Showing papers in "Optical Engineering in 1990"

ESPRIT-estimation of signal parameters via rotational invariance techniques

Abstract: High-resolution signal parameter estimation is a problem of significance in many signal processing applications. Such applications indude direction-of-arrival estimation, system identification, and time series analysis. A novel approach to the general problem of signal parameter estimation is described. Although discussed in the context of directionof- arrival estimation, ESPRIT can be applied to a wide variety of problems. It exploits an underlying rotational invariance among signal subspaces induced by an array of sensors with a translational invariance structure. The technique, when applicable, manifests significant performance and computational advantages over previous algorithms such as Burg's maximum entropy method, Capon's maximum likelihood method, and Schmidt's multiple signal classification.

Atmospheric wavefront simulation using Zernike polynomials

Abstract: An algorithm is described that simulates atmospherically distorted wavefronts using a Zernike expansion of randomly weighted Karhunen- Loeve functions. Its performance is presented and analyzed, and the program is then used to forecast resulting structure function and Strehl resolution for adaptive optics systems.

Theory and design of the liquid crystal TV as an optical spatial phase modulator

Abstract: An expression is derived for the complex amplitude transmittance of the liquid crystal TV (LCTV) and verified experimentally. It is shown that spatial phase modulation is realizable with good linearity, high transmission efficiency, and small intensity distortion. Although the commercial LCTV does not provide a sufficiently large dynamic range for continuous phase modulation, a dynamic range of 2'rr can be obtained by making appropriate changes in the design.

Surface-Emitting Microlasers for Photonic Switching and Interchip Connections

Abstract: Vertical-cavity electrically pumped surface-emitting microlasers are formed on GaAs substrates at densities greater than two million per square centimeter. Two wafers were grown with ln0.2Ga0.8As active material composing three 80 ? thick quantum wells in one and a single quantum well (SQW) 100 ? thick in the other. Lasing was seen in devices as small as 1 .5 ?m diameter with <0.05 ?m3 active material. SQW microlasers 5 x 5 ?m square had room-temperature cw current thresholds as low as 1.5 mA with 983 nm output wavelength. 10 x 10 ?m square SQW microlasers were modulated by a pseudorandom bit generator at 1 Gb/s with less than 10-10 bit error rate. Pulsed output >170 mW was obtained from a 100?m square device. The laser output passes through the nominally transparent substrate and out its back side, a configuration well suited for micro-optic integration and photonic switching and interchip connections.

Improved Fourier transform profilometry for the automatic measurement of 3D object shapes

Abstract: By using a defocused optical field with a Ronchi grating to get a quasi-sine optical field and by using a grating ii phase shifting technique, the fundamental spectrum modulated by the object height distribution can theoretically be extended from 0 to 2f0 ( f0 is the carrier frequency of the optical field) without overlapping the zero component and other higher spectra. Consequently, the measurable slope of height variation can be extended to nearly three times that of the unimproved Fourier transform profilometry method. This paper gives the theory and experiment results of the improved Fourier transform profilometry.

Cone-beam tomography: recent advances and a tutorial review

Abstract: Cone-beam tomography is the science of forming images by inverting three-dimensional divergent cone-beam ray-sum data sets. The impetus for its application is its three-dimensional data collection abilities, which result in (1) significant reduction in the time needed to collect a sufficient number of data to produce a three-dimensional image and (2) elimination of the inaccuracy due to misalignment of cross sectional images. On the other hand, the divergence of cone-beam data has hindered its application. Because of the divergence, the theory that has been developed for fan-beam and parallel two- and three-dimensional tomography does not provide a totally adequate means for analyzing or inverting cone-beam data. Consider the following: In practice, as the data are collected, the vertex of the cone is movedalong some path about the object. Which paths, if any, provide enough information to make an inversion possible? Suppose by some means enough information has been obtained. How does one derive an exact formula for inverting this data? To answer these questions a new theory that takes into account the threedimensional divergence of cone-beam data needs to be developed. In 1985, a paper was published in which several advances in the theory of cone-beam tomography were made. A tutorial review of the results given in the 1985 paper [B. D. Smith, "Image reconstruction from cone-beam projections: necessary and sufficient conditions and reconstruction methods," IEEE Trans. Med Imag. MI-4, 14-28 (1985)] will be given here. This review will include the advances that have been made since that time. Additionally, a brief review of the contributions made by a number of other researchers will be given.

