Showing papers by "Norman S. Kopeika published in 1998"

A System Engineering Approach to Imaging

Abstract: This textbook addresses imaging from the system engineering point of view, examining advantages and disadvantages of imaging in various spectral regions. Focuses on imaging principles and system concepts, rather than devices. Intended as a senior-year undergraduate or graduate level engineering textbook. Solution manual included.

Direct method for restoration of motion-blurred images

Abstract: We deal with the problem of restoration of images blurred by relative motion between the camera and the object of interest. This problem is common when the imaging system is in moving vehicles or held by human hands, and in robot vision. For correct restoration of the degraded image, it is useful to know the point-spread function (PSF) of the blurring system. We propose a straightforward method to restore motion-blurred images given only the blurred image itself. The method first identifies the PSF of the blur and then uses it to restore the blurred image. The blur identification here is based on the concept that image characteristics along the direction of motion are affected mostly by the blur and are different from the characteristics in other directions. By filtering the blurred image, we emphasize the PSF correlation properties at the expense of those of the original image. Experimental results for image restoration are presented for both synthetic and real motion blur.

Experimental investigation of the influence of the relative position of the scattering layer on image quality: the shower curtain effect

Abstract: The imaging quality of optical systems in a turbid environment is influenced not only by the content of the turbid layer between the object and the optical receiver but also by the inhomogeneity of that medium. This is important, particularly when imaging is performed through clouds, nonhomogeneous layers of dust, or over vertical or slant paths through the atmosphere. Forward small-angle scattering influences image quality and blur more severely when the scattering layer is closer to the receiver. In this study it is the influence of the relative position of the scattering layer on the image quality and modulation transfer function (MTF) that is investigated. The scattering layer in controlled laboratory experiments consists of calibrated polystyrene particles of known size and quantity in a small cuvette. A point source was imaged by a computerized imaging system through a layer containing polystyrene particles, and the point-spread function (PSF) was recorded. The aerosol MTF was calculated using the measured PSF. The MTF was measured as a function of changing relative distance of the scattering layer from the receiver, whereas the object-plane-to-receiver distance was constant. The experimental results were compared to theoretical shower curtain effect models based on the solution from radiative transfer theory under the small-angle approximation. Although the general trend of the experimental results certainly agrees with the theoretical models, it could be that the small-angle approximation method might be of limited validity at such low spatial frequencies. Aggregation also causes some disagreement with predictions from theory.

Causes of atmospheric blur: comment on Atmospheric scattering effect on spatial resolution of imaging systems

Abstract: A paper by Ben Dor [J. Opt. Soc. Am. A14, 1329 (1997)] concludes that the blur we measured in our experiments was not atmospherically scattered light and that our theoretical model is incorrect because it violates the rules of linearity. Their work is based in part on “lack of raw data” in one of our experimental papers [J. Opt. Soc. Am A12, 970 (1995)]. We present here the raw data measured in the experiments in question, which show clearly the measured atmospherically scattered light. Similar raw data has also been published elsewhere regarding other experiments. We also clarify some rules of linear systems that justify our conceptual approach, which is shown to be similar to that of turbulence modulation transfer function. A review of several dozen experiments and analyses by other investigators all over the world that directly contradict the Ben Dor et al. results and conclusions is presented. The well-known significance of aerosol blur in imaging through the atmosphere from satellites is discussed, and pictorial examples of satellite imagery are shown for different atmospheric optical depths. It is noted that atmospheric point-spread-function analyses in the remote-sensing literature generally neglect turbulence blur altogether and deal with aerosol blur only, which is often called the adjacency effect, and that such phenomena are well supported by many different types of experiments and many different Monte Carlo simulations for many different aerosol and instrumentation parameter situations. The Monte Carlo simulation results of Ben Dor et al. are shown also to contradict everyday reality such as the solar aureole. This wealth of literature by others strongly contradicts the results of Ben Dor et al. and confirms our conclusion that forward scatter of light by aerosols is indeed a significant source of blur in imaging through the atmosphere, especially if atmospheric optical depth is on the order of unity or more. This can be confirmed, too, by any observer looking through binoculars at the moon and surrounding moonlight even on a clear night. A broad system engineering approach involving both aerosol and turbulence blur is called for.

