This issue and the next (June 1986) of Optical Engineering contain about half of the papers that were presented at the International Conference on Speckle, held in San Diego, August 20-23, 1985.* The aim of the conference was to provide a forum for the discussion of all aspects of speckle and its applications. Experts from 11 countries gathered to exchange knowledge and to interact with each other. There were papers on speckle with laser light, speckle from astronomical objects, and speckle with ultrasounds, microwaves, and the infrared. Research and review papers were presented on the statistical properties of speckle, speckle me trology, speckle in imaging and radar systems, speckle in astronomy, and speckle interferometry. Many new results were presented for the first time. The conference showed that speckle is a lively and rapidly evolving field, not only in the study of speckle phenomena but also for its practical applications. Speckle metrology and speckle astronomy in particular have matured to the point where commercial equipment is being built especially for those applications, yielding results that can be obtained by no other methods. Because of deadline considerations and requirements of the refereeing and revision processes, it was not possible to group all the papers on one subject together in one issue, although all the speckle astronomy papers will appear in the June issue. The papers included in these two special lissuues of Optical Engineering represent a snapshot of the lively field of speckle as it exists today.
The salient features of "classical" speckle patterns are reviewed. The relaxation of assumptions leading to the classical model yields statistical properties that differ from those associated with conventional speckle. Attention is turned to a summary of those new statistical aspects of speckle that have been discovered or explored since the Applications of Speckle Phenomena conference held in August 1980 (Proc. SPIE Vol. 243).
A rigorous new electromagnetic theory has been developed to compute the speckle pattern of 2D random microrough surfaces with perfect conductivity, when the width of the irregularities is less than 10 um. The computer code enables us to compute rigorously the speckle pattern, whatever the number of irregularities illuminated by the laser beam. Our first numerical results are compared with some predictions of the speckle theory developed in optics.
The polarization characteristics of speckle patterns are investigated by a novel ellipsometric approach. The method is based on the illumination of the scattering object by a plane-polarized light beam with its polarization plane rotating. The speckle pattern produced by the scattered light is then investigated for its polarization characteristics. An approximate theory is outlined and compared with experimental measurements demonstrating the usefulness of the method as a powerful surface analytic tool.
If a rough surface is illuminated by a coherent light wave of wavelength Al, it is not possible to determine the surface profile from the phases of the speckle field formed by the scattered light. However, if the rough surface is illuminated by an additional coherent wave of wavelength A2, the phase differences between the two speckle fields do contain information about the macro-scopic surface profile even though subject to a statistical error. We present the pertinent statistical properties of dichromatic speckle fields and show that (1) the macroscopic surface profile may be determined from the phase differences if the effective wavelength A = Al X2/I Al - A21 is sufficiently larger than the standard deviation of the microscopic profile of the illuminated surface and (2) the statistical error is reasonably small if the phase measurements are obtained from speckles with sufficient intensity.
Characteristics of the phase of Gaussian speckles are studied theoretically in both image and diffraction fields. The general analysis for these studies is given by introducing an "optical system function," and the results are applied to the analysis of the speckle phases in both fields. The speckle phase distributions are evaluated by a phase angle defined as the extent of the equiprobability density ellipse stretched from the origin in the complex plane of the speckle amplitude. In addition, several peculiar properties of the equiprobability density ellipse are summarized.
Speckle appearing in synthetic aperture radar (SAR) images is generated by coherent interference of radar echoes from target scatters. Basically, speckle noise has the nature of a multiplicative noise. In this paper procedures for defining and verifying a statistical noise model are developed, and two multiplicative noise-smoothing algorithms are pre-sented. These two algorithms are computationally efficient and have the potential of achieving real-time or near-real-time processing. Several SEASAT SAR and SIR-B (Shuttle Imaging Radar) images are used for illustration.
Equations for laser radar heterodyne efficiency are derived by assuming that heterodyne detection is degraded by phase-front distortion produced by target speckle and atmospheric turbulence. These equations are numerically evaluated for representative target detection scenarios to illustrate the effects of laser speckle on coherent laser radar detection.
A nonlinear speckle filter based on geometric concepts is defined, and an example of its effectiveness on synthetic aperture radar imagery is shown. Comparison with look-averaging is made using artificial imagery with synthetic speckle.
