David Tofsted

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As a research scientist at the US Army's Atmospheric Sciences Laboratory that later became the Battlefield Environment Division of the US Army Research Laboratory, I worked on numerous projects aimed at understanding how we look through the atmosphere. Studies included propagation at various wavelengths through atmospheric turbulence and aerosols, including aerosol absorption and scattering effects.

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Modeling the target acquisition performance of active imaging systems

Optics Express, 2007

Recent development of active imaging system technology in the defense and security community have... more Recent development of active imaging system technology in the defense and security community have driven the need for a theoretical understanding of its operation and performance in military applications such as target acquisition. In this paper, the modeling of active imaging systems, developed at the U.S. Army RDECOM CERDEC Night Vision &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp; Electronic Sensors Directorate, is presented with particular emphasis on the impact of coherent effects such as speckle and atmospheric scintillation. Experimental results from human perception tests are in good agreement with the model results, validating the modeling of coherent effects as additional noise sources. Example trade studies on the design of a conceptual active imaging system to mitigate deleterious coherent effects are shown.

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<title>Visible through near-IR solar loading influences on characterization</title>

Proceedings of SPIE, Jul 14, 1999

The ability of sensors to discriminate objects in scenes depends on the scene composition and int... more The ability of sensors to discriminate objects in scenes depends on the scene composition and interactions with the available incident radiation. As sensors and camouflage techniques become more complex the nature of the energy interactions become more important to model accurately. Specific areas of interest are the influences of fluctuations in incident total solar loading radiation on terrain surfaces. The means used to produce 3D radiative calculations over the solar spectrum involves coupling the Air Force's Moderate- resolution Transmission (MODTRAN) code to the Army's 3D Atmospheric Illumination Module (AIM). The solar loading outputs calculated by these coupled codes are then used as input to the Army Smart Weapons Operability Enhancement (SWOE) thermal models. Variations in incident radiation produce surface temperature variations of up to 8 degrees Celsius. In the paper we describe the means of evaluating solar loading effects using a correlated-k-distribution-like algorithm to compress spectral processing, and show comparisons between measured and modeled results.

Navigation Through Fog: A Mathematical Analysis of Folded Path Propagation

Wendall Watkins of the U.S. Army Research Laboratory has a currently pending patent, which provid... more Wendall Watkins of the U.S. Army Research Laboratory has a currently pending patent, which provides a technique for solving the problem of navigation through fog. This technique entails the use of laser illumination of retroreflective targets on airfield runways, stereo detection of the return signal, and deblurring processing of the received imagery. The current analysis attempts a mathematical analysis of this same problem. Here, the propagation equations necessary to model the propagation of a Gaussian beamwave through a forward scattering fog aerosol are developed. Interaction of the propagated wave with a retroreflective material is postulated, and the returned reflected energy is evaluated. Also, backscattered diffuse energy is analyzed. Through this analysis, it should be possible to predict the angular structure of energy returning from a retroreflector embedded within the fog field. It should also be possible to predict the level of diffuse radiation which must be dealt with in order to discriminate between the directly reflected radiation and the diffusely scattered radiation, both in the forward and backward hemispheres.

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Outer-scale effects on beam-wander and angle-of-arrival variances

Applied optics, Sep 20, 1992

Single-integral equations are developed for the angle-of-arrival and beam-wander variances of pro... more Single-integral equations are developed for the angle-of-arrival and beam-wander variances of propagated gaussian and uniform-intensity beams through refractive turbulence with a finite outer scale. The equations developed for Gaussian beams include initial wave-front curvature, diffraction, and turbulent spreading effects. Form-fitting expressions are used in both cases to reduce second-and third-order integral equations into equivalent single-integral forms. Numerical calculations with these new approximations give results within a few percent of the results obtained by using the more rigorous multiple integral expressions. Spectral analysis indicates that the refractive-index turbule size with the greatest influence on beam wander and angle of arrival is proportional to the outer-scale size.

