Paul S. Idell

Image synthesis from nonimaged laser-speckle patterns

We demonstrate that unspeckled images of coherently illuminated, diffuse objects can be formed from measurements of backscattered laser-speckle intensity. The theoretical basis for this imaging technique is outlined, and results of computer experiments that successfully construct images from digitally simulated laser-speckle measurements are presented.

Practical issues in wave-front sensing by use of phase diversity

We present the results of the phase-diversity algorithm applied to simulated and laboratory data. We show that the exact amount of defocus distance does not need to be known exactly for the phase-diversity algorithm on extended scene imaging. We determine, through computer simulation, the optimum diversity distance for various scene types. Using laboratory data, we compare the aberrations recovered with the phase-diversity algorithm and those measured with a Fizeau interferometer that uses a He-Ne laser. The two aberration sets agree with a Strehl ratio of over 0.9. The contrast of the recovered object is found to be ten times that of the raw image.

Image synthesis from nonimaged laser-speckle patterns: experimental verification.

We have performed laboratory experiments that demonstrate that unspeckled images of coherently illuminated, diffuse objects can be formed from measurements of backscattered laser-speckle intensity. We show that images revealing gross target features can be successfully recovered with relatively few snapshots of the observed laser-speckle pattern.

Image synthesis from nonimaged laser-speckle patterns: comparison of theory, computer simulation, and laboratory results.

The performance of an imaging technique relying on the spatial correlation of laser-speckle intensity measurements is evaluated on the basis of theoretical analysis, computer simulation, and laboratory results. A theoretical expression for the signal-to-noise ratio of the recovered imaging targets power spectrum is used to estimate the imaging performance expected in the computer simulation and laboratory experiment. Power-spectrum estimates for an imaging target, obtained both in the laboratory and through simulation, are compared with the theoretical results and with the true spectrum of the target. Images recovered from the simulation data and the laboratory data are also compared. Our results suggest that the signal-to-noise ratio expression provides an accurate means for estimating the recoverable frequency content of a simple target.

Imaging Correlography With Sparse Arrays Of Detectors

Imaging correlography is a technique for constructing high resolution images of laser-illuminated objects from measurements of back-scattered (nonimaged) laser speckle intensity patterns. In this paper, we investigate the possibility of implementing an imaging correlography system with sparse arrays of intensity detectors. The theory underlying the image formation process for imaging correlography is reviewed, emphasizing the spatial filtering effects that sparse collecting apertures have on the reconstructed imagery. We then demonstrate image recovery with sparse arrays of intensity detectors through the use of computer experiments in which laser speckle measurements are digitally simulated. It is shown that the quality of imagery reconstructed using this technique is visibly enhanced when appropriate filtering techniques are applied. The signal-to-noise ratio of the process and its dependency on array redundancy and number of speckle pattern measurements is also discussed.

Image synthesis from wave-front sensor measurements of a coherent diffraction field

We demonstrate that images of laser-illuminated objects can be formed from measurements of the wave-front slope (gradient) associated with the backscattered, coherent laser-speckle field. A digital wave-front recovery and image synthesis procedure is described, and the results of computer-simulation experiments are presented in which coherent images are reconstructed from digitally simulated Fourier-plane laser-speckle measurements. Images are recovered from noisy wave-front-difference data to illustrate the effect that measurement noise has on recovered image quality.

Illumination coherence effects in laser-speckle imaging: modeling and experimental demonstration

Coherent imaging techniques rely on the coherence properties of backscattered radiation to form an image of an illuminated object. These techniques are sensitive to the degree of coherence of the illuminating source. We present an approach for simulating the effects that partial temporal coherent illumination has on these techniques. Computer simulation and laboratory experimental results are presented that illustrate illumination coherence effects on imagery obtained with the technique known as imaging correlography.

Cramér–Rao analysis of orientation estimation: viewing geometry influences on the information conveyed by target features

A methodology for analyzing an imaging sensors ability to assess target properties is developed. By the application of a Cramér-Rao covariance analysis to a statistical model relating the sensor measurements to the target, a lower bound can be calculated on the accuracy with which any unbiased algorithm can form estimates of target properties. Such calculations are important in understanding how a sensors design influences its performance for a given assessment task and in performing feasibility studies or system architecture design studies between sensor designs and sensing modalities. A novel numerical model relating a sensors measurements to a targets three-dimensional geometry is developed in order to overcome difficulties in accurately performing the required numerical computations. The accuracy of the computations is verified against simple test cases that can be solved in closed form. Examples are presented in which the approach is used to investigate the influence of viewing perspective on orientation accuracy limits. These examples are also used to examine the potential accuracy improvement that could be gained by fusing multiperspective data.

Coherent image synthesis from wave-front sensor measurements of a nonimaged laser speckle field: a laboratory demonstrations

We report what are to our knowledge the first coherent images recovered in the laboratory from measurements made with a Shack-Hartmann wave-front sensor of the phase and amplitude of a laser speckle wave front. We discuss the design of our wave-front sensor, which can obtain the phase and amplitude of an optical field with a single intensity measurement, and we point out a particular type of phase jump that cannot be detected by the Shack-Hartmann sensor. We also discuss implementations of this technique that may permit near-diffraction-limited imaging through turbulent media.

Cramer-Rao bound analysis of target characterization accuracy limits for imaging systems

A methodology for analyzing an imaging sensors ability to assess target properties is developed. By applying Cramer- Rao covariance analysis to a statistical model relating the sensor measurements to the target, a bound on the accuracy with which target properties can be estimated can be calculated. Such calculations are important in understanding how a sensors design effects its performance for a given assessment task, and in performing feasibility studies or trade studied between sensor designs and sensing modalities. A novel numerical model relating a sensors measurements to a targets three-dimensional geometry is developed in order to overcome difficulties in accurately performing the required numerical computations. An example use of the approach is presented in which the influence of viewing perspective on orientation accuracy limits is analyzed. The example is also used to examine the potential for improving the accuracy bound by fusing multi-perspective data.