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Biomedical subjects

S G Lipson

Publications and source records attributed to S G Lipson.

12 recordsLinked to original sources
PubMed2006

Production of radially and azimuthally polarized polychromatic beams.

We describe a system that efficiently provides radially or azimuthally polarized radiation from a randomly polarized source. It is constructed from two conical reflectors and a cylindrical sheet of polarizing film. Envisaged applications include a microscope illuminator for high-resolution surface plasmon resonance microscopy, illumination for high-resolution microlithography, and efficient coupling of a laser source to hollow optical fibers. The angular coherence function of light polarized by the device was measured to evaluate its usefulness for these applications.

Journal Article↗
PubMed2006

Measurement of the spatial response of a detector pixel.

In order to measure the internal spatial response of a pixel in a detector, it is scanned by a beam smaller than its size. This becomes difficult as the wave length grows and becomes comparable to the pixel size, such as in the infra red. To overcome this difficulty, a special phase mask which makes the beam narrower was designed, constructed, and tested successfully. The mask was made from five alternating transparent rings, where the rings had half a wave phase difference between them. The beam was scanned with and without the mask in two dimensions in fine steps by a much smaller detector and its response was taken. The spot width dropped by 19% at half its height and by 42% at tenth its height, a significant narrowing. The scan was repeated with the full detector pixel. That beam scan served as a deconvolution kernel and allowed us to find the pixel point spread function (spatial response), the pixel modulation transfer function and the optical cross talk between the pixels.

Journal Article↗
PubMed2006

Simulated annealing in ocular adaptive optics.

We present what is to our knowledge a first hardware realization of a simulated annealing algorithm in an adaptive optics system designed to image the retina of the human eye. The algorithm is applied to the retinal image itself without the need for wavefront sensors in the system. We find that this optimization algorithm can be an alternative to the traditional Hartmann-Shack sensing. We also compare the simulated annealing algorithm to the stochastic parallel gradient descent algorithm.

Algorithms↗
PubMed2005

Banding of suspended particles in a rotating fluid-filled horizontal cylinder.

Non-Brownian particles suspended at low volume concentration in a rotating horizontal cylinder filled with a low-viscosity fluid are observed to segregate into well-defined periodic axial bands. We present an experimental investigation of the dependence of the phenomenon on particle characteristics, tube diameter and length, and fluid viscosity. A theoretical explanation of the phenomenon is suggested, in which the segregation occurs as a result of mutual interaction between the particles and inertial waves excited in the bounded fluid. This leads to the result that macroscopic suspended particles accumulate in alternate nodes of the wave excitation, which is in agreement with the experiments, and leads to two degenerate band patterns for each mode. Under some conditions the observed pattern oscillates between the two possible band configurations. The mechanism underlying the oscillations is unclear. A confirmation of the theoretical approach was obtained by means of a photographic capture of the flow field resulting from the inertial waves.

Journal Article↗
PubMed2004

How does a thin volatile film move?

When a water film evaporates from a mica substrate, an interface similar to a solidification front develops, separating two films of different thicknesses. We show experimentally that the evolution dynamics is controlled mainly by material diffusion through the vapor phase rather than by hydrodynamic flow through the film. Our results illustrate the role of different contributions to pattern formation of volatile liquid films.

Letter↗
PubMed2004

Oscillatory axial banding of particles suspended in a rotating fluid.

A nonviscous fluid, completely filling a tube rotating about its horizontal axis, contains a suspension of macroscopic particles. The particles are observed to distribute themselves spontaneously in bands distributed periodically along the axis, with a band separation dependent only on the tube radius and length. In many cases, the bands oscillate periodically between two interleaving patterns. We explain this banding phenomenon as arising from the excitation of inertial standing waves in the rotating fluid.

Journal Article↗
PubMed2003

Symbiosis of different-sized drops.

We demonstrate that the usual situation of coarsening in crystal growth or Ostwald ripening in evaporating liquid drops is not universal, and when the drops coexist with a microscopically thin continuous surface film, a different behavior is observed. The predicted behavior was investigated experimentally and supported by numerical simulations.

Journal Article↗
PubMed2003

Improvement of optical resolution in far-field imaging by optical multiplication.

Based on the idea of the complex filters suggested by Toraldo di Francia in 1952, super-resolved imaging has been achieved by using interferometric image multiplication. The question of whether super-resolution implies a limited field of view is discussed, and an analytical criterion to decide this question is obtained. Three different imaging configurations are investigated, all of which gave a spatial resolution exceeding that corresponding to the full open aperture of the optical system. A resolution limit 55% of the Rayleigh limit was achieved for incoherently illuminated objects. One price that must be paid for the super-resolution is inefficient use of the light source. A second price is a limitation to the usable field of view.

Fourier Analysis↗
PubMed2003

Why is super-resolution so inefficient?

Many optical imaging systems have been designed to achieve spatial resolution exceeding the limit proposed by Abbe theory, namely lambda/2NA. These systems invariably use light inefficiently. We discuss the application of information theory to this problem, in an effort to find an upper limit to the theoretical efficiency of a super-resolving system, as a function of the degree to which its capabilities exceed the Abbe limit.

Image Enhancement↗
PubMed2002

Superresolution in far-field imaging.

We demonstrate the feasibility of achieving superresolved images by using Fourier-plane phase masks and image multiplication, which together create effective point-spread functions that are not positive definite and therefore cannot be created by any single Fourier plane mask in a linear system. Three different configurations were investigated, all of which gave a spatial resolution exceeding that corresponding to the full open aperture of the optical system. One price that must be paid for the superresolution is inefficient use of the light source.

Journal Article↗