Optimum transmitter optics aperture for satellite optical communication

Abstract: An important aspect in satellite optical communication is to obtain minimum bit error rate (BER) using minimum power. This aim can be achieved with very small transmitter beam divergence angles. The disadvantages of too narrow divergence angle is that the transmitter beam may sometimes miss the receiver satellite, due to pointing vibrations. A mathematical model of communication and tracking systems that optimize the BER as function of the transmitter gain is derived.

Performance limitations of a free-space optical communication satellite network owing to vibrations: heterodyne detection

Abstract: Free-space optical communication between satellites in a distributed network can permit high data rates of communication between different places on Earth. To establish optical communication between any two satellites requires that the line of sight of their optics be aligned during the entire communication time. Because of the large distance between the satellites and the alignment accuracy required, the pointing from one satellite to another is complicated because of vibrations of the pointing system caused by two fundamental stochastic mechanisms: tracking noise created by the electro-optic tracker and vibrations derived from mechanical components. Vibration of the transmitter beam in the receiver plane causes a decrease in the received optical power. Vibrations of the receiver telescope relative to the received beam decrease the heterodyne mixing efficiency. These two factors increase the bit-error rate of a coherent detection network. We derive simple mathematical models of the network bit-error rate versus the system parameters and the transmitter and receiver vibration statistics. An example of a practical optical heterodyne free-space satellite optical communication network is presented. From this research it is clear that even low-amplitude vibration of the satellite-pointing systems dramatically decreases network performance.

Adaptive suboptimum detection of an optical pulse-position-modulation signal with a detection matrix and centroid tracking

Abstract: In some applications of optical communication systems, such as satellite optical communication and atmospheric optical communication, the optical beam wanders on the detector surface as a result of vibration and turbulence effects, respectively. The wandering of the beam degrades the communication system performance. In this research, we derive a mathematical model of an optical communication system with a detection matrix to improve the system performance for direct-detection pulse-position modulation. We include a centroid tracker in the communication system model. The centroid tracker tracks the center of the beam. Using the position of the beam center and an a priori model of the beam spreading, we estimate the optical power on each pixel (element) in the detection matrix. Using knowledge of the amplitudes of signal and noise in each pixel, we tune adaptively and separately the gain of each individual pixel in the detection matrix for communication signals. Tuning the gain is based on the mathematical model derived in this research. This model is defined as suboptimal, owing to some approximations in the development and is a suboptimum solution to the optimization problem of n multiplied by m free variables, where n, m are the dimensions of the detection matrix. Comparison is made between the adaptive suboptimum model and the standard model. From the mathematical analysis and the results of the comparison it is clear that this model significantly improves communication system performance.

Experimental comparison of three target acquisition models

Abstract: The role of the atmosphere in target acquisition modeling is investigated experimentally. Three models are compared to experimental results measured on the Golan Heights, israel. Concepts considered are atmospheric attenuation versus atmospheric blur, and contrast- limited (blur-limited) versus noise-limited imaging. Results indicate that the role of the atmosphere in target acquisition is blur rather than attenuation and that for ranges of the order of a few kilometers, modern sensors are limited by atmospheric blur rather than by noise. A significant portion of the atmospheric blur derives from small angle forward scattering by aerosols, which actually increases measured temperature differences for ranges up to a few kilometers.

