Arlon J. Hunt

University of California, Berkeley, Berkeley, California, United States

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Publications (14)9.88 Total impact

  • Light Scattering by Nonspherical Particles : Theory, Measurements, and Applications. 01/2000;
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    ABSTRACT: The intensity and polarization of light scattered from marine aerosols affect visibility and contrast in the marine atmospheric boundary layer (MABL). The polarization properties of scattered light in the MABL vary with size, refractive index, number distributions, and environmental conditions. Laboratory measurements were used to determine the characteristics and variability of the polarization of light scattered by aerosols similar to those in the MABL. Scattering from laboratory-generated sea-salt-containing (SSC) [NaCl, (NH(4))(2) SO(4), and seawater] components of marine aerosols was measured with a scanning polarization-modulated nephelometer. Mie theory with Gaussian and log normal size distributions of spheres was used to calculate the polarized light scattering from various aerosol composition models and from experimentally determined distributions of aerosols in the marine boundary layer. The modeling was verified by comparison with scattering from distilled water aerosols. The study suggests that polarimetric techniques can be used to enhance techniques for improving visibility and remote imaging for various aerosol types, Sun angles, and viewing conditions.
    Applied Optics 08/1997; 36(21):5168-84. · 1.69 Impact Factor
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    ABSTRACT: The angle- and polarization-dependent light scattering were measured for oriented first-year and multiyear sea ice taken from the Chukchi Sea near Pt. Barrow, Alaska. The entire Mueller matrix for these samples was determined at 532 nm. Mueller matrices were also determined for artificially grown saline ice samples and melted samples of the respective ice types. Phase functions for thin-slab samples are qualitatively consistent with calculations for scattering from brine inclusions in a solid ice medium and depend strongly on the shape of the scattering sample. Small orientation-dependent effects are observed for scattering from oriented sea ice. A simple model is used to describe qualitatively some features of the measured sea ice Mueller matrices. This model combines the effects of scattering from spherical inhomogeneities and the intrinsic birefringence of pure water ice. A set of Mueller matrix inequalities is presented and used to obtain physical insight into the measurement results.
    Applied Optics 03/1997; 36(6):1278-88. · 1.69 Impact Factor
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    ABSTRACT: The coupled-dipole approximation has been used to model polarized light-scattering data obtained from the sperm of the octopus Eledone cirrhosa. Mueller scattering-matrix elements (which describe how a sample alters the intensity and degree of polarization of scattered light) were measured as a function of angle. The sample was modeled as a helical fiber believed to correspond to a DNA protein complex. It was necessary to propose an inherent anisotropy in the polarizability of the fiber in order to fit the data. The direction of the principle axes of the polarizability were determined by comparing the model with experimental data. The results suggest that the 2-nm DNA fibers are perpendicular to the thick fiber that defines the helical geometry of the octopus sperm head.
    Applied Optics 08/1994; 33(24):5733-44. · 1.69 Impact Factor
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    ABSTRACT: We have developed a bistatic, polarization-modulated nephelometer for making in situ measurements of angle- and polarization-dependent light scattering in sea ice. The instrument sits directly and noninvasively on the surface of the sea ice, and can also be used for studying scattering in seawater. A visible laser beam is directed into the medium and the scattered light is detected by the analyzer optics. Both laser and detector angles can be varied. The technique of ac polarimetry is used to accurately obtain phase function and polarization information of the scattered light. This instrument was tested by scattering from monodisperse polystyrene (latex) spheres in the laboratory. Results of Mie calculations are in good qualitative agreement with scattering measured in the laboratory. This instrument was used to measure the angle- and polarization-dependent scattering in sea ice at Pt. Barrow, Alaska May 1994. Preliminary findings indicate a large polarization signal associated with the orientational ordering of the sea ice crystallites.
    Proc SPIE 01/1994;
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    ABSTRACT: This paper investigates the variation in the polarization properties of light scattered by aerosols in the MBL arising from various aerosol sources and environmental conditions. Mie scattering models of Gaussian and log normal distributions of spheres and combinations thereof were used to calculate the polarization properties of light scattered from various models and from experimentally determined distributions of aerosols in the marine boundary layer. The results of the models were verified by comparison to scattering from (NH4)2SO4 particles, a component of aerosols in the MBL that is nearly spherical and readily generated in the laboratory. Calculations indicate that scattering by aerosols is distinctly not Rayleigh-like. The polarization properties of light scattered in the MBL vary with size, refractive index, and number distributions of particles in the MBL. These differences should be instrumentally detectable and could be important for visibility.
    Proc SPIE 01/1994;
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    ABSTRACT: The consequences of light scattering from both spherical and non-spherical particles on the propagation of light in the ocean were investigated. The scattering from an ensemble of non- spherical micro-organisms is calculated using the coupled-dipole approximation with an orientational average over Euler angles using Gauss-Legendre integration. Mie calculations provide rigorous solutions for spherical particles and are considerably less computer intensive than the coupled-dipole approximation. Furthermore, they have been shown to accurately predict the scattering for marine organisms that are nearly spherical. Scattering matrix elements calculated using the coupled-dipole approximation were compared with those obtained using Mie calculations in the limit as an ellipsoidal object approaches a sphere in order to assess the