Showing papers by "Norman S. Kopeika published in 1994"
TL;DR: A method of calculating numerically the optical transfer function appropriate to any type of image motion and vibration, including random ones, has been developed and an analytical approximation to the probability density function for random blur has been obtained.
Abstract: A method of calculating numerically the optical transfer function appropriate to any type of image motion and vibration, including random ones, has been developed. We compare the numerical calculation method to the experimental measurement; the close agreement justifies implementation in image restoration for blurring from any type of image motion. In addition, statistics regarding the limitation of resolution as a function of relative exposure time for low-frequency vibrations involving random blur are described. An analytical approximation to the probability density function for random blur has been obtained. This can be used for the determination of target acquisition probability. A comparison of image quality is presented for three different types of motion: linear, acceleration, and high-frequency vibration for the same blur radius. The parameter considered is the power spectrum of the picture.
93 citations
TL;DR: A model for reliability of digital optical communication in a particulate-scattering environment is presented and an adaptive method to improve and in some cases to make possible communication is suggested.
Abstract: Decreased signal-to-noise ratio and maximum bit rate as well as increased in error probability in optical digital communication are caused by particulate light scatter in the atmosphere and in space. Two effects on propagation of laser pulses are described: spatial widening of the transmitted beam and attenuation of pulse radiant power. Based on these results a model for reliability of digital optical communication in a particulate-scattering environment is presented. Examples for practical communication systems are given. An adaptive method to improve and in some cases to make possible communication is suggested. Comparison and analysis of two models of communication systems for the particulate-scattering channel are presented: a transmitter with a high bit rate and a receiver with an avalanche photodiode and a transmitter with a variable bit rate and a new model for an adaptive circuit in the receiver. An improvement of more than 7 orders of magnitude in error probability under certain conditions is possible with the new adaptive system model.
41 citations
TL;DR: In this article, a Monte Carlo simulation model for the temporal characteristics of optical pulse propagation through clouds is presented. But the model is not suitable for the propagation of optical pulses through clouds.
Abstract: Clouds, if part of an optical communication channel, cause temporal widening and attenuation of optical pulse power. Space optical communication from satellite to earth (ground or airplane) occasionally involves clouds as part of the optical channel. Here, based upon Monte Carlo simulations, mathematical models are developed for the temporal characteristics of optical pulse propagation through clouds. These include temporal impulse response, transfer function, bandwidth, received energy and bode analysis. The method presented here can be used as an inclusive framework for developing other mathematical models of other characteristics of radiation propagating through clouds, as required. Several conclusions of this work are obtained. One is that simple prediction models can be applied to adaptive methods of optical communication. Another is that using shorter wavelengths such as 0·532 μm yields least temporal widening and maximum received power, and is thus preferable for optical communication. In ...
40 citations
TL;DR: In this paper, the authors developed a mathematical model to characterize propagation through a turbid medium at three different wavelengths in the visible and near infrared spectral range, based on relations between the temporal, angular, and spatial spread of electromagnetic unpolarized radiation, geometrical path length, particle size distribution, and the medium's propagation parameters such as Mie scattering, and absorption coefficients, Mie phase-function, and optical thickness.
Abstract: Mathematical models are developed to characterize propagation through a turbid medium at three different wavelengths in the visible and near infrared spectral range. These models are based upon relations between the temporal, angular, and spatial spread of electromagnetic unpolarized radiation, geometrical path length, particle size distribution, and the medium's propagation parameters such as Mie scattering, and absorption coefficients, Mie phase-function, and optical thickness. Calculations of the radiation characteristics were carried out using Monte Carlo simulations. Here, atmospheric particulates are used to model turbid media for optical thickness between 1 and 6, emphasizing optical communication applications, The advantage of this work is the ability to predict simply and in real time important radiation parameters relevant to any optical communication system. Results indicate very high correlation between optical thickness and propagation characteristics. For transmission, comparison is...
35 citations
TL;DR: In this article, a method for restoration of thermal images distorted by the atmosphere is presented based on atmospheric MTF analysis, both theoretical and experimental, both for 3- to 5- and 8- to 12-μm wavelengths.
