Showing papers on "Spatial filter published in 1983"

Application of Three-Dimensional Filtering to Moving Target Detection

Abstract: The standard approach to the detection of a stationary target immersed within an optically observed scene is to use integration to separate the target energy from the background clutter. When the target is nonstationary and moves with fixed velocity relative to the clutter, the procedure for integrating the target signal is no longer obvious. In this paper it is shown that the problem of tracking a target having a fixed velocity can be cast into a general framework of three-dimensional filter theory. From this point of view, the target detection problem reduces to the problem of finding optimal three-dimensional filters in the three-dimensional transform domain and processing the observed scene via this filtering. The design of these filters is presented, taking into account the target, clutter, and optical detection models. Performance is computed for a basic clutter model, showing the effective increase in detectability as a function of the target velocity. The three-dimensional transform approach is readily compatible with VLSI array processing technology.

Real-time Fourier transformation in dispersive optical fibers.

Abstract: The general concept of temporal Fourier transformation in dispersive media is analyzed. The real-time optical Fourier transformer is shown to be realizable by using dispersive single-mode fibers and chirping lasers.

Resonant array bandpass filters for the far infrared.

Abstract: An array of square apertures in a conducting film behaves as a high-pass filter element. By contrast, an array of cross-shaped apertures exhibits bandpass behavior. We have investigated experimentally how variations in the pattern periodicity, the crossarm width, and the separation between the crosses alter the spectral behavior. We find that these bandpass filters can have excellent peak transmission and good shortwave rejection. The wavelength of peak transmission is determined by the length of the crossarm (and not by the array period itself), while the bandwidth is determined chiefly by coupling between the crosses. These results are qualitatively consistent with a coupled-dipole model. This model appears to have more utility for practical filter design than the commonly used transmission-line model.

Six formal properties of two-dimensional anisotropie visual filters: Structural principles and frequency/orientation selectivity

Abstract: Six formal properties of anisotropic linear two-dimensional filters are noted which are relevant for modeling the mechanisms of spatial visual information extraction. These properties concern the relationship between the organizational principle of a two-dimensional anisotropic spatial filter or neural receptive field (such as elongation, or concatenation of subunits, or differential operators mediated by lateral inhibition in neural laminae) and the resulting general consequences for spatial frequency and orientation selectivity. These properties are demonstrated without assuming particular two-dimensional filter functional forms; rather, they are shown as general principles associated with certain broad categories of two-dimensional filters. Such an analysis enhance theoretical understanding of the two-dimensional receptive field organization of neurons in the visual cortex and permits the rejection of some candidate organizational principles on two-dimensional spectral grounds.

Image sharpening for mixed spatial and spectral resolution satellite systems

Abstract: Two methods of image sharpening (reconstruction) are compared. The first, a spatial filtering technique, extrapolates edge information from a high spatial resolution panchromatic band at 10 meters and adds it to the low spatial resolution narrow spectral bands. The second method, a color normalizing technique, is based on the ability to separate image hue and brightness components in spectral data. Using both techniques, multispectral images are sharpened from 30, 50, 70, and 90 meter resolutions. Error rates are calculated for the two methods and all sharpened resolutions. The results indicate that the color normalizing method is superior to the spatial filtering technique.

Dual energy computerized tomography system

Abstract: A dual energy method for CT scanning. Scanning is carried out at low and high energy levels, S 2 and S 1 respectively, each at a preselected dose level. Detected electronic signals (I 2 ,I 1 ) are digitized. A look up table is used to generate a pair of values from each pair of signals, one a photoelectric component and the other a Compton component. Each of these values are subjected to spatial filtering by a convolution filter function for each signal specially designed to increase the signal to noise ratio in the resultant combined image by exploiting the correlation in noise in the two values.

