Matched Spatial Filter Research Articles

A technique for making holograms easily and for measuring simultaneously the behaviour of particles of different sizes and/or shapes

A new technique for rapidly discriminating shapes and/or sizes of micrometre size particles which are spatially distributed on a large scale has been developed. The technique is based on multiplexed matched spatial filtering, which is a kind of Fourier holographic filtering technique. Using the technique, large view visualization of the spatial distribution and the spatial behaviour of specific particles (e.g. aerosols, allergen particles, red blood cells, etc) can be realized. To make the multiplexed matched spatial filter (MMSF), a new material for hologram recording has been applied. The material is a photoconductor plastic which is processed by a solvent vapour and a corona discharge. The method of hologram recording is a dry process, which processes the material in several minutes at the initial settings of the device. Therefore, the MMSF can be made very easily in a short time. In the research, the discrimination of spatial behaviour of moving particles of different shape and/or size has been carried out by the MMSF made by the photoconductor plastic material.

Adaptive multiple filtering

The method described represents an attractive compromise between the use of a single filter for a number of image distortions and one filter for each image or distortion of an image. The goal of this work is the generation of a number of filters, each of them being able to recognize a number of distortions. One of the main problems of the filter design is its high sensitivity to internal noise of the system, optical aberrations, etc. In this work, the most common case of image distortion invariance has been considered together with system noise invariance. The simulation results indicate the absence of a false alarm and good identification. The filter generation is based on a learning process in an electro-optical pattern recognition system. The genetic algorithm serves as an optimization method. A binary filter has been selected as the spatial filter. The comparison between a traditional matched spatial complex filter and this adaptive binary filter performance indicates the significant benefits of the latter. Statistical tools were used to estimate and compare the significance of the difference between the output average of a rejection and recognition image sets.

Performance of a spatio-temporal matched filter receiver for DS/SSMA communications

A spatio-temporal matched filter receiver for direct-sequence spread-spectrum multiple-access (DS/SSMA) communications with aperiodic random quadriphase spreading sequences is derived, and the system performance is analyzed. It is shown with the method of characteristic functions that the cross-correlation coefficients between the desired user's and the interfering users' spreading sequences tend, in distribution, to independently and identically distributed circularly symmetric complex Gaussian random variables as the processing gain goes to infinity. Based on this Gaussian approximation, the structure of the spatio-temporal matched filter receiver is derived and a bit error rate formula is obtained. Using Monte-Carlo simulations, as well as analytical methods, it is shown that the spatio-temporal matched filter receiver achieves a significant performance improvement over the conventional, temporal, and spatial matched filter receivers by effectively suppressing the multiple access interference.

Optical processing of particle tracking velocimetry under deformeddouble exposure

A new technique which can quantitatively visualize the spatialdistribution of velocity in flows using optical processing of particletracking velocimetry (PTV) under deformed double exposure has been developed.Tracer particles seeded in the flow are irradiated by a sheet of light from anargon-ion laser under deformed double exposure. Under deformed doubleexposure, each locus of the particles displaced in the flow visualizes thedirection and the magnitude of the velocity. The loci of displaced particlesshow the spatial distribution of the flow velocity in a single image frame.The image is taken continuously in real time by a CCD camera. The deformeddouble exposure has been synchronized with the scanning frame of the CCDcamera. The spatial distributions of the loci of the particles have beendiscriminated at certain magnitudes and directions simultaneously andinstantaneously by a multiplexed matched spatial filter (MMSF). The MMSF is aholographic filter which can discriminate the shapes of the loci using opticalcross-correlation. Cross-correlation of the shapes has been calculatedoptically and the correlation peaks which are the discriminated signal appearskew-symmetrically relative to the position of the shapes in eachdiscrimination area. Using this technique, the number and the position ofloci, i.e. spatial distributions of velocity in the flow, are visualized. TheMMSF technique has been tested for synthetic images and experimental images.The synthetic images have the overlapped and deformed loci of particles inflow. The experimental images are loci of the cavity flow with a high Reynoldsnumber. From the results, it has been demonstrated that PTV images which havehigh density tracer particles and deformed streaks can be measured by thistechnique.

On the blind implementation of the ultimate source separators for arbitrary noisy mixtures

Since the beginning of the 1980s the blind second-order (SO) and higher-order (HO) narrow-band (NB) methods of source separation have received a great interest. However, despite this fact, the situations of practical interest for which these blind methods may blindly implement the ultimate ones, such as the spatial matched filter (SMF) or the weighted least square (WLS) source separators, are not still identified for general noisy situations, which may limit the use of these blind methods in operational contexts. Assuming instantaneously mixed NB sources, the purpose of this paper is to enlighten the previous important problem through the analysis of the SO correlation and the FO cross-cumulants at the outputs of the SMF and the WLS separators for arbitrary noisy mixtures of sources.