Abstract: Restoration of thermal images distorted by the atmosphere is presented. The method is based on atmospheric MTF analysis, both theoretical and experimental. Thermal IR atmospheric MTF measurements performed simultaneously in both atmospheric IR windows (3- to 5- and 8- to 12-μm wavelengths) are also presented. The MTFs were evaluated via point spread function measurements, under various meteorological conditions and different SNRs. Results are analyzed and shown to be in very good agreement with theoretical predictions.
32 citations
TL;DR: In this paper, an experimental investigation of the atmospheric coherence diameter is presented in order to examine the relative effects of turbulence and aerosol forward scattering, and it is shown that aerosols affect light coherence identically for both short and long exposures.
Abstract: An experimental investigation of the atmospheric coherence diameter is presented in order to examine the relative effects of turbulence and aerosol forward scattering. The investigation includes measurements through the open atmosphere for path lengths of several kilometres. In addition to turbulence degradation of the atmospheric coherence diameter, it is shown here that aerosol forward scattering also causes severe limitations, particularly for short exposures. Two methods, direct (spatial domain) and indirect (spatial frequency domain), for measuring the atmospheric coherence diameter are presented. The methods are theoretically and experimentally independent. The results of both methods are in very good agreement, emphasizing measurement reliability. It is shown that, in contradiction to turbulence, aerosols affect light coherence identically for both short and long exposures. Experimental results during rather extreme atmospheric conditions such as fog are presented too. The results here are...
30 citations
TL;DR: In contradiction to the conventional concept that absorption gives rise to constant attenuation, it is shown here that the particulate-absorbed irradiance is spatial frequency dependent.
Abstract: A new approach to studying the effects of absorption by aerosols and molecular particulates of electromagnetic radiation is presented. In contradiction to the conventional concept that absorption gives rise to constant attenuation, it is shown here that the particulate-absorbed irradiance is spatial frequency dependent. An analytically corrected model of the aerosol modulation transfer function and the aerosol mutual coherence function is presented. An important application of this model is in thermal imaging, in which particulate-absorption effects are very significant.
22 citations
TL;DR: In this article, the angular spatial frequency dependence of the MTF was investigated and a theoretical explanation was discussed, based on aerosol forward scattering and absorption effects, which is shown to be angular spatio-temporal frequency dependent and yields MTF results similar to those measured.
Abstract: Atmospheric modulation transfer function (MTF) measurements carried out simultaneously in both thermal imaging atmospheric windows using both passive and actively heated targets are presented. Results indicate rather significant angular spatial frequency dependence of the MTF, in contradiction to the conventional approach, which assumes contrast transfer is atmospheric transmission only. A theoretical explanation is discussed, based on aerosol forward scattering and absorption effects, which is shown to be angular spatial frequency dependent and yields MTF results similar to those measured. This means that small details are blurred much more than large details by the atmosphere, thus also affecting target acquisition probabilities.
21 citations
29 Jun 1994
TL;DR: In this paper, a real-time high resolution imaging through the atmosphere is presented based on the knowledge of average atmospheric MTF at the time the image is received, which can be used to help overcome the jitter characteristics of turbulence.
Abstract: A new method of real-time high resolution imaging through the atmosphere is presented. This technique is based on the knowledge of average atmospheric MTF at the time the image is received. Atmospheric effects are modeled by a noisy spatial frequency filter including an average component described by the average atmospheric modulation transfer function, and a noisy component modeled by the atmospheric point spread function's power spectral density. Analytical results are accompanied by experimental image restoration examples, indicating significant image quality improvement based upon knowledge of average atmospheric MTF. This method can be used to help overcome the jitter characteristics of turbulence, and is capable of yielding real-time image restoration with resolution limited essentially only by the hardware itself.
13 citations
TL;DR: In this paper, a theoretical and experimental investigation of image quality through an inhomogeneous turbulent medium under controlled laboratory conditions is presented, and a theoretical analysis based on simple physical principles leads to the same 5/3 power theoretical result obtained 25 years ago by Fried [1] on the basis of phase covariance and phase structure function.
Abstract: A theoretical and experimental investigation of image quality through an inhomogeneous turbulent medium under controlled laboratory conditions is presented. In addition, a theoretical analysis based on simple physical principles leads to the same 5/3 power theoretical result obtained 25 years ago by Fried [1] on the basis of phase covariance and phase structure function. The present analysis, however, can be expanded to indicate how effects of turbulence depend upon field of view. This work is applicable to determining optimum elevation for imaging through inhomogeneous turbulence near the surface of the Earth.