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$Research paper thumbnail of Optical refraction in the atmospheric surface layer$

Optical refraction in the atmospheric surface layer

In AGARD, Feb 1, 1992

Modeling Optical Turbulence Effects on Centrally Obscured Aperture Systems

Passive adaptive imaging through turbulence

Proceedings of SPIE, May 13, 2016

Standard methods for improved imaging system performance under degrading optical turbulence condi... more Standard methods for improved imaging system performance under degrading optical turbulence conditions typically involve active adaptive techniques or post-capture image processing. Here, passive adaptive methods are considered where active sources are disallowed, a priori. Theoretical analyses of short-exposure turbulence impacts indicate that varying aperture sizes experience different degrees of turbulence impacts. Smaller apertures often outperform larger aperture systems as turbulence strength increases. This suggests a controllable aperture system is advantageous. In addition, sub-aperture sampling of a set of training images permits the system to sense tilts in different sub-aperture regions through image acquisition and image cross-correlation calculations. A four sub-aperture pattern supports corrections involving five realizable operating modes (beyond tip and tilt) for removing aberrations over an annular pattern. Progress to date will be discussed regarding development and field trials of a prototype system.

Test Report on the November 2005 NATO RTG-40 Active Imager Land Field Trials

Public reporting burden for this collection of information is estimated to average 1 hour per res... more Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

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Modeling optical turbulence in the atmospheric boundary layer

Proceedings of SPIE, May 5, 2006

Modeling optical turbulence in the atmospheric boundary layer. [Proceedings of SPIE 6239, 62390G ... more

<title>Characterizing the effects of natural clouds on scene simulations</title>

Proceedings of SPIE, Jun 20, 1997

In modeling and simulations the importance of the natural environment has always been recognized ... more In modeling and simulations the importance of the natural environment has always been recognized with regard to its influence on contrast transmission. However, the variability of surface illumination and solar loading due to broken clouds, the resulting impact of dynamic range on recognition, and clouds as backgrounds, along with the traditional influences due to transmission and path radiance, are emerging areas of relevance due to improvements in the modeling of these effects. The Air Force LOWTRAN model has been the traditional choice for multi- waveband analysis of spectral atmospheric effects on systems performance. But this code only has spatially varying effects in the vertical direction. Dynamic range impacts of horizontally variable illumination conditions cannot be addressed. We describe a series of codes designed to allow the linking of predictions of cloud fractions, base heights, layer depths, and layer cloud types with a model to predict the cloud density structure. These results are coupled to a radiative transfer model. We describe the salient features of this physics based model. We then describe the point-to- point calculation method to produce path radiance and transmittance statistics at multi-channel resolution. The weighted spectra are used to describe the effects on a given sensor channel. We further describe the perspective view generation method used to render cloudy scenes from a variety of observer positions. The radiative transfer model is robust in the sense that its results are not limited to low cloud densities. The spectral region covered is the same as that treated by LOWTRAN and LOWTRAN output is used to initialize the upper boundary for incident direct (solar/lunar) and diffuse radiation source and used to determine the background molecular absorption (by modeled layer) of the scattering volume. Typical scattering volumes treated have an 8 km X 8 km footprint and are either 4 km, 8 km, or 16 km high. These volume choices can be used for addition of clouds as scene elements in simulations, usage of the surface illumination information as a positionally varying solar loading or brightness data set, and for path characterization for contrast transmission calculations.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

<title>Visualization of dense cloud radiation data in modeling and simulations</title>

