Samuel T. Thurman

Efficient subpixel image registration algorithms

Three new algorithms for 2D translation image registration to within a small fraction of a pixel that use nonlinear optimization and matrix-multiply discrete Fourier transforms are compared. These algorithms can achieve registration with an accuracy equivalent to that of the conventional fast Fourier transform upsampling approach in a small fraction of the computation time and with greatly reduced memory requirements. Their accuracy and computation time are compared for the purpose of evaluating a translation-invariant error metric.

Controlling the spectral response in guided-mode resonance filter design

Techniques for controlling spectral width are used in conjunction with thin-film techniques in the design of guided-mode resonance (GMR) filters to provide simultaneous control over line-shape symmetry, sideband levels, and spectral width. Several factors that could limit the minimum spectral width are discussed. We used interference effects for passband shaping by stacking multiple GMR filters on top of one another. A design is presented for a 200-GHz telecommunications filter along with a tolerance analysis. Compared with a conventional thin-film filter, the GMR filter has fewer layers and looser thickness tolerances. Grating fabrication tolerances are also discussed.

Phase retrieval with signal bias

The effect of a uniform measurement bias, due to background light, stray light, detector dark current, or detector offset, on phase retrieval wavefront sensing algorithms is analyzed. Simulation results indicate that the root-mean-square error of the retrieved phase can be more sensitive to an unaccounted-for signal bias than to random noise in practical scenarios. Three methods for reducing the impact of signal bias are presented.

CHARACTERISTICS OF A 128 X 128 LIQUID-CRYSTAL SPATIAL LIGHT MODULATOR FOR WAVE-FRONT GENERATION

Spatial and temporal characteristics of a 128x128 zero-twist nematic liquid-crystal spatial light modulator are investigated for wave-front generation at a wavelength of 632.8 nm. The modulator is found to have the capability of producing at least eight phase levels between 0 and 2pi , and the rate of arbitrary phase modulation is limited to approximately 4.5 Hz. Wave-front generation of the first 55 Zernike polynomials is demonstrated.

Amplitude metrics for field retrieval with hard-edged and uniformly illuminated apertures.

In field retrieval, the amplitude and phase of the generalized pupil function for an optical system are estimated from multiple defocused measurements of the system point-spread function. A baseline field reconstruction algorithm optimizing a data consistency metric is described. Additionally, two metrics specifically designed to incorporate a priori knowledge about pupil amplitude for hard-edged and uniformly illuminated aperture systems are given. Experimental results demonstrate the benefit of using these amplitude metrics in addition to the baseline metric.

Correction of anisoplanatic phase errors in digital holography

The quality of coherent images computed from digital holography or heterodyne array data is sensitive to phase errors of the reference and/or object beams. A number of algorithms exist for correcting phase errors in or very near the hologram plane. In the case of phase errors introduced a nonnegligible distance away from hologram plane, the resulting imagery exhibits anisoplanatism. A feature of coherent imaging is that such phase errors may be corrected by simply propagating the aberrated fields (from the object) from the hologram plane to the plane where the phase errors were introduced and applying the phase-error correction algorithms to the fields in that plane. We present experimental results that demonstrate correction of such anisoplanatic phase errors.

Wiener reconstruction of undersampled imagery.

We derive a Fourier-domain Wiener filter for the reconstruction of undersampled imagery. The filter differs from previous implementations in that it permits adjustment of the trade-offs between sharpness of the reconstruction, noise amplification, and aliasing artifact suppression. Additionally, a net transfer function that characterizes the combined effects of the imaging system and the reconstruction process is derived. This net transfer function is valid for both unaliased and aliased spatial frequencies. The expression for the net transfer function is applicable to more general linear image sharpening algorithms.

Complex pupil retrieval with undersampled data.

The ability to retrieve the complex-valued, generalized pupil function of an imaging system from undersampled measurements of the defocused system point spread function (PSF) is examined through numerical simulations. The ability to do so degrades as the detector pixel pitch increases when using a fixed number of PSF measurements. Two strategies for obtaining better results with undersampled data are demonstrated using additional PSF measurements with (i) random shifts due to system pointing fluctuations and (ii) intermediate amounts of defocus.

Multi-aperture Fourier transform imaging spectroscopy: theory and imaging properties

Fourier transform imaging spectroscopy (FTIS) can be performed with a multi-aperture optical system by making a series of intensity measurements, while introducing optical path differences (OPDs) between various subapertures, and recovering spectral data by the standard Fourier post-processing technique. The imaging properties for multi-aperture FTIS are investigated by examining the imaging transfer functions for the recovered spectral images. For systems with physically separated subapertures, the imaging transfer functions are shown to vanish necessarily at the DC spatial frequency. Also, it is shown that the spatial frequency coverage of particular systems may be improved substantially by simultaneously introducing multiple OPDs during the measurements, at the expense of limiting spectral coverage and causing the spectral resolution to vary with spatial frequency.

Method of obtaining wavefront slope data from through-focus point spread function measurements

A method is described for analyzing point source imagery collected with various amounts of defocus to obtain wavefront slope data at the exit pupil of an imaging system. Integration of this slope data yields the system wavefront aberration function. The method is based on a geometric optics interpretation of intensity point spread function measurements in the caustic region near focus. Algorithm performance is examined through Monte Carlo simulations. Application of the method to segmented-aperture systems is also explored. The proposed method is used to generate initial wavefront estimates for an iterative phase retrieval algorithm, significantly improving the capture range over a blind phase retrieval approach when segment tilt errors are large.