Showing papers on "Spatial filter published in 1997"

Localization of brain electrical activity via linearly constrained minimum variance spatial filtering

Abstract: A spatial filtering method for localizing sources of brain electrical activity from surface recordings is described and analyzed. The spatial filters are implemented as a weighted sum of the data recorded at different sites. The weights are chosen to minimize the filter output power subject to a linear constraint. The linear constraint forces the filter to pass brain electrical activity from a specified location, while the power minimization attenuates activity originating at other locations. The estimated output power as a function of location is normalized by the estimated noise power as a function of location to obtain a neural activity index map. Locations of source activity correspond to maxima in the neural activity index map. The method does not require any prior assumptions about the number of active sources of their geometry because it exploits the spatial covariance of the source electrical activity. This paper presents a development and analysis of the method and explores its sensitivity to deviations between actual and assumed data models. The effect on the algorithm of covariance matrix estimation, correlation between sources, and choice of reference is discussed. Simulated and measured data is used to illustrate the efficacy of the approach.

Deriving object visibilities from interferograms obtained with a fiber stellar interferometer

Abstract: A method is given for extracting object visibilities from data provided by a long baseline interferometer, where the beams are spatially filtered by single-mode fibers and interferograms are obtained as scans around the zero optical pathlength difference. It is shown how the signals can be corrected from the wavefront perturbations caused by atmospheric turbulence. If the piston perturbations are also removed, then the corrected data contain both spatial and spectral information on the source (double Fourier interferometry). When the piston cannot be removed, object phase and spectral information are lost, and the observable (free of detector noise bias) is the squared modulus of the coherence factor, integrated over the optical bandpass. In a fiber interferometer this quantity leads to very accurate object visibility measurements because the transfer function does not involve an atmospheric term. The analysis also holds for a more classical pupil plane interferometer which does not take advantage of the spatial filtering capability of single-mode fibers. In that case however, the transfer function includes a turbulence term that needs to be calibrated by statistical methods.

Video postfiltering with motion-compensated temporal filtering and/or spatial-adaptive filtering

Abstract: A postfiltering process for improving the appearance of a video image includes motion compensated temporal filtering and spatial adaptive filtering. For each target pixel being filtered, the temporal filtering uses multiple motion vectors and one or more pixel values for a prior frame to determine one of more reference values for the target filter. In one embodiment, a weighted average of multiple motion vectors for blocks near or containing the target pixel value provides a filter vector that points to a pixel value in the prior frame. This pixel value is a reference value for the target pixel value and is combined with the target pixel value in a filter operation. Alternatively, multiple motion vectors for blocks near or containing the target pixel value point to pixel values in the prior frame that are averaged to determine a reference value for the target pixel value. In each alternative, the weighting for the average is selected according to the position of the target pixel value. The spatial filtering determines a dynamic range of pixel values in a smaller block containing the target pixel value and a dynamic range of pixel values in a larger block containing the target pixel value. The two dynamic ranges suggest the image context of the target pixel, and an appropriate spatial filter for the target pixel is selected according to the suggested context.

Improvement of spatial resolution in surface-EMG: a theoretical and experimental comparison of different spatial filters

Abstract: In the present study, different isotropic and anisotropic filters have been compared by means of theoretical field simulations and experiments in volunteers. A tripole model for an excited motor unit (MU) was used as the basis for simulating the spatial extension of the filter response for each of the investigated filters. The spatial extension is an indicative of the spatial resolution. For the experimental validation, the total number of single motor units was not directly investigated, but the signal-to-noise ratio (SNR) has been determined. Therefore, the potential distribution generated on the skin surface during maximum voluntary contraction has been simultaneous spatially filtered with each of the investigated filters. The simulations show that an isotropic spatial filtering procedure reduces the spatial extension of the filter response and improves the spatial resolution of the electromyography (EMG)-recording arrangement in comparison to anisotropic spatial filters up to 30%. In other words, the spatial selectivity of the arrangement is increased. This improvement in the filter performance is more pronounced for MU's located close to the skin surface than for MU's more distantly located. Additionally, this theoretical improvement in selectivity depends on the direction of the excitation spread relative to the filter alignment. However, the investigations also show that isotropic filters offer an advantage, compared to anisotropic filters, only when the investigated MU is located extremely close to the filter input. The results of the simulations can be confirmed by the experimental investigations. An improvement of 11% in the SNR, relative to anisotropic spatial filters, can be established when using an isotropic spatial filter. This experimental improvement in selectivity is less than the theoretical improvement because the experimentally investigated MU's have less portion in the anisotropic range of the filters than the simulated one at best.

