Showing papers on "Spatial filter published in 1986"

Digital holographic interference-phase measurement using the Fourier-transform method

Abstract: A method for accurate phase determination in holographic interferometry using a one- or two-dimensional Fourier transform is described. The method calculates the interference phase pointwise, even between fringe extrema, and thus has advantages over conventional fringe-finding and -tracking methods. Only one interference pattern may be used, although the use of two patterns reconstructed with a mutual phase shift permits an easier phase unwrapping and determination of nonmonotonic fringe-order variations. Additionally, the method offers a means for filtering out disturbances such as speckle noise and background variations.

Nonlinear space-variant postprocessing of block coded images

Abstract: An important application of spatial filtering techniques is in the postprocessing of images degraded by coding. Linear, space-invariant filters are inadequate to reduce the noise produced by block coders. The noise in block coded images is correlated with the local characteristics of the signal, and such filters are unable to exploit this correlation to reduce the noise. We propose a new nonlinear, space-variant filtering algorithm which smooths jagged edges without blurring them, and smooths out abrupt intensity changes in monotone areas. Edge sharpness is preserved because near edges the filtering of the signal is negligible. Consequently, in-band noise is not reduced, but the well-known masking effect reduces the visibility of this in-band noise. The algorithm is only slightly more complex to implement than simple linear filtering. We present examples of processed images and SNR figures to demonstrate that a significant improvement in subjective and objective quality is achieved.

Optical computing using hybrid encoded shadow casting

Abstract: The parallel processing property of optics has been recog­ nized as the main driving force behind digital optical computing. A parallel pattern logic operation, first proposed by Tanida and Ichioka, overlaps spatially coded 2-D bina­ ry pixel patterns situated in an optical input plane. These patterns, when illuminated by divergent light beams ema­ nating from a group of LEDs, form different interlaced pro­ jections (shadows) representing different parallel logic oper­ ations in the optical output plane. Since the pattern overlap corresponds to a spatial domain filtering process, it is also known as optical shadow casting (OSC). Using OSC, a large number of 2-D binary or multiple-valued logic inputs can be parallel processed. Another pattern logic method, proposed by Bartelt et al., uses theta modulation to encode the signal grey-level values into different grating orienta­ tions. The combination of these grey-level-dependent grat­ ings form the logic inputs that are to be manipulated by a coherent optical processor. Using spatial-frequency domain filtering, different optical logic functions can be generated. Recently, Yatagai described another pattern logic method in which the spatially encoded patterns are overlapped with an operational mask. Instead of using the OSC LED pat­ terns, this method switches an operational mask for the different logic operations. However, in all these pattern logic methods, either the spatial filtering or pixel casting process is performed by either a transparent or opaque screen. In this communication, the use of a polarization encoding and filtering method to perform lensless OSC logic operations is proposed. Both linear orthogonally polarized and hybrid form polarizations with transparent/opaque mask input, logic signals are used. Using this polarization or hybrid encoded OSC (POSC), doubleor triple-instruction logic operations can be performed. This technique can be extended to generate multivariate binary as well as twovariable multiple-valued logic functions and can also be used in conjunction with the Yatagai's pattern logic method. Pertinent examples such as the design of a binary fulland a ternary half-adder are presented.

Eye-position sensor

Abstract: An eye-position sensor for use in an eye-activated optical transducer in which a spatial filter is used to modify light reflected from the eye to form a substantially rectangular pattern on a quadrantal array of contiguous sensors. This arrangement provides a substantially linear change in the output signal from the sensors in response to an equivalent movement of the eye.

Combined staring and scanning photodetector sensing system having both temporal and spatial filtering

Abstract: A photodetector sensing system which combines staring and scanning features. By providing both temporal and spatial filtering, many system benefits are obtained. Each pixel in the viewed scene has a ''dedicated'' filter, which receives signals only from that pixel. By integrating time spaced signals from the same pixel, the filter maintains the staring effect of a two-dimensional detector array. A signal detector (22) may view a plurality of pixels (32), thereby improving resolution. A single pixel may be viewed by a plurality of detectors, thereby providing redundancy to correct for detector failures. The signal from each detector, after amplification, is first sent through a spatial, high frequency filter (34), and then is sent to one of a plurality of parallel temporal, low frequency filters (36), which time share the detector. Synchronizing means are provided for ensuring that each temporal filter always receives its time spaced signals from the same source. Any suitable scanning mechanism may be used. The system may be used either to distinguish a moving target from a non-moving background, or to distinguish a non-moving target from a moving background.

Variation of the count-dependent Metz filter with imaging system modulation transfer function.