Proceedings of SPIE, Jul 15, 1997

Visualization technologies are improving our ability to assess the effectiveness of the warfighte... more Visualization technologies are improving our ability to assess the effectiveness of the warfighter on today's battlefield. Increasingly, our ability to predict the behavior and performance of competing systems is being facilitated by simulations. These predictions typically involve visual and sensor simulations, but they may also be used for mission performance generalizations. A key link in this analysis involves the assessment of real optics, ATR algorithms, and observers under the changing influence of the natural atmosphere. The effects of the atmosphere can be as diverse as target contrast degradation, dynamic range influences of light and shadows in the viewed field, cloud free lines of sight, and optical turbulence. However, in general, the modeling and simulation community has only treated limited versions of the full influences of the atmosphere. In some cases these influences have been modeled using cartoon-like emulations of reality, bereft of physical content. Physics-based solutions are usually bypassed in the drive for near-realtime results. In this paper, we discuss the need for three-dimensional (3-D) solutions to near-earth atmospheric representation, describe a set of physics-based programs designed to generate cloudy- hazy atmospheric scenarios, run a robust 3-D radiative transfer (RT) model, and present this representation for visualization by a perspective view generator of the resulting radiance fields. The cloud fields are generated with a stochastic model that uses cloud layer height information, cloud type, and vertical sounding profile data. The output from this model is coupled to standard vertical haze profiles to produce a 3-D field of atmospheric properties. The official Army Research Laboratory discrete ordinates method (DOM) RT code, contained in the WAVES modeling package, has difficulties with dense cloud conditions. Here, we discuss a recommended upgrade to WAVES in the form of a specially designed RT code that accesses these cloud/haze data and is insensitive to cloud density variations. This feature allows it to effectively simulate effects in and around natural clouds. Further processing compresses and interprets the outputs of the RT code for a given sensor spectral response. Point to point calculations can then be performed on the resulting database for path characterization, including path radiance and contrast transmission calculations. These can be used in assessing system performance for each color channel of a sensor, or in visualizing the cloud fields themselves.

<title>Biorthogonal wavelet transform and compression of image data</title>

Proceedings of SPIE, Apr 3, 1997

A set of bi-orthogonal wavelet transforms are developed based on an extension of Sweldens; liftin... more A set of bi-orthogonal wavelet transforms are developed based on an extension of Sweldens; lifting method for variable support wavelet bases. The bi-orthogonal wavelet has advantages of compact support for both the transform and inverse transform process, and, like the Zernike and Legendre polynomials, can be designed to pass localized polynomial variations of orders m up to the limit, depending on the width of support of the lesser support width of either the wavelet or the scaling function. Features of this wavelet set include symmetry of the wavelet and scaling functions. The wavelet and scaling functions of a given order m are related to their pair of duals through simple relations involving position shifts and sign changes. A general method for producing transform functions is given, and results are shown for up to m equals 3, which treats up to 7th order polynomial insensitivity. The set of transforms is tested against a sample image and results show the possibilities for compression. It appears the lowest order wavelet yields the best performance for simple compression techniques on the image used. This m equals 0 transform shows the least degradation from truncation and tends to treat small regions effectively. Despite the advantages of higher order wavelets with respect to fluctuations, these tend to artificially create noise information which is passed on to the next processing stage.

A re-analysis of short-exposure turbulent effects on passive imaging

Proceedings of SPIE, Apr 23, 2010

In 1966, D. L. Fried developed a theoretical framework for describing the modulation transfer fun... more In 1966, D. L. Fried developed a theoretical framework for describing the modulation transfer function (MTF) of optical turbulence effects for short-exposure imaging, thereby extending Hufnagel and Stanley's model for longexposure imaging through turbulence. In his analysis Fried assumed the independence of phase perturbations about the mean tilt factor and this tilt factor. In this reanalysis this assumption is deleted, resulting in a new term in the derivation. This term exhibits several new features, including superresolution at high spatial frequencies under moderate turbulence levels, and turbulence strength dependent degradation of the MTF at high turbulence levels.

Terahertz target illumination fluctuation estimates derived from field measurements of atmospheric water vapor

Proceedings of SPIE, Apr 3, 2008

Many terahertz imaging systems under development will be employed in outdoor environments, where ... more

<title>Analysis of speckle interferometry images</title>

Proceedings of SPIE, Jan 12, 1993

Speckle interferometry provides a basis for analyzing the dynamic motions of materials through di... more Speckle interferometry provides a basis for analyzing the dynamic motions of materials through differencing of the field scattered from objects at two different times. Phase differences in the signals measured at different times, inferred from fringe patterns, indicate the degree of deformation present. Automatic analysis of differences images requires significant preprocessing to enhance the contrast of fringe regions. Often fringes that are evident to the human eye cannot be perceived automatically because the fringes usually consist of widely separated high intensity spikes. Median or averaging filters are ineffective at enhancing these patterns. Adaptive filtering similar to that used in SAR image analysis is capable of enhancing the fringe area contrast. A two step process is detailed. In a first phase a filter based on window mean and variance suppresses noise and generates a greater cohesion of high intensity points in the fringe areas. In a second phase the image is average filtered to smooth the intensities in the fringes. An auxiliary routine used to count fringes is discussed. Comparisons with median filtered results show the greater ability to automatically count fringes using this two step method.

