Biomedical subjects

Nicholas George

Publications and source records attributed to Nicholas George.

10 recordsLinked to original sources

PubMed2006

Light-emitting diode illumination design with a condensing sphere.

An illumination system is described that comprises a glass or plastic sphere, an embedded light-emitting diode (LED) source, and a reflecting or scattering mirror. The rays from the LED are either refracted in the forward direction or totally internally reflected by the sphere. The mirror breaks the total internal reflection and thus sends the rays in the forward direction. The illumination pattern and efficiency of the system are analyzed. This system is extremely compact and efficient and is a good starting point for a complex illumination system design.

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PubMed2005

Sampling-theory approach to eigenwavefronts of imaging systems.

We present a direct method based on the sampling theorem for computing eigenwavefronts associated with linear space-invariant imaging systems (including aberrated imaging systems). A potential application of the eigenwavefronts to inverse problems in imaging is discussed. A noise-dependent measure for the information-carrying capacity of an imaging system is also proposed.

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PubMed2004

Scanned-picture-beam holography for nonrigid objects.

A technique is described for holographically recording nonrigid objects by scanning the picture beam in a small, bright spot across the object in a series of short exposures. The reference beam is incident across the entire recording medium during each exposure. This technique can freeze the motion of objects subject to ambient forcings, such as acoustic vibrations. Experiments are described in support of this method. Exposures are in the 1- to 10-ms range with 100 separate object regions. An analysis of certain factors affecting image quality, such as diffraction efficiency, is also presented.

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PubMed2004

Analysis of multitone holographic interference filters by use of a sparse Hill matrix method.

A theory is presented for the application of Hill's matrix method to the calculation of the reflection and transmission spectra of multitone holographic interference filters in which the permittivity is modulated by a sum of repeating functions of arbitrary period. Such filters are important because they may have two or more independent reflection bands. Guidelines are presented for accurately truncating the Hill matrix, and numerical methods are described for finding the exponential coefficient and the coefficients of the Floquet-Bloch waves within the filter. The latter calculation is performed by use of a computational technique known as inverse iteration. The Hill matrix for such problems is sparse, and thus, even though the matrix can be quite large, it may be efficiently stored and processed by a desktop computer. It is shown that the results of using Hill's matrix method are in close agreement with numerical calculations based on thin-film decomposition, a transfer-matrix technique. An important result of this research is the demonstration that Hill's matrix method may, in principle, be used to analyze any multiperiodic problem, so long as the periods are known to finite precision.

Journal Article↗

PubMed2004

Fractional finite Fourier transform.

We show that a fractional version of the finite Fourier transform may be defined by using prolate spheroidal wave functions of order zero. The transform is linear and additive in its index and asymptotically goes over to Namias's definition of the fractional Fourier transform. As a special case of this definition, it is shown that the finite Fourier transform may be inverted by using information over a finite range of frequencies in Fourier space, the inversion being sensitive to noise. Numerical illustrations for both forward (fractional) and inverse finite transforms are provided.

Journal Article↗

PubMed2003

Fourier optical analysis of gradient-index array imaging.

We present theory and experiments in which Fourier optics and the generalized Fresnel-zone integral form are used to characterize the imaging both of a single gradient-index rod and an array of nine rods. For the unity magnification SELFOC imager, we show that it has both internal and external Fourier transform planes. Careful treatment of the aperture function for off-axis points leads to a theoretical point-spread function that is in close agreement with experiments. Experiments are presented to illustrate the Fourier transform patterns as well as the imaging for single rods and for the entire array.

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PubMed2003

Direct coarse sampling of electronic holograms.

We describe a series of experiments in order to test a new sampling criterion for wavefront reconstruction from a carrier-frequency signal as obtained in electronic holography. Both a Fourier-transform configuration and a Fresnel-zone form of a digital holographic microscope are described. Direct coarse sampling of holograms with no phase-shifting elements can be used to obtain wavefront reconstruction, with the number of samples required reduced by the ratio f0/(2Bs), where f0 is the carrier frequency and 2Bs is the signal bandwidth.

Journal Article↗

PubMed2003

Holographic interference filters for infrared communications.

We demonstrate that high-quality interference filters for the wavelength range 1300-1600 nm can be holographically fabricated in DuPont HRF-800X001 photopolymer material by use of visible laser illumination. We also summarize a chain-matrix technique, which we call thin-film decomposition, that is useful for modeling multilayer films with an arbitrary index profile n(z). We use the thin-film-decomposition technique to create design curves that allow one to choose the proper exposure angle and film thickness with which to fabricate a holographic interference filter with a desired transmission efficiency and bandwidth at a particular wavelength. These curves are of general utility and are not confined to any particular holographic recording medium. Excellent agreement between theory and experiment is found.

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PubMed2003

Inactivation of ompR promotes precocious swarming and flhDC expression in Xenorhabdus nematophila.

The response regulator OmpR is involved in numerous adaptive responses to environmental challenges. The role that OmpR plays in swarming behavior and swarm-cell differentiation in the symbiotic-pathogenic bacterium Xenorhabdus nematophila was examined in this study. Swarming began 4 h sooner in an ompR mutant strain than in wild-type cells. Precocious swarming was correlated with elevated expression of fliC, early flagellation, and cell elongation. The level of flhDC mRNA was elevated during the early period of swarming in the ompR strain relative to the level in the wild type. These findings show that OmpR is involved in the temporal regulation of flhDC expression and flagellum production and demonstrate that this response regulator plays a role in the swarming behavior of X. nematophila.

Bacterial Proteins↗

PubMed2003

Computational imaging with the logarithmic asphere: theory.

A theory for an integrated system is described that combines a logarithmic aspheric imaging lens with maximum-entropy digital processing to extend the depth of field ten times over that of a conventional lens and to provide near-diffraction-limited resolution. Two types of logarithmic aspheres are derived that are circularly symmetric lenses with controlled continuous radial variation of focal length. The details of an iterative maximum-entropy algorithm are also presented. The properties of convergence and speed of the algorithm are greatly improved by introducing a metric parameter to adjust the weight of different pixel values of the recovered picture in each loop properly.

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