Adaptive bandwidth for satellite optical communication

Abstract: Satellites in free space suffer from periods of high displacement amplitude vibrations. Most of the vibration is caused by satellite internal subsystems (such as thruster firing or solar array drive mechanism) and controlled by the satellite computer. To utilise the advantages of optical communication in space, very narrow divergence transmitted beams are used. The high-amplitude vibrations of the transmitter satellite cause a decrease in received signal power in the receiver satellite due to mispointing of the transmitted beam. In the paper, the authors derive a model of a communication system that adapts the communication system parameters to changes in received signal caused by changes in vibration amplitude. The purpose of this model is to keep the bit error rate (BER) low and constant by adapting the system bandwidth and the receiver parameters to the vibration amplitude. This model is useful for communication systems with two or more priorities of real time – for example, telephone calls and electronic mail. This means that, when the bandwidth shrinks, electronic mail messages may be delayed but the phone calls can continue. Comparison and analysis of the performance of practical standard and adaptive models of communication systems for variable vibration amplitude are presented.

General restoration filter for vibrated-image restoration.

Abstract: Mechanical vibrations are often the principal cause of image degradation. Low temporal-frequency mechanical vibrations involve random image degradation that depends on the instant of exposure. Exact restoration requires the calculation of a specific filter unique to each vibrated image. To calculate the restoration filter for each image, one needs the specific optical transfer function unique to the motion in the image. Therefore the instant of exposure and the motion function have to be measured or estimated by some other means. We develop a restoration filter for individual images blurred randomly by low-frequency mechanical vibrations. The filter is independent of the instant of exposure. The filter is designed to give its best performance averaged over a complete ensemble of vibrated images. Although when applying the new filter to any vibrated image the restoration achieved is slightly poorer than that achieved with an exact filter unique to the specific motion function, the new filter has the advantage of simplicity.

Bandwidth maximization for satellite laser communication

Abstract: Free space optical communication between satellites networked together can make possible high speed communication between different places on earth. The basic free space optical communication network includes at least two satellites. In order to communicate between them, the transmitter satellite must track the beacon of the receiver satellite and point the information optical beam in its direction. The pointing systems for laser satellite communication suffer during tracking from vibration due to electronic noise, background radiation from interstellar objects such as sun, moon, earth and stars in the tracking field of view, and mechanical impact from satellite internal and external sources. Due to vibrations the receiver receives less power. This effect limits the system bandwidth for given bit error rate. In this research we derive an algorithm to maximize the communication system bandwidth using the transmitter telescope gain as a free variable based on the vibration statistics model and the system parameters. Our model makes it possible to adapt the bandwidth and transmitter gain to change of vibration amplitude. We also present an example of a practical satellite network which includes a direct detection receiver with an optical amplifier. A bandwidth improvement of three orders of magnitude is achieved in this example for certain conditions, as compared to an unoptimized system.

Relative effects of distortion and noise on target acquisition: the advisability of image restoration

Abstract: Any image acquired by optical, electro-optical, or electronic means is likely to be degraded by the environment. The resolution of the acquired image depends on the total modulation transfer function (MTF) of the system and the additive noise. Image restoration techniques can improve image resolution significantly; however, as the noise increases, improvements via image processing become more limited because im- age restoration increases the noise level in the image. We characterize the influence of the MTF and noise level on human target acquisition probability to ascertain the advantages, if any, of image restoration. Con- ditions when restoration would be advisable are determined. © 1998 So- ciety of Photo-Optical Instrumentation Engineers. (S0091-3286(98)01007-1)

Imaging vertically through the atmosphere: restoration of satellite images based on atmospheric MTF evaluation