Maximum likelihood image and blur identification: a unifying approach

Abstract: A number of different algorithms have recently been proposed to identify the image and blur model parameters from an image that is degraded by blur and noise. This paper gives an overview of the developments in image and blur identification under a unifying maximum likelihood framework. In fact, we show that various recently published image and blur identification algorithms are different implementations of the same maximum likelihood estimator resulting from different modeling assumptions and/or considerations about the computational complexity. The use of the maximum likelihood estimation in image and blur identification is illustrated by numerical examples.

Survey of recent developments in digital image restoration.

Abstract: We present a tutorial review of recent developments in restoring images that are degraded by both blur and noise. We consider three fundamental aspects of digital image restoration: modeling, identification algorithms, and restoration algorithms. An overview of modeling the degradations, and certain properties of images are given first. We then survey the methods that identify these models. Image restoration algorithms are surveyed in two categories: general algorithms and specialized algorithms. We briefly discuss present and future research topics in the field. Our emphasis here is on fundamental concepts and ideas rather than mathematical details.

Self-organizing optical neural network for unsupervised learning

Abstract: One of the features in neural computing must be the ability to adapt to a changeable environment and to recognize unknown objects. This paper deals with an adaptive optical neural network using Kohonen's self-organizing feature map algorithm for unsupervised learning. A compact optical neural network of 64 neurons using liquid crystal televisions is used for this study. To test the performance of the self-organizing neural network, experimental demonstrations and computer simulations are provided. Effects due to unsupervised learning parameters are analyzed. We show that the optical neural network is capable of performing both unsupervised learning and pattern recognition operations simultaneously, by setting two matching scores in the learning algorithm. By using a slower learning rate, the construction of the memory matrix becomes more organized topologically. Moreover, the introduction of forbidden regions in the memory space enables the neural network to learn new patterns without erasing the old ones.

Optimal estimation of the regularization parameter and stabilizing functional for regularized image restoration

Abstract: Regularization is an effective method for obtaining satisfactory solutions to image restoration problems. The application of regularization necessitates a choice of the regularization parameter as well as the stabilizing functional. For most problems of interest, the best choices are not known a priori. We present a method for obtaining optimal estimates of the regularization parameter and stabilizing functional directly from the degraded image data. The method of generalized cross-validation (GCV) is used to generate the estimates. Implementation of GCV requires the computation of the system eigenvalues. Certain assumptions are made regarding the structure of the degradation so that the GCV criterion can be implemented efficiently. Furthermore, the assumptions on the matrix structure allow the regularization operator eigenvalues to be expressed as simple parametric functions. By choosing an appropriate structure for the regularization operator, we use the GCV criterion to estimate optimal parameters of the regularization operator and thus the stabilizing functional. Experimental results are presented that show the ability of GCV to give extremely reliable estimates for the regularization parameter and operator. By allowing both the degree and the manner of smoothing to be determined from the data, GCV-based regularization yields solutions that would otherwise be unattainable without a priori information.

Image identification and restoration based on the expectation-maximization algorithm

Abstract: In this paper, the problem of identifying the image and blur parameters and restoring a noisy blurred image is addressed. Specifying the blurring process by its point spread function (PSF), the blur identification problem is formulated as the maximum likelihood estimation (MLE) of the PSF. Modeling the original image and the additive noise as zeromean Gaussian processes, the MLE of their covariance matrices is also computed. An iterative approach, called the EM (expectation-maximization) algorithm, is used to find the maximum likelihood estimates ofthe relevant unknown parameters. In applying the EM algorithm, the original image is chosen to be part of the complete data; its estimate is computed in the E-step of the EM iterations and represents the restored image. Two algorithms for identification/restoration, based on two different choices of complete data, are derived and compared. Simultaneous blur identification and restoration is performed by the first algorithm, while the identification of the blur results from a separate minimization in the second algorithm. Experiments with simulated and photographically blurred images are shown.

