Barbara Tehan Landesman

Sheared beam imaging in the presence of space-time distortions

In sheared beam imaging, a target is coherently illuminated with three sheared and modulated beams. Both target motion and the temporal distribution of the laser illumination affect image recovery. Movement of the target will cause a corresponding motion of the targets speckle pattern on the ground. As the speckle pattern travels across the detector, each integration period, or bin, in a single laser pulse, or frame, will record a slightly different speckle distribution. This phenomenon effects a smearing of the speckle formation from bin to bin within a single frame, and subsequently degrades the recovered image. Image recovery is further complicated by temporally nonuniform illumination. Both smearing and nonuniform pulse shapes create distortions in the spatial and temporal distributions of the speckle patterns that garble image recovery. This paper documents the derivation of the governing equations for the simulation of sheared beam imaging in the presence of these space-time distortions. In addition, we present algorithms for alleviating these distortions prior to image reconstruction. We conclude with simulation results showing the effect of the space-time distortions on sheared beam image recovery and the improvement achieved with the post-detection deblurring and pulse correction algorithms.

Image transfer through cirrus clouds. I. Ray trace analysis and wave-front reconstruction

A new technique for modeling image transfer through cirrus clouds is presented. The technique uses a ray trace to model beam propagation through a three-dimensional volume of polydisperse, hexagonal ice crystals. Beyond the cloud, the technique makes use of standard Huygens-Fresnel propagation methods. At the air-cloud interface, each wave front is resolved into a ray distribution for input to the ray trace software. Similarly, a wave front is reconstructed from the output ray distribution at the cloud-air interface. Simulation output from the ray trace program is presented and the modulation transfer function for stars imaged through cirrus clouds of varying depths is discussed.

Imaging and localization in turbid media

We apply out previously-developed turbid-media backpropagation algorithm to imaging extended objects imbedded in turbid media such as clouds. Although the backpropagation algorithm was developed initially for biomedical applications, the underlying development is general enough to encompass imaging objects imbedded in any sort of turbid media whose scattering properties dominate their absorption properties. For non-biomedical applications, imaging data is usually obtained only for a limited number of view angles. As a result, we look at the potential of the backpropagation algorithm to reconstruct an image of an object, imbedded in a cloud, from a single view. Using both computer-simulated data and laboratory data, we show that the backpropagation algorithm successfully increases resolution in these types of images. Because the backpropagation algorithm incorporates a depth-dependent deconvolution filter, it turns out that the optimal image quality obtained in the reconstruction occurs for the deconvolution filter which corresponds to the location of the object in the medium. This surprising result permits object localization in the range dimension even when the illuminating radiation is continuous-wave illumination, such as sunlight.

Image transfer through cirrus clouds. II. Wave-front segmentation and imaging

A hybrid technique to simulate the imaging of space-based objects through cirrus clouds is presented. The method makes use of standard Huygens-Fresnel propagation beyond the cloud boundary and a novel vector trace approach within the cloud. At the top of the cloud, the wave front is divided into an array of input gradient vectors, which are in turn transmitted through the cloud model by use of the Coherent Illumination Ray Trace and Imaging Software for Cirrus. At the bottom of the cloud, the output vector distribution is used to reconstruct a wave front that continues propagating to the ground receiver. Images of the object as seen through cirrus clouds with different optical depths are compared with a diffraction-limited image. Turbulence effects from the atmospheric propagation are not included.

Simulation of imaging of space-based objects through cirrus clouds

This paper discussed a simulation fo the imaging of a space- based object through cirrus clouds. The wavefront reflected by the object is propagated to the top of the cloud using Huygens-Fresnel propagation theory. At the top of the cloud, the wavefront is divided into an array of input rays, which are in turn transmitted through the cloud model using the CIRIS-C software. At the bottom of the cloud, the output ray distribution is used to reconstruct a wavefront that continues propagating to the ground receiver. Images of the object as seen through cirrus clouds with different optical depths are compared to a diffraction-limited image. Turbulence effects from the atmospheric propagation are not included.

Modulation transfer function through multiple realizations of a cirrus cloud model

Statistical variations in the size, position, shape and orientation of hexagonal ice crystals in a 3D volume complicates the modeling of image transfer through cirrus clouds. These variations will give rise to fluctuations in image quality as measured by the MTF of a single realization of a cloud/receiver combination. Computing the average MTF from several realizations of the cloud/receiver combination, allowing for a different cloud composition on every realization can alleviate these fluctuations. In this paper, we present the result of the multiple MTF for clouds of different optical depths, distinguishing the separate contributions of scattered as well as unscattered light. Finally, we apply the MTF thus generated to create images of objects as seen through the cloud.

Wavefront propagation and imaging through cirrus clouds

The authors present a wavefront reconstruction technique for beams forward scattered and back scattered through cirrus clouds. The technique uses ray distributions from the Coherent Illumination and Ray Trace Imaging Software for Cirrus which traces the propagation and E field vectors through a 3D volume of ice crystals in the shape of columns, plates, bullets, and bullet rosettes with random positions and polydisperse sizes and orientations. The wavefronts are then propagated to a telescope receiver on the ground and imaged in the receiver focal plate. A modification transfer function for each of these images is calculated and compared to the MTF for a diffraction-limited system.

Simulation of imaging in the presence of cirrus clouds

The effect of atmospheric phase perturbations on the diffractive and coherent properties of the uplink and downlink paths of an active imaging illumination beam has been studied in some detail. Similarly, the scattering and depolarization induced by water and ice cloud particles in the path of coherent laser illumination is currently an area of much production research. In contrast, the effect of cloud particles on the diffractive properties of a laser illumination beam has not received as much attention due primarily to the daunting mathematics of the physical mode. This paper seeks to address some of the mathematical issues associated with modeling the interaction of a coherent illumination beam with a cloud of ice particles. The simulation constructs a 3D model of a cirrus cloud consisting of randomly oriented hexagonal ice crystals in the shape of plates, columns, and bullet rosettes. The size, shape, and vertical distribution of the crystals are modeled after measured particles concentrations and distributions. An illumination pattern, in the form of grid of rays, is traced through the cloud, and the properties of the exiting wavefronts are analyzed.

Speckle-size metric to improve reconstruction of coherent speckle images

Robust reconstruction of coherent speckle images from non- imaged laser speckle patterns in the aperture plane of an optical system requires adequate sampling of the speckle intensity at the focal plane. Although detector size cannot be changed dynamically in the course of an experiment to achieve the necessary sampling in every frame, a measure of speckle size could be used to accept or reject individual frames in post-processing software to improve the final reconstructed image. This paper investigates the use of a speckle size metric to gauge the integrity of speckle sampling in each frame of a series of coherent speckle images. Frames containing inadequate sampling are sorted out of the final reconstructed image. The quality of the final recovery for a variety of targets and imaging conditions are compared for sorted and non-sorted reconstructions.

Deconvolution of physical effects in speckle imaging using power-averaged speckle size

The speckle size metric known as power-averaged speckle size (PASS), which is based on the integral of the power spectral density of the data, is insensitive to target asymmetry. PASS is defined as the inverse of the median frequency of the power spectral density of the reconstructed pupil. Implementation of the metric using simulated data from different targets under a variety of imaging conditions illustrates the impact of the environment. In this paper, we deconvolve the imaging environment using power spectral density of the detected speckle in the error function. Different targets are simulated in the presence of a variety of imaging situations as well as noise and we compare the resultant deconvolved images with undistorted images as well as with the unimproved ones.