Showing papers on "Spatial filter published in 1992"
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03 Jan 1992
TL;DR: An iterative implementation is shown which successfully computes the optical flow for a number of synthetic image sequences and is robust in that it can handle image sequences that are quantified rather coarsely in space and time.
Abstract: Optical flow cannot be computed locally, since only one independent measurement is available from the image sequence at a point, while the flow velocity has two components A second constraint is needed A method for finding the optical flow pattern is presented which assumes that the apparent velocity of the brightness pattern varies smoothly almost everywhere in the image An iterative implementation is shown which successfully computes the optical flow for a number of synthetic image sequences The algorithm is robust in that it can handle image sequences that are quantified rather coarsely in space and time It is also insensitive to quantization of brightness levels and additive noise Examples are included where the assumption of smoothness is violated at singular points or along lines in the image
901 citations
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04 Mar 1992TL;DR: In this paper, a decimated array is used to detect and track frequency hopped radio signals in a dense electromagnetic environment by using cross-spectrum of two signals obtained from antenna elements spaced by one-half an RF wavelength.
Abstract: : We studied optical processing techniques to detect and track frequency hopped radio signals in a dense electromagnetic environment by using cross-spectrum of two signals obtained from antenna elements spaced by one-half an RF wavelength. We decimate the array by retaining only every Mth element and scan the cross-spectrum past the decimated array. We therefore reduce the circuit complexity, but suffer some loss in system performance because we require more photodetector bandwidth to accommodate the scanning action. An extension of the decimated array concept is to decimate the reference waveform in the heterodyne spectrum analyzer. In this case, we generate only 64 optical probes in the Fourier domain instead of the 2048 that are normally required. The advantage is that all of the optical power in the reference beam can be concentrated into the reduced number of optical probes, thereby offsetting some of the intrinsic loss in performance experienced by the cross-spectrum analyzer. (JHD)
276 citations
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TL;DR: In this article, the authors show that extending an electric dipole can reduce the static effect due to confined resistivity anomalies smaller than a dipole length, which is a modification of the sensor characteristics that involves a spatial filtering process.
Abstract: Electromagnetic Array Profiling (EMAP) is an adaptation of magnetotellurics to overcome spatial aliasing effects associated with the sampling of the surface electric field. Undersampling lateral electric field variations can result in misleading geoelectric interpretations of the subsurface, particularly under the common presence of static distortion. In the EMAP field procedure, electric dipoles are positioned end‐to‐end along a continuous survey path; this configuration, in addition to reducing aliasing effects, lends itself to low‐pass filtering of the lateral electric field variations. We show that lengthening an electric dipole can reduce the static effect due to confined resistivity anomalies smaller than a dipole length. This modification of the sensor characteristics involves a spatial filtering process in which the cutoff wavenumber is inversely proportional to the length of the dipole. However, excessively long dipoles may not prove appropriate at high frequencies where the objective is to sense...
176 citations
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TL;DR: Measurements of the point spread function at stages in the visual system that precede the generation of this distortion product were similar to those obtained with simultaneous presentation of the two fringes, implying that the aftereffect of light adaptation is extremely local, no larger than the dimensions of single cones.
173 citations
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07 Feb 1992
TL;DR: In this paper, the authors describe an apparatus used to inspect patterned wafers and other substrates with periodic features for the presence of particles, defects and other aperiodic features in which a spatial filter placed in the Fourier plane is used in combination with either broadband illumination, angularly diverse illumination or both.