Thin film filter performance for extreme ultraviolet and x-ray applications

Abstract: Substantial work has been done to characterize filter materials for the vacuum ultraviolet and x-ray regions of the electromagnetic spectrum. This paper summarizes the theoretical basis for predicting performance and compiles the results of different measurement programs for comparison. Recent work that better quantifies transmission as a function of wavelength for various filter and window materials is reported. Other applications of thin films in which these optical properties are important include photocathodes and x-ray laser targets. Of particular interest are figures giving linear absorption coefficients as a function of wavelength for commonly used filter materials. Also included are recent data on the effect of aging on aluminum filters, plus test data and comments on the use of composite materials designed to adjust the bandpass of a filter to meet particular research requirements. The data are presented so that the reader may more easily design and predict the performance of filters and windows for specific applications.

Wavefront fitting with discrete orthogonal polynomials in a unit radius circle

Abstract: Zernike polynomials have been used for some time to fit wavefront deformation measurements to a two-dimensional polynomial. Their orthogonality properties make them ideal for this kind of application. The typical procedure consists of first obtaining the fitting using x-y polynomials and then transforming them to Zernike polynomials by means of a matrix multiplication. Here, we present a new method for making this fitting faster by using a set of orthogonal polynomials on a discrete base of data points on a unitary circle.

Sinusoidal phase modulating laser diode interferometer with a feedback control system to eliminate external disturbance

Abstract: We propose a sinusoidal phase modulating laser diode interferometer that is insensitive to vibrations of the optical components and fluctuations in the optical wavelength of the laser diode. These external disturbances cause fluctuations in the phase of the interference signal. After we analyze the sinusoidal phase modulation in a laser diode interferometer, we describe the method of the feedback control of the injection current of the laser diode to eliminate the phase fluctuations of the interference signal. We construct two sinusoidal phase modulating interferometers for movement measurements and surface profile measurements, respectively. The experimental results make it clear that the interferometers work well in mechanically noisy circumstances.

Agile beam steering using binary optics microlens arrays

Abstract: Agile steering of a helium-neon laser beam (X = 632.8 nm) has been demonstrated using a complementary pair of 5-cm-aperture binary optic microlens arrays in the Galilean telescope geometry. Segmentation of a collimated input beam by illuminating approximately 60,000 /5 microlenses of 200-jim diameter and parabolic phase profile results in nearly aberration free beam steering over an 1 1° field of view for 100- pm lateral displacements of one array relative to the other. Wavefront quality and steering efficiency of the deflected beam has been measured as a function of steering angle and is compared to a simple theoretical model.

Review of image-blur models in a photographic system using the principles of optics

Abstract: When a photographic imaging system has a focus error or a motion blur, it is not possible to correct the error by conventional optical printing. Digital image processing offers a chance to restore the blurred images by reversing the blurring processes under certain constraints. Although restoration algorithms vary in detail, they all need a model of the image-blurring process. This paper reviews image-blur models of each individual component in an image chain consisting of camera, film, and scanner. The models include image blur caused by focus error, camera shutter, object motion, light scattering in the film emulsion, film interimage effect, scanner optics, and scanner aperture. The emphasis is on mathematical simplicity and practical usefulness.

Application of a noise-adaptive contrast sensitivity function to image data compression

Abstract: The visual contrast sensitivity function (CSF) has found increasing use in image compression as new algorithms optimize the display- observer interface in order to reduce the bit rate and increase the perceived image quality. In most compression algorithms, increasing the quantization intervals reduces the bit rate at the expense of introducing more quantization error. The CSF can be used to distribute this error as a function of spatial frequency such that it is undetectable by the human observer. Thus, instead of being mathematically lossless, the compression algorithm can be designed to be visually lossless, with the advantage of a significantly reduced bit rate. However, the CSF is strongly affected by image noise, changing in both shape and peak sensitivity. This work describes a model of the CSF that includes these changes as a function of image noise level by using the concepts of internal visual noise. The model is tested in the context of image compression with an observer study.