Designs of optoelectronic trinary signed-digit multiplication by use of joint spatial encodings and optical correlation

Trinary signed-digit (TSD) symbolic-substitution-based (SS-based) optical adders, which were recently proposed, are used as the basic modules for designing highly parallel optical multiplications by use of cascaded optical correlators. The proposed multiplications perform carry-free generation of the multiplication partial products of two words in constant time. Also, three different multiplication designs are presented, and new joint spatial encodings for the TSD numbers are introduced. The proposed joint spatial encodings allow one to reduce the SS computation rules involved in optical multiplication. In addition, the proposed joint spatial encodings increase the space-bandwidth product of the spatial light modulators of the optical system. This increase is achieved by reduction of the numbers of pixels in the joint spatial encodings for the input TSD operands as well as reduction of the number of pixels used in the proposed matched spatial filters for the optical multipliers.

Recognition of color patterns using a photorefractive LiNbO 3:Fe crystal

In this paper, we used a LiNbO 3:Fe crystal to produce a matched spatial filter and realize recognition of color patterns. Our method is simple and sensitive to variation of the shape and color distribution of the color patterns.

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

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)

Contrast enhancement of underwater images with coherent optical image processors

The use of coherent optical processors for improving the quality of degraded underwater images is discussed, and a holographic filter suitable for the enhancement of contrast in underwater images is described. The filter is a modified matched spatial filter, and it performs a nonlinear local contrast enhancement. The theory of the operation of the coherent optical image processor with this new filter is presented and experimentally verified. The removal of the backscattered light from underwater images is demonstrated under laboratory conditions. The results show an improvement in the overall image contrast.

Fiducial markers for increasing the versatility of optical correlation in the measurement of faults on integrated-circuit wafers

A microscope-coherent optical processor is used for the measurement of the registration errors on integrated-circuit wafers. The measurements are obtained from the optical correlation of wafers with reference wafer patterns by use of matched spatial filters. Previously, the intricate pattern of the active circuit area of wafers has been used in the correlation process, and a new matched spatial filter had to be created for each different integrated circuit. Here, the results of using comparatively plain fiducial markers on a wafer for the registration-error measurement are presented, and these show that the measurements can be made independent of the design of the integrated circuit while maintaining the advantages and accuracy of the optical correlation technique.

Cylindrical Liquid Crystal Lens and Its Applications in Optical Pattern Correlation Systems

The optical characteristics of a cylindrical liquid crystal (LC) lens are investigated with voltages applied to LC cells. We show that the focusing performance of cylindrical LC lenses, such as focal length, can be varied by voltage application. With this kind of cylindrical LC lens, optical correlation architectures are achieved for the first time using a holographic matched spatial filter. The influence on correlation results of different reference light distributions from a LC lens under various applied voltages is also discussed.

Tuneable edge enhancement filters for optical correlation

Difference of Gaussian (DOG) filters, a form of wavelet filter, are an extremely useful preprocessing tool for the enhancement of image edge data. The DOG filter is highly tuneable by control of the standard deviations of its constituent Gaussian distributions. These can be used in the frequency plane of optical correlators and implemented in the form of a static non-updateable filter. This article reports on a simple optical technique using photorefractive materials whereby a filter, updateable in real time and very similar to the DOG filter, is implemented by tuning the intensity of the hologram reference wave to give enhanced modulation in a selectable frequency band. This is called a tuneable photorefractive (TPR) filter. The results of the DOG and TPR filters are compared with those of the phase-only filter and classical matched spatial filter with respect to the criteria of signal-to-noise ratio, Horner efficiency and discrimination capability.

Noise robustness of tuneable photo-refractive filters

Noise robustness of tuneable photo-refractive (TPR) filters, which can be implemented for real time pattern recognition, is assessed and compared with phase-only and classical matched spatial filters. The bandpass selectivity of the TPR filters results in sharp correlation peaks even when the input scene signal-to-noise ratio is decreased significantly.

Character recognition by feature extraction using cross-correlation signals from a matched filter

A hybrid pattern recognition system is described. It is based on a matched spatial filter (MSF) synthesized not by conventional template-matching patterns but by feature-extracted reference patterns. Some features are extracted from target objects used to recognize unknown objects. The optimum reference patterns for the filter are selected according to computer simulations. Through more than 30 simulations, four best patterns are chosen for recognition of 10 kinds of digits by the MSF. Recognition is performed not by conventional autocorrelation peaks but by cross-correlation signals because the reference patterns are not wholly the same as the input. The proposed system shows feasibility for feature-extracted pattern recognition using the multiplexed MSF.