In a recent paper we investigated the problem of reconstructing the magnitude of a 2-D complex signal f from samples of the Fourier transform of f lying in a small region offset from the origin. The primary application of interest was synthetic aperture radar. We showed that high quality speckle reconstructions are possible so long as the phase of f is highly random. In this paper we explore the possibility of Fourier-offset reconstruction from just the phase of the Fourier transform. We provide and compare a large number of computer-simulated image reconstructions from phase plus magnitude, phase only (constant magnitude), magnitude only (zero phase), and magnitude plus quantized phase. A number of conclusions are drawn regarding Fourier offset phase-only reconstruction, and several topics are suggested for further research.
After a homomorphic transformation has been used to transform speckle noise into additive signal-independent noise, classic techniques are used to evaluate the number of discernible gray levels and the information capacity of images degraded by fully developed speckle noise. The probability distributions that we developed earlier are used to evaluate the required probabilities of error. Expressions are derived for square and for circular apertures. The information capacity is found to have a maximum of about 0.2 bits per speckle. The spatial frequency resolution required to obtain a given signal-to-noise ratio by spatial integration is evaluated.
An improved version of the laser speckle strain gauge is presented in which speckle displacement caused by deformation of a laser-illuminated object is detected in real time using new photodetectors in place of the linear image sensors and microcomputer in the conventional version. The detector, called a spatial filtering detector with an electronic scanning facility, produces a voltage directly proportional to speckle displacement. Therefore, this strain gauge, which utilizes differential speckle displacement, delivers a voltage that is proportional to surface strain. It has a strain sensitivity of a few millivolts per microstrain. We applied this gauge to strain measurements of high-polymer films in various directions under loading at frequencies up to several tens of hertz and were able to evaluate their Poisson ratios.
This paper describes real-time processing with a hologram that generates a phase-conjugate wavefront to compensate the aberration through a phase-distorting medium. Good-quality reconstructed images are obtained. Comparisons with the conventional holographic procedure, using silver halide emulsion, and the degenerate four-wave mixing phase conjugation process are presented.
We present a computer code in FORTRAN 77 for the calculation of the mutual coherence function (MCF) of a plane wave normally incident on a stochastic half-space. This is an exact result. The user need only input the path length, the wavelength, the outer scale size, and the structure constant. This program may be used to calculate the MCF of a well-collimated laser beam in the atmosphere.
A feasibility study to develop a fiber-optic probe for measuring features of an internal thread has been carried out. The principle used in this study is the optical triangulation mapping technique. A design of the gage including the optical system is discussed. The design of the optical components is evaluated by use of a computer ray-tracing pogram. In addition, signal detection error analysis is performed to estimate the accuracy of the measurement. Results indicate that the gage accuracy is well within ±2.5 um, which is the desired goal that is meaningful for applications in quality control, quality assurance, and metrology. This accuracy includes the effect of surface roughness, which is minimized by selecting an appropriate diameter of the probing light spot.
Some of the major accomplishments of infrared technology in the decade 1975-84 are reviewed. The trend most influential on technology was development of sensors for use in space. Advances in technology to make these accomplishments possible include evolution of (1) longer-wavelength detectors, (2) techniques to make large lightweight and adaptive optics, (3) mosaic focal plane arrays using CCDs, and (4) staring sensors. Likely major challenges for the next decade are strategic defense from space, global resource management, and understanding of ecology and astronomy preparatory to space colonization. Technology changes to support these challenges are estimated.
This paper presents an overview of the application of deep-level transient spectroscopy (DLTS) for characterizing and identifying electronic defects in semiconductors. After a review of general principles, the range of defect problems that has been studied by DLTS is illustrated with results from crystalline semiconductors, semiconductor-insulator interfaces, and amorphous semiconductors.
For many years now, virtually every issue of Optical Engineering has been a "special issue," an issue in which a majority of the papers have been devoted to one or two "special" topics related to the field of optical engineering. Although some minor changes have been instituted in this regard, these special issues will continue to be the backbone of the journal for some time to come. Our current plan is to have ten special issues every year, each containing about ten papers related to the special topic, with about ten additional contributed papers on various other topics included in each journal. The remaining two issues, which will be the June and December issues beginning in 1987, will be reserved exclusively for contributed papers and will have about twenty papers each. Consequently, we anticipate that approximately 100 papers related to special topics and 140 contributed papers will be published in Optical Engineering each year.