Noise estimation and filtering using block-based singular value decomposition

Abstract: Preprocessing of image and video sequences with spatial filtering techniques usually improves the image quality and compressibility. We present a block-based, nonlinear filtering algorithm based on singular value decomposition and compression-based filtering. Experiments show that the proposed filter preserves edge details and can significantly improve the compression performance.

Spatial and temporal filtering mechanism for digital motion video signals

Abstract: Frames of a digital video signal are spatially filtered to remove impulse and other noise from the video signal. The spatially filtered frame is temporally filtered to further remove noise from the digital video signal. The spatial filter is adaptive, heavily filtering portions of low detail in the represented subject matter while lightly filtering or not filtering portions of moderate to high detail in the represented subject matter. As a result, clarity and detail in the subject matter of the digital video signal are preserved. The temporal filter is similarly adaptive and determines if and to what degree the subject matter of portions of the spatially filtered frame correlate with corresponding portions of a previous frame. By previously spatially filtering the frame, determinations regarding the correlation of the current frame with corresponding portions of the previous, which is also spatially and temporally filtered, are more accurate. Accordingly, temporal filtering can be performed more aggressively without temporally blurring the digital video signal. The result of spatial and temporal filtering of the digital video signal is significant reduction in noise without degradation of the signal and improved compression of the digital video signal.

Computationally efficient low-artifact system for spatially filtering digital color images

Abstract: A computationally efficient system spatially filters a digital input image to sharpen the image. A process is disclosed to spatially filter pixels of an RGB image by applying a high pass filter to the image. First, signals representative of luminance are extracted from the input image. Then, the extracted luminance signals are filtered. Quotient signals are then developed, representing the ratio between each filtered luminance signal and the corresponding original luminance signal. Quotient signals are multiplied by the corresponding pixels of each plane of the original image to provide an output image. The invention may also be embodied by an image editor to implement the spatial filtering process of the invention, where the image editor includes stored programming instructions executed by a processor, discrete circuit components, an ASIC, or another suitable hardware device.

Optimality of the maximum average correlation height filter for detection of targets in noise

Abstract: A statistical analysis is provided for the properties of the re- cently developed maximum average correlation height (MACH) filter (Mahalanobis et al. 1994). It is shown that the MACH filter can be inter- preted as an optimum filter for the detection of targets in additive noise. A rationale is given for using a popular peak-to-sidelobe ratio metric to characterize the output of the MACH filter. Finally, the performance of the MACH filter is compared to that of the matched spatial filter (MSF) in terms of the relation between the probabilities of correct detection and false alarm, which is represented as a receiver operating characteristic (ROC) curve. © 1997 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(97)00910-0)

Mechanical filtering effect of elastic cover for tactile sensor

Abstract: Tactile sensors are ordinarily covered with an elastic cover to protect the sensor from being damaged by shock or chemical contamination. The cover, however, greatly decreases the sensor's spatial resolution. This effect becomes serious in the fabrication of a high-spatial-resolution sensor, even if the cover is only 0.2 mm thick, when the sensor requires a spatial resolution of less than 1 mm. This paper analyzes the low-pass spatial filtering effect of the cover, calculates the filtering gain for different types of elastic cover materials using the finite-element method, and gives preliminary experimental results.

Improved vessel visualization in MR angiography by nonlinear anisotropic filtering.

