Showing papers on "Spatial filter published in 1991"

White light enhanced color field sequential projection

Abstract: A method for increasing the brightness of a scene in a color projection system 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 lights modulator (18), or a spatial light modulator could be dedicated to that area to use white light

Elimination of systematic error in subpixel accuracy centroid estimation [also Letter 34(11)3347-3348(Nov1995)]

Abstract: An analysis of the properties of the centroid method for subpixel accuracy image feature location is presented. This method is free of systematic error if the maximum spatial frequency of the image incident on the image sensor is less than the sensor's sampling frequency. This can be achieved by using a lens aperture setting such that the modulation transfer function cut-off frequency due to diffraction is appropriately small. Both simulation and experimental tests of this prediction are presented for the case of the location of the center lines of the images of projected light stripes in a triangulation-based three-dimensional shape measurement system.

Propagation invariance and self-imaging in variable-coherence optics

Abstract: The concept of propagation invariance in partially coherent optics is introduced. Explicit expressions are given for the cross-spectral density and the angular correlation function (cross-angular spectrum) characterizing a class of fields that are propagation invariant in the sense that their correlation properties in the space-frequency domain are exactly the same in every transverse plane. The so-called diffraction-free beams are shown to be members of this new, wider class of wave fields, which itself is a subset of a generalized class of partially coherent self-imaging fields. The existence of partially coherent propagation-invariant fields with a sharp correlation peak is verified experimentally by considering radiation from a planar J0 Bessel-correlated source.

Real-time holography using the high-resolution LCTV-SLM

Abstract: The real-time holography system with a liquid-crystal television spatial light modular (LCTV- SLM) is presented. In the system, the LCTV-SLM is used as a phase hologram device. On the recording, the in-line holography method is used to reduce the spatial frequency of the fringe pattern of the hologram. The CCD camera detects the fringe pattern, and the fringe-pattern signal is transferred to the LCTV-SLM through the video-signal line. On the reconstruction process, the LCTV-SLM is illuminated by an He-Ne laser (633nm) and the spatial filtering technique is used to remove the zero and the higher-order diffracted light. Thus the holographic image of the real object can be observed continuously in real-time.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Spatial filtering for depth of focus and resolution enhancement in optical lithography

Abstract: Spatial filtering at the lens pupil can achieve amplitude superposition for multiple images along the light axis and phase control between them. This enhances depth of focus while maintaining high resolution capability in optical lithography. Compared to conventional methods, three times larger depth of focus is expected for hole patterns with 20% improved resolution limit. For general pattern features, a 1.5–1.7 times larger depth of focus is expected at Rayleigh’s resolution limit.

A Model-Based

Abstract: A model-based approach to improve the spatial resolution of a buried inhomogeneous object using an active microwave imaging system is discussed. First, by numerically solving an integral equation representing the ideal scattered field of the object, we generate the model. Next using this model we develop an optimal spatial filter to process the measured field. Finally, the so-called inverse problem is solved using an imaging algorithm based on diffraction tomography and Fourier transforms. Experimental results indicate a reasonable perfor- mance of the approach.

Full-complex spatial filtering with a phase mostly DMD

Abstract: A new technique for implementing fully complex spatial filters with a phase mostly deformable mirror device (DMD) light modulator is described. The technique combines two or more phase-modulating flexure-beam mirror elements into a single macro-pixel. By manipulating the relative phases of the individual sub-pixels within the macro-pixel, the amplitude and the phase can be independently set for this filtering element. The combination of DMD sub-pixels into a macro-pixel is accomplished by adjusting the optical system resolution, thereby trading off system space bandwidth product for increased filtering flexibility. Volume in the larger dimensioned space, space bandwidth-complex axes count, is conserved. Experimental results are presented mapping out the coupled amplitude and phase characteristics of the individual flexure-beam DMD elements and demonstrating the independent control of amplitude and phase in a combined macro-pixel. This technique is generally applicable for implementation with any type of phase modulating light modulator.

Phase-space beam summation for time-dependent radiation from large apertures: continuous parameterization

Abstract: We extend the study of alternative phase-space formulations of time-harmonic radiation from extended but truncated aperture source distributions to the time domain. Included are nonwindowed continuous forms spanning the space–time (configuration) domains, wave-number-frequency (spectrum) domains, and windowed (local beam-type) continuous forms. Synthesized in the frequency domain by nonwindowed or windowed Fourier transforms, field synthesis in the time domain involves nonwindowed or windowed radon transforms combined with the theory of analytic signals. Because the properties of suitable wave objects used in the analysis and synthesis of the field are strongly tied to relevant configurational and spectral parameters, the incorporation of these aspects into the various formats is referred to as phase-space parameterization. In the continuous parameterization the resulting time-dependent field radiated from the aperture is expressed as a superposition of pulsed beams whose phase-space parameters are their initiation time, initiation location, and initial direction. The properties of these formulations are discussed in detail, within a rigorous format and also with more physically transparent asymptotic approximations. As in the time-harmonic case, major stress is placed on localization in the phase space, which is achievable with various alternatives, and on the corresponding implications. Specific examples include analytic δ windows that yield as propagators complex-source pulsed beams, and numerical implementation of field synthesis for nonfocused and focused pulsed aperture distributions.