Abstract: A systematic investigation was conducted of how a number of parameters which alter the system modulation transfer function (MTF) influence the count-dependent Metz filter. Since restoration filters are most effective at those frequencies where the object power spectrum dominates that of the noise, it was observed that parameters which significantly degrade the MTF at low spatial frequencies strongly influence the formation of the Metz filter. Thus the radionuclide imaged and the depth of the source in a scattering medium had the most influence. This is because they alter the relative amount of scattered radiation being imaged. For low-energy photon emitters, the collimator employed and the distance from the collimator were found to have less of an influence but still to be significant. These cause alterations in the MTF which are more gradual, and hence are most pronounced at mid to high spatial frequencies. As long as adequate spatial sampling is employed, the Metz filter was determined to be independent of the exact size of the sampling bin width, to a first approximation. For planar and single photon emission computed tomographic (SPECT) imaging, it is shown that two-dimensional filtering with the Metz filter optimized for the imaging conditions is able tomore » deconvolve scatter and other causes of spatial resolution loss while diminishing noise, all in a balanced manner.« less

Spatial filtering of the fetal electrocardiogram.

Abstract: One of the basic problems in the analysis of the rhythm and the shape of the fetal ECG (FECG) is the interference caused by the electrical activity of the maternal heart (MECG). Methods devised for suppressing the maternal signals in the abdominally recorded leads obviously have to rely on differences in the characteristics of both kinds of signals. In the past various differences have been employed for this purpose such as differences in wave shape, frequency content and source location. The method presented here is based on the latter principle. The electrical sources of the fetal and maternal heart are clearly separated in space. By employing multiple abdominal leads, a linear combination of the recorded signals can be formed which acts effectively as a spatial filter. As such the filtering characteristics depend exclusively on the separation inside the human body (the electrical volume conductor) and, as such, can be expected to be independent of the temporal aspects of the recorded signals. In particular, the obtained filter is unaffected by the occurrence of ectopic beats in either the fetal or the maternal heart. The determination of the required filter (weighting coefficients) is carried out by using the technique of singular value decomposition (SVD) of the data matrix representing the sampled input signals. The filter effectively suppresses the maternal component in the filter output and optimizes the fetal component. The properties of this filter are demonstrated.

Parallel spatial processing: A cure for signal cancellation in adaptive arrays

Abstract: A spatial processing algorithm with parallel structure is presented for the prevention of signal caneellation phenomena in conventional adaptive arrays. This algorithm basically uses a parallel structure with a spatial averaging effect to combat coherent jamming. It results in a spatially smoothed maximum-likelihood estimate of the desired signal when the adaptive beamformer converges. Simulations have been conducted which verify the effectiveness of the proposed structure.

Modified liquid crystal television as a spatial light modulator

Abstract: The spatial light modulator utilizes a modified liquid crystal television for modulating a laser beam so as to produce a transformed hologram image on a film plate which is identical with the video information being fed into the liquid crystal TV. Once a image film has been produced the film is developed and placed back in the system so that the system will now act as a comparator and tracking device. With the reference beam blocked and a new video input fed into the liquid crystal TV the system now will have an output only when the scene on the TV is identical to the scene recorded on the film. Indicator means give a signal output from this comparison that is identical.

Method and apparatus for testing integrated circuit susceptibility to cosmic rays

Abstract: Apparatus and method for testing the susceptibility of an integrated circuit (10) to single event upsets caused by high energy heavy ions, such as are found in cosmic rays. The integrated circuit is mounted in a three axes manipulator (20) by which it is positioned at a desired target point. A light pulse generated by source (32) is filtered and collimated by spatial filter (34), and is focused as a spot on the integrated circuit by optics (60). Preferably, the diameter of the focused spot is 5 microns or less and its wavelength is in the range of 850-1100 nanometers. The susceptibility of any point on the integrated circuit is determined by operating the circuit and monitoring it for errors after it is subjected to the focused light pulse. Timing of the light pulse may also be controlled with respect to integrated circuit operation, to determine susceptibility as a function of time.

Method for improvidng the spatial resolution in an integrated adaptive optics apparatus

Abstract: Apparatus is disclosed for correcting wavefront errors in adaptive optics systems. A modified liquid crystal light valve is used as an integrated wavefront sensing and wavefront control system. A remote reference aberrated wavefront 10 first reflects off the liquid crystal substrate 20 and is then transferred by beam-splitters 14 and 15a, b and reflector 19 to the rear surface ofo the liquid crystal light valve 12. A one-to-one imaging system composed of lenses 70a, b and spatial filter 72 provide a new diffraction-limited optical transfer system which allows full use of the extremely high resolution capabilities of the liquid crystal light valve 12. The wavefront 10, after being imaged by the optical transfer system, is combined with a local reference plane 16, the interference pattern from which strikes the rear surface photoconductor 26. The photoconductor electrons liberated serve to alter the voltage across the liquid crystal at those points where the interference pattern has right maxima. Using a tunable birefringent liquid crystal layer 20, a voltage change causes a commensurate refractive index change in the liquid crystal substrate. These refractive index changes alter the optical path length of particular portions of the aberrated wavefront and the device's inherent negative feedback drives these phase errors to zero, at which point the wavefront 18 is completely corrected. A second laser beam 11 may be sent out of the device, reflecting off the corrective liquid crystal layer and predistorting it in order to correct for atmospheric path disturbances. The present invention provides an adaptive optics correction system possessing extraordinarily high spatial resolution.

