Showing papers on "Spatial filter published in 1985"

Uncertainty relation for resolution in space, spatial frequency, and orientation optimized by two-dimensional visual cortical filters.

Abstract: Two-dimensional spatial linear filters are constrained by general uncertainty relations that limit their attainable information resolution for orientation, spatial frequency, and two-dimensional (2D) spatial position. The theoretical lower limit for the joint entropy, or uncertainty, of these variables is achieved by an optimal 2D filter family whose spatial weighting functions are generated by exponentiated bivariate second-order polynomials with complex coefficients, the elliptic generalization of the one-dimensional elementary functions proposed in Gabor’s famous theory of communication [ J. Inst. Electr. Eng.93, 429 ( 1946)]. The set includes filters with various orientation bandwidths, spatial-frequency bandwidths, and spatial dimensions, favoring the extraction of various kinds of information from an image. Each such filter occupies an irreducible quantal volume (corresponding to an independent datum) in a four-dimensional information hyperspace whose axes are interpretable as 2D visual space, orientation, and spatial frequency, and thus such a filter set could subserve an optimally efficient sampling of these variables. Evidence is presented that the 2D receptive-field profiles of simple cells in mammalian visual cortex are well described by members of this optimal 2D filter family, and thus such visual neurons could be said to optimize the general uncertainty relations for joint 2D-spatial–2D-spectral information resolution. The variety of their receptive-field dimensions and orientation and spatial-frequency bandwidths, and the correlations among these, reveal several underlying constraints, particularly in width/length aspect ratio and principal axis organization, suggesting a polar division of labor in occupying the quantal volumes of information hyperspace. Such an ensemble of 2D neural receptive fields in visual cortex could locally embed coarse polar mappings of the orientation–frequency plane piecewise within the global retinotopic mapping of visual space, thus efficiently representing 2D spatial visual information by localized 2D spectral signatures.

Model of human visual-motion sensing

Abstract: We propose a model of how humans sense the velocity of moving images. The model exploits constraints provided by human psychophysics, notably that motion-sensing elements appear tuned for two-dimensional spatial frequency, and by the frequency spectrum of a moving image, namely, that its support lies in the plane in which the temporal frequency equals the dot product of the spatial frequency and the image velocity. The first stage of the model is a set of spatial-frequency-tuned, direction-selective linear sensors. The temporal frequency of the response of each sensor is shown to encode the component of the image velocity in the sensor direction. At the second stage, these components are resolved in order to measure the velocity of image motion at each of a number of spatial locations and spatial frequencies. The model has been applied to several illustrative examples, including apparent motion, coherent gratings, and natural image sequences. The model agrees qualitatively with human perception.

Noise and filtration in magnetic resonance imaging.

Abstract: Noise in two-dimensional Fourier transform magnetic resonance images has been investigated using noise power spectra and measurements of standard deviation. The measured effects of averaging, spatial filtering, temporal filtering, and sampling have been compared with theoretical calculations. The noise of unfiltered images is found to be white, as expected, and the choice of the temporal filter and sampling interval affects the noise in a manner predicted by sampling theory. The shapes of the imager's spatial frequency filters are extracted using noise power spectra.

Optical-data-processing properties of a liquid-crystal television spatial light modulator.

Abstract: The potential of the extremely inexpensive Radio Shack liquid-crystal television (LCTV) as a two-dimensional spatial light modulator has been investigated. The LCTV modulates the transmission of coherent or incoherent light and can either be electronically addressed through a microcomputer or optically addressed with a TV camera. We have measured the transmission characteristics of the device, examined its diffraction pattern, and tested its use as an input device for an optical correlator. We have discovered that, with proper modifications, it has potential for optical-data-processing applications.

Operation of individual diode lasers as a coherent ensemble controlled by a spatial filter within an external cavity

Abstract: Five diode gain elements (diode lasers with one facet antireflection coated) have been controlled to operate coherently with each other by a spatial filter placed between the antireflection coated facet and the mirror of an external cavity which provides the feedback for laser operation The locking of the lasers is relatively insensitive to phase adjustment in each laser path Locking has occurred with lasers whose output wavelength when operating individually differed by 60 A It is expected that the maximum tolerance to wavelength difference is much larger because lasers used were selected so simple individual temperature control could bring them into wavelength coincidence These results point to the feasibility of placing well over a thousand diode lasers in an external cavity with coherent output

Spatial filter for increasing the depth of focus.

Abstract: The Strehl ratio, in the form of McCutchen’s theorem, is employed to design a spatial filter that increases the depth of focus. Computer-simulated images show the increment in focal depth.