1-F Binary Joint Transform Correlator.

Abstract: A one lens focal length binary joint transform correlator is described. This correlator uses a magneto-optic spatial light modulator, lens, and standard 8-bit resolution CCD camera. Computer simulations and experimental results of the effects of changes in scale, in-plane rotation, out-of-plane rotation, target/reference separation, and multiple targets are discussed. The performance using actual sensor imagery containing clutter is presented.

Electronic speckle pattern interferometry system for in situ deformation monitoring on buildings

Abstract: An electronic speckle pattern interferometry system has been developed for in situ measurements of microdeformations on buildings or monuments. Design of the optomechanical setup received special attention to allow firm and steady contact between object and optical head. Features are the miniaturization of the light source (laser diode) and pickup (CCD camera). Image processing was carried out with a personal computer. Deformation monitoring was demonstrated successfully over periods of several weeks in the study of decay mechanisms of walls in an historic church.

Optical distortion coefficients of high-power laser windows

Abstract: This paper concerns the problem of describing and evaluating thermal lensing phenomena that occur as a result of the absorption of laser light in solid windows. The aberration function expansion method is applied for deriving the two optical distortion coefficients x+ and xthat characterize the degradation in light intensity at the Gaussian focus of an initially diffraction-limited laser beam passing through a weakly absorbing stress-birefringent window. In a pulsed mode of operation, the concept of an effective optical distortion coefficient Xeff, which properly combines the coefficients x+ and x- in terms of potential impact on focal irradiances, then leads to the definition of a figure of merit for distortion. The theory and calculations presented in this and earlier papers provide simple analytical tools for predicting the optical performance of a windowmaterial candidate in a specific system's environment.

Development of an active truss element for control of precision structures

Abstract: An active structural element for use in precision control of large space structures is described. The active member is intended to replace a passive strut in a truss-like structure. It incorporates an eddy current displacement sensor and an actuator that is either piezoelectric (PZT) or electrostrictive (PMN). The design of the device is summarized. Performance of separate PZT and PMN actuators is compared for several properties relevant to submicrometer control of precision structures.

Performance and head movements using a helmet-mounted display with different sized fields-of-view

Abstract: Seventeen subjects searched for, and then monitored, 3, 6, or 9 stationary targets in order to detect and shoot 3 threats. The targets were located in an area 1200 left and right and 900 upward, from straight ahead. They were viewed on a helmet-mounted display (HMD) by making appropriate head movements. The HMD had five different sizes of field-of view (FOV), ranging in width from 200 to 1 200. Subjects hit fewer threats and were threatened for longer durations with small FOVs than with large FOVs. The decrement in performance was dependent on the number of targets. Optimum performance with 3, 6, and 9 targets required FOVs of 200, greater than 200, and greater than 600, respectively. Subjects moved their head less, but faster, with large FOVs than with small FOVs. Also, there was an apparent inverse relation between head velocity while performing the task and error at replacing the targets afterward. It was interpreted that head velocity was mediated by the certainty with which subjects remembered target locations. It appeared that performance at the task was limited by how well subjects integrated information about target locations and not by how fast or how much they could move their heads.

Visual issues in the use of a head-mounted monocular display

Abstract: A miniature display device, recently available commercially, is aimed at providing a portable, inexpensive means of visual information communication. The display is head mounted in front of one eye with the other eye's view of the environment unobstructed. Various visual phenomena are associated with this design. The consequences of these phenomena for visual safety, comfort, and efficiency of the user were evaluated: (1) The monocular, partially occluded mode ofoperation interrupts binocular vision. Presenting disparate images to each eye results in binocular rivalry. Most observers can use the display comfortably in this rivalrous mode. In many cases, it is easier to use the display in a peripheral position, slightly above or below the line of sight, thus permitting normal binocular vision of the environment. (2) As a head-mounted device, the displayed image is perceived to move during head movements due to the response of the vestibulo-ocular reflex. These movements affect the visibility of small letters during active head rotations and sharp accelerations. Adaptation is likely to reduce this perceived image motion. No evidence for postural instability or motion sickness was noted as a result of these conflicts between visual and vestibular inputs. (3) Small displacements of the image are noted even without head motion, resulting from eye movements and the virtual lack of display persistence. These movements are noticed spontaneously by few observers and are unlikely to interfere with the display use in most tasks.