Spatial-temporal correlation filter for in-plane distortion invariance.

Abstract: A new space and time correlation filter is described. This filter achieves full in-plane distortion invariance (shift, scale, and rotation) as well as intensity invariance. The use of nonconstant temporal signatures for multiclass recognition is suggested. Initial discrimination results as well as the results demonstrating the ability to estimate the orientation of the input object are provided.

Influence of external mirror on antireflection‐coated phased‐array semiconductor lasers

Abstract: We have analyzed a phased‐array semiconductor laser coupled to an external cavity containing a spatial filter. The results indicate that the external cavity‐spatial filter combination couples the natural supermodes of the array. Thus if a single supermode were to be emitted by the antireflection‐coated facet, the reflected field would couple into many supermodes of the same parity. By self‐consistently modeling the laser‐cavity configuration we show that substantial mode mixing occurs in practical cases and its effect is more pronounced when the coupling coefficient is small.

Generalization of some incoherent spatial filtering techniques.

Abstract: A two-channel incoherent spatial filtering system is generalized by considering two object transparencies, one in each of the two channels. Various special cases result, including two that have previously been described and others not previously given. The system can be either linear in irradiance (the basic incoherent case) or linear in field (the basic coherent case) even though the illumination is incoherent in either case. In particular, we show spatial filtering with the object in one channel and the spatial filter in the other channel.

Discrete spatial filtering with SQUID gradiometers in biomagnetism

Abstract: First‐, second‐, and third‐order gradiometers used in detecting biomagnetic signals are analyzed as spatial filters. Their transfer functions independent of the source to be measured are presented and both the magnitude and phase characteristics of the transfer functions are analyzed. The distortion introduced by the gradiometer can be estimated from these characteristics. In order to treat the signal in that approach, the spatial Fourier transform of a magnetic signal produced by a current dipole at a given distance is discussed.

Efficient master-oscillator-amplifier system utilizing copper vapor laser active elements

Abstract: An investigation was made of the temporal, spatial, and energy characteristics of a system consisting of a master oscillator, a spatial filter, and a power amplifier constructed on the basis of GL-201 and UL-102 sealed commercial laser active elements. The questions of raising the operating efficiency of such a system and obtaining high-power radiation with the diffraction-limited divergence were considered. When GL-201 active elements were used in the oscillator and amplifier, the radiation output power in a beam having a divergence close to the diffraction limit was 30-35 W. This was 3-4 times higher than the output power of the oscillator alone.

Direct-Referencing Automatic Two-Points Reticle-To-Wafer Alignment Using A Projection Column Servo System

Abstract: An improved version of the Philips/ASM-L phase grating alignment system is described. The new system can be used in combination with standard, single-sided telecentric projection lenses, by using a spatial filter external to the lens. In addition, the alignment-detection has been extended to a double system which enables direct control of magnification and die rotation. Long-term frame instability and changes in environmental conditions such as temperature and barometric pressure have no influence on the overlay accuracy. The new external spatial filtering technique gives the same proven benefits as the earlier spatial filtering which was built internal to the lenses. A comparison of this alignment scheme with other methods is given, showing that this new scheme is superior in terms of dependence on external influences.

Grating shearing interferometer with variable shear and fringe orientation.

Abstract: A lateral shear interferometer using Ronchi rulings, spatial filtering, and moire technique is described. Two gratings of different spatial frequency are placed on opposite sides of the focus of the beam under test. First diffraction orders of the first grid after being diffracted at the second grating and subsequent spatial filtering form the carrier frequency lateral shear interferogram. It is visualized using the moire fringe technique which simultaneously provides arbitrary reference fringe orientation and number. The experimental verification of principles is given.

Sampling and reconstruction of a turbulence-distorted wave front

Abstract: This paper examines the problem of linear least-mean-squares reconstruction of a wave front distorted by turbulence described by the Kolmogorov 5/3 law. The wave-front sensors are assumed to be of the Hartmann imaging type, with circular apertures of arbitrary size and location relative to the beam to be reconstructed and having a four-quadrant photodiode detector in the focal plane. The weighting function for x1 and x2 slopes across the sensor aperture is determined, and the sensor signal-to-noise ratio, modulation transfer function, and cross-correlation matrix are calculated. Some examples involving various sensor configurations and the interpolation functions for their outputs are presented as well as the least-squares estimation of a low-order Zernike coefficient.