Physical Characterization Of The Photorefractive Incoherent-To-Coherent Optical Converter

Abstract: Volume holographic storage in photorefractive Bit 2S i020 (BSO) crystals is utilized to perform dynamic incoherent-to-coherent image conversion by means of selective spatial erasure of a uniform grating with white (or quasi-monochromatic) light. Physical characterization of device performance is de-scribed, with emphasis on resolution, sensitivity, and temporal response. The photorefractive incoherent-to-coherent optical conversion process is analyzed in terms of a simple model that relates the diffracted intensity to the space-variant effective modulation ratio.

Optical system for wide angle IR imager

Abstract: The invention relates to a high performance optical system for a wide angle IR imager requiring a cryogenic environment for the IR detector array. The wide angle requirement makes it necessary for one lens to be behind a "cold aperture" within the cryogenic environment where focusing is difficult and performance of the aperture reduced. These objectives are minimized by use of a telecentric lens pair, one member of which is within the cryogenic environment. The telecentric lens pair refocuses the primary image formed by an objective lens, permitting a high quality image to be formed on the IR detector array. The usual errors in positioning the internal lens, which might impair the quality of the focused image, are avoided by a compensatory repositioning of the externally accessible objective lens. The use of collimated bundles of light, characteristic of telecentric lens pairs, permits maximum aperture efficiency, and also provides a fourier transformation plane in which spatial filtering may be used to reduce IR background of low spatial frequency.

Array mode selection utilizing an external cavity configuration

Abstract: We report operation of a ten‐stripe, gain‐guided, phase‐locked diode laser in an external cavity configuration. The laser radiates in a single narrow (1°) lobe. Such lasers generally lase in the highest order array mode, L=10, which radiates in a twin‐lobe far‐field pattern. With one antireflection‐coated facet and a slit spatial filter, the laser has been operated in the L=1, 2, 3 or 10 array modes. A theoretical explanation of the spatial filter function is included.

Recursive optical filter system

Abstract: An optical notching or filtering system in which a beam of optical radiation is passed through the optical filtering means a multiplicity of times. The optical filtering means has an optical Fourier transform means (12), a spatial filter (14), optical inverse Fourier transform means (16), and a reflector array (18). After an input optical signal beam undergoes a filtering pass by being directed through the transform means, the filter, and the inverse transform means, the reflector array re-introduces the beam for a multiplicity of subsequent filtering passes. Four embodiments of the system of the invention having various configurations of the reflector arrays for use with one- and two-dimensional spatial filters are disclosed. The various embodiments are set forth as being used in a heterodyning optical notching filter systsm.

Transfer functions and spatial filtering in grating interferometers.

Abstract: Diffraction grating interferometers, under extended source monochromatic illumination and with appropriate spatial filtering, are seen to be essentially imaging systems, linear in amplitude. By broadening the spectrum of the illumination, the system becomes linear in irradiance.

Boundary effects on transient radiation fields from vibrating surfaces

Abstract: The transient radiation or diffraction from a planar source imbedded in an infinite baffle is analyzed for three different baffle conditions (rigid, free‐space, and soft). For an excitation separable in time and space, it is shown that the field is related to the normal derivative of the input field only. A method is also given for computing the transient fields based on a wave decomposition in the spatial frequency domain. This method is a time generalization of the angular spectrum theory that presents transient wave propagation as a time‐varying spatial filter, allowing a linear systems interpretation of the diffraction. The formalism is shown to easily include the effects of a finite receiving aperture. The method is amenable to computing the field from an arbitrary time excitation and an arbitrary spatial distribution. The method is valid for arbitrary distances to the observation point and uses computationally efficient FFT calculations.

Optically Addressed Membrane Spatial Light Modulator

Abstract: The architecture, operation, and performance of a new two-dimensional, optically addressed, membrane spatial light modulator are described. The modulator, the optical-to-optical deformable mirror device, consists of an array of 128 X128 deformable mirror elements on 25 .im centers integrated on silicon. The device exhibits a sensitivity of 1.7 jJ/cm2 at 525 nm, write times of 25 i. is, and erase times of 40 I.Ls. Images are read out with near-IR illumination. The device can be used to convert incoherent images to coherent images for optical information processing applications.

Fourier plane recursive optical filter

Abstract: An optical recursive filtering system in which a signal beam of optical radiation is passed through the optical filtering means a multiplicity of times. The filtering system has an optical Fourier transform means, a spatial filter, optical inverse Fourier transform means and a pair of reflectors in the Fourier plane of the spatial filter. An input signal beam to be filtered is optically Fourier transformed by the transform means and is passed through the spatial filter where unwanted frequencies of the signal beam are attenuated. The filtered beam is reflected by one of the reflectors back through the filter and the second mirror reflects it back through the filter a third time. Subsequent reflections back through the filter with a consequent further filtering of the signal beam can be accomplished by appropriately positioning the reflectors. After multiple filtering passes, the filtered beam is extracted for utilization. By maintaining the recursions of the beam in the Fourier plane of the filter, which preferably is of a programmable type, the optical throw of the filter system can be significantly reduced and a flexible imaging system without the restraints of external recursion is realizable in a robust structure with fewer components. An embodiment of the system being used in a heterodyning RF optical filtering system is described.