12 citations
TL;DR: In this paper, effects of acceleration are considered for two important concerns: image quality and target acquisition and a comparison between the effects of uniform velocity and accelerated motion is presented.
Abstract: One of the most apparent aspects of motion in real-time airborne systems is acceleration. In this paper, effects of acceleration are considered for two important concerns: image quality and target acquisition. A comparison between the effects of uniform velocity and accelerated motion is presented. There are two opposing considerations when flying over hostile territory: The pilot must fly as fast as possible so as not to be detected or attacked by the enemy, but sufficiently slow so that the degradation of the image quality will not be too severe. Mathematical tools developed recently permit a quantitative analysis of the effects of acceleration on the image quality and acquisition of the target.
08 Jun 1994
TL;DR: This work defines typical characteristics of optical pulse propagation through clouds and proposes a model for optimum performance of digital optical communication through clouds using Monte-Carlo simulation.
Abstract: Optical space communication will use clouds as part of communication channels. Propagation of optical pulses through clouds causes widening and deformation in the time domain and attenuation of the pulse radiant power. These effects decrease the received signal and limit the information bandwidth of the communication system. This work defines typical characteristics of optical pulse propagation through clouds. Characteristics of the optical pulses are calculated using Monte-Carlo simulation. Based on these characteristics a model for optimum performance of digital optical communication through clouds is presented. Examples for practical communication systems are given. An adaptive method to improve and in some cases to make possible communication is suggested. Comparison and analysis of two models of communication systems in cloud channels are presented: (1) adaptive transmitter and standard receiver (semi-adaptive system) and (2) adaptive transmitter and receiver (adaptive system). An improvement of more than eight orders of magnitude in bit error rate under certain conditions is possible with the new adaptive system model.
08 Jul 1994
TL;DR: In this paper, the CNVEO target acquisition search model is revised to include atmospheric modulation transfer function (MTF) spatial frequency dependence, and it is shown how the target acquisition probabilities and the range at which targets can be detected are changed by the inclusion of atmospheric effects.
Abstract: Recent investigation of atmospheric modulation transfer function (MTF) in the thermal range has shown significant spatial frequency dependence. This dependence is related to aerosol forward scatter as well as absorption effects, shown to be primary contributors to the blur in thermal imaging through the atmosphere. In this paper, the night vision laboratory (CNVEO) target acquisition search model is revised to include atmospheric MTF spatial frequency dependence. It is shown how the target acquisition probabilities and, conversely, the range at which targets can be detected are changed by the inclusion of atmospheric effects. Under weak turbulence conditions target acquisition probabilities are better than predicted by the CNVEO model at shorter ranges. For very strong turbulence, as at midday on hot days, target acquisition probabilities decrease. The improvement possible by correcting for this blur automatically in real time is significant.
29 Jun 1994
TL;DR: In this article, an experimental investigation of atmospheric coherence diameter is presented in order to examine relative effects of turbulence and aerosol forward scattering, and it is shown that, in contradiction to turbulence, aerosols affect light coherence identically for both short and long exposures.
Abstract: Experimental investigation of atmospheric coherence diameter is presented in order to examine relative effects of turbulence and aerosol forward scattering. The investigation includes measurements through the open atmosphere for path lengths of several kilometers. In addition to turbulence degradation of atmospheric coherence diameter, it is shown here that aerosol forward scattering also causes severe limitations, particularly for short exposure. Two methods, direct (spatial domain) and indirect (spatial frequency domain), for measuring atmospheric coherence diameter are presented. The methods are theoretically and experimentally independent. Results of both methods are in very good agreement, emphasizing measurement reliability. It is shown that, in contradiction to turbulence, aerosols affect light coherence identically for both short and long exposures. Experimental results during rather extreme atmospheric conditions such as fog are presented too. The results here are applicable to cost-effective imaging system design, and to predicting imaging system performance through the atmosphere.
