Richard A. Carreras

Intelligent sensors research using pulse-coupled neural networks for focal plane image processing

An important difference between biological vision systems and their electronic counterparts is the large number of feedback signals controlling each aspect of the image collection process. For every forward path of information in the brain, from sensor to comprehension, there appears to be several neural bundles which send information back to the sensor to modify the way the information is collected. In this paper we will examine the role of such feedback signals and suggest algorithms for intelligent processing of images directly on the focal plane, using feedback. We consider first what form these signals might take and how they can be used to implement functions common to conventional image processing with the objective of moving the computation out of the digital domain and place much of its on the focal plane, or analog processing close to the focal plane. While this work falls under the general heading of artificial neural networks, it goes beyond the static processing of signals suggested by the McCulloch and Pitts model of the neuron and the Laplacian image processing suggested by Carver Mead by including the dynamics of temporal encoding in the analysis process.

Demonstration of an optically phased telescope array

The first successful demonstration of actively phase matching, coaligning, and agilely steering a number of mutually coherent optical wavefronts through an array of transmitting telescopes has been performed on the Phasar experiment at the Air Force Weapons Laboratory. In the Phasar experiment, modern wavefront sensing and feedback techniques are used to control wavefronts transmitted through an assembly of three independent optical telescopes. Mutually coherent wavefronts transmitted through the telescope array are controlled to produce an effective single aperture wavefront. The multiple phased wavefronts produce a far-field energy distribution equivalent to the diffraction pattern of a single aperture whose diameter and obscurations match the geometry of the transmitting array. The Phasar experiment demonstrates absolute phasing of a broadband source in the presence of dynamic disturbances. The technologies developed facilitate the construction of large aperture, high quality optical systems by the modular combination of relatively small, inexpensive telescopes.

Phase diversity as an on-line wavefront sensor: experimental results

We present the preliminary results of a laboratory experiment using phase diversity as a wavefront sensor. Computer simulations of this experiment were also performed. The phase diversity algorithm used the ordinary finite-difference method to solve the transport equation of intensity and phase. This method of phase diversity retrieves the phase directly and may prove to be useful for low light level applications and for extended objects. This entertains the possibility of using phase diversity as an on-line wavefront sensor for adaptive optics.

Implementation Of Optical Path Length And Tilt Control In A Phased Array System

A description is presented of a system designed to control the path length and tilt components of the optical wavefronts in a multiple-aperture transmitting system. The system has been implemented in the three-telescope Phasar testbed at the Air Force Weapons Laboratory. The system hardware and software are described, and measurements of the system performance are presented. The measurements indicate that under laboratory conditions the current system can maintain an optical path length difference between pairs of telescopes of approximately X/15 and a tilt control error on each telescope of about 40 nrads. The band-widths of the path length control and tilt control subsystems have been measured at 130 Hz and 950 Hz, respectively.

Concurrent computation of Zernike coefficients used in a phase diversity algorithm for optical aberration correction

This paper describes a method to compute the optical transfer function, in terms of Zernike polynomials, one coefficient at a time using a neural network and gradiant decent. Neural networks, which are a class of self-tutored non-linear transfer functions, are shown to be appropriate for this problem as a closed form solution does not exist. A neural network provides an approximation to the optical transfer function computed from examples using gradient descent methods. Orthogonality of the Zernike polynomials allow image wavefront aberrations to be described as an ortho-normal set of coefficients. Atmospheric and system distortion of astronomical observations can introduce an unknown phase error with the observed image. This phase distortion can be described by a set of coefficients of the Zernike polynomials. This orthogonality is shown to contribute to the simplicity of the neural network method of computation. Two paradigms are used to determine the coefficient description of the wave front error to provide to a compensation system. The first uses a phase diverse image as input to a feedforward backpropagation network for generation of a single coefficient. The second method requires the transfer function to be computed in the Fourier domain. Architecture requirements are investigated and reported together with saliency determination of each input the the network to optimize computation and system requirements.