Optical processing and holography with polychromatic point source illumination.

Abstract: An achromatic Fourier transforming system is analyzed in a manner which gives new physical insight. A matched spatial filtering experiment is performed comparing the noise performance of a coherently produced matched filter and one made using broad spectrum light in an achromatic Fourier transforming system. The broad spectrum light is obtained by scanning a dye laser through its tuning range, thereby simulating a polychromatic point source. The system is applied to the construction of low noise holographic optical elements.

Measurements of flow velocity in a microscopic region using a transmission grating: a differential type

Abstract: A method is described for measuring flow velocity in a microscopic region by means of a differential-type laser velocimeter with a transmission grating. The principle of this velocimeter is described on the basis of spatial filtering of the transmission grating. The filtering characteristics are theoretically analyzed using a power spectral density function. The velocimeter is used to measure flow velocities and their spatial distributions in a small (700-μm diam) glass tube. The theoretical and experimental results show that this differential-type velocimeter is more suitable and practical than the conventional one for measuring the flow velocity in a small probe.

Analysis of errors caused by optical interference effects in wavelength-diverse CO2 laser long-path systems.

Abstract: The measurement errors in a CO2 laser differential absorption system caused by optical interference effects are analyzed. Major sources of errors are identified and design suggestions to improve the system accuracy are made. The effects of temperature changes, short measurement paths, and spectral features of the measured species are taken into account.

Realization of a uniform circular source using a two-dimensional binary filter.

Abstract: A uniform-intensity circular light source has been realized from a Gaussian laser beam by using an area-modulated binary filter. The technique preserves the wave front, and as a result the uniform circular source can be located at a plane perpendicular to the axis of propagation over a wide area.

Incoherent spatial filtering with grating interferometers.

Abstract: The combination of a two-channel spatial filtering system in tandem with a grating interferometer adjusted for broad-source operation results in a highly effective system for generating arbitrary transfer functions under incoherent illumination.

Adaptive microwave spatial filter operating on-reflection, and a corresponding method

Abstract: The invention pertains to a modulation method at pick-up of the amplitude of the secondary lobes of the radiation pattern for a hyperfrequency antenna and the method application for sensing and eliminating the jamming effects of jammers. According to the invention, we place as a filter (4) close to the reflector (1) which reflects the transmission-pick-up beam of the antenna, the filter having at least one network of conductive wires loaded with variable controllable resistors, such as diodes. During transmission, we make the filter (4) transparent by having strong equal currents travel through the wires, while at pick-up we modulate the amplitude of the currents traveling through the wires in order to obtain the desired distortions of the pattern. The invention especially applies to the sensing and elimination of the jamming effects produced by jammers.

Automatic Recognition And Tracking Of Targets From Visible Or Thermal Imagery, Using Optical Processing

Abstract: (0272) 693831 Ext. 171/177AbstractA real -time Vander Lugt correlator for automatic recognition and tracking of targetsfrom visible or thermal data is described. A modification to the classical systemarchitecture was the use of difference -of- Gaussian spatial filtering, which providedgreater tolerance to scale and aspect variations in targets.Incoherent -to- coherent conversion was achieved using a Hughes LCLV, and the results ofa rigorous evaluation study of the device are discussed.IntroductionRecent research and development in the area of real -time spatial light modulators hasled to an upsurge of interest in optical information processing, in areas such as opticallogic, signal processing, and optical pattern recognition_A system is described which utilises a Hughes Liquid crystal light valve (LCLV) toimpose imagery, in near real time, onto the coherent beam of an optical correlator, withthe aim of achieving automatic recognition and tracking of targets. This is based uponthe classic Van der Lugt architecture (reference 1), but incorporates an intermediatespatial filtering stage, which provides a convenient means of modifying the Fourier spectraof both reference and input functions, in order to achieve increased tolerance to scale andorientation variations in targets.System operationA schematic diagram of the optical system is shown in Figure 1. There are two stagesinvolved in its use. The first is production of the matched spatial filter (MSF), and thesecond is correlation with input data. For production of the MSF, a reference function,either in the form of a suitably apertured target from within the field -of -view of theinput sensor, or a computer -generated, weighted combination function of a number of viewsof the target (reference 2), is introduced at plane P1, via a fibre -optic faceplate C.R.T.The Fourier transform of the reference function is formed at P2, where it is modifiedby the difference -of- Gaussian (DOG) filter, and the relay lens L then refocuses themodified Fourier transform at P