A scanning system for chest radiography with regional exposure control: Theoretical considerations

Abstract: Conventional chest radiography is limited by the presence of scattered radiation and the small useful exposure range of radiographicfilm. A computer‐assisted scanning system to minimize these two effects is outlined. The system uses a small beam of radiation swept over the patient’s chest in a raster pattern to expose a conventional film cassette, while a slit collimator scanning between the patient and the film serves to reject scatteredphotons. A microcomputer measures beam attenuation by the patient with a detector placed behind the film which in turn automatically adjusts the x‐ray tube output to minimize excursions in film exposure as the beam scans. A formalism which relates the patient transmission and film exposure distribution is developed and a system transfer function is given. It is shown that such a system operates as a spatial filter which attenuates film contrast for structures of spatial frequency less than the inverse scanning beam width. By manipulating the software parameters of the feedback network, it is possible to alter this filter and produce radiographs with low spatial frequency enhancement, attenuation, or contrast inversion.

Read/write system for optical disc apparatus with fiber optics

Abstract: A read/write system is disclosed for optical disc apparatus. The light source is removed from the head and optically coupled to the disc by a single-mode fiber which acts as a spatial filter. Adverse effects on the light source due to reflection from the disc are avoided by making the fiber length greater than one-half the coherence length of the source. A sparing switch and combiner component may be included in the system. An array of discrete light sources, each coupled to an associated fiber, may also be employed for multiple track accessing.

Coherent optical production of the Hough transform

Abstract: The Hough transform (HT) is an efficient shape detector that maps straight lines into a two-parameter feature space. Recently it has been pointed out that the forward Radon transform (FRT), well known from the theory of computed tomography, and the HT are equivalent for binary images. In this paper, analog coherent optical implementation of the FRT is discussed. The FRT will not only be of use in implementing the HT shape descriptors but also act as a coherent optical preprocessor for the implementation of multidimensional convolution, correlation, and spectral analysis using 1-D acoustooptical signal processing devices. Several different coherent optical FRT architectures are presented. Experimental results using conventional coherent Fourier transform configuration are given. The relationship between the coherent optical implementation of the FRT and the inverse Radon transform, an important tool in computed tomography, is also detailed.

Frequency multiplexed optical spatial filter

Abstract: A frequency multiplexed optical spatial filter comprises a thin film of a photochemical hole burning material. The filter stores a plurality of images in the photochemical hole burning medium. The images are formed at different optical frequencies and coexist in the same volume of the storage medium. Individual images are accessed by changing the frequency of the illuminating radiation.

Optical correlator using electronic image preprocessing

Abstract: An optical processing system uses spatial filtering to recognize two-dimensional functions and images. Images of real objects are captured with a television camera. The electronic signals representing the two-dimensional function or image are electronically preprocessed, such as by edge enhancement techniques or by applying a curvature function, and displayed on a narrow-band-phosphor cathode ray tube or television monitor. The monitor image is used directly as the input to an incoherent holographic correlator. Alternatively, the monitor image is used to modulate a source of coherent radiation such as a laser via a spatial light modulator to generate a modulated optical signal which is the input to a coherent holographic correlator. Analyzer circuitry with an optical detector at the correlation plane analyzes the shape of the correlation function to determine the intensity and position of its peak. The input to the CRT or monitor may also be in the form of non-optically generated electronic signals representative of a two-dimensional function for display in graphic form by the CRT or monitor.

Color signal correlation detection by matched spatial filtering

Abstract: A polychromatic correlation detection technique by complex spatial filtering is presented. This technique utilizes a diffraction grating and three co-lineared (red, green, and blue) coherent sources. The technique of complex spatial filter synthesis for the polychromatic corelation detection is demonstrated. This technique offers true color correlation detection which is very suitable for color signal recognition and identification. The correlation diffraction efficiency with this technique is generally higher than that of the wavelength-multiplexed technique. Several interesting experimental demonstrations of this color signal correlation detection scheme are provided. Finally we note that this color signal detection technique is a simple and versatile processing technique which has broad range of applications of complex color signal detection, recognition and identification.

Phase-only optimization for the generation of wide deterministic nulls in the radiation pattern of phased arrays

Abstract: Some analytical and experimental results of the phase-only optimization of each array element for the generation of wide deterministic nulls in the sidelobe region of the radiation pattern of a phased-array antenna are presented. The technique derives from the adaptive null-steering theory and is applied in the radar field to the spatial filtering of strong clutter echoes or jammers. A wide deterministic null has been realized in the elevation pattern of a phased-array antenna, with electronic scanning in elevation, without any hardware change; the calculated performances are compared to the measured data.