Hyperspectral observations of space objects

In this paper we present (1) the optical system design and operational overview, (2) laboratory evaluation spectra, and (3) a sample of the first observational data taken with HYSAT. The hyperspectral sensor systems which are being developed and whose utility is being pioneered by the Phillips Laboratory are applicable to several important SOI (space object identification), military, and civil applications including (1) spectral signature simulations, satellite model validation, and satellite database observations and (3) simultaneous spatial/spectral observations of booster plumes for strategic and surrogate tactical missile signature identification. The sensor system is also applicable to a wide range of other applications, including astronomy, camouflage discrimination, smoke chemical analysis, environmental/agricultural resource sensing, terrain analysis, and ground surveillance. Only SOI applications will be discussed here.

Image restoration using nonlinear optimization techniques with a knowledge-based constraint

An image restoration problem will be formulated in the context of nonlinear programming using the conjugate gradient algorithm. The formulation of the objective function used in the conjugate gradient routine is presented. Situations may occur when there is a great deal already known about a certain object of interest which have been optically blurred because of the atmosphere or system imperfections. This paper shows a new and innovative way to incorporate a priori, perfect, partial knowledge of an object into the nonlinear optimization procedure. The topics discussed include the steps which led to the development of this procedure, the incorporation of the a priori knowledge into the nonlinear optimization problem, an analytical, mathematical approach which shows how the improvement should occur, and finally, data from simulated results which demonstrate the improvement using the developed diagnostic metrics.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Use of electro-optical devices for optical path-length (OPL) compensation

We present the results of some laboratory experiments of the use of electro-optical (EO) devices to control the optical path length (OPL) of an interferometric array. One of the most important problems in interferometric beam combination is the control of the path length; this is coupled with the need for partial wavefront compensation in order to increase the sensitivity of the interferometer. Traditional approaches to such problems are often very expensive and sometimes impractical. For this reason we started an effort, both theoretically and experimentally, in order to investigate if less costly and more effective techniques can be applied. In our experiments we used single-cell LCDs in order to eliminate piston terms in a two- aperture interferometer. We used phase diversity techniques for extracting the phase information. Although the experimental results are still partial we believe that there is enough evidence that such devices can be used for the OPL control and partial wavefront compensation. Further testing is needed in order to assess the real capabilities of commercially available LCDs and the need, if any, of customization.

Image restoration using nonlinear optimization techniques for partially compensated imaging systems

This paper shows the development of the imaging restoration problem entirely in the frequency domain, then solves for the analytical solution. The analytical solution is found to be ill-posed, thus, a good approach for the solution is via nonlinear optimization. The image recovery problem is thus formulated in the context of nonlinear optimization using Fourier data. Several examples are shown using unconstrained optimization techniques with the incorporation of the conjugate gradient algorithm. These examples basically are the inverse filter solutions. Three main diagnostic metrics are shown and discussed as possible candidates for evaluation of the results.

Phasar Nonlinear Optical Experimentation: Phased Far-Field Return From Multiple Telescopes By Optical Wavefront Reversal Of A Reference Signal

A multiple telescope array, Phasar, is used to collect light for a phase-conjugate mirror of barium titanate, forming a hybrid receiving/ transmitting system. The observed fidelity of the system output is independent of the type and amount of aberration introduced into the optical path. In all test cases, the measured far-field peak intensity indicates that the fidelity of the return from the three telescopes approaches the fidelity required to yield the ideal in-phase superposition of waves in the far field. The system demonstrates high fidelity phase conjugation for a telescope piston mismatch of up to ±5 Am, for tilt error up to ±700 grad, and for higher order aberrations. A scheme implemented to compensate for depolarizing and polarization-altering effects boosts the amplitude of the point spread function considerably. Near-field diagnostics confirm the fidelity of the phase-conjugate output.