Reanalysis of turbulence effects on short-exposure passive imaging

Optical Engineering, 2011

<title>Empirical modeling of laser propagation effects in intermediate turbulence</title>

Proceedings of SPIE, Jul 1, 1991

Low energy laser propagation through moderate turbulence is considered. Propagation of lasers thr... more Low energy laser propagation through moderate turbulence is considered. Propagation of lasers through the surface boundary layer at ranges greater than a kilometer for visible wavelengths often involves intermediate level turbulence effects. Although propagation through weak turbulence is described through a series of integral equations and propagation through strong turbulence is understood in the asymptotic theory, propagation through intermediate level turbulence is usually described through heuristic theories. In this paper, techniques are described to simulate beam spread, wander, scintillation saturation, aperture- averaging, and an aperture-averaged scintillation distribution. Inner-scale effects are discussed throughout. Hill and Clifford theory is parameterized and adjusted to obtain an irradiance variance saturation curve that compares well with measured data of Coles and Frehlich, and Churnside. An empirical aperture-averaging curve compared well with intermediate turbulence data collected by Churnside.

<title>Active imaging applied to navigation through fog</title>

Proceedings of SPIE, Jul 19, 1999

Navigation, especially in aviation, has been plagued since its inception with the hazards ofpoor ... more Navigation, especially in aviation, has been plagued since its inception with the hazards ofpoor visibility conditions. Vehicular ground movement is also hampered at night or in low visibility even with night vision augmentation because of the lack of contrast and depth perception. For landing aircraft in fog, the visible and near-infrared have been discounted because of the large backscatter coefficients in favor of primarily radar that penetrates waterladen atmospheres. Aircraft outfitted with an Instrumentation Landing System (ILS) can land safely on an aircraft carrier in fog. Landing at an airport with an ILS is not safe because there is no way to detect small-scale obstacles that do not show up on radar but can cause a landing crash. We have developed and tested a technique to improve navigation through fog based on chopped active visible laser illumination and wide baseline stereo (hyperstereo) viewing with real-time image correction of backscatter radiation and forward scattering blur. The basis of the approach to developing this active hyperstereo vision system for landing aircraft in fog is outlined in the invention disclosure ofthe Army Research Laboratory (ARL) patent application ARL-97-72, filed Dec. 1997. Testing this concept required a matched pair of laser illuminators and cameras with synchronized choppers, a computer for near real-time acquisition and analysis of the hyperstereo imagery with ancillary stereo display goggles, a set of specular reflectors, and a fog generator/characterizer. The basic concept of active hyperstereo vision is to compare the imagery obtained from alternate wings ofthe aircraft while illuminating only from the opposite wing. This produces images with a backscatter radiation pattern that has an increasing gradient towards the side with the illumination source. Flipping the imagery from one wing left to right and comparing it to the opposite wing imagery will allow the backscattered radiation pattern to be subtracted from both sets of imagery. Use of specular reflectors along the sides of the runway will allow the human stereo fusion process to fuse the forward scatter blurred hyperstereo imagery of the array of specular reflectors with backscatter eliminated and allow the appropriate amount of inverse point spread function deblurring to be applied for optimum resolution of scene content (i.e., obstacles on the runway). Results of this testing will be shown.

Three-Dimensional Radiative Transfer Modeling of Tropospheric Atmospheres

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Physics-based visualization of dense natural clouds II Cloud-rendering algorithm

Applied optics, Nov 20, 1998

We discuss the representation of aerosol-scattering properties, boundary information, and the use... more We discuss the representation of aerosol-scattering properties, boundary information, and the use of these results in line-of-sight rendering applications for visualization of a modeled atmosphere based on a discrete ordinates three-dimensional radiative-transport method. The outputs of the radiativetransfer model provide spatial and angular distributions of limiting path radiance, given an input density distribution and external illumination conditions. We discuss the determination of the direct attenuated radiance, integrated path radiance, and background radiance for each pixel in the rendered scene. Orthographic and perspective projection approaches for displaying these results are described, and sample images are shown.

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David Tofsted

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