Smart visual sensors for real-time image processing and pattern recognition based upon human visual system characteristics (matrix inversion, subtractive inhibition, multiple spatial filtering)

Abstract: To design lightweight smart sensor systems which are capable of outputting motion-invariant features useful for automatic pattern recognition systems, we must turn to the simultaneous image processing and feature extraction capability of the human visual system (HVS) to enable operation in real time, on a mobile platform, and in a "natural environment." This dissertation studies the smart sensor problem, develops new concepts, supported by simulation, and discusses research areas for further exploration. An n('2) parallel data throughput architecture implemented through a hardwired algotecture which accomplishes, without computation, an equivalent logarithmic coordinate mapping is presented. The algotecture mapping provides, at the sensor level, the ability to change scales and rotations in the input plane to shifts in the algotecture mapped space. The resulting invariant leading edge is shown to possess an intensity preserving property for arbitrary variations of image size. The sensitivity of the algotecture to center mismatch is discussed in terms of the difference between coordinate and functional transformation methods. A mathematical link between the lateral subtractive inhibition (SI) and multiple spatial filtering (MSF) mechanisms of the HVS coexist and function simultaneously. The feature extraction filter in visual neurophysiology, known as the novelty filter, is identified to be the first feedback term of an iterative expansion of the sensory mapping point spread function. The use of the algotecture space combined with the image plane MSF approach is explored for detection and classification using template crosscorrelation methods of recognition. The concept of using a three spatial frequency band model, based upon HVS physiological and psychophysical data, for an intra-class, and inter-class, and a membership identification classification scheme is introduced. This concept is extended to represent the feature vector entries for, not only each spatial frequency band, but for each image view angle in the recognition library. A new algorithm for implementing an intelligent enormous matrix inversion is proposed. Such an inverse problem exists in the solution of the negative feedback equation for SI and has a form that lends itself easily to parallel processing. The solution provides for a means to solve the inversion even though one of the partitioned submatrices is singular. Procedures are given to construct a partition tree which is analogous to quadtree partitioning methods in image processing. Finally a simple matrix inversion example is worked out to demonstrate how the algorithm works.

Inverse problem with quasi-homogeneous sources

Abstract: The problem is discussed of determining the distribution of the intensity and of the degree of spectral coherence of a planar, secondary quasi-homogeneous source from the cross-spectral density function of the field measured over any plane that is parallel to the source. A solution to this problem is presented under the assumption that the degree of spectral coherence g(ρ1 − ρ2, ω) of the quasi-homogeneous source does not vary appreciably across the source over distances |ρ1 − ρ2| that are of the order of or less than the wavelength λ corresponding to the frequency to. The results are illustrated by computational reconstruction of Gaussian-correlated quasi-homogeneous sources.

Fiber-optic sensor for distance and velocity measurements using speckle dynamics.

Abstract: A fiber-optic sensor for distance and velocity measurements is described which is based on dynamic properties of speckles formed by two illuminating laser beams. This sensor is composed of two optical-fiber laser guides and an optical-fiber-array spatial filter consisting of linearly arrayed endfaces of optical fibers. The measurable ranges of distance and velocity and their accuracies are expressed in terms of the sensor's configuration parameters. The configuration for accurate sensing over a wide range is discussed. The usefulness of this sensor is confirmed by experiments.

A course of experiments with He-Ne-laser

Abstract: Preface List of Experiments INTRODUCTION TO He-Ne LASER: Laser Beam Parameters Interference of Light Diffraction of Light Polarisation of Light Holography Speckle Phenomenon Spatial Filtering Laser Doppler Anemometry Appendices Suggested Readings Index.

Realization of Fourier Images without Using a Lens by Sampling the Optical Object

Abstract: The realization of the Fourier image of a two-dimensional object without using a lens is described. The two-dimensional Fourier transform is generated optically by means of a periodic array of pin-holes (the sampling filter). The object is illuminated by a monochromatic, coherent plane wave and sampled by the pin-hole array. Multiple Fourier images of the object appear in certain planes behind the sampling filter. The simple theory of this phenomenon, together with experimental results, is given.

Optical processing technique for spot-size measurements in single-mode fibres

Abstract: A new, simple and very promising technique for spot-size measurements in single-mode fibres is proposed, based on a spatial filter which modulates the optical beam at the output of the fibre. Its inherent simplicity, accuracy and high efficiency make it attractive for laboratory and factory measurements.