09 Oct 1994
TL;DR: A method of calculating numerically the optical transfer function appropriate to any type of image motion and vibration, including random ones, has been developed and this method has been verified experimentally, and the close agreement justifies implementation in image restoration for blurring deriving from any typeof image motion.
Abstract: A method of calculating numerically the optical transfer function appropriate to any type of image motion and vibration, including random ones, has been developed. This method has been verified experimentally, and the close agreement justifies implementation in image restoration for blurring deriving from any type of image motion. The goal of this research is to recover the original image from its degraded version. There are many methods of image restoration based on the point spread function. One of the common methods is the Wiener filter. Here, some image restorations of physically degraded images are presented, based on a constrained least squares improvement of the original Wiener filter. The key to restoration is determination of the optical transfer function unique to each particular image motion and vibration.
TL;DR: In this article, a novel near ultraviolet (UV) gas-filled detector which is operated in the pre-breakdown regime is described, where the detection mechanism is photoionization of the excited state.
Abstract: A novel near ultraviolet (UV) gas-filled detector which is operated in the prebreakdown regime is described. The detection mechanism is photoionization of the excited state. Modulated continuous UV light was used to study experimentally the detector responsivity and the spectral response for typical electrode materials such as Mo, Ti, W and Ta at around 200 nm wavelength. Of the materials investigated, molybdenum yielded the highest relative responsivity. This is attributed to the secondary electron emission and, in particular, to the initial electron energy after secondary emission, which contributes to internal signal gain and to improved quantum efficiency through increased excitation. For molybdenum electrodes, the detector performance was studied with various electrode geometries. Of the electrode configurations considered, two straight parallel wires which are located parallel to the incoming UV photon direction yields the best responsivity. This is attributed to maximum electric field nonuniformity and photon interaction depth between the electrodes. Effective quantum efficiency, including detector internal gain, is found to be about 5570% per photon at 200 nm wavelength. This leads to an effective yield of 9 A W-1, including internal gain, at 200 nm wavelength. The noise equivalent power NEP is estimated to be approximately 10-15 W Hz-12/.
25 Oct 1994
TL;DR: It appears from both experiments that image motion increases the detection time of a target by the observer as the complexity of the original image increases, detection time is more affected, increasing more rapidly with blur radius in the first experiment and with velocity in the second experiment.
Abstract: The effect of linear image motion and high frequency vibrations on human performance for target acquisition is considered. Two clutter metrics, one local and the other global, are combined to one metric of signal-to-clutter ratio (SCR). The SCR is used as a parameter in the model for actual target acquisition results. Two experiments involving human observers are considered. A static experiment is developed with spatial filters representing image motion, and a dynamic experiment is described which imitates the operation of a scanning camera with a constant velocity. It appears from both experiments that image motion increases the detection time of a target by the observer. As the complexity of the original image increases, detection time is more affected, increasing more rapidly with blur radius in the first experiment and with velocity in the second experiment.
08 Jul 1994
TL;DR: In this article, the effect of low frequency mechanical vibrations in the image plane on thermal imaging target acquisition is considered and a model is described that takes as input the mechanical vibration data such as amplitude and frequency and the physical characteristics of the target.
Abstract: The effect of low frequency mechanical vibrations in the image plane on thermal imaging target acquisition is considered. A model is described that takes as input the mechanical vibration data such as amplitude and frequency and the physical characteristics of the target. The output is the probability of detection as a function of time. Analysis indicates that even low amplitude vibration greatly affects the predicted target detection times and, hence, the utility of the IR system in realistic scenarios.
29 Jul 1994
TL;DR: In this article, the influence of image vibrations and motion on the probability of acquiring the target with an automatic target recognition (ATR) system is analyzed, including factors that characterize the relationship existing between the target and its background, enabling efficient performance in the prediction and evaluation of any ATR system.
Abstract: The resolution capability of imaging systems is effected by a blurring effect due to the vibration and motion recorded in the image. This disturbance is often more severe than electronic and optical limitations inherent in the system. This fact must be considered when dealing with the development and analysis of automatic target recognition (ATR) systems for military applications. The aim of this research is to analyze the influence of image vibrations and motion upon the probability of acquiring the target with an ATR system. The analysis includes factors that characterize the relationship existing between the target and its background. A high level of correlation is expected between these factors and the probability of target detection, enabling efficient performance in the prediction and evaluation of any ATR system. The results of this research can be implemented in military applications as well as in developing image restoration procedures for image-blur conditions.