Abstract: This paper deals with a preprocessing technique of magnetic resonance angiography (MRA) images, applied before maximum-intensity-projection (MIP). The purpose was to recover small low-intensity vessels, visible in individual slices, but lost in MIP images that usually have higher background level than the individual slices. The authors have developed a nonlinear three-dimensional spatial filtering technique (called HD filter) based on anisotropic smoothing. The filter first searches for the local orientation of the vessel. It then performs a nonlinear smoothing in the vessel's local direction so as to avoid blurring its boundaries. Noise level reduction, contrast enhancement, and improved small vessel visibility achieved by this filter are illustrated on dynamic contrast-enhanced subtraction MRA images of the lower limbs.

Very thick holographic nonspatial filtering of laser beams

Abstract: Philip Hemmer, MEMBER SPIE Rome Laboratory RL/EROP, 63 Scott Road Hanscom Air Force Base, Massachusetts 01731 Abstract. A novel device, the nonspatial filter, is described for laser beam cleanup. It is based on the Bragg selectivity of thick holograms. Unlike pinhole and fiber spatial filters, which employ lenses and apertures in the transform plane, nonspatial filters operate directly on the laser beam. This eliminates the need for laser beam focusing, which is the source of many of the material and alignment instabilities and laser power limitations of spatial filters. Standard holographic materials are not suitable for this application because differential shrinkage during processing limits the maximum Bragg angle selectivity attainable, and because they are generally too thin. New technologies that eliminate the problem of differential shrinkage are described. These technologies are based either on the use of a rigid porous substrate material, such as porous glass, filled with a light-sensitive material, such as holographic photopolymers or dichromated gelatin, or on the use of a thick photopolymer with diffusion amplification (PDA). We report results of holographic nonspatial filtering of a laser beam in one dimension, with an angular selectivity of better than 1 mrad. © 1997 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(97)01606-1]

Controlling extended systems with spatially filtered, time-delayed feedback

Abstract: We investigate a control technique for spatially extended systems combining spatial filtering with a previously studied form of time-delay feedback. The scheme is naturally suited to real-time control of optical systems. We apply the control scheme to a model of a transversely extended semiconductor laser in which a desirable, coherent traveling-wave state exists, but is a member of a nowhere stable family. Our scheme stabilizes this state and directs the system towards it from realistic, distant, and noisy initial conditions. As confirmed by numerical simulation, a linear stability analysis about the controlled state accurately predicts when the scheme is successful and illustrates some key features of the control including the individual merit of, and interplay between, the spatial and temporal degrees of freedom in the control.

Apparatus and method for generating diffractive element using liquid crystal display

Abstract: Apparatus and method for recording diffractive high resolution graphical information is suited to recording information that would be difficult to counterfeit. In one embodiment, light is selectively passed by a shutter (52), and through spatial filter (54) to remove noise. Liquid crystal display (68) or similar display device receives a data stream from computer (20) and displays periodic structure(s), diffracting the incident beam into one or more diffracted beams (59). Lens (78) focuses an image of the macroscopic pattern formed of these periodic structures to a recording material in image plane (80) where diffracting structure(s) caused by the overlap of some or all the diffracting beams and, optionally, the undiffracted beam, is created. Optional mask (76) blocks some or all of the diffracted and undiffracted beams. The recording material is moved and the process repeated with another pattern of periodic structures until the desired macroscopic image is composited.

Computing illumination-invariant descriptors of spatially filtered color image regions

Abstract: Spatial filters provide a useful and efficient means of analyzing an input color image into components that capture different spatial properties. Representations based on spatial filtering have restricted usefulness for recognition, however, because the output of a spatial filter across an image depends on the scene illumination conditions. We use a physically accurate linear model for spectral reflectance to derive invariants of distributions in spatially filtered color images that do not depend on the scene illumination. These invariants can be used for the illumination-invariant recognition of regions following an arbitrary linear filtering operation. We describe a method for illumination correction based on color distributions and introduce an illumination change consistency constraint that is useful for verifying matches obtained using the invariants. We show, using a set of classification experiments, that the filtered distribution invariants can significantly improve the capability of a recognition system in environments where illumination cannot be controlled.

