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 …

I and i

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.

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A Parametric Texture Model Based on Joint Statistics of Complex Wavelet Coefficients

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.

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Optical coherence tomography - principles and applications

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.

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Survey on Free Space Optical Communication: A Communication Theory Perspective

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.

/pdf/free-space-optical-communication-through-atmospheric-48mj86nu81.pdf

Free-space optical communication through atmospheric turbulence channels

The effect of low frequency mechanical vibrations in the image plane on thermal imaging target acquisition is considered. A model is described that takes as input the mechanical vibration data such as amplitude and frequency and the physical characteristics of the target. The output is the probability of detection as a function of time. Analysis indicates that even low amplitude vibration greatly affects the predicted target detection times and, hence, the utility of the IR system in realistic scenarios.

Target acquisition probabilities for image motion and vibration

There are two kinds of video image sequence distortions caused by vibration of the camera. The first is the vibration of the line- of-sight, causing location changes of the scene in successive frames. The second distortion is the blur of each frame of the sequence due to frame motion during its exposure. The relative effects of these two types of degradations on the ability of observers to recognize targets are in- vestigated. This study is useful for evaluating the amount of effort re- quired to compensate each effect. We found that the threshold contrast needed to recognize a target in a vibrating video sequence under certain conditions is more affected by the motion blur of each frame than the oscillation of the line-of-sight. For digital sequence restoration methods, this study determines the required precision of the deblurring and regis- tration processes. It shows that the deblurring process should not be neglected, as it often is. © 2001 Society of Photo-Optical Instrumentation Engineers.

Influence of severe vibrations on the visual perception of video sequences

Miniature inexpensive neon indicator lamp plasma glow discharge detectors (GDD) are excellent candidates to serve as
room temperature, low cost, terahertz (THz) radiation detectors and as pixels in THz imaging systems.
Heterodyne amplification of low power signals via higher power reference beams is very important for THz imaging
systems because it permits detection of much lower object beam intensities. An experimental result of 300GHz
heterodyne detection by a single commercial GDD device costing about 30 cents is presented here. In heterodyne image
detection a picture is taken of interference fringes or a hologram deriving from a coherent reference wave and a coherent
wave reflected from or transmitted through an object. Transmission with in-line or zero angles between those two waves
is important to widen the fringes. The GDD detector is transparent, so that it's possible to receive radiation from both
sides, at 0 and 180 degree. This permit receiving the wave reflected from or transmitted through an object at 0 degree
and the reference wave from the opposite direction at 180 degree. Such interference fringe widening can permit
heterodyne direct imaging of the object instead of imaging the interference pattern.

Low-cost plasma terahertz heterodyne image detection

A number of serious consortiums develop satellite communication networks The objective of these communication projects is to service personal communication users almost everywhere on earth The inter satellite links (ISL) in those project use microwave radiation as the carrier Free space optical communication between satellites networked together can make possible high speed communication between different places on earth The advantages of an optical communication system instead of a microwave communication system in free space are: a) smaller size and weight, b) less transmitter power, c) larger bandwidth, d) higher immunity to interference, and e) smaller transmitter beam divergence The use of optical radiation as a carrier between the satellites creates very narrow beam divergence angles Due to the narrow beam divergence angle and the large distance between the satellites the pointing from one satellite to another is complicated The problem is more complicated due to vibration of the pointing system caused by two stochastic fundamental mechanisms 1) tracking noise created by the electrooptic tracker and 2) vibrations created by internal and external mechanical mechanisms The vibrations displace the transmitted beam in the receiver plane Such movement of the transmitted beam in the receiver plane decreases the average received signal which decreases increases the bit error rate In this paper we review: 1) the present status of satellite networks 2) developing efforts of optical satellite communication around the world, 3) performance results of vibration effects on different kinds of optical communication satellite networks and 4) seven approaches to overcome the problems caused by transmitter pointing vibration

Free-space optical communication satellite networks-vibration effects and possible solutions

An effort to quantify effects of weather on coarse aerosol concentration and total cross- sectional area per unit volume, so as to permit their prediction according to weather forecast, has begun. Correlations of prediction with measurement are on the order of 92% and 88% respectively. Relative humidity is the dominant parameter. Based on this work, further development is planned so as to predict overall size distribution, scattering coefficients, and aerosol MTF, according to weather forecast.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Norman S. Kopeika

Papers

Target acquisition probabilities for image motion and vibration

Influence of severe vibrations on the visual perception of video sequences

Low-cost plasma terahertz heterodyne image detection

Free-space optical communication satellite networks-vibration effects and possible solutions

Prediction of coarse aerosol statistics according to weather forecast