Design and fabrication of a miniaturized optical correlator

A miniature Vander Lugt optical correlator has been built to demonstrate this technology for guidance and navigation applications. Using a liquid crystal light valve as the input spatial light modulator, a cemented set of Porro prisms to fold the light path, and a combination holographic matched spatial filter and optical element, this rugged assembly is a prototype seeker head for an Army missile. The correlator is battery operated and mounts in a precision two-axis gimbal. The system correlates on aerial imagery and guides the missile to its preselected ground target. The correlator is 105 mm in diameter and 90 mm long. Including input optics, it weighs 2.3 kg.

Optimal template masks for detecting signals with varying background level

This paper studies the problem of designing templates for detecting signals which will be subject to a varying background level B. An approach based on use of noise-whitening and spatial matched filters shows that templates must be adjusted towards zero mean: a formal mathematical analysis specifies precisely the required forms of the masks. It is shown that such templates are required in several applications - e.g. when objects in images are subject to conditions of variable lighting and shadow, or radar signals are subject to baseline drift. Studiert wird das problem des Musterentwurfs zur Detektion von Signalen, die einer Variation des Hintergrundniveaus B unterworfen sind. Ein Ansatz auf der Grundlage einer Rausch-Dekorrelation und räumlicher Optimalfilter zeigt daß die Muster in Richtung auf einen verschwindenden Mittelwert getrimmt werden müssen: eine formale mathematische Analyse legt die erforderliche Form der Masken der Masken exakt fest. Es wird gezeigt, daß solche Muster in verschiedenen Anwendungen nötig sind - z.B. dann wenn Bildobjekte wechselnden Licht- und Schatten-Bedingungen unterworfen sind oder Radarsignale eine Baseline-Drift erfahren. Cet article étudie le probléme de la conception de gabarits pour la détection de signaux qui seront sujets à un niveau variable B de bruit de fond. Une approche basée sur l'utilisation de filtres de blanchissement de bruit et de filtres adaptés spatialement montre que les gabarits doivent être ajustés à moyenne nulle: une analyse mathématique formelle spécifie précisément les formes requises pour les masques. Nous montrons que de tels gabarits sont requis dans plusieurs applications - p.e. lorsque des objets dans des images sont soumis à des conditions d'éclairage variable et d'ombre, ou lorsque des signaux de radars sont soumis à une dérive de la ligne de base.

Procedure for generating template masks for detecting variable signals

Abstract This paper is concerned with the use of template masks for locating objects and their features in digital images. In an ideal case suitable masks would take the form of spatial matched filters for the chosen object or feature profile. However, in practical situations, not only will noise be present but also the signals themselves may vary. This paper analyses the situation and formulates a procedure for generating suitable masks in such cases. The overall filter can be construed as a special type of spatial matched filter with integral noise-whitening. The method is applied to edge, corner and hole detection, and tradeoff formulae are derived connecting accuracy, sensitivity and computational load, with number of masks as the single free parameter.

Properties of quasi phase-only matched spatial filters

A new type of matched spatial filter referred to as the quasi phase-only filter (QPOF) has been recently introduced as an optical solution for the phase-only filtering concept. The high-efficiency attribute has been found to link the QPOFs and phase-only filters (POFs). Using computer simulation experiments, the properties of QPOFs are discussed in terms of a signal-to-noise ratio, a peak-to-sidelobe ratio, ability to discriminate, and also the general behavior of the output correlation signals.

Spherical particle sizing based on the VanderLugt correlator

We present a flexible architecture for particle sizing based on the VanderLugt correlator with a multiplexed matched spatial filter. Theoretical and experimenal data obtained using opaque spherical particle projections demonstrate the potential uses and simplicity of the architecture. The system is capable of handling spherical and nonspherical particles and shows potential for handling high particle concentrations.

Amplitude compensated matched filters using circular harmonic expansion and a Mellin transform

The new techniques of amplitude compensated matched filtering using circular harmonic expansion and a Mellin transform are presented. These techniques yield much better discrimination and sharper autocorrelation peaks compared with the classical matched spatial filtering. Aside from these advantages, the amplitude compensated circular harmonic expansion matched filtering can also yield rotation invariance, and the amplitude compensated Mellin transform matched filtering can yield scale invariance. By computer simulation, we have verified the unique advantages of the new methods. We also provide 3-D graphs of autocorrelation and cross correlation.