Imaging apparatus having a spatial filter and image shifting mechanism controller based on an image mode

Abstract: A single imaging apparatus is adapted to pick up both a motion image and a still image. In an imaging apparatus 1, image light from an object is separated into multiple monochromatic image light components by an optical system 3 and then imaged by an imaging means 4 for generation of an original image signal. The imaging apparatus 1 switches an imaging mode depending upon an output mode. The optical system 3 switches the sate of a variable spatial filter 18 depending upon an imaging mode thereby changing an effect for limiting the spatial frequency. In an output mode for motion images, an interpolative synthesizer circuit 7 generates the output image signal from a single original image signal. In an output mode for still image, an image shift mechanism 19 parallely shifts the optical axis of the incoming image light while the imaging means captures the original image light multiple times so that an output image is produced from these original image signals. An amount of attenuation of the filter 18 in the output mode for motion images is greater than that for an equivalent spatial frequency component in the output mode for still image.

Improvement in the rejection rate of a nulling interferometer by spatial filtering.

Abstract: We propose applying the techniques of spatial filtering to the concept of interferometric coronography. In such a system, provided that the object being studied is not resolved by the individual apertures of the interferometric array, the beams can be considered as coherent or, more exactly, single mode. Hence spatial filtering allows one to cleanse the beams of imperfections generated by defects on the optical components of the interferometer and thus to obtain very high rejection rates in the destructive output of the interferometer (coronographic output) for an on-axis star. Numerical simulations show that the very stringent constraints on the optical quality of a space IR interferometer aimed at detecting extrasolar planets can be relaxed to values achievable with current technology. In particular, we show that the difficulties induced by dust scattering, small micrometeorite impacts on the primary mirror, and high-frequency ripples of polishing residuals can be eliminated by simple pinhole spatial filtering. The effects, however, will be less dramatic on large-scale defects such as coating defects and pointing errors in the telescopes.

Jamming suppression of spread spectrum antenna/receiver systems

Abstract: A jamming suppression system which includes a plurality of receivers for receiving electromagnetic radiations and a separate notch filter coupled to each receiver for filtering selected frequencies from electromagnetic radiations received by each of the receivers. In accordance with one embodiment of the invention, a spatial combiner responsive to the outputs of each of the notch filters extracts essentially jam-free information from the electromagnetic radiations. In accordance with a second embodiment of the invention, a plurality of spatial combiners, each responsive to the outputs of each of the notch filters, extracts essentially jam-free information from the electromagnetic radiations. The notch filters are adaptive notch filters for on-line selection of their filtering action. One of the notch filters is a master filter and the remainder of the notch filters are slave filters controlled by the master filter. In both embodiments, spatial filtering action is accomplished using a generalized sidelobe canceler that permits gain (relative to an isotropic antenna) to be preserved in the direction of one or more satellites while casting spatial mulls in the directions of inadvertent friendly or hostile jamming signals.

Single-mode vertical cavity surface emitting laser by graded-index lens spatial filtering

Abstract: 45 mW of power in a single spatial mode has been obtained from a vertical cavity surface emitting laser (VCSEL) in an external cavity setup, using a graded index (GRIN) lens with a spatial filtering high reflecting aperture deposited on its endface The spatial filter on the GRIN lens endface forces a larger single transverse mode in the VCSEL than is obtained without it A brightness of 51×105 W/cm2 str is demonstrated, which is 91% of the maximum achievable

Programmable standard for use in an apparatus and process for the noninvasive measurement of optically absorbing compounds

Abstract: The present invention provides a programmable filter for use as a standard in correlation spectrometers. Also provided is the use of the programmable standard in a process for determining the concentration of an optically absorbing compound. Also provided is a method and apparatus for noninvasively determining the concentration of an optically absorbing biological sample which incorporates the programmable standard of the invention. In one embodiment the programmable standard (20) contains a dispersive element (106a) and a transmissive spatial light modulator (108). Because the position of each light beam correlates to its wavelength, spatial filter (108) controllably and selectively attenuates desired wavelengths of the input beam.