Abstract: An apparatus used to inspect patterned wafers and other substrates with periodic features for the presence of particles, defects and other aperiodic features in which a spatial filter placed in the Fourier plane is used in combination with either broadband illumination, angularly diverse illumination or both. In contrast to prior devices that direct light from a single monochromatic source through a pinhole aperture stop, embodiments are describes that illuminate a patterned substrate using (1) a single monochromatic source with a slit-shaped aperture stop for angularly diverse illumination, (2) a single broadband source with a pinhole aperture stop for broadband illumination, (3) a single broadband source with a slit-shaped aperture stop for both broadband and angularly diverse illumination, or (4) multiple sources with an aperture stop for each source for at least angularly diverse illumination. The spatial filters for these illumination systems are characterized by opaque tracks in an otherwise transmissive filter for blocking the elongated bands produced by diffraction from the periodic features on the illuminates substrate. The filter may be made photographically by exposing high contrast film placed in or near the Fourier plane to the diffracted light from a defect and particle frame substrate having only periodic features. Light scattered from the aperiodic features is able to substantially pass through the filter and be imaged onto, a CCD array, vidicon camera or TDI sensor.
110 citations
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TL;DR: In this article, a single ring from a Fabry-Perot transmission fringe pattern was used to create a non-fracting beam and the transverse and axial intensity distributions of this beam were measured and found to be in good agreement with previously existing and newly derived theoretical expressions.
Abstract: An experiment was performed in which a single ring from a Fabry–Perot transmission fringe pattern was used to create a nondiffracting beam. The transverse and axial intensity distributions of this beam were measured and found to be in good agreement with previously existing and newly derived theoretical expressions. The diffraction-free range was found from calculation to be proportional to the cavity finesse and length, and the central-spot radius of the beam was theoretically shown to be proportional to the square root of the wavelength and cavity length.
91 citations
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09 Mar 1992
TL;DR: In this article, an approach for increasing the sensitivity of a high-resolution measurement device is described. But the method is not suitable for high-dimensional data, and it cannot be used in the case of high dimensional data.
Abstract: An approach for increasing the sensitivity of a high resolution measurement device 50 is disclosed. The device includes a laser 52 for generating a probe beam 54 which is tightly focused onto the surface of the sample 58. A detector 66 is provided for monitoring a parameter of the reflected probe beam. In accordance with the subject invention, a spatial filter is provided for reducing the amount of light energy reaching the detector that has been reflected from areas on the surface of the sample beyond the focused spot. The spatial filter includes a relay lens 68 and a blocking member 70 located in the focal plane of the lens. The blocking member 70 includes an aperture 72 dimensioned to block light reflected from the surface of the sample beyond a predetermined distance from the center of the focused spot. In this manner, greater sensitivity to sample characteristics within the highly focused spot is achieved.
84 citations
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TL;DR: In this paper, the Gerchberg-Saxton algorithm was used for phase retrieval from a set of intensity measurements, which is based on application and further minimization by gradient procedures of the functional of reconstruction error in all the planes of recording.
Abstract: The method for phase retrieval from a set of intensity measurements is described and thoroughly investigated. This method is based on application and further minimization by gradient procedures of the functional of reconstruction error in all the planes of recording. The method is shown to result in the well-known Gerchberg–Saxton algorithm [ Optik (Stuttgart)35, 237 ( 1972)] if two intensity measurements (in the pupil and the focal planes of a lens) are used. Numerical simulation revealed the following advantages of the method: the convergence of iterative procedures is improved, the range of reconstruction turns out to be wider, and the stability of procedures with respect to the additive noise in intensity measurements is enhanced. Experimental data confirming the conclusions of numerical simulation are presented. A nonlinear optical spatial filter is also described. The possibility of applying this filter to the problem of recovering phase from intensity measurements in the input and the output planes of the filter is shown.
81 citations
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26 Oct 1992TL;DR: The problem of processing electroencephalographic (EEG) data to monitor the time series of the components of a current dipole source vector at a given location in the head is addressed.
Abstract: The problem of processing electroencephalographic (EEG) data to monitor the time series of the components of a current dipole source vector at a given location in the head is addressed. This is the spatial filtering problem for vector sources in a lossy, three-dimensional, zero delay medium. Dipolar and distributed sources at other than the desired location are cancelled or attenuated with an adaptive linearly constrained minimum variance (LCMV) beamformer. Actual EEG data acquired from a human subject serve as the interference in a case where the desired source is simulated and superimposed on the actual data. It is shown that the LCMV beamformer extracts the desired dipole time series while effectively canceling the subject's interference. >
69 citations
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TL;DR: In this paper, the problem of controlling the superresolution in adaptive beamformers is treated, and a straightforward method that works for both narrowband and broadband arrays is presented, based on forming the blocking matrix in a general sidelobe canceller structure using a spatial FIR filter.
