Showing papers by "Norman S. Kopeika published in 1987"

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 required so 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. These data can be used to statistically define expected performance of high resolution systems and to aid accordingly in sensor selection. The probability of achieving higher resolution improves noticeably as relative exposure time is decreased.

Imaging Through The Atmosphere For Airborne Reconnaissance

Abstract: Turbulence, atmospheric background, and aerosol forward scattering modulation transfer functions (MTFs) are analyzed with regard to both low elevation remotely piloted vehicles and high elevation reconnaissance applications. Turbulence is seen to limit image quality only at very high spatial frequencies, where degradation is likely to take place anyway as a result of vibration and diffraction. Background and aerosol MTFs limit low spatial frequency contrast as well. However, this can be overcome somewhat by proper selection of the imaging wavelength and of operation timing. This analysis can aid in sensor selection for system design from the standpoints of both wavelength selection and sensor resolution. Because this analysis includes the effects of weather changes on image propagation through the atmosphere, it also can aid in selecting operation timing on the basis of weather forecasts, with a view toward optimizing expected resolution.

Non-Optogalvanic Signal Characteristic Times in Prebreakdown Discharges

Abstract: The prebreakdown regime, which exists in a gas cell during the time interval of free charge density growth from first electron-ion pair generation until the onset of breakdown, is characterized by low bias and incident field intensities In our experiment, dc biased Ne and Ar gas cells at 90 torr each were illuminated with laser pulses Both the incident laser field and dc bias field were smaller, sometimes much smaller, than those required for gas breakdown by each alone Under these conditions, clear-cut electrical signals of amplitude on the order of tens of millivolts are observed as a response to laser pulse irradiation at wavelengths non resonant to any gas atomic transitions We called these non-breakdown responses “prebreakdown or precursor signals” In 1,2 such prebreakdown signal magnitude, polarity and occurrence probability as a function of incident field intensities were discussed

Space as an Adverse Environment: Vacuum Surface and Gamma Ray Irradiation Effects on LED's and Photodiodes

Abstract: Vacuum surface and gamma-ray irradiation effects on photodiodes and LED's play a significant role in the optical and electronic properties of these diodes, particularly in a space environment, which involves both ionizing radiation as well as vacuum conditions. Exposure of GaAs light emitting diodes (LED's) to vacuum gives rise to desorption of surface-adsorbed gases and subsequent free charge diffusion and redistribution that significantly alter diode properties. Changes in photodiode properties due to vacuum operation and y-radiation have also been observed. These changes are given, and a model explaining the changes is presented.

Imaging Through The Atmosphere For Airborne Reconnaissance

Abstract: Turbulence, atmospheric background, and aerosol forward scattering MTFs are presented and analyzed with regard to both low elevation rpv and high elevation reconnaissance applications. Turbulence is seen to limit image quality only at very high spatial frequencies where degradation effects are likely to take place anyway as a result of vibrational and diffraction effects. Background and aerosol MTFs limit low spatial frequency contrast as well. However, this can be overcome somewhat by proper selection of imaging wavelength. This analysis can aid in sensor selection for system design from the standpoints of both wavelength selection and sensor resolution.

Optoelectronic Diodes And CCDs In Vacuum

Abstract: Electrical and optical properties of light-emitting diodes and photodiodes can change significantly in vacuum. These changes involve alterations in forward and reverse current-voltage and capacitance-reverse voltage characteristics, time response, spectral characteristics, quantum efficiency, diode ideality factor, etc. Here, attention is centered on alterations in electrical properties of imaging CCDs in vacuum. Such alterations include increased dark signal, increased responsivity at longer wavelengths in particular, and non-linearity of response.