Waveguide grating router with maximally flat passband produced by spatial filtering

Abstract: A wavelength router with a maximally flat passband produced by spatial filtering is described. The router is suitable for long haul and local area networks. It is realised in integrated form using silica-over-silicon technology. It features a wide passband, low crosstalk and an excess loss close to 2.5 dB.

Controlling optical turbulence

Abstract: A robust global control strategy, implemented as a spatial filter with delayed feedback, is shown to stabilize and steer the weakly turbulent output of a spatially extended system. The latter is described by a generalized complex Swift-Hohenberg equation [J. Lega, J. V. Moloney, and A. C. Newell, Phys. Rev. Lett. 73, 2978 (1994); Physica D 83, 478 (1995)], which is used as a generic model for pattern formation in the transverse section of semiconductor lasers. Our technique is particularly adapted to optical systems and should provide convenient experimental control of filamentation in wide-aperture lasers.

Flared semiconductor optoelectronic device

Abstract: As to a first feature, a semiconductor optoelectronic device includes a resonator having an optical cavity between opposite end facets, a larger portion of a length of the resonator cavity comprising a single mode confining region for propagation of light and a smaller portion of a length of the resonator cavity comprising a tapered region for permitting propagation of light with a diverging phase front to a first of the end facets, which first facet is the light beam output. The tapered region provides a sufficiently large aperture to prevent catastrophic optical mirror damage (COD) at the first end facet while reducing the amount of required astigmatism correction while the single mode confining region provides spatial filtering to maintain diffraction-limited beam at the output. This structure therefore, more readily lends itself for incorporation into existing device packages designed for linear stripe laser diodes devices. As to a second feature, a semiconductor optoelectronic device includes a gain region having a region permitting propagation of light with a diverging phase front to a first end facet of the device, which is its output, and a single mode region is coupled to an inner end of the gain region extending from the inner end to a second end facet of the device permitting propagation of light with an adiabatic phase front to the second end. The significantly smaller taper of the single mode region permits retained maintenance of single mode operation while reducing optical density of the propagating beam at the second end facet.

Thermal lens determination of end-pumped solid-state lasers using primary degeneration modes

Abstract: A simple method using the singularities of an optical resonator to determine the thermal lens of end-pumped laser crystals is reported in this letter. At certain resonator parameters the transverse structures of the laser beam become a superposition of many transverse cavity modes of equal resonance frequencies. Because the resonator parameters depend on the thermally induced lens in the active medium, the strength of the lens can be determined by means of measuring the shift of the degeneration resonator length depending on the pumping power. The method is applied to an end-pumped Nd:YAG laser.

High-resolution multidimensional displacement monitoring system

Abstract: The possibility of using quadrant detectors to develop a new optical system that can monitor all six degrees of freedom of mechanical workpieces with very high resolution is investigated. A prototype system based on this approach has been designed and built. Although the system is not fully optimized, our proposed system has already demonstrated some promising results. Using a thermally compensated laser source together with a pinhole spatial filtering system, we have demonstrated that lateral resolution better than 50 nm and angular displacement resolution better than 0.25 ?rad is achievable with this system.

Image input apparatus having a spatial filter controller

Abstract: The Invention is an image input apparatus equipped with means of image shifting which is capable of inputting images of different resolutions, and enables the transmission band of the spatial filter to be switched in accordance with resolution in order to prevent moire. When a flat transparent refraction plate 22 is inclined relative to the optical axis 10 by which light from the object is introduced, the position where the image is formed on the imaging surface of the CCD 14 is shifted, thus making image shifting possible. High resolution images are obtained by shifting the image a distance shorter than the distance between the photosensitive parts of the CCD 14, and then combining the images. The transmission band of the spatial filter 12 for preventing moire can be switched by changing the relative angles of the double refraction plates 20, 21. The transmission band of the spatial filter 12 is switched to the high frequency side when inputting high resolution images by means of the image shift mechanism 13, thus obtaining images of good resolution.