Abstract: The problem of controlling the superresolution in adaptive beamformers is treated. A straightforward method is presented that works for both narrowband and broadband arrays. The method is based on forming the blocking matrix in a general sidelobe canceller structure using a spatial FIR filter. The suppression of this spatial filter and the implicit noise of the leaky least-mean-square algorithm together determine the beamformer. >
65 citations
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29 Jan 1992
TL;DR: In this paper, an apparatus for performing an ophthalmic operation on an eye by photocoagulation using a laser beam while allowing continuous observation of the eye to be treated, includes a light source for producing the laser beam, an observing system having a first inherent optical path including a slit-lamp microscope, a laser optical system with a second inherent path, and a contact lens having predetermined characteristics positioned in front of and contacting the eye.
Abstract: An apparatus for performing an ophthalmic operation on an eye by photocoagulation using a laser beam while allowing continuous observation of the eye to be treated, includes a light source for producing the laser beam, an observing system having a first inherent optical path including a slit-lamp microscope, a laser optical system having a second inherent optical path, means for transmitting the laser beam from the light source to the laser optical system, means for introducing the laser beam from the second optical path of the laser optical system to the first optical path of the observing system, a contact lens having predetermined characteristics positioned in front of and contacting the eye to be treated, first optical means provided in the first inherent optical path of the observing system for adjusting an objective plane observable from the slit-lamp microscope, means for shifting the objective plane to a desired position along the optical path while observing the objective plane, and second optical means provided in the second inherent optical path of the laser optical system for adjusting a focal point, the laser beam being controlled while adjusting the focal point along the second optical path of the laser optical system, wherein the first optical means controls the movement of the objective plane observed from the observing system according to the predetermined characteristics of the contact lens and the second optical means adjusts the focal point to direct the laser beam on the objective plane.
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TL;DR: In this article, a spatial filter is proposed to cope with the striping noise at frequencies of 1/6, 1/3, and 1/2 cycles per line in the Landsat multispectral scan.
Abstract: The removal of striping noise encountered in the Landsat Multispectral Scanner (MSS) images can be generally done by using frequency filtering techniques. Frequency domain filtering has, however, several problems, such as storage limitation of data required for fast Fourier transforms, ringing artifacts appearing at high-intensity discontinuities, and edge effects between adjacent filtered data sets. One way for circumventing the above difficulties is to design a spatial filter to convolve with the images. Because it is known that the striping always appears at frequencies of 1/6, 1/3, and 1/2 cycles per line, it is possible to design a simple one-dimensional spatial filter to take advantage of this a priori knowledge to cope with the above problems.
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TL;DR: The outlines of a model for edge finding in human vision are proposed, where two-component plaid components are processed through cortical, orientationselective filters that are subject to attenuation by forward masking and adaptation, and zero crossings in the combined output are used to determine edge locations.