Abstract: When carrying out satellite images by imaging vertically through the atmosphere, distortions and blur arise as a result of turbulence and aerosols. Contrast is reduced by path radiance. The recently developed atmospheric Wiener filter, which corrects for turbulence blur, aerosol blur, and path radiance simultaneously, is implemented in digital restoration of Landsat imagery over seven wavelength bands of the satellite instrumentation. A required input is weather. Restoration is most impressive for high optical depth situations, which cause larger blue. Restoration improves both smallness of size of resolvable detail and contrast. Turbulence modulation transfer function (MTF) is calculated from meteorological data. Aerosol MTF is consistent with optical depth. The product of the two yields atmospheric MTF, which is implemented in the atmospheric Wiener filter. Turbulence blue, aerosol blur, and path radiance contrast loss are all corrected simultaneously, as if there were in intervening atmosphere. The primary source of atmospheric blur is seen to be aerosol forward scatter of light. Restorations are shown for various wavelength bands and are quite apparent even under clear weather conditions.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Causes of blur in imaging through the atmosphere: a system engineering approach to imaging

Abstract: Aerosol blur, often referred to as the adjacency effect, is well-established as the primary and perhaps only source of atmospheric blur in remote sensing imaging from satellites. However, much of the propagation community considers turbulence blur only in interpreting experiments, and then notes discrepancies with turbulence theory without considering how aerosol blur may have affected their experiments, Because of the complexities of atmospheric and meteorological processes a broad system engineering approach is called for, which includes aerosols, turbulence, absorption, and any other atmospheric effects. In general, turbulence is most significant at low elevations up to a few meters above earth's surface, and aerosol blur is most significant at higher elevations, especially if optical depth is on the order of unity or more.

Causes of atmospheric blur in remote sensing: a system engineering approach to imaging

Abstract: Aerosol blur, often referred to as the adjacency effect, is well-established as the primary and perhaps only source of atmospheric blur in remote sensing imaging from satellites. However, much of the propagation community considers turbulence blur only in interpreting experiments. Because of the complexities of atmospheric and meteorological processes a broad system engineering approach is called for, which includes aerosols, turbulence, absorption, and other atmospheric effects. In general, turbulence is most significant at low elevations up to a few meters above earth's surface, and aerosol blur is most significant at higher elevations, especially if optical depth is on the order of unity or more. However, turbulence and aerosol effects increase in the stratosphere.

Role of the atmosphere in target acquisition: models versus experiment

Abstract: The role of the atmosphere in target acquisition modeling is investigated experimentally. Three models are compared to experimental results measured on the Golan Heights, Israel. Concepts considered are atmospheric attenuation versus atmospheric blur, and contrast limited (blur-limited) versus noise limited imaging. Results indicate that the role of the atmosphere in target acquisition is blur rather than attenuation, and that for ranges on the order of a kilometer or more, target acquisition is limited by atmospheric blur rather than by hardware. A significant portion of the atmospheric blur derives from small angle forward scattering by aerosols, which actually increases measured temperature differences for ranges up to a few kilometers.

Diffraction-Limited Imaging

Abstract: After a brief introduction to Fourier optics in the previous chapter, we now return to imaging with incoherent light. In Chapter 2 geometrical optics was used to describe image formation and location. Diffraction effects were neglected, and distortions from ideal image quality were attributed to chromatic and spherical aberrations. At this point, another limitation to image quality can be considered, namely, diffraction. Diffraction causes parallel light rays incident on the edges of optical elements such as lenses or mirrors to be deflected away from the focal point, thus degrading image quality. This causes spreading of point images and loss of resolution as described on p. 244 and in Section 7.4 for coherent light. Here we consider primarily incoherent light which is normally used in imaging. Diffraction-limited imaging refers to a situation in which image quality is limited by such diffraction. It is very common, particularly when geometrical optics aberrations are minimized through use of achromatic elements and parabolic rather than spherical surfaces. Moreover, diffraction-limited imaging is a convenient method in which to introduce the use of system concepts such as impulse response and transfer function to characterize image quality. These system concepts are introduced here and emphasized throughout the remainder of this book.