Theoretical and experimental study of metal grid angular filters for sidelobe suppression

Abstract: The synthesis, analysis, and experimental measurements of a metal grid angular filter are described. Angular filters are devices that can be added to existing antennas in order to reduce the radiation over a specified angular region while allowing radiation over other angular sectors to pass relatively unaffected. The behavior of this angular filter, both in frequency and angle, is analyzed in this paper, and these results are compared with measurements.

Image transmission through a turbulent medium using a point reflector and four-wave mixing in BSO crystal.

Abstract: A method of one-way image transmission through a turbulent medium is presented using four-wave mixing in BSO crystal. In this method, a pointlike reflector is placed on the observation plane to return part of the incident wave, giving the instantaneous wave propagation function of the turbulent medium to the image transmission side. The returned wave together with the plane reference wave and the Fourier-transformed input image wave is impinged in BSO crystal to modulate the image wave by the phase-conjugate wave, so that transmission of the image with reduced turbulence effects is achieved. The principle and laboratory experimental results are given.

Image data processing apparatus for performing spatial filtering of image data

Abstract: In a digital radiographic technique, a spatial filtering process has been widely introduced. The image data processing apparatus includes a first storage device for storing original image data in a digital form which is used for medical diagnosis. A filtering device is provided for performing a filtering function to the original image data so as to control spatial frequency characteristics of the original image data. A second storage device is used for storing the filtered image data derived from the filtering device, and a first multiplexer device is provided for multiplying the filtered image data which is stored in the second storage device, by a first parameter so as to give a first weighted value to the filtered image data. A second multiplier device is used for multiplying the original image data which is stored in the first storage means, by a second parameter so as to give a second weighted value to the original image data, and a parameter input terminal device is provided for producing the two parameters and delivering the same to the first and second multiplier devices respectively. An adder is provided for performing a linear combination operation between the first and second multiplied image data so as to obtain an added image data whose spatial frequency characteristics are controlled to be different from that of the original image data.

Closed-cavity solutions with partially coherent fields in the space-frequency domain

Abstract: Closed or stable optical cavities, used frequently to determine the efficiency of high performance chemical laser nozzles, are designed primarily for maximum multimode power extraction from the medium. The very large (>500) Fresnel numbers associated with such cavities have in the past necessitated their analytical modeling by representing them as plane-parallel Fabry-Perot or rooftop cavities. In this paper, a rigorous 2-D scalar diffraction formalism of the closed cavity is presented in which quasi-monochromatic partially coherent fields in the space-frequency domain are used to obtain quasi-steady state but stable solutions using a simplified gain model. Small power fluctuations in the numerical iterative solution history that displays no monotonic increasing or decreasing trends are interpreted as the redistribution of energy from one degenerate set of high-order transverse modes into another. The degree of coherence in the second-order spatial correlation function (or the mutual coherence function) required of the input fields which permit such solutions is presented. Further, it is shown that the upstream/downstream coupling in this closed cavity occurs as a natural consequence of the physical model itself rather than through some artificial geometrical means, such as that introduced in the rooftop model. The axial variation in the resulting mode width is in excellent agreement with the Hermite-Gaussian distribution predicted for the particular geometry of interest. The computed closed-cavity power variation with mode width using a simplified gain model shows qualitative agreement with experimentally observed trends; quantitative agreement is poor and is ascribed to the rudimentary nature of the gain model. In the limiting case of small Fresnel numbers (NF ∼ 1) this procedure yields, in the bare cavity, the well-known fundamental mode of the cavity when appropriate symmetry constraints are applied.

Nonlinear local image preprocessing using coherent optical techniques.