Abstract: The experiments examined the perceived spatial structure of plaid patterns, composed of two or three sinusoidal gratings of the same spatial frequency, superimposed at different orientations. Perceived structure corresponded well with the pattern of zero crossings in the output of a circular spatial filter applied to the image. This lends some support to Marr & Hildreth’s ( Proc. R. Soc. Lond . B 207, 187 (1980)) theory of edge detection as a model for human vision, but with a very different implementation. The perceived structure of two-component plaids was distorted by prior exposure to a masking or adapting grating, in a way that was perceptually equivalent to reducing the contrast of one of the plaid components. This was confirmed by finding that the plaid distortion could be nulled by increasing the contrast of the masked or adapted component. A corresponding reduction of perceived contrast for single gratings was observed after adaptation and in some masking conditions. I propose the outlines of a model for edge finding in human vision. The plaid components are processed through cortical, orientationselective filters that are subject to attenuation by forward masking and adaptation. The outputs of these oriented filters are then linearly summed to emulate circular filtering, and zero crossings (zcs) in the combined output are used to determine edge locations. Masking or adapting to a grating attenuates some oriented filters more than others, and although this changes only the effective contrast of the components, it results in a geometric distortion at the zc level after different filters have been combined. The orientation of zcs may not correspond at all with the orientation of Fourier components, but they are correctly predicted by this two-stage model. The oriented filters are not ‘orientation detectors’, but are precursors to a more subtle stage that locates and represents spatial features.
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TL;DR: In this paper, the authors applied the concept of a synthetic acousto-optic hologram to convert a Gaussian laser beam into a partially coherent anisotropic Gaussian Schell-model (AGSM) beam.
Abstract: The recently introduced concept of a synthetic acousto-optic hologram [ J. Appl. Phys.67, 49 ( 1990)] is applied to convert a Gaussian laser beam into a partially coherent anisotropic Gaussian Schell-model (AGSM) beam. Real-time reconfigurability of the coherence properties is achieved by this technique, which features scattering of the laser beam by an electronically synthesized, digitally phase-modulated volume grating that propagates in an acousto-optic Bragg cell. The coherence and intensity distributions of the fields obtained by different types of phase modulation are investigated theoretically. We demonstrate some particularly interesting AGSM sources and fields: a secondary elliptical AGSM source with a circularly symmetric far-field intensity distribution and an AGSM field that retains the eccentricity of its intensity profile in the propagation through any centrosymmetric (paraxial) optical system.
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TL;DR: A new type of Nd:YAG laser is described that uses two spatial filters that are placed against the mirrors in the focal planes of a converging lens (Fourier planes) to obtain a uniphase beam with a flat spatial profile and minimal beam divergence in the free-running regime.
Abstract: A new type of Nd:YAG laser is described that uses two spatial filters that are placed against the mirrors in the focal planes of a converging lens (Fourier planes). With the appropriate filters, we experimentally obtained a uniphase beam with a flat spatial profile and minimal beam divergence in the free-running regime.
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TL;DR: In this article, a CCD camera reads holographic interference fringe pattern and its signal is transferred to the LCTV-SLM by video signal line, a spatial filtering technique is applied to remove zero-order diffracted light on reconstruction.
Abstract: A matrix-addressed liquid crystal display device like LCTV-SLMs is one of the important devices for our electro-holography system. In this system, a CCD camera reads holographic interference fringe pattern and its signal is transferred to the LCTV-SLM by video signal line. The LCTV-SLM works as a dynamic holography device. A spatial filtering technique is applied to remove zero-order diffracted light on reconstruction. A comparison between amplitude-type and phase-type holograms and reconstruction by white light are described. The possibility of fine-pixel LCDs are also discussed.
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TL;DR: A Fourier optics analysis for a recently developed lateral shearing interferometry-coherent gradient sensing-is presented and several examples demonstrating the applicability of the method to quasistatic and dynamic crack-growth problems is presented.
Abstract: A Fourier optics analysis for a recently developed lateral shearing interferometry-coherent gradient sensing-is presented. The governing equations for the method are explicitly derived. The method of coherent gradient sensing is particularly suitable for investigating the mechanics of fracture of transparent and opaque solids. Several examples demonstrating the applicability of the method to quasistatic and dynamic crack-growth problems is presented.
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21 May 1992TL;DR: In this paper, a light source, a condenser lens, a nonlinear optical crystal element, and a pair of reflecting means for constituting a laser resonator are considered.