A flexible high resolution tactile imager with video signal output

Abstract: A high-resolution and sheetlike form imaging tactile sensor with video signal output has been developed. The sensor has a 64*64 array of sensing elements on a flexible PC board with 1-mm spatial resolution. Since the sensor outputs pressure distribution as a video signal, a real-time tactile image can be observed using a TV monitor. As the sensor is made up of a sheet of pressure-sensitive conductive rubber and stripe electrodes, there are undesirable currents passing between sensing elements. Thus these undesirable current passes must be cut within a short scanning period (500 ns for each element). The authors used a ground potential method and proved it useful under such a high-speed scanning condition. The properties of pressure-sensitive conductive rubber, including hysteresis and creep effects, are presented. A spatial filtering effect of an elastic cover for a tactile sensor is analyzed. >

Measurement of optical flow by a generalized gradient scheme

Abstract: We describe a procedure for recovering the global, two-dimensional velocity of translation of an image by incorporating spatial filtering, and, optionally, temporal filtering, into a scheme that employs a novel and generalized version of the gradient algorithm of motion detection. Motion within a patch is analyzed in parallel by six different spatiotemporal filters derived from two linearly independent spatiotemporal kernels. Advantageous features of this scheme are that (a) the average velocity within the patch is determined in a single step and without recourse to constraints imposed by neighboring calculations or assumptions about the global structure of the pattern; (b) there is no need to impose a smoothness constraint on the optical flow; (c) the need to compute spatial derivatives directly is obviated by our combining the outputs of the kernel filters with the outputs of other filters whose weighting functions are partial derivatives, in space, with respect to the first set; (d) there is no need to compute second derivatives, and thus the scheme is potentially more resistant to noise than certain other schemes; (e) the spatiotemporal kernels can be chosen almost completely arbitrarily and can therefore be tailored to maximize signal reliability; and (f) the measurement of velocity can be made as local or as global as desired by altering the size of the patch that is viewed by the filters. The validity of the scheme is demonstrated on a computer by application to a variety of real, moving images.

Performance evaluation of minimum average correlation energy filters

Abstract: A new SDF type correlation filter referred to as the minimum average correlation energy (MACE) filter has been recently described in the literature. In this paper, we experimentally address the distortion tolerance and noise properties of this filter. The MACE filter has attractive properties that include: easily detectable peaks, distortion invariance, simplified training set selection, solutions to input bias effects, performance in noise and real background clutter, and less clutter with its reduced number of training set images. Each of these properties is investigated in detail in this paper.

Ultrasonic holographic imaging apparatus having an improved optical reconstruction system

Abstract: A preferred embodiment of the optical reconstruction assembly 50 in which the assembly 50 includes an assembly housing 52 that is a unitary unit for supporting a spatial filter 72 and a light source 74 in a single plane at the optical length "L" from a collimating lens 86. A liquid container 88 is mounted to the assembly housing 52 containing the holographic liquid 90. The light source 74 and the spatial filter 72 are spaced a fixed distance "A" which is less than the optical diameter "D" of the collimating lens 86.

Method and apparatus for homogenizing a collimated light beam

Abstract: A method and apparatus for smoothing and homogenising the cross-sectional intensity distribution of the output from an excimer laser for improved application of excimer lasers to optical laser surgery to the cornea. The laser beam is focused by means of a cylindrical lens or mirror and then filtered using a narrow gap spatial filter to smooth the intensity distribution by removing high spatial frequencies. The resulting output beam may be collimated by means of a second cylindrical lens or mirror for application to further optical processes.

Operation of five individual diode lasers as a coherent ensemble by fiber coupling into an external cavity

Abstract: Five diode lasers that each have one facet antireflection coated were fiber coupled into an external cavity and operated as a coherent ensemble by the use of a spatial filter placed at the Fourier plane of the cavity. The resulting output was shown to be in a single spectral line with a linewidth less than the instrumental resolution (7.5 MHz) of the Fabry–Perot spectrum analyzer used. The effect of rotating the polarization of the radiation inside one of the fibers is described.

