Showing papers by "Norman S. Kopeika published in 1985"

Image Resolution Limits Resulting From Mechanical Vibrations

Abstract: High resolution airborne or vehicular imaging systems are often limited in performance by mechanical vibrations. High vibration frequency MTF is known. Low vibration frequency MTF is a random process analyzed here. Average and ideal maximum spatial frequency limitations are calculated. Plots are presented to describe the number of independent images of the same object that are required in order that at least one "lucky shot" with a given spatial frequency requirement is obtained with a given probability. Examples for short and long relative exposures are included. This data can be used to statistically define expected performance of high resolution systems and to aid accordingly in sensor selection. Probability of achieving higher resolution improves noticeably if relative exposure time is decreased.

Laser-triggered dynamic breakdown of gases and laser-induced prebreakdown signals

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.

Dynamic nonoptogalvanic signal polarity and magnitude in prebreakdown gas discharges

Abstract: Nitrogen laser pulse irradiation of prebreakdown discharges in Ne and Ar result in pulse responses strikingly similar to those reported for dynamic optogalvanic signals. For the latter, response polarity depends primarily upon atomic transition. Here, it depends primarily upon bias. Nevertheless, analysis of the results points to similar internal processes within the gas concerning metastable generation and destruction. Photoionization-assisted electron heating is an additional photon process relevant here which changes relative populations of excited states and plays a key role in reversing prebreakdown signal response polarity. It can also explain the mechanism of reported electron temperature increase in optogalvanic experiments which cannot be explained on the basis of atomic transitions.

Effects of aerosols on imaging through the atmosphere: a review of spatial frequency and wavelength dependent effects

Abstract: Experiments carried out over diagonal lines of sight through the entire atmosphere support the concept of spatial coherence degradation through forward scattering as described by an aerosol transfer function that strongly affects the wavelength dependence of imaging through the atmosphere. Airborne particulate size and concentration are affected strongly by wind strength and soil moisture. Changes in weather that result in changes in average particulate size of airborne soil-derived particulates also strongly change the wavelength dependence of resolution through the atmosphere as a result of changes in the wavelength dependence of the scattering coefficient. Knowledge of such effects can therefore permit prediction of spectral regions most suitable for imaging through the atmosphere. The roles of aerosols in determining optimum spectral regions for imaging through the atmosphere are quite significant since the turbulence modulation transfer function displays very little spectral dependence.

A simple qualitative model of surface versus bulk effects in gamma-ray irradiated p-n diodes

Abstract: Various Si and Ge rectifier diodes are tested at frequent intervals of γ‐radiation doses for changes in ideality factor η and minority carrier lifetime τ. Although both parameters are well known to decrease with dosage, here they are probably for the first time measured to also subsequently increase, decrease, increase, etc., at higher dosages. Examination of diodes in vacuum prior to and following even modest irradiation levels indicates experimentally that noticeable changes in surface properties have been induced by the irradiation. Such experimental techniques permit greater insight into the basic structure of surface phenomena long suspected to play a significant role in diode changes brought about by nuclear irradiation. Utilization of such surface changes leads to a broad general concept to explain the reversals and changes in the dosage dependences of η and τ in terms of bulk versus surface effects.

Short wavelength responsivity improvement and long wavelength responsivity degradation in photodiodes as a result of gamma irradiation

Abstract: Changes in surface and bulk properties of United Detector Technology pin-05D photodiodes as a result of 1.3 Mrad gamma irradiation are compared. As suspected by previous investigators but not verified until now, changes in surface properties are seen experimentally to significantly alter overall device characteristics. Changes in device properties include increases in surface conductivity, improved quantum efficiency at visible wavelengths, decreased dark current at very low reverse bias, decreased diode ideality factor, decreased infrared response, and decreased minority carrier lifetime. The first four results are new and permit differentiation between surface and bulk effects. A model consistent with all of these measurements is presented to explain the changes. The model is based upon gamma ray photodesorption of surface impurities.