Aerosol light scatter vs turbulence effects in image blur

Abstract: A recent paper by Belen'kii proposed an inner scale turbulence MTF theory model to explain one of the many types of aerosol MTF experimental results by Dror, Sadot, and Kopeika. A broad comparison is made here between Belen'kii's inner scale turbulence MTF and our practical instrumentation-based aerosol MTF models and experiments. Belen'kii's model is strongly contradicted by those experimental results themselves, as well as by the many other published results not considered by Belen'kii. The Dror, Sadot, Kopeika experiments all contain comparisons of practical instrumentation-based aerosol MTF modeling which was calculated from aerosol size distributions actually measured during the experiments. These aerosol MTF calculations validated rather accurately the practical aerosol MTF model measurements. A summary of 9 independent different types of measurements and analyses which show we measured aerosol MTF and not any form of turbulence MTF is included, as well as specific contradictions between Belen'kii's model and our actual measurements. These also indicate our measurements of turbulence MTF were indeed correct, and that a broad system engineering approach to atmospheric optics should be encouraged instead of the narrow pure turbulence, pure aerosol, or pure absorbing atmospheric models often used.

Blur in imaging through the atmosphere: a system engineering approach to imaging

Abstract: Aerosol blur, often referred to as the adjacency effect, is well-established as the primary and perhaps only source of atmospheric blur in remote sensing imaging from satellites. However, much of the propagation community considers turbulence blur only in interpreting experiments, and then notes discrepancies with turbulence theory without considering how broad system engineering approach is called for, which includes aerosols, turbulence, absorption, and many other atmospheric effects. In general, turbulence is most significant at low elevations up to a few meters above earth's surface, and aerosol blur is most significant at higher elevations, especially if optical depth is on the order of unity or more.

Comparison of direct methods for restoration of motion-blurred images

Abstract: Simple filters used to restore blurred images require knowledge of the point spread function (PSF) of the blurring system. Unfortunately such knowledge is usually not available when the blur is caused by relative motion between the camera and the scene. Various methods addressing this problem were developed in the last four decades. These methods can be divided into two types: direct methods whereby the restoration process is performed in a one step fashion, and indirect methods whereby the restoration process is performed by an iterative technique. Direct methods usually require identification of the PSF as a first step, and then use it to restore the blurred image with a simple filter. Lately, a new direct method was developed. As a result of this development, direct restoration methods (given only a single blurred image) are studied and compared in this paper for a variety of motion types. Various criteria such as quality of restoration, sensitivity to noise and computation requirements are considered.

Contrast-Limited Resolution and Target Acquisition

Abstract: In the previous two chapters the optical transfer function was introduced and its physical implications concerning contrast derived. In the present chapter, these results are applied toward determining image resolution in a quantitative manner. By resolution we mean the smallest size detail that can be resolved. This can be related to system optical transfer function and spatial frequency bandwidth. However, in reality image quality and resolution are very subjective parameters, varying considerably with each person’s visual system characteristics and its dependence on background irradiance—system signal-to-noise ratio. Therefore, the quantitative formulations presented in this and the next few chapters should be viewed as representative rather than as absolute quantities. A more appropriate approach is stochastic rather than deterministic and is presented as well. System design integrating the material from the previous eight chapters is emphasized strongly in the exercises and solutions.

Using satellite vibrations to improve performance of free-space satellite laser communication

Abstract: In some of the future laser communication satellites, it is plausible to assume that tracking and communication receivers will be using the same array of detector. The reason for such detector dual use is to design simpler and cheaper satellites. Satellites vibrate continually due to operation of their subsystems and environmental sources. The vibrations cause non-uniform spreading of the received energy on the detector array. In view of this, we use the information from the tracking system in order to adapt individually the communication signal gain of each of the detectors in the array. This adaptation of the gains improves communication system performance. It is important to emphasize that the communication performance improvement is achieved by only gain adaptation. Any additional vibrations decrease the tracking and laser pointing system performances, which decrease the return communication performances (two-way communication). A comparison of practical communication systems is presented. The novelty of this research is the utilization of natural satellite vibrations to improve the communication system performance.