Abstract: A laser light generating apparatus comprises a light source for generating an exciting light beam, a condenser lens for converging the exciting light beam from the light source, a laser medium for generating a laser light of first wavelength by the radiation of the exciting light beam supplied thereto through the condenser lens, a nonlinear optical crystal element for generating a second laser light by the radiation of the first laser light generated from the laser medium, a pair of reflecting means for constituting a laser resonator together with at least the laser medium and the nonlinear optical crystal element, a deflecting element for deflecting an optical axis of the second laser light emitted from reflecting means provided at the emitting side of the pair of reflecting means, detecting means for detecting a part of a light beam output from reflecting means disposed at the output side of the pair of reflecting means, control means for controlling the light source on the basis of a detected output from the detecting means, a base on which there are disposed at least the light source, the condenser lens, the laser medium, the nonlinear optical crystal element, the deflecting element and the detecting means and a housing for housing therein at least the light source, the condenser lens, the laser medium, the nonlinear optical crystal element, the deflecting element, the detecting means and the base.
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12 May 1992
TL;DR: In this paper, a planar array of individually addressable light valves is used as a spatial filter in an imaged Fourier plane of a diffraction pattern, with valves having a stripe geometry corresponding to positions of members of the diffraction patterns, blocking light from those members.
Abstract: An inspection apparatus for a light diffracting surface employs a planar array of individually addressable light valves for use as a spatial filter in an imaged Fourier plane of a diffraction pattern, with valves having a stripe geometry corresponding to positions of members of the diffraction pattern, blocking light from those members. The remaining valve stripes, i.e. those not blocking light from diffraction order members, are open for transmission of light. Light directed onto the surface, such as a semiconductor wafer, forms elongated curved diffraction orders from repetitive patterns of circuit features. The curved diffraction orders are transformed to linear orders by a Fourier transform lens. The linear diffraction orders from repetitive patterns of circuit features are blocked, while light from non-repetitive features, such as dirt particles or defects is allowed to pass through the light valves to a detector. Patterns of stripes can be recorded corresponding to the repetitive features of different integrated circuits. Different filters may be rapidly switched electronically in synchronization with a beam scanning a patterned surface inspecting different light diffracting patterns in different positions, allowing scattered or diffracted light from non-repetitive features to pass through the filter to a detector. A logical AND combination of two filters may be used so that two regions may be inspected in a single scan of the beam.
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27 Aug 1992
TL;DR: In this article, an optical filter 10 for filtering spatial frequencies from an output of an optical imaging system comprises a substantially transparent, or reflective, optical element 12 having a plurality of light refracting or reflecting, segments 18 arranged on a surface thereof.
Abstract: An optical filter 10 for filtering spatial frequencies from an output of an optical imaging system comprises a substantially transparent, or reflective, optical element 12 having a plurality of light refracting, or reflecting, segments 18 arranged on a surface thereof. Each of the plurality of segments 18 is provided with a predetermined angle of refraction, or reflection, distinct from adjacently located segments 18, wherein the different angles of refraction, or reflection, and the arrangement of the segments 18 on the optical element 12, are determined based on a desired spatial frequency transfer function for the filter 10. The optical spatial filter 10 reduces errors in optically formed images by accurately confining a broadened Point Spread Function (PSF) to a desired spot size.
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TL;DR: Significant improvement in the signal-to-noise ratio has been shown over conventional linear or averaging filters when using spatial and adaptive filtering, both when used separately and when used together.
Abstract: A comparison of signal-to-noise ratios and rise times was performed on several myoelectric filters used for muscle-force estimation and prosthesis control. Linear, averaging, and adaptive filters were compared using single as well as multiple electrode pairs (spatial filtering). The filters were matched for having the same rise time (0-95%) and the signal-to-noise ratios were measured off-line using the same myoelectric signal recording. The linear filter was a low-pass filter with a time constant of 80 ms. The averaging filter had an averaging time of 250 ms. The adaptive filter was the same as is used in the Utah Artificial Arm. The adaptive filter varied its time constant according to the rate of change of the signal mean. If the rate was high, the time constant was set low. If the rate was low, the time constant was set high. Spatial filtering is where the myoelectric signals from four cutaneous sites over the same muscle were summed, that is, spatially filtered, and the resultant signal was smoothed by the linear, averaging, or adaptive filter. Significant improvement in the signal-to-noise ratio has been shown over conventional linear or averaging filters when using spatial and adaptive filtering, both when used separately and when used together.