Enhancement of the reflectivity of multilayer x-ray mirrors by ion polishing

Abstract: The performance of multilayer x-ray mirrors is improved by smoothing the boundaries within the multilayer stack with ion bombardment at grazing angles of incidence. The process is applied to Co-C and to RhRu-C mirrors for normal incidence telescopes at X = 63.5and 114 A, and the reflectivity is increased by a factor of 2 in both cases. Accumulation of roughness during deposition is eliminated. An estimate ofthe presently achievable reflectivity for normal incidence mirrors in the X = 44 to 1 20 A wavelength range is given.

Autonomous real-time object tracking with an adaptive joint transform correlator

Abstract: A hybrid optical/digital system for tracking an object in a sequence of images is described. Since a joint transform correlator does not require a matched spatial filter in the correlation process, object tracking can be carried out by continuously updating the reference image with the object image in the previous frame. This adaptive property of a joint transform correlator, together with the parallelism and high processing speed of an optical system, ensure high correlation between objects in two sequential frames. The relative position of the object can then be determined based on the location of the correlation peak. System performance is evaluated and experimental demonstrations are presented.

Constrained sinogram restoration for limited-angle tomography

Abstract: Tomographic reconstruction from incomplete data is required in many fields, including medical imaging, sonar, and radar. In this paper, we present a new reconstruction algorithm for limited-angle tomography, a problem that occurs when projections are missing over a range of angles. The approach uses a variational formulation that incorporates the Ludwig- Helgason consistency conditions, measurement noise statistics, and a sinogram smoothness condition. Optimal restored sinograms, therefore, satisfy an associated Euler-Lagrange partial differential equation, which we solve on a lattice using a primal-dual optimization procedure. Object estimates are then reconstructed using convolution back-projection applied to the restored sinogram. We present results of simulations that illustrate the performance of the algorithm and discuss directions for further research.

New binarization techniques for joint transform correlation

Abstract: In this paper the application of joint transform correlator (JTC) techniques for use in automatic target recognition using actual sensor data is addressed. The problem of interest is the detection and classification of objects in forward-looking infrared (FLIR) images. A JTC architecture using a single magneto-optic spatial light modulator and a chargecoupled device camera is tested. Unique techniques for binarizing the JTC input and the subsequent Fourier transform fringe pattern, commonly called the joint transform power spectrum (JTPS), are presented. Computer simulations and experimental results are provided.

Refractometry by minimum deviation: accuracy analysis

Abstract: We analyze the accuracy achieved when evaluating high refractive indices by minimum deviation deflectometry.

Analysis of spatial pseudodepolarizers in imaging systems

Abstract: The objective of a number of optical instruments is to measure the intensity accurately without bias as to the incident polarization state. One method to overcome polarization bias in optical systems is the insertion of a spatial pseudodepolarizer. Both the degree of depolarization and image degradation (from the polarization aberrations of the pseudodepolarizer) are analyzed for two depolarizer designs: (1) the Cornu pseudodepolarizer, effective for linearly polarized light, and (2) the dual Babinet compensator pseudodepolarizer, effective for all incident polarization states. The image analysis uses a matrix formalism to describe the polarization dependence of the diffraction patterns and optical transfer function.

Complex ternary matched filters yielding high signal-to-noise ratios

Abstract: Complex ternary matched filters (CTMFs) can be implemented optically using detour phase and a single ternary spatial light modulator or a Mach-Zehnder arrangement and two ternary signal light modulators. In this paper, we present the design of CTMFs that yield high signal-tonoise ratios (SNRs) and show with the help of simulation examples that their performance (in the sense of SNRs) is close to that of optimal matched filters.