Modulation Contrast Function

Abstract: In Chapter 8 the optical transfer function was introduced. It was used first to describe limitations on image quality stemming from diffraction by various shape and size apertures. Such diffraction at aperture edges gives rise to deflection of light incident on such aperture edges. This deflection causes rays incident on aperture edges to take exit paths different from those of rays incident on the middle portions of the aperture. The latter rays behave according to Snell’s law and form the image. The diffracted rays give rise to image blur. In Section 8.9, it was suggested that the transfer function approach can be used to describe image quality in general for an imaging system and for its various components and not only for diffraction-limited imaging.

Optical Properties of the Atmosphere

Abstract: Many properties of the atmosphere affect the quality of images propagating through it. There are atmospheric phenomena that give rise to attenuation of the irradiance of the propagating image, thus reducing the contrast of the final image. There are also atmospheric phenomena that cause blurring of detail. Both types of phenomena prevent small detail from being resolved in the final image, thus degrading image quality. Phenomena that give rise to attenuation are electromagnetic (EM) wave absorption and scattering by the constituent gases and particulates of the atmosphere and airborne particulates. Scattering of photons by airborne particulates is manifested as deflections of the photons to directions other than that of original propagation. If such scattering causes the deflected photons to miss the imaging receiver, then the scattering is manifested as attenuation. The received irradiance of the image propagated through the atmosphere is correspondingly diminished from that in the object plane. However, if the light scattering is at very small angles with respect to the original directions of propagation, and several such small-angle scattering events take place, then forward-scattered radiation can take round-about paths and still be received by the imaging system together with the unscattered radiation. The net effect is image blurring caused by a multitude of angles of arrival at the imaging receiver of radiation, emanating from the same point in the object plane, for one multiply forward-scattered ray and one unscattered ray. Many such multiple-scattering paths give rise to a relatively large blurred-point image rather than a fine sharp-point image. Several adjacent object plane points can then appear as a single blurred image plane point, thus degrading image resolution.

Optical Transfer Functions for Image Motion and Vibration

Abstract: Image blurring caused by vibrations is a factor whose influence on resolution is often significant in imaging systems that involve mechanical motion. For example, an unstabilized, airborne system mounted on vibration isolators experiences an angular velocity typically on the order of 60,000 μrad/s. On the other hand, a stabilized system, which obtains image motion corrections by moving optical elements so as to counteract sensor motions, has a residual error typically on the order of 2500 μrad/s involving pitch, roll, yaw, and forward motion compensation errors. The greatest errors result usually from rotational vibration. A stabilized mount-type system has a combined error typically on the order of 100 μrad/s [14.1]. Even where no motion is involved, as in large fixed-position astronomical telescopes, vibrations deriving from thermal gradients in the walls and other parts of the telescope can be the limiting factor in image resolution.

Turbulence Modulation Transfer Function

Abstract: There are basically two techniques used to correct for turbulence degradation. One is adaptive optics, whose purpose is to prevent turbulence-derived distortions from being recorded in the image. The other technique is image restoration using typically digital computers. In the latter case, turbulence-derived blur has already been recorded in the image but is removed through image processing.

Motion-distorted composite frame restoration

Abstract: Several imaging systems produce pictures by superimposing the two fields of frames of interlaced sequences. The pictures obtained in this way, which are termed composite frames, are severely degraded if relative motion between the camera and the scene occurs. In the presence of motion the composite frame is affected by two types of distortions: the edge `staircase effect' due to the fact that objects appear at different positions in successive fields, and motion blur de to the scene motion during each field exposure. Motion- deinterlacing methods previously proposed to recover the `staircase effect' neglect the motion blur. However the motion blur may be significant, especially in systems designed for low intensity radiometric imaging which use long exposures, or even in short exposure systems which happen to be in moving vehicles such as tanks, planes, ships, etc.. In this paper we introduce an algorithm for restoration of the two types of distortions in a composite frame degraded by linear uniform motion.