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17 Dec 1992TL;DR: In this article, a color video display system using a white light source and attaining increased brightness is disclosed, where the brightness can be increased in both temporal and spatial filtering systems.
Abstract: A color video display system using a white light source and attaining increased brightness is disclosed. The brightness can be increased in both temporal and spatial filtering systems. In a temporal system, a portion of the total illumination time is allocated to white light to raise the overall brightness. In a spatial system, part (26) of the illuminated area (18) is allocated to white light. The parts (20, 22, 24, 26) could be allocated as regions on a spatial light modulator (18), or a spatial light modulator could be dedicated to that area to use white light. Control means determine a minimum brightness level and operate the spatial light modulator to achieve the minimum brightness during projection of the white light.
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TL;DR: Results obtained using a fine-resolution, three-dimensional imaging method that consists of flood illuminating an extended object with a laser beam and recording the scattered light as the laser frequency is varied are presented.
Abstract: Experimental results obtained using a fine-resolution, three-dimensional imaging method are presented. The method consists of flood illuminating an extended object with a laser beam and recording the scattered light as the laser frequency is varied. An image is recovered by three-dimensional Fourier transformation of the recorded data. For the results presented here, a tunable dye-laser source is used, and the obtained range resolution is 287 μm.
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10 Jan 1992
TL;DR: In this paper, the beam expansion optics magnifies the beam with each pass through the resonator and is accomplished with a negative lens at one end of the resonators and a telescope at the other end, producing a diverging beam through a laser medium in one direction and recollimating the beam in the reverse direction.
Abstract: An unstable laser with improved beam quality has a laser medium and a pair of crossed Porro prism end reflectors positioned to form a resonator defining a light path through the laser medium. Beam expansion optics magnify the beam with each pass through the resonator and is accomplished with a negative lens at one end of the resonator and a telescope at the other end. The negative lens produces a diverging beam through a laser medium in one direction and the telescope recollimates the beam in the reverse direction. Radially varying birefringent compensation is accomplished in the resonator with a crystalline waveplate. Output coupling is achieved through the use of the waveplate in conjunction with polarizer optics.
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17 Jul 1992
TL;DR: In this paper, a self-referencing Mach-Zehnder interferometer is proposed for accurately measuring laser wavefronts over a broad wavelength range (for example, 600 nm to 900 nm).
Abstract: A self-referencing Mach-Zehnder interferometer for accurately measuring laser wavefronts over a broad wavelength range (for example, 600 nm to 900 nm). The apparatus directs a reference portion of an input beam to a reference arm and a measurement portion of the input beam to a measurement arm, recombines the output beams from the reference and measurement arms, and registers the resulting interference pattern ("first" interferogram) at a first detector. Optionally, subportions of the measurement portion are diverted to second and third detectors, which respectively register intensity and interferogram signals which can be processed to reduce the first interferogram's sensitivity to input noise. The reference arm includes a spatial filter producing a high quality spherical beam from the reference portion, a tilted wedge plate compensating for off-axis aberrations in the spatial filter output, and mirror collimating the radiation transmitted through the tilted wedge plate. The apparatus includes a thermally and mechanically stable baseplate which supports all reference arm optics, or at least the spatial filter, tilted wedge plate, and the collimator. The tilted wedge plate is mounted adjustably with respect to the spatial filter and collimator, so that it can be maintained in an orientation in which it does not introduce significant wave front errors into the beam propagating through the reference arm. The apparatus is polarization insensitive and has an equal path length configuration enabling measurement of radiation from broadband as well as closely spaced laser line sources.