Abstract: Two coherent optical systems are described that can realize local nonlinear preprocessing operators such as the Sobel edge-enhancement function in parallel on a 2-D input image. By local, we refer to the size of the image region rather than a nonstationary process. Realization of such operators using a multiple-exposure matched spatial filter and a computer-generated hologram is discussed. Experimental results using these techniques for 3 × 3 and 5 × 5 Sobel operators, respectively, are presented. Our techniques can be extended to larger window sizes and other edge-enhancement operators. These new operators are achieved by novel coherent systems using complex arithmetic with magnitude evaluation of the output pattern.

Hybrid incoherent optical pattern recognition system.

Abstract: A pattern recognition system that uses incoherent spatial filtering to recognize images directly from a narrowband phosphor television monitor is described. Images of real objects are captured with a television camera. These images are then edge-enhanced electronically and displayed on the TV monitor. The monitor output is used directly as the input to a holographic correlator. An optical multichannel analyzer at the correlation plane is used to analyze the shape of the correlation function and to determine the position of its peak. Experimental results agree well with theory. Concepts for handling rotation, aspect angle, and scale variations of the input are discussed.

Pencil beam interferometer

Abstract: An interferometer which provides for the precise figure measure of optical surfaces through the interference of two pencil beams, reflected off the optical surface, comprises a laser for generating a laser beam which is split into two parallel beams by a beam splitter and a mirror, the two pencil beams are reflected off a second beam splitter, through an alignment invariant optical device and onto the optical surface to be measured. The two pencil beams are reflected and back-trace through the alignment invariant optical device, propagate through the second beam splitter and enter an optical lens which is capable of focusing two beams in its back focal plane where the interference of the two pencil beams takes place. This information is then relayed through optical spatial filter and optional micro objective into the readout section.

Self-imaging through a Fabry-Pérot interferometer

Abstract: The spatial filtering property of a Fabry-Perot interferometer is shown to transform the spatial spectrum of an arbitrary image into a form which satisfies, approximately, the conditions for self-imaging. The limitations of the technique are studied and checked experimentally.

Analysis of optical spectra by Fourier methods: filtering and least-squares regression in reciprocal space

Abstract: We describe a method for accurately determining critical point parameters from optical spectra in which digital filtering in real (energy) and reciprocal (Fourier-coefficient) space is treated on an equivalent basis. Experimental and theoretical line shapes are also filtered in parallel, thereby eliminating systematic errors that can arise in the standard approach in which only the data are processed. Real-space filtering is done using false data to isolate individual or groups of critical points in complicated spectra, to provide a more accurate representation of the data in reciprocal space, and to minimize the effects of end-point discontinuities and truncation errors on the Fourier coefficients calculated from these spectra. Reciprocal-space filtering is done by numerically differentiating the data to maximize the amplitudes of the Fourier coefficients carrying the critical point information, followed by truncating low- and high-order coefficients to minimize artifacts that are due to baseline effects and noise. The optimum order of differentiation (not necessarily integral) is determined from the coefficients themselves. We show that a least-squares regression (LSR) analysis of a restricted interval of equally weighted points in reciprocal space is equivalent to the LSR analysis of all data points equally weighted in real space, making LSR particularly useful for analyzing higher-derivative spectra, where the real-space line shapes rapidly approach zero outside the central structure. For a specific example discussed here, maximum accuracy is obtained if the data are analyzed in the form of a third derivative, as was previously concluded empirically from numerical processing in real space.

Fourier transformation using an electroabsorptive CCD spatial light modulator

Abstract: The use of a GaAs CCD as a spatial light modulator is described and its application to coherent optical Fourier transformation is analyzed. In this device, the transmission through the two-dimensional buried-channel CCD may be electroabsorption modulated near the GaAs cutoff wavelength since the electric field in each storage well is controlled by the transferred charge. One of the primary advantages of this modulator is the ability to electrically address the device at high speed. Analysis of the two-dimensional modulator with a silicon CCD detector array yields a projected dynamic range approximately equal to the number of modulator array elements. For arrays containing greater than 1000 elements, detector performance and nonuniformities can limit the maximum range to 30-40 dB. The device can also be optically addressed, and in this mode of operation it has a comparable dynamic range to the electrically addressed structure with an optical write energy an order of magnitude lower than liquid crystal or photorefractive light valves. An alternative mode of device operation is a waveguide mode in which the light propagates along an epitaxial layer and is modulated as it passes under a one-dimensional CCD. The detection is done by a second linear CCD. The higher modulation efficiency results in a dynamic range approximately one hundred times the number of elements but is again limited to 30-40 dB because of detector response and nonuniformities.