Noise reduction using adaptive spatial filtering in photorefractive two-beam coupling

Abstract: We present an optical signal-processing technique for additive noise reduction that uses the noisy signal and a Gaussian reference beam to produce an adaptive Wiener filter. We experimentally demonstrate an improvement from 1 to 8 in the signal-to-noise ratio by using nonlinear gain in two-beam coupling in barium titanate to transmit 50% of the signal and 6% of the noise.

Time reconstructions in light-in-flight recording by holography.

Abstract: When light-in-flight recording by holography is used two different sorts of apparent distortion of the wavefronts exist. The first distortion is common to all types of ultrafast gating viewing system and like relativistic phenomena it is caused by the limited speed of light used for observation. The second distortion is produced by the holographic process itself and is caused by the limited speed of the light pulse used as a reference beam. By using the second distortion to compensate for the first it is possible to manipulate or eliminate apparent wavefront tilts or distortions so that measurement of the 3-D shape of wavefronts or other objects is facilitated. A reconstruction beam that is the conjugate of the reference beam results in three interesting effects, one of which is the reemission of the recorded pulse.

Active microwave imaging: a model-based approach

Abstract: A model-based approach that improves the spatial resolution of a buried inhomogeneous object using an active microwave imaging system is discussed. Algorithms are developed for quasi-real-time processing using a microwave camera specifically constructed for civil engineering applications using reflection mode imaging. The model is generated by numerically solving an integral equation representing the ideal scattered field of the object. The model is used to develop an optimal spatial filter to process the measured field. Finally, the inverse problem is solved using an imaging algorithm based on diffraction tomography and Fourier transforms. Experimental results indicate a reasonable performance of the approach. >

Image processing using the HSI color space

Abstract: Traditional interfield spatial filtering in the YIQ color space is compared with new spatial filtering techniques developed using the HSI color space. Results based on an extended version of the CCIR impairment scale are presented. The present experiments are based on renderings of computation grids obtained from the MSU/NFS Engineering Research Center for Computational Field Simulation. It is shown that there are three significant points to be considered when performing spatial filtering in the HSI color space for the purpose of flicker reduction. First, sharpness degradations are normally considered more objectionable than flicker degradations. Second, luminance and intensity filtering are visually equivalent. Third, saturation processing can be used to introduce appropriate colors into the gray scale fringe regions that are generated by intensity processing, but hue processing must be used in conjunction with this to avoid the red fringe effect. Therefore, the marginal improvement in image quality does not normally justify the additional computational effort required. >

Operation of a coherent ensemble of five diode lasers in an external cavity

Abstract: A theoretical analysis of the coherent external cavity semiconductor laser ensemble is presented. The theoretical analysis lends insight to the limits of stability of the coherent ensemble for different spatial filter designs and indicates differences in operating behavior expected for different current drives to the gain elements. The theoretical analysis obtained from considerations of both spectral control and stability predicts an experimental observation that spatial filter duty cycles of 30-40% optimize system behavior. The measurements demonstrate the effects on both the intensity and spectrum of the laser output that results from changing the DC and pulse drive currents to the elements in the array. With current pulses applied to only two of the elements, single-spectral-mode pulse output is possible with a linewidth of less than 7.5 MHz. Approximately 100% modulation depth pulses in a single spectral mode are reported, with measured linewidths of approximately 9 MHz compared to the 7.5 MHz instrument resolution. >

Modal discrimination in leaky-mode (antiguided) arrays (diode lasers)

Abstract: Presents on the fabrication of a ten-element antiguided (leaky-mode) diode laser array that operates in an in-phase mode with a diffraction-limited far-field pattern at drive currents up to 8.7 times the threshold current (pulsed) and an output power of 400 mW (both facets). This device evidences good discrimination against the out-of-phase leaky mode without the benefit of a 'Talbot' spatial filter, relying instead on mode-sensitive interelement losses in an overlying GaAs layer. This device has been studied using two numerical models. Results from the more realistic of the two models predict in-phase-mode operation for certain restricted ranges of waveguide dielectric constant, in agreement with experiment. This behavior is shown to result from resonances between the waveguide modes and the modes of the active region. These results suggest that improved yields would result from a different type of leaky-mode array employing smaller, rectangularly shaped waveguides. >

Digital phase-encoded inverse filter for optical pattern recognition.

Abstract: Optical pattern recognition can profit from the progress in coding theory and technology that has been made in digital holography. The calculation of a phase-encoded inverse filter is described. This filter allows compromises between discrimination capability and diffraction efficiency. Phase quantization facilitates the materialization of the filter.

Optical phase-only spatial filter

Abstract: An optical, phase-only spatial filter for filtering spatial frequencies from the output of an optical imaging system is described which comprises an optical element comprised of a pattern of transparent (or reflective, in the case of a mirror) segments, each of which produces an optical path length generally different from that of other segments, and the design of which and the pattern they form dictated by the desired system optical transfer function.