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23 Jun 1992TL;DR: In this article, a foreign particle inspection apparatus includes a detection optical system (4) for condensing scattered light generated by slant illumination (2) by an optical system with a NA of more than 0.4 from the rear side of a sample using a transparent or semitransparent substrate having an opaque circuit pattern.
Abstract: A foreign particle inspection apparatus includes a detection optical system (4) for condensing scattered light generated by slant illumination (2) by an optical system (41) with a NA of more than 0.4 from the rear side of a sample using a transparent or semitransparent substrate having an opaque circuit pattern. The circuit pattern, such as a reticle, etc., has a phase shift film for improving the patterning resolution, for shielding diffracted light from the circuit pattern by a spatial filter (44) mounted on the Fourier transform plane, and for forming images on a detector (51). A circuit (113) is also provided for correcting detected values of the detector according to uneven illumination, and a circuit for obtaining the added value of detected values of 2 by 2 pixels. A circuit (114) is provided for obtaining the maximum value of four added values which are shifted pixel by pixel in the four directions around each detector pixel. A circuit (112) is provided for storing the detected result in a memory where the substrate sample is divided into blocks every several hundreds pixels. By this arrangement small foreign particles of the order of submicrons adhered on the substrate can be separated and detected easily and stably from the circuit pattern principally using a simple optical structure.
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13 Aug 1992TL;DR: In this paper, a monolens optical detection system for on-column optical detection with capillary separation columns is presented. But the method is not suitable for the detection of capillary separations.
Abstract: On-column optical detection apparatus and method for use with capillary separation columns are shown. The apparatus and method minimize the adverse effects of light scattered from the walls of the column, thereby improving detection sensitivity and providing greater dynamic range. Light from a conventional source is focussed onto a column. Spatial filter means are positioned in front of the column to prevent at least some light from striking the column wall. Another spatial filter is positioned in front of a light collecting means which gathers light emanating from the column. In a specific embodiment, a unique monolens design is shown comprising collecting and focussing lens formed as an integral unit with a bore for a capillary column.
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TL;DR: An iterative method for generating holograms on spatial light modulators is based on measuring the reconstructed complex image and on-line correction of the hologram and may become a useful tool for applications such as adaptive optics and reconfigurable interconnection networks.
Abstract: An iterative method for generating holograms on spatial light modulators is based on measuring the reconstructed complex image and on-line correction of the hologram. Apart from recording complex amplitude distributions, the new procedure may become a useful tool for applications such as adaptive optics and reconfigurable interconnection networks.
01 Jan 1992
TL;DR: The focus in this paper is on the application to aircraft flight flutter testing of a spatial filtering technique used to uncouple multiple response measurements into modal coordinate responses.
Abstract: Ongoing development of an automated system for monitoring the frequency and damping of time varying structural systems is discussed. A spatial filtering technique called the discrete modal filter is used to uncouple multiple response measurements into modal coordinate responses. Since the modal coordinate responses are the responses of single-degree-of-freedom systems, the frequency and damping may be accurately estimated with short data records. An on-line monitoring system is discussed which may be useful in a variety of applications. The focus in this paper is on the application to aircraft flight flutter testing.
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TL;DR: In this article, the authors investigated the noise characteristics and low-light-level imaging capabilities of a stimulated Raman amplifier and found that approximately 1−4 photons per spatial mode are needed to produce images with a signal-to-noise ratio of 1.
Abstract: We have investigated the noise characteristics and the low-light-level imaging capabilities of a stimulated Raman amplifier. Diffraction-limited Stokes beams as well as Stokes beams carrying spatial structure have been amplified with a collimated pump with a Fresnel number in the range 3–17. Both direct imaging and Fourier-transform imaging of the Stokes signal through the Raman amplifier were used. Results indicate that approximately 1–4 photons per spatial mode are needed to produce images with a signal-to-noise ratio of 1. Amplification of the Fourier transform of the object effectively provides spatial filtering of the noise and gives the best signal-to-noise ratio at the lowest input levels for our experiments.