29 Jun 1994
TL;DR: In this article, the size distribution parameters measured by optical counters are related to weather parameters, which can be used to predict under which conditions the MODTRAN aerosol models have good or poor accuracy.
Abstract: Predictions of atmospheric transmittance in desert aerosol environments using MODTRAN code diverge significantly from measured data. Good prediction of the desert particulate size distribution is required in order to predict atmospheric scattering and absorption parameters. It is also essential to the prediction of the aerosol atmospheric modulation transfer function which is often the dominant component of the overall atmospheric MTF. Recently an effort to predict statistics but not size distribution according to simple weather parameters has been made for coarse desert aerosols. A quantitative analysis of the desert particulate size distribution models was also performed. In this research the size distribution parameters measured by optical counters are related to weather parameters. Known statistical and analytical models such as MODTRAN relate the size distribution parameters only to relative humidity for continental atmospheres. Although humidity has a significant role in the prediction of aerosol size statistics, other weather parameters are seen here to strongly influence also the size distribution parameters. Comparisons such as the above can be used to predict under which conditions the MODTRAN aerosol models have good or poor accuracy. It is also hoped that they will lead to improvements in MODTRAN, improving the accuracy of the humidity dependence as well as by incorporating other meteorological parameters into the MODTRAN prediction models.
14 Sep 1994
TL;DR: In this article, a general and inclusive framework for understanding and characterizing the atmospheric effects in the thermal infrared in imaging systems is presented This includes separation of the atmospheric distortions to their main two ingredients: optical turbulence, and scattering and absorption by atmospheric particulates.
Abstract: In this paper, a general and inclusive framework for understanding and characterizing the atmospheric effects in the thermal infrared in imaging systems is presented This includes separation of the atmospheric distortions to their main two ingredients: optical turbulence, and scattering and absorption by atmospheric particulates A basic and correct understanding of those effects enables the development of prediction models for the atmospheric turbulence and aerosol MTFs Here, both models are implemented for real-time thermal image restoration, with the prior knowledge of standard meteorological parameters, and specifications of the imaging system Examples of such restorations are presented and the uniqueness of the restoration method is discussed with the inclusion of atmospheric degradation of the received image with and without subsequent image restoration The most important conclusion is that knowledge of the expected atmospheric MTF is crucial for the system designer
08 Jul 1994
TL;DR: In this paper, the restoration of thermal images distorted by the atmosphere, received on a focal plane array (FPA) thermal imaging system, is presented based upon atmospheric modulation transfer function (MTF) analysis.
Abstract: 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.
30 Sep 1994
TL;DR: In this article, the restoration of thermal images distorted by the atmosphere, detected with a focal plane array (FPA) Pt-Si thermal imaging system, is presented based upon atmospheric modulation transfer function (MTF) analysis.
Abstract: Restoration of thermal images distorted by the atmosphere, detected with a focal plane array (FPA) Pt-Si 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 degradation are modeled. Restoration results indicate significant improvement in image quality. However, it is critical to include the unique shape of aerosol MTF when modeling atmospheric MTF in order to obtain good restoration.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
17 Oct 1994
TL;DR: In this paper, an active method for measuring atmospheric transmittance with an imaging system is presented, which has the advantage of immunity to background noise, independence of atmospheric conditions such as solar radiation, and improved capability to evaluate effects of turbulence on the measurements.
Abstract: An active method is presented for measuring atmospheric transmittance with an imaging system. In comparison to other measurement methods, this method has the advantage of immunity to background noise, independence of atmospheric conditions such as solar radiation, and an improved capability to evaluate effects of turbulence on the measurements. Other significant advantages are integration over all particulate size distribution effects including very small and very large particulates whose concentration is hard to measure, and the fact that this method is a path-integrated measurement. In this implementation attenuation deriving from molecular absorption and from small and large particulate scatter and absorption and their weather dependences are separated out. Preliminary results indicate high correlation with direct transmittance calculations via particle size distribution measurement, and that even at 10.6 micrometers wavelength atmospheric transmission depends noticeably on aerosol size distribution and concentration.