Showing papers by "Norman S. Kopeika published in 2008"

Propagation of electromagnetic waves in Kolmogorov and non-Kolmogorov atmospheric turbulence: three-layer altitude model

Abstract: Turbulence properties of communication links (optical and microwave) in terms of log-amplitude variance are studied on the basis of a three-layer model of refractive index fluctuation spectrum in the free atmosphere. We suggest a model of turbulence spectra (Kolmogorov and non-Kolmogorov) changing with altitude on the basis of obtained experimental and theoretical data for turbulence profile in the troposphere and lower stratosphere.

Laser beam wander in the atmosphere: implications for optical turbulence vertical profile sensing with imaging LIDAR

Abstract: A summary of different approaches for laser beam wander statistics estimation is presented. The principles of an imaging LIDAR technique for remote measurements of refractive turbulence vertical profile based on image motion analysis of a secondary source created by a laser beam at a given altitude are described. The turbulence-induced beam displacement statistics and angle-of-arrival variance of backscattered wave-front are analyzed for different sensing configurations. This study has implications for airborne reconnaissance image restoration, optical satellite communication, adaptive optics, and atmospheric effects on laser weaponry.

Slant-path generalized atmospheric MTF

Abstract: The quality of imaging systems is usually expressed using the modulation transfer function (MTF). MTF is also used to express the quality of transmitted images through a random medium. The resolution, as well as MTF, is essentially dependent on the properties of such media. Transmitted images are often affected by the presence of particulate matter (aerosol, dust, fog) and turbulence between object and receiver. It is usually assumed that the turbulent field is of the Kolmogorov type and, in imaging systems, the long- and short-exposure MTF characterization is based on this type of turbulence. However, experimental data indicates that the turbulence can be different from Kolmogorov's case. Influence of Kolmogorov and non-Kolmogorov turbulence statistics on imaging system performance is analyzed for different propagation scenarios, and the generalized MTF is presented.

Relatively inexpensive terahertz imaging

Abstract: The advantages of imaging at terahertz [THz] frequencies are well known for homeland security applications. It is possible to image through non-highly conducting media, and there is no known biological hazard. Thus, it is possible to image concealed weapons and explosives. There are also many biomedical applications of THz radiation. One problem limiting widespread use is the expense of such equipment, much of which revolves around the detector. One solution is the use of miniature neon indicator lamps as detectors. Such devices cost about 30 cents each, and certain models exhibit noise equivalent powers to terahertz radiation similar to those of Golay cells and bolometers. These miniature lamps are biased to an abnormal glow discharge, and illumination with THz radiation increases the discharge current.2 In practical use, the THz radiation is amplitude modulated, and the neon glow discharge detector [GDD] detects the THz modulation envelope. Internal signal amplification on the order of a million can arise through ionizing collisions of signal electrons with neutral gas atoms. The low price of GDD lamps, the electronic ruggedness, and their THz sensitivity make them an attractive choice as detectors for novel focal plane array THz cameras.3 Indeed, such devices are currently under development. Preliminary images using 4×4 GDD arrays at 100 GHz have already been obtained. VLSI boards for a small array are being built for imaging at 300 GHz, and further development to 64×64 arrays is being planned. Image processing to improve THz image quality is also planned.

Comparing statistical and spatial characteristics of urban and rural infrared images, part 1: data analysis

Abstract: Machine vision of specific objects on natural backgrounds in the IR is an extensively studied subject. Characterizing the clutter is essential in order to evaluate a sensor's performance under various conditions. The Ben-Yosef model is the main one used for the characterization and parameterization of rural background IR images in terms of image statistics and texture. However, to the best of our knowledge, no such parameterization of urban images has been established. The aim of this work is a comparison between statistical and spatial characteristics of urban and rural scenes in the IR and their diurnal dynamics. We conclude that the Ben-Yosef model cannot fully describe the urban scene characteristics, mainly due to the model assumptions regarding the uniform spatial structure of the emissivity and of the magnitude of the solar flux over the scene. Experimental results show that, although daytime urban scenes have high variance in the IR, they have a less complex spatial structure than nighttime images, which are characterized by much lower variance.

Comparing statistical and spatial characteristics of urban and rural infrared images, part 2: background simulation

Abstract: In the previous article, a comparison between the statistical and the spatial properties of IR images of rural and urban background was presented. Analyzing the characteristics of these two backgrounds yielded noticeable differences in most of the extracted parameters and their diurnal patterns. Furthermore, the experimental data pose a remarkable deviation from those predicted by the most accepted model for desert terrain IR images (Ben-Yosef model). The effect that might be responsible for these discrepancies is the local scene topography, which is clearly enhanced in an urban environment. To investigate this hypothesis, we introduce a simple modification of the Ben-Yosef model that incorporates a simulated urban background. The comparison between the simulation and experiment shows good agreement. We conclude that the scene's topography in an urban background is the most important parameter that governs its statistical and spatial characteristics in the IR band.

Novel mm-wave and THz radiation active imaging system based on glow discharge detector (GDD) pixel

Abstract: A novel imaging system for mm wavelength and THz radiation is presented in this paper. The imaging system is based upon an inexpensive neon indicator lamp or glow discharge detector (GDD) that serves as a pixel in a focal plane array (FPA). It was shown in previous investigations that inexpensive neon indicator lamp GDDs are quite sensitive to mm wavelength and THz radiation. The diameter of the GDD lamp is 6 mm and thus the FPA can be diffraction limited. Using such neon lamps we realize a 4X4 FPA. We used a Polyethylene lens to focus the radiation on the FPA. First imaging results of the novel mm-wave and THz imaging system are presented here. They images are of decent quality. Increasing the resolution of the FPA to 8X8 or 16X16 will improve significantly the quality of the photos in the mm-wave and THz radiation. Our goal is to construct a 128X128 imaging system using the GDD technology.