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

We describe novel architecture for a real-time image restoration system of live TV signals. No DSP is involved. The spatial filtering is obtained from two electronic analog filters, one for the raster lines and one for the columns. The very fast response of analog filters is the key for truly real-time video frame rate performance. The digital part of the system serves the purpose of pipe-lined parallel data conversion and flow, but not that of image processing at all. Despite the lack of DSP, this architecture exhibits some very important advantages. It does not need any computational source, it is very fast, and it is much cheaper. Also our 'parallel analog computer' can be easily incorporated in any complex system with video signal data as a simple 'plug-in' between the camera and monitor. An important aspect is that the system carries lower digitalization noise than DSP, thus yielding better SNR characteristics at a lower price. The system is not bound to nay specific kind of spatial frequency filtering and can be electronically tuned to obtain exact performance parameters. Because of these advantages, this architecture is promising for a wide variety of system such as supermarket multicamera security, military and aerospace vision systems, and medical diagnostics.

Real-time two-dimensional electronic image filtering and live TV restoration

Spectral effects of desert dust on imaging through the atmosphere

Calculations are made of regression coefficients for relative humidity, air temperature, and windspeed, with respect to atmospheric contrasts in various wavelength bands over visible and near IR wavelengths as measured over a 3 year period. Significant changes are noted between summer and winter, including some sign changes and opposing wavelength dependences. Analysis of spatial frequency data indicates in the rainy season, when the atmosphere is freer of airborne soil-derived particulates, turbulence is dominant in limiting imaging resolution through the atmosphere, with wavelength dependence determined primarily by background and forward scattering effects associated with humidity. Resolution is best in the near infrared. However, in the dry season image quality is limited primarily by large airborne particulates and their effects on atmospheric background and spatial frequency-dependent multiple forward scattering phenomena. As a result, resolution is best at short wavelengths. The strong wavelength dependences on small and large radii aerosol related effects suggest the possibility of predicting imaging resolution spectral dependence in advance in accordance with meteorological predictions. Analysis of regression coefficients in the spatial frequency domain permits quantitative determination of effects of each meteorological parameter on each type of atmospheric MTF, i.e., background, aerosol, and turbulence MTF's separately. In this way insight is gained as to not only the extent to which each meteorological parameter effects imaging resolution, but also the mechanism of the effect.

Meteorological Effects On Image Quality: Reconnaissance Considerations

Many properties of the atmosphere affect the quality of images propagating through it by blurring it and reducing its contrast, as well as blur. Use of the standard Wiener filter for correction of atmospheric blur is often not effective because, although aerosol MTF (modulation transfer function) is rather deterministic, turbulence MTF is random. The atmospheric Wiener filter is one method for overcoming turbulence jitter. The recently developed atmospheric Wiener filter, which corrects for turbulence blur, aerosol blur, and path radiance simultaneously, is implemented here in digital restoration of Landsat TM (thematic mapper) imagery over seven wavelength bands of the satellite instrumentation. Turbulence MTF is calculated from meteorological data or estimated if no meteorological data were measured. Aerosol MTF is consistent with optical depth. The product of the two yields atmospheric MTF, which is implemented in the atmospheric Wiener filter. Restoration improves both smallness of size of resolvable detail and contrast. Restorations are quite apparent even under clear weather conditions. Techniques for high resolution restoration involving more versatile filtering techniques, such as Kalman's and adaptive methods, are considered by filter comparison.

Satellite image restoration filter comparison

A miniature neon indicator lamp, also known as a Glow Discharge Detector (GDD), costing about 50 cents was found
to be an excellent room temperature THz radiation detector. Down conversion detection using the GDD for 300 GHz
radiation is demonstrated in this study. Previous results with the GDD at 10 GHz showed 40 times better sensitivity
using down conversion detection compared to direct detection. Preliminary results at 300 GHz showed better
sensitivity by at least one order of magnitude using down conversion compared to direct detection. This can be
improved by increasing reference beam power.
In order to realize a down-conversion set up we used two synchronized THz sources based on RF multipliers.
The first is a 300 GHz source and the second is a 300 GHz+Δf source, where Δf stands for the frequency difference
between the two sources. Using a beam splitter configuration we combine the two frequencies for Δf=20 kHz and
directed them to the GDD. Due to the unique detection mechanism of the GDD and its linear response, the difference
frequency Δf was detected by the electronics circuits. We anticipate significant improvement in detection performance
for higher values of Δf due to lower detector noise at higher frequencies.

Norman S. Kopeika

Papers

Real-time two-dimensional electronic image filtering and live TV restoration

Spectral effects of desert dust on imaging through the atmosphere

Meteorological Effects On Image Quality: Reconnaissance Considerations

Satellite image restoration filter comparison

Down-conversion detection in 300 GHz radiation using Glow Discharge Detector (GDD)