Image processing for a scanning acoustic microscope that measures amplitude and phase

Abstract: Several image-processing techniques for a low-frequency (3 to 10 MHz) scanning acoustic microscope (SAM) that measures amplitude and phase are described. This microscope is capable of measuring both the amplitude and phase of the reflected and transmitted signals, in contrast with most earlier implementations that only measure the amplitude. By measuring phase, the authors can carry out quantitative nondestructive evaluation (NDE) and image processing that cannot be done with amplitude or phase alone. The effective 2-D point spread function of the microscope is modified by spatial filtering of the digitized complex images. In various images, the transverse resolution is improved by about 20%, aberration of images of subsurface features is corrected, and surface features are numerically defocused. The last process is used to remove the obscuring effect of surface roughness from images of subsurface features. >

Spatial filter type speed measuring apparatus

Abstract: A speed measuring apparatus includes a light projecting unit having a light source for emitting light, a projection optical system for projecting the light from the light source onto a relatively moving object and a first polarizer included in a part of the projection optical system for allowing the passage of polarized light from the light source as projected light. A light receiving unit includes a second polarizer allowing the passage of polarized light in a direction at right angles to the direction of polarization of the projected light reflected from the relatively moving object and a spatial filter detector for taking out a predetermined spatial frequency of the light passed through the second polarizer and outputting an electrical signal representing the frequency. The relative speed of the relatively moving object is calculated on the basis of the signal from the spatial filter detector.

Ultrasound imaging through time-domain diffraction tomography

Abstract: Time-domain diffraction tomography, a technique for imaging with acoustic (and other) fields in which a medium parameter, such as density, can be mapped from scatter data collected from one pulse, is discussed. When Born approximations hold, the technique provides an exact inversion of the acoustical scattering equations. Computer simulation of the time-domain diffraction tomography equations indicates that under ideal conditions, and when the Born approximation is valid, the method can reconstruct maps of parameter variations. However, when data are collected from an incident pulse whose bandwidth is limited, the reconstruction is no longer perfect. A simple question is derived that characterizes the performance of time-domain diffraction tomography, and the limitations are explained as the effect of a spatial filter that eliminates some of the spatial frequencies. Relations between the object parameters, pulse bandwidth, and reconstruction accuracy are investigated with numerical experiments. >

Application of spatial filters to DOA estimation of coherent sources

Abstract: Problems associated with spatial smoothing which relate to the rate of decorrelation of correlated sources are discussed. Although a small improvement is attained, if sources have small differences in their directions-of-arrival (DOAs), they will remain highly correlated and the problem is not significantly diminished. A method which is based on the spatial filtering of an array vector is proposed. A procedure is developed that significantly reduces the correlations among sources and results in a nearly diagonal source covariance matrix. The design of the proposed spatial filters requires preliminary estimates of DOAs. This requirement, however, is not a drawback to this method, as is shown. Simulation studies are carried out for a 12-element linear array with quarter-wavelength element spacing. In all cases, 100 snapshots are used to estimate the array covariance matrix. In each case, 10 different simulations, with different noise sequences, are performed to present the variability of the resultant spectrum. The number of sources is set to 3 with -60 degrees , 30 degrees , and 40 degrees DOAs. >

Spatial filter for an image processing system

Abstract: An medical imaging system includes a photodetector array which converts an X-ray image into a series of picture elements arranged in rows and columns of a two dimensional matrix. The picture elements are read from the array on a column by column basis. A spatial filter for the picture elements includes a first filter that applies one filter function to the pixels in each column of the image. The partially filtered pixels are stored in a first memory and then read therefrom row by row in a field interlaced order. The rows of picture elements are sent to a second filter that applies another filter function to each row. The fully filtered picture elements from the second filter either are stored or converted to a video signal for display.

Fabrication of phase structures with continuous and multilevel profile for diffraction optics

Abstract: This paper presents the investigation results of a new photolithographic fabrication method of highly effective diffractive optical elements (DOE). The method, unlike the well-known multilevel one, does not require a set of masks prepared and aligned. It is demonstrated that the application of half-tone image binarization technique allows one to fabricate a DOE with continuous phase profile and high diffractive efficiency, which can be achieved with a single raster mask and an optical projection system of a photolithography setup used as low-pass spatial filter. Using the algorithm of pulse-width modulation experimental samples of DOE aberration correctors and kinoform lens array with the numerical aperture NA equals 0.1 were fabricated. The diffraction efficiency of these elements, fabricated with conventional photolithography equipment, was more than 80%.