Optical system for a hematology analytical instrument

Abstract: An optical illuminator assembly for an analytical instrument, such as a clinical hematology or a flow cytometer instrument, and method of aligning the components of the illuminator assembly (130,381) and orienting the illuminator assembly (130,381) The illuminator assembly (130,381) includes a plurality of optical components, such as a laser source (131), eg, a laser diode (131), optionally a spatial filter (185,190,195), a beam shaping aperture (201A,220A), and a focusing lens (220,387b) The optical components are mounted in or to a housing (121A,3020) and are internally oriented with respect to the housing (121A,3020) to produce a focussed laser beam output The housing is in turn mounted on an alignment mechanism (3040) which can move the focussed beam in four degrees of freedom, thereby to direct the focussed beam to a particular location Thus, a factory alignment and a low cost prealigned optical illuminator assembly is obtained, which can be easily oriented for use in an instrument

Energy weighted parameter spatial/temporal filter

Abstract: In an ultrasound imaging system, the mean velocity of a fluid flow, variance of velocity of the fluid flow and energy value of Doppler ultrasound signals from the flow in response to transmitted ultrasound are first estimated from autocorrelation values of the signals to form a set of parameters for a corresponding sampling time or a corresponding sampling location. An averaging process is then performed on two sets of the estimated parameters corresponding to a plurality of different sampling times in temporal filtering or on two sets of the estimated parameters corresponding to a plurality of different locations in spatial filtering. The filtered parameters are then compared to thresholds in a discrimination process to reject noise and clutter.

Laser code symbol scanner employing optical filtering system having narrow pass-band characteristics and spatially-separated optical filter elements with laser light collection optics arranged along laser light return path disposed therebetween

Abstract: A laser bar code symbol scanner embodying a narrow band-pass optical filtering system of novel construction. A first optical filter element is installed over the light transmission aperture of the scanner housing, and has wavelength selective properties which transmit only light having wavelengths from slightly below a predetermined wavelength in the visible band of the electromagnetic spectrum (e.g., greater than slightly below 670 nanometers). A second optical filter element is installed within the housing, along the focused laser return light path between the light focusing element and the light detecting element, and transmits only light having wavelengths from slightly above the predetermined wavelength (e.g., less than slightly above 670 nanometers). Collectively, the first and second optical filter elements cooperate to form a narrow wavelength band-pass filtering system centered about the predetermined wavelength, providing improved signal-to-noise ratio. As a result of the present invention, aesthetically unappealing electro-optical components mounted within the scanner housing are hidden from plain view, while the optical filtering elements of the system can be easily and inexpensively manufactures and used without compromising the signal-to-noise ratio performance of the laser scanner.

End stopping and length tuning in psychophysical spatial filters

Abstract: Psychophysical spatial filters or channels are usually modeled after simple-cell receptive fields, although many cortical cells are end stopped and length tuned. Using psychophysical masking, we demonstrate an analog to receptive field end stopping and length tuning in psychophysical spatial filters tuned to a wide range of spatial frequencies. Specifically, masking is maximal when the mask is approximately 5–6 arcmin longer than the target but is reduced when the mask exceeds this length, consistent with the properties of end-stopped cells. The strength and the extent of psychophysical end stopping appear to be determined by the filter’s spatial-frequency tuning, but length tuning varies with target length. The latter implies that spatial filters tuned to the same spatial frequency could have different length tuning and that there is no fixed length-to-width ratio of filter size. Phase effects suggest linear length summation but nonlinear psychophysical end stopping, which suggests that both first- and second-order visual processing is involved in end-stopped spatial filters.