Noise reduction in laser amplifiers

Abstract: Amplified spontaneous emission is substantially reduced in a novel optical amplifier wherein the gain medium is disposed within a converging region of the coherent signal, which converging region terminates in a waist at or near a limiting stop or saturable absorber. In contrast to the converging coherent signal flux, the amplified spontaneous emission flux is nonconverging and therefore most of the latter is removed by a spatial filter or saturable absorber.

White-light speckle method for obtaining an equi-velocity map of a whole flow field

Abstract: The surface of a flow seeded with aluminium oxide particles is photographed with two pulsed illuminations of whitelight. When the resultant double-exposure specklegram is optically processed using a spatial filtering technique, an equivelocity pattern can be obtained. Especially, a coloured display of the pattern by the white-light reconstruction gives many useful informations about the flow state.

Frequency multiplexed optical spatial filter based upon photochemical hole burning

Abstract: A frequency multiplexed optical spatial filter is based upon a thin film of a photochemical hole burning material. The filter stores a plurality of images in the photochemical hole burning medium. The images are formed at different optical frequencies and coexist in the same volume of the storage medium. Individual images are accessed by changing the frequency of the illuminating radiation.

Map transformations by optical anamorphic processing.

Abstract: A map transformation is a willful geometrical distortion of an image. Examples are rotation, shearing, and local stretching of the coordinate system. We describe an optical setup that performs a map transformation by spatial filtering with two phase-only filters. The system is able to perform in x- and y-linear distortions (e.g., shears) as well as certain x- and y-nonlinear distortions. The distorting filters introduce no aberrations. The object may radiate coherently, incoherently, or partially coherently. Some experimental results are presented.

Multichannel Bragg cells: compensation for acoustic spreading.

Abstract: In some applications, the performance of multichannel Bragg cells is compromised by the spreading of the acoustic waves as they propagate; the spreading causes the signals in the channels to overlap. The overlapping can be significantly reduced by a spatial filter in a Fourier–image plane. The spatial filter is shown to be a cylindrical lens whose power is a function of the distance from the transducer. The effects of changes in the drive frequency as well as those of displacements of the filter are calculated. The reduction in the modulation transfer function as a function of propagation distance is calculated, and some bounds on the time–bandwidth product and the number of channels are dserived. In general, the overall performance can be improved by increasing the center frequency of the Bragg cell while keeping the bandwidth fixed.

Intracavity spatial filtering in unstable ring resonator geometries: Part I - Passive cavity mode theory

Abstract: The general theory of attaining enhanced transverse mode discrimination in an unstable ring resonator by employing an intracavity spatial fiber is developed. The theory presented is for the passive cavity mode structures of both the forward and reverse traveling wave fields supported by the optical cavity. Of particular interest here is the transverse mode behavior in the presence of strong spatial filtering wherein the spatial filter aperture completely dominates the diffractive mode structure formation processes. Both a circular focal point aperture and a pair of orthogonal focal line apertures are considered. The succeeding parts to this first paper describes the numerical and experimental results obtained for several unstable ring laser geometries with intracavity spatial filtering. The analysis presented here provides the fundamental understanding of these results.

Nd:YAG Holographic Interferometer For Aerodynamic Research

Abstract: A holographic interferometer system has been installed in the NASA Ames 2- by 2-Foot Transonic Wind Tunnel. The system incorporates a modern 10 pps, Nd:YAG pulsed laser which provides reliable operation and is easy to align. The spatial filtering requirements of the unstable resonator beam are described, as well as the integration of the system into the existing schlieren system. A two-plate holographic interferometer is used to reconstruct flow field data. For static wind tunnel models, the single exposure holograms are recorded in the usual manner; however, for dynamic models such as oscillating airfoils, synchronous laser hologram recording is used.