Anamorphic optical processing of multiple-exposure speckle photographs.

Abstract: Previous methods for analyzing double-exposed speckle photographs provide point-by-point velocity information by using two-dimensional Fourier analysis, or constant velocity contours by using spatial filtering techniques. A new method using an anamorphic optical system to measure one component of the velocity throughout a section of the flow is analyzed and demonstrated. A laser sheet, thin in the x direction and extended in the y direction, is used to probe a line of the photograph of which the anamorphic optical system forms a one-dimensional Fourier transform in the x direction and images the speckle pattern in the y direction for measuring the x-velocity component. This results in curved fringes, which have a local spacing inversely proportional to the x-velocity component at that point. Thus it is possible to measure a velocity component along a selected line in the flow. This differs from spatial filtering techniques that produce contours showing the points where a selected velocity occurs.

The use of wire chambers in structural biology

Abstract: Position-sensitive neutron detectors with high detection efficiency and high spatial resolution are essential for neutron protein crystallography as well as for small-angle neutron scattering on large biological assemblies. Here we report on the performance of two wire chambers recently developed by the Brookhaven Instrumentation Division. Both detectors have a spatial resolution of about 1.3 mm, a detection efficiency of about 80%, and excellent differential and integral linearities. Using small-angle data as an example, we show that the new detectors, furthermore, have excellent temporal stability and can provide difference spectra of unparalleled accuracy. The Biology small-angle spectrometer, because of its location on the constrained reactor floor, has a short secondary flight path of 2 m. High detector resolution is therefore essential to preserve the ability to measure at small angles and to record narrowly spaced diffraction fringes. In addition, we demonstrate in detail that the state-of-the-art detector makes it possible to extend the precision and resolution of a protein single-crystal data set. By measuring beam divergence, wavelength spread, and the crystal mosaic in auxilliary experiments, a set of parameters is obtained for the calculation of a spatial filter for each diffraction spot.

Exposure apparatus including an optical system for aligning a reticle and a wafer

Abstract: A reduction projection type alignment and exposure apparatus having a light source, for alignment, a reticle having at least a first grating, first lens system, a spatial filter disposed around a Fourier spectral plane of the first lens system, second lens system, a wafer having at least a second grating, and a photo-detector for detecting light intensity of superimposed beams appearing on the spatial filter An optical system for light exposure is provided separately from the optical system for alignment which includes the light source for alignment, first and second lens system, spatial filter, etc The light beam generated from the light source for alignment is applied to the reticle at which it is divided into a plurality of difracted light beams by the first grating, and the diffracted light beams are applied through the first lens system, spatial filter and second lens system onto the wafer so that the diffracted light beams are re-diffracted by the second grating, and the re-diffracted light beams are superimposed with the diffracted light beams and the light intensity of the superimposed beams detected by the photo-detector

Optical Parametric Fluorescence In Ti:LiNbO 3 Channel Waveguides

Abstract: The optical parametric fluorescence was observed for the first time in a guided wave structure. In contrast to bulk optics three processes occur: signal and idler photons belong both to guided modes or both to radia-ting substrate modes or one to a guided mode and the other to substrate modes. These processes were investigated experimentally in Ti:LiNb03-channel waveguides. Using a cw-argon laser (λ = 514 nm) or a pulsed dye laser (580 nm < λ < 615 nm) as pump source, the fluorescence was studied as function of the pump wavelength, the pump power, the waveguide temperature and of spatial filtering.

Electromagnetic wave spatial filter with circular polarization and a Cassegrain antenna comprising such a filter

Abstract: A spatial filter is provided for electromagnetic waves with circular polarization comprising three parallel conducting networks, the central network being totally reflecting for a rectilinear polarization of given direction and having a non zero reflection coefficient for a rectilinear polarization having a direction perpendicular to the preceding one.

The Fourier Optical Analysis Of Aberrations In Focused Laser Beams

Abstract: Methods are presented for calculating the radiation intensity in the focal region resulting from the focusing of a laser beam by a spherical surface lens. An approach based upon Fourier optics is employed to include the effects of aberrations as well as diffraction and interference, and digital signal processing techniques are used to speed the calculations. Calculated results are displayed on a digital image processor to aid in visual interpretation. Sample calculations are presented for intensity fields in the transverse and meridional planes of the focal region for circular, gaussian and annular laser beams incident on the lens perpendicularly and at small oblique angles.