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Norman S. Kopeika

Bio: Norman S. Kopeika is an academic researcher from Ben-Gurion University of the Negev. The author has contributed to research in topics: Image restoration & Optical transfer function. The author has an hindex of 36, co-authored 371 publications receiving 5221 citations. Previous affiliations of Norman S. Kopeika include Ariel University & University of Pennsylvania.


Papers
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Journal ArticleDOI
TL;DR: In this paper, the effects of bias and incident laser intensity are seen to be complementary, with and without absorption of very short duration incident N 2 laser pulses, and the effect of laser illumination of the interelectrode gap causes gas breakdown at the cathode to take place at a faster rate and at lower threshold bias than without the illumination.
Abstract: Dynamic breakdown of Ne and Ar gases biased to the prebreakdown stage, with and without absorption of very short duration incident N 2 laser pulses, is studied. Effects of bias and incident laser intensity are seen to be complementary. Laser illumination of the interelectrode gap causes gas breakdown at the cathode to take place at a faster rate and at lower breakdown threshold bias than without the illumination. Breakdown pulse shape varies according to gas composition and bias, and is much different from simple nonbreakdown "prebreakdown" responses to the laser pulses. The prebreakdown signals are attributed to photon-enhanced ionization in the focal volume between the electrodes, while the laser-triggered breakdown pulses are attributed to photon-enhanced excitation and diffusion of such neutral atoms to the high field gradient region near the cathode, where cascade ionization collisional effects are amplified.

16 citations

Proceedings ArticleDOI
TL;DR: An imaging LIDAR system for measuring vertical profile of the atmospheric refractive index turbulence has been developed and its performance demonstrated in this paper, where the turbulence profile retrieval technique is based on image motion analysis.
Abstract: An imaging LIDAR system for measuring vertical profile of the atmospheric refractive index turbulence has been developed and its performance demonstrated. The turbulence profile retrieval technique is based on image motion analysis. In the present work LIDAR measurements of C n 2 vertical profiles are demonstrated. Unlike the existing turbulence models, the experimental results show the various strata and layers in the vertical turbulence profiles.

16 citations

Journal ArticleDOI
TL;DR: In this paper, a mathematical model of spatial widening of the optical beam is derived using Monte Carlo simulation at three different wavelengths in the visible and the near IR, and it is shown that using shorter wavelengths such as 0532 μm results in least spatial widening and maximal received power, and is thus preferable for optical communication.
Abstract: As part of a communication channel, clouds cause spatial widening and attenuation of optical pulse power Free-space optical communication from satellite to earth (ground or airplane) occasionally involves clouds over part of the optical channel Most of the energy of optical pulses propagating through thin clouds passes through the clouds The propagating energy is concentrated around the center of the beam The distribution of the energy relative to the center of the beam is not uniform Using the received energy in an efficient way reduces the transmitter power needed for given bit error rate The advantages of low transmitter power are less radiation exposure and greater immunity to eavesdropping To use the received energy efficiently, a mathematical model of spatial widening of the optical beam is derived using Monte Carlo simulation The simulation is carried out at three different wavelengths in the visible and the near IR Important aspects of this work include the fact that (1) using shorter wavelengths such as 0532 μm results in least spatial widening and maximal received power, and is thus preferable for optical communication, and (2) the mathematical model derived is a basis for adaptive communication with less transmitted energy consumption

16 citations

Proceedings ArticleDOI
TL;DR: In this paper, both active and passive atmospheric modulation transfer function (MTF) measurements were carried out simultaneously in both thermal imaging atmospheric windows and results indicate rather significant angular spatial frequency dependence of the MTF, in contradiction to the conventional approach which assumes contrast transfer is atmospheric transmission only.
Abstract: Both active and passive atmospheric modulation transfer function (MTF) measurements carried out simultaneously in both thermal imaging atmospheric windows are presented. Results indicate rather significant angular spatial frequency dependence of the MTF, in contradiction to the conventional approach which assumes contrast transfer is atmospheric transmission only. A theoretical explanation is discussed based upon aerosol forward scattering and absorption effects which are shown to be angular spatial frequency dependent and yield MTF results similar to those measured. This means that small targets are blurred much more than large targets by the atmosphere, thus also affecting target acquisition probabilities.

16 citations

Proceedings ArticleDOI
TL;DR: In this article, a new method of numerical calculation of MTF for image motion in one-dimensional space is presented, which is applicable in principle to any type of motion and can be expanded to two-dimensional motion.
Abstract: A new method of numerical calculation of MTF is presented here for image motion in one- dimension. The method is applicable in principle to any type of motion and can be expanded to two-dimensional motion. It is applied here to uniform velocity motion and to sinusoidal vibrations. Comparison to known analytical methods is made where possible, and agreement is excellent. This supports its implementation to any kind of random motion, particularly where no unique analytical MTF is possible.

15 citations


Cited by
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[...]

08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

33,785 citations

Journal ArticleDOI
TL;DR: A universal statistical model for texture images in the context of an overcomplete complex wavelet transform is presented, demonstrating the necessity of subgroups of the parameter set by showing examples of texture synthesis that fail when those parameters are removed from the set.
Abstract: We present a universal statistical model for texture images in the context of an overcomplete complex wavelet transform. The model is parameterized by a set of statistics computed on pairs of coefficients corresponding to basis functions at adjacent spatial locations, orientations, and scales. We develop an efficient algorithm for synthesizing random images subject to these constraints, by iteratively projecting onto the set of images satisfying each constraint, and we use this to test the perceptual validity of the model. In particular, we demonstrate the necessity of subgroups of the parameter set by showing examples of texture synthesis that fail when those parameters are removed from the set. We also demonstrate the power of our model by successfully synthesizing examples drawn from a diverse collection of artificial and natural textures.

1,978 citations

Journal ArticleDOI
TL;DR: OCT as discussed by the authors synthesises cross-sectional images from a series of laterally adjacent depth-scans, which can be used to assess tissue and cell function and morphology in situ.
Abstract: There have been three basic approaches to optical tomography since the early 1980s: diffraction tomography, diffuse optical tomography and optical coherence tomography (OCT). Optical techniques are of particular importance in the medical field, because these techniques promise to be safe and cheap and, in addition, offer a therapeutic potential. Advances in OCT technology have made it possible to apply OCT in a wide variety of applications but medical applications are still dominating. Specific advantages of OCT are its high depth and transversal resolution, the fact, that its depth resolution is decoupled from transverse resolution, high probing depth in scattering media, contact-free and non-invasive operation, and the possibility to create various function dependent image contrasting methods. This report presents the principles of OCT and the state of important OCT applications. OCT synthesises cross-sectional images from a series of laterally adjacent depth-scans. At present OCT is used in three different fields of optical imaging, in macroscopic imaging of structures which can be seen by the naked eye or using weak magnifications, in microscopic imaging using magnifications up to the classical limit of microscopic resolution and in endoscopic imaging, using low and medium magnification. First, OCT techniques, like the reflectometry technique and the dual beam technique were based on time-domain low coherence interferometry depth-scans. Later, Fourier-domain techniques have been developed and led to new imaging schemes. Recently developed parallel OCT schemes eliminate the need for lateral scanning and, therefore, dramatically increase the imaging rate. These schemes use CCD cameras and CMOS detector arrays as photodetectors. Video-rate three-dimensional OCT pictures have been obtained. Modifying interference microscopy techniques has led to high-resolution optical coherence microscopy that achieved sub-micrometre resolution. This report is concluded with a short presentation of important OCT applications. Ophthalmology is, due to the transparent ocular structures, still the main field of OCT application. The first commercial instrument too has been introduced for ophthalmic diagnostics (Carl Zeiss Meditec AG). Advances in using near-infrared light, however, opened the path for OCT imaging in strongly scattering tissues. Today, optical in vivo biopsy is one of the most challenging fields of OCT application. High resolution, high penetration depth, and its potential for functional imaging attribute to OCT an optical biopsy quality, which can be used to assess tissue and cell function and morphology in situ. OCT can already clarify the relevant architectural tissue morphology. For many diseases, however, including cancer in its early stages, higher resolution is necessary. New broad-bandwidth light sources, like photonic crystal fibres and superfluorescent fibre sources, and new contrasting techniques, give access to new sample properties and unmatched sensitivity and resolution.

1,914 citations

Journal ArticleDOI
TL;DR: An up-to-date survey on FSO communication systems is presented, describing FSO channel models and transmitter/receiver structures and details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits are provided.
Abstract: Optical wireless communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers, i.e., visible, infrared (IR), and ultraviolet (UV) bands. In this survey, we focus on outdoor terrestrial OWC links which operate in near IR band. These are widely referred to as free space optical (FSO) communication in the literature. FSO systems are used for high rate communication between two fixed points over distances up to several kilometers. In comparison to radio-frequency (RF) counterparts, FSO links have a very high optical bandwidth available, allowing much higher data rates. They are appealing for a wide range of applications such as metropolitan area network (MAN) extension, local area network (LAN)-to-LAN connectivity, fiber back-up, backhaul for wireless cellular networks, disaster recovery, high definition TV and medical image/video transmission, wireless video surveillance/monitoring, and quantum key distribution among others. Despite the major advantages of FSO technology and variety of its application areas, its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading and sensitivity to weather conditions. In the last five years or so, there has been a surge of interest in FSO research to address these major technical challenges. Several innovative physical layer concepts, originally introduced in the context of RF systems, such as multiple-input multiple-output communication, cooperative diversity, and adaptive transmission have been recently explored for the design of next generation FSO systems. In this paper, we present an up-to-date survey on FSO communication systems. The first part describes FSO channel models and transmitter/receiver structures. In the second part, we provide details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits. Specific topics include advances in modulation, channel coding, spatial/cooperative diversity techniques, adaptive transmission, and hybrid RF/FSO systems.

1,749 citations

Journal ArticleDOI
TL;DR: The use of ML detection in spatial diversity reception to reduce the diversity gain penalty caused by correlation between the fading at different receivers is described.
Abstract: In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. We describe several communication techniques to mitigate turbulence-induced intensity fluctuations, i.e., signal fading. These techniques are applicable in the regime in which the receiver aperture is smaller than the correlation length of fading and the observation interval is shorter than the correlation time of fading. We assume that the receiver has no knowledge of the instantaneous fading state. When the receiver knows only the marginal statistics of the fading, a symbol-by-symbol ML detector can be used to improve detection performance. If the receiver has knowledge of the joint temporal statistics of the fading, maximum-likelihood sequence detection (MLSD) can be employed, yielding a further performance improvement, but at the cost of very high complexity. Spatial diversity reception with multiple receivers can also be used to overcome turbulence-induced fading. We describe the use of ML detection in spatial diversity reception to reduce the diversity gain penalty caused by correlation between the fading at different receivers.

1,490 citations