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.

/pdf/a-parametric-texture-model-based-on-joint-statistics-of-2p5nbgtfy6.pdf

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.

/pdf/optical-coherence-tomography-principles-and-applications-17y32cabbm.pdf

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.

/pdf/survey-on-free-space-optical-communication-a-communication-239v68hfyw.pdf

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

Ofer Hadar, MEMBER SPIE

Restoration of thermal images distorted by the atmosphere, received on a focal plane array (FPA) thermal imaging system, is presented. The restoration method is based upon atmospheric modulation transfer function (MTF) analysis. Using turbulence and aerosol MTF prediction models atmospheric distortions and image corruption are modeled. Preliminary restoration results indicate significant improvement in image quality.

Restoration of thermal images distorted by the atmosphere, using predicted atmospheric modulation transfer function

The advantages of terahertz (THz) imaging are well known. They penetrate well most
non-conducting media and there are no known biological hazards, This makes such
imaging systems important for homeland security, as they can be used to image
concealed objects and often into rooms or buildings from the outside. There are also
biomedical applications that are arising. Unfortunately, THz imaging is quite
expensive, especially for real time systems, largely because of the price of the
detector. Bolometers and pyroelectric detectors can each easily cost at least hundreds
of dollars if not more, thus making focal plane arrays of them quite expensive.
We have found that common miniature commercial neon indicator lamps costing
typically about 30 cents each exhibit high sensitivity to THz radiation [1-3], with
microsecond order rise times, thus making them excellent candidates for such focal
plane arrays. NEP is on the order of 10-10 W/Hz1/2. Significant improvement of
detection performance is expected when heterodyne detection is used
Efforts are being made to develop focal plane array imagers using such devices at 300
GHz. Indeed, preliminary images using 4x4 arrays have already been obtained. An
8x8 VLSI board has been developed and is presently being tested. Since no similar
imaging systems have been developed previously, there are many new problems to be
solved with such a novel and unconventional imaging system.
These devices act as square law detectors, with detected signal proportional to THz
power. This allows them to act as mixers in heterodyne detection, thus allowing NEP
to be reduced further by almost two orders of magnitude.
Plans are to expand the arrays to larger sizes, and to employ super resolution
techniques to improve image quality beyond that ordinarily obtainable at THz
frequencies.

Active terahertz imaging with Ne indicator lamp detector arrays

Target acquisition is important especially in the military sector, with the evolution of automated systems for target acquisition there is an enormous importance to digital target detection models. Johnson's criterion is based on research with large numbers of observers. The model divides resolution into 4 stages: Detection, Orientation, Recognition and Identification. These stages are separate from one another and involve different levels of resolution. Moving from one stage to a higher stage is by increasing resolution (the number of line pairs is increased). The Johnson model was based on analog images, which are not sampled. An Image converter was the sensor in his experiments. We examined whether Johnson's criteria comply with sampled images as well as with analog ones, or if they should be changed when sampled images are used. Perception experiments with 120 observers were used to check our assumptions and the results were summarized.

Effect of sampling on target detection

Wavelength tuning via shallow junction GaAs LEDs as a result of y-irradiation is increased significantly when the irradiated LEDs are operated in vacuum Vacuum operation is seen to be essentially equivalent to increased y-ray doseage for low irradiation levels as a result of desorptive processes common to both phenomena They give rise to increased radiative surface recombination photon emission It is this spectrum which is shifted according to changes in surface potential and forward voltage deriving from alterations in surface state populations This technique is, in principle, a general technique independent of semiconductor material It suggests the possibility of wavelength tuning via surface band bending changes deriving from surface electric field changes, as is done with MIS devices Examination of irradiated diode properties in vacuum and under pressure permits greater insight into the basic nature of surface phenomena long suspected to play a significant role in the diode electronic property changes brought about by nuclear irradation© (1984) COPYRIGHT SPIE--The International Society for Optical Engineering Downloading of the abstract is permitted for personal use only

Norman S. Kopeika

Papers

Ofer Hadar, MEMBER SPIE

Restoration of thermal images distorted by the atmosphere, using predicted atmospheric modulation transfer function

Active terahertz imaging with Ne indicator lamp detector arrays

Effect of sampling on target detection

Gamma ray-irradiated LEDs in vacuum - Surface emission and significant wavelength tuning via surface states