z component of the magnetic field produced by the current was measured using the magnetic resonance technique, and a spatial filtering technique was used to find the current density (J). The spatial resolution with which one can measure electric current density depends strongly on the distance at which the magnetic field from these currents can be measured. Magnetic resonance imaging offers a way to measure magnetic fields noninvasively and thus the possibility of reducing this distance to zero (if the current flows in an appropriate medium). While less sensitive than a superconducting quantum interference device (SQUID), the MR imaging method might, under certain circumstances, be capable of higher spatial resolution. >

Full-wave analysis of discontinuities in planar waveguides by the method of lines using a source approach

Abstract: An accurate full-wave analysis of discontinuities in shielded microwave structures by the method of lines is presented for use in microwave integrated circuit design The input currents are derived from a sinusoidal source representation and modeled by inhomogeneous boundary conditions Interacting discontinuities can be treated without segmentation Inhomogeneous boundary conditions are introduced to account for the excitation Analysis shows that in discretizing the differential operators additional source terms which result in inhomogeneous differential equations are obtained A deterministic equation for the current distribution is derived For higher flexibility, nonequidistant discretization is used The scattering parameters are presented for the microstrip step discontinuity and for a single-filter structure with interacting discontinuities, namely a coupled line filter The filter characteristics are compared with measurements Measurements verify the filter calculations >

Single-ended dual spatial filter detector for the passive measurement of winds and turbulence aloft

Abstract: The disclosed invention is a measurement device for use in determining wind turbulence in the atmosphere, such as the cross-wind velocity and strength of refractive turbulence in a localized region. In the preferred embodiment, the device is comprised of: a telescope for collimating the light received from the light source, the telescope placed in front of a pair of spatially filtered optical receivers and focussed at infinity; a pair of gratings; a collecting lens, placed behind the gratings to collect the spatially filtered light; a photodetector to collect the light; an amplifier to amplify the signal received from the photodetector, and; a spectrum analyzer to analyze the interference pattern created by the gratings. The device uses the information contained in the light as it is scattered into its various Fourier components as the light passes through the atmosphere. This information, by means of the two filters, is processed to determine the turbulence, whose wavenumber is the difference of the spatial filter wavenumber of the first and the second spatial filters.

Optical correlator performance using a phase-with-constrained-magnitude complex spatial filter.

Abstract: Certain kinds of spatial light modulators (SLMs) can be used in a mode where the phase transmittance at a given point has the desired value, measured modulo-2π, and the magnitude transmittance has approximately the desired value. Computer simulations, based on a specific model of a deformable-mirror SLM, show that these phase-with-coupled-magnitude SLMs perform better for the particular cases studied, in terms of discrimination and signal-to-clutter ratio, than phase-only and binary phase-only SLMs in coherent optical pattern-recognition correlators where the input is distorted by variable illumination, rotation, and noise relative to the template pattern.

Image enhancement techniques using selective amplification of spatial frequency components

Abstract: An optical image enhancer includes a beam of coherent light for carrying an input image and a first lens for receiving the beam and performing a spatial Fourier transform of the input image. A photorefractive crystal at the Fourier plane receives the transformed image, while a coherent pump beam illuminates selected spatial frequency components of the transformed image at the Fourier plane, thereby transferring energy to the selected frequencies by two-beam coupling. A second lens receives the transformed image from the Fourier plane and performs a spatial Fourier transform of the transformed image, thereby reproducing the input image in an output image which contains intensified features corresponding to the selected spatial frequency components. A spatial light modulator may be used to cause the pump beam to illuminate the selected spatial frequency components of the transformed image at the Fourier plane. The pump beam can carry the Fourier transform of a selected portion of the input image, such that the intensity of the selected portion is increased in the output image.

Spatial filter for optically based defect inspection system

Abstract: In an imaging system (10) for detecting defects in a specimen (14) having a repetitive pattern (16), a spatial filter (50) receives a spatial frequency spectrum produced by a Fourier transform lens (34) and blocks preselected spatial frequency components thereof. The spatial filter includes an array of substantially parallel opaque stripes (70a-70c) that are positioned on a substantially transparent substrate (72). In one embodiment, the stripes are spaced apart by equal distances (78) and are of increasing widths (76a-76c) that correspond to the orders of diffraction of the Fourier transform pattern (45) produced by the Fourier transform lens. The spatial filter can be used to filter light spots forming a Fourier transform pattern for specimens having repetitive pattern sizes included within a specified range of sizes.

Control of transverse spatial modes in transient stimulated Raman amplification

Abstract: We have studied the transverse modes of a transient stimulated Raman signal generated in unsaturated Raman amplifiers with Fresnel numbers from one to seven, both with and without a seed Stokes pulse. In the absence of a seed Stokes pulse and for a pump Fresnel number greater than 1.5, the spatial intensity pattern of the stimulated Stokes signal is highly variable from shot to shot, reflecting the random nature of the quantum-noise source. However, when a separately generated and sufficiently strong seed Stokes pulse with a Gaussian spatial profile is injected into the amplifier, the spatial mode of the output Stokes beam assumes a smooth Gaussian spatial pattern that is significantly smaller in diameter than either the pump or the seed Stokes beam. This spatial mode control, which is due to the injected Stokes signal, persists down to a level of approximately 300 seed Stokes photons per spatial mode and, surprisingly, is observed to suppress the noise-initiated signal by at least a factor of 10 in the outer regions of the amplified Stokes beam. We have also studied the Stokes pulse-energy statistics over a range of Fresnel numbers. Data taken with systems having Fresnel numbers between 4 and 7 follow the same pattern as the spatial-mode-control results in that the statistical character of the Stokes pulse energies changes rapidly from that characteristic of a quantum-noise-initiated process to one characteristic of a stable source as the level of the injected seed Stokes is increased. However, for interaction regions with Fresnel numbers of 1 or smaller, the Stokes pulse-energy statistics show a gradual change as the level of the seed Stokes signal is increased and do not become stabilized until the seed Stokes is more than 100 times the total level needed to control the spatial character of the output Stokes in the larger-Fresnel-number cases. This slow change in the statistics for the Fresnel 1 system, and the suppression of the quantum-noise-initiated signal in the larger-Fresnel-number systems, is not fully understood at this time.

Write with light optical notching filter

Abstract: An optical notching filter is disclosed in which an optical spatial filter therein is formed by a spatial light modulator which is written into by light. The optical notching filter system of the present invention provides a linear phase and amplitude response with applications for high speed adaptive filtering to filter out noise and signals of no interest. In one particular disclosed embodiment, the write with light optical notching filter system of the present invention is designed to adaptively excise interference from Radio Frequency (RF) signals, such as are utilized in communications or radar systems. The technical approach of the present invention provides a more compact system design than similar electronically addressed optical notching filtering systems, and also significantly reduces the complexity and attendant power requirements of the system. Different embodiments of a write with light optical notching filter system can operate with either nonochromatic or polychromatic light sources. Moreover, the write light source can be independent of the read light source, such that any suitable write or read light source can be utilized therein, either with single pass or recursive optical arrangements.

Adaptive three-dimensional spatio-temporal filtering techniques for infrared clutter suppression

Abstract: The detection of weak targets with an infrared surveillance system is often complicated not only by a severe clutter environment but also by background and platform motion effects. Conventional sequentially applied algorithms combining frame—to—frame registration, clutter rejection filtering, and adaptive thresholding detection simply overwhelm the track processor in a weak target scenario, due to the required lowering of detector block thresholds. To address this problem, we have developed a 3—D filter/"track—before—detect" signal processing approach in which an adaptive spatio—temporal filter is used for clutter suppression and a Viterbi "track—before—detect" block is used for noncoherent target integration. This paper discusses a 3—D adaptive filtering technique which combines time and spatial filtering (in both azimuth and elevation directions) to achieve simultaneous frame—to-frame registration, background clutter suppression, and target preservation/enhancement. In addition, this 3-D filtering procedure whitens the data, thus greatly facilitating the "track-before—detect" processing block task. Unlike other commonly employed procedures, this technique neither entails the suboptimal sequential application of filtering procedures (e.g., spatial followed by temporal filtering) nor demands very accurate subpixel-level registration or exact knowledge of the target's velocity characteristics. The only requirements are that data frames should be roughly aligned (so the offsets are contained within the filter window) and that the assumption of the moving target indicator (MTI) is valid. In this paper simulation results of the 3—D filtering procedure using real, scanned sensor array data are presented, and the procedure performance and implementation complexity are traded off versus adaptive spatial filtering, adaptive temporal, and sequentially applied time/spatial filtering techniques. Also, modification and simulation results are presented for an extension of the 3—D adaptive spatiotemporal filtering technique, which accommodates both MTI and non-MTI case scenarios.

Application of spatial filter theory to kriging

Abstract: Data-processing requirements for remotely sensed, digital images include spatial filtering to suppress image noise, enhance edges/contacts, and improve image clarity. Spatial filter theory demonstrates that the addition of a high-pass filtered image to a low-pass filtered image yields the original digital image. Application of this principle in kriging can be accomplished by using the same covariance matrix to solve for two weighting vectors to yield a result analogous to low- and high-pass filtering. The addition of kriged estimates calculated using both weighting vectors is analogous to summing high-, and low-pass filtered digital images. This modified method of kriging yields estimates associated with less smoothing compared to ordinary kriging. Statistical moments of original sample data are better preserved through estimation by this method.

Modified two-focal-length optical correlator.

Abstract: We present a modification to our previously reported 2f optical correlator. This modification removes a lens from the system and maintains a variable Fourier transform scale property while providing an equal correlation performance.

Laser protection system

Abstract: The present invention pretains to a device for protecting the human eye by means of a narrow bandwith interference filter which filters out one or more specific wavelengths of light emitted in the form of a coherent, concentrated beam that is laser radiation and in coherent light, an image intensifier for amplifying an image passing through the narrow bandwidth filter to an observable light level, and a neutral density filter which reduces image reflected illuminance to avoid detection.

Adaptive optics system performance approximations for atmospheric turbulence correction

Abstract: Analysis of adaptive optics system behavior often can be reduced to a few approximations and scaling laws. For atmospheric turbulence correction, the deformable mirror (DM) fitting error is most often used to determine a priori the interactuator spacing and the total number of correction zones required. This paper examines the mirror fitting error in terms of its most commonly used exponential form. The explicit constant in the error term is dependent on deformable mirror influence function shape and actuator geometry. The method of least squares fitting of discrete influence functions to the turbulent wavefront is compared to the linear spatial filtering approximation of system performance. The author finds that the spatial filtering method overestimates the correctability of the adaptive optics system by a small amount. By evaluating fitting error for a number of DM configurations, actuator geometries, and influence functions, fitting error constants verify some earlier investigations. Limitations of the approximations and scaling laws are evaluated and compared to wave optics ground-to-space propagations. Results include determination of a multiplicative conversion factor of 1.06 that should be applied to the actuator spacing whenever the spatial filtering method is used for adaptive optics system performance analysis.

Iterative generation of holograms on spatial light modulators

Abstract: Iterative learning procedures on a hybrid electro-optic system can be employed to generate holograms on inexpensive liquid-crystal-television spatial light modulators. The algorithm takes into account random electronic noise in the system and compensates for spatial-light-modulator distortions. Experimental results are given for the reconstruction of intensity distribution, and computer simulations demonstrate the possibility of a complete complex amplitude reconstruction.

Method and device for measuring particle size of extremely small particle by light scattering method

Abstract: PURPOSE:To measure the particle size of extremely small particles nondestructively without contacting by irradiating the surface of a sample which moves at an equal speed nearby a 1st focus of an elliptic condenser with converged laser light, detecting scattered light which is converged on a 2nd focus, and processing a detection signal CONSTITUTION:The laser light 2 emitted by a laser 1 is passed through a chopper 3, a spatial filter 4, a collimator lens 5, a polarizing prism 6, a beam splitter (BS) 7, and a lambda/4 plate 8, and reflected by a parabolic mirror 9 to travel backward, and the light is reflected by a BS 7 and converged in a spot The laser light which is made incident from an incidence opening 13 is converged on the 1st focus 11 of an elliptic surface mirror 10, reflected by the surface of the sample 15 in equal-speed motion, and reflected by the elliptic surface mirror 10, and the light is converged on a 2nd focus 12 and made incident on a detector E through a projection opening 14 and a parabolic light condenser 22 Its output is inputted to a computer 30 through a detector 29, whose pulse train output is integrated and the maximum value of the output voltage is compared with correlation data of the particle size and the maximum voltage to calculate the particle size of the extremely small particles sticking on a wafer 15

Use of a spatial filter for phase locking of a laser array

Abstract: An experimental investigation was made of a new method for phase locking of a periodic laser array. The advantages of the method include an afocal position of a spatial filter and the ease of variation of the coefficient representing the coupling between the lasers in the array, which makes it possible to select the optimal value of this coefficient to ensure the highest total radiation power.

Real-time optical generation of circular or Mellin radial-harmonic filters

Abstract: Invariant pattern-recognition systems use spatial filters that are, in most cases, computer-generated holograms. A novel approach for the optical generation of circular-harmonic or radial-harmonic filters is presented, together with experimental results.

On spatial filtering for angle-of-arrival estimation in a scattering environment by eigendecomposition-based methods

Abstract: A method of spatial filtering is proposed as a preprocessor to restore the high resolution performance of eigendecomposition-based methods when used with a linear array for angle-of-arrival (AOA) estimation in the presence of spatially distributed coherent interference (SDCI). Applying the conventional bandpass filters used in the spatial frequency domain, the spatial filtering method effectively suppresses the unwanted coherence interference outside a specified frequency region. Therefore, the success of the method hinges on the assumption that direct arrival and its coherent interference are well separated in spatial frequency (in angle). Under this assumption, the eigendecomposition methods with a spatial filtering preprocessor can give accurate AOA estimates of direct arrivals in spite of the presence of SDCI. Simulation results are shown for the case of scattering from an infinite circular cylinder, using discrete prolate spheroidal sequences as the optimum band-pass preprocessing filters, and a uniformly spaced linear array in a conjunction with the MUSIC algorithm. >

Small-aperture, high-resolution, two-channel imaging system.

Abstract: A method that utilizes incoherent light interferometry is used to form images through vanishingly small apertures. The method utilizes the increased channel capacity produced by reduction of spatial coherence, but in a way that improves the resolution instead of the signal-to-noise ratio.

Self-induced photorefractive spatial frequency filter

Abstract: A Bi12SiO20 crystal, cut in the transverse orientation as for holographic applications, is used as a spatial filtering device in the Fourier plane of a coherent single-beam optical processor with λ = 633 nm. Frequency selection is obtained by adjusting a polarizing compensator to suppress or to select some part of the spectrum, which is characterized by a certain ellipticity of polarization, depending on its intensity.

An adaptive technique for digital noise suppression in on-line portal imaging

Abstract: Two complementary approaches to the noise suppression problem in on-line portal imaging have been analysed. Temporal filtering by image summation can substantially reduce the amount of noise in an image. In many cases, however, movements of the patient or the radiation source limit the time period over which the averaging can be done. Any remaining noise has to be dealt with by applying spatial filtering. The adaptive Lee filter is particularly suitable for portal imaging applications. The authors have proposed a modification to the basic Lee technique which permits the calculation of the noise variance locally by utilising the information contained in intermediate images acquired during frame averaging. Unlike the original Lee formulation, no a priori knowledge of the noise variance is required, and in contrast to Mastin's approach (Mastin 1985), the variance may vary with position in the image. The tests of performance of the modified Lee filter, carried out using on-line images, have shown its superiority in comparison with the original Lee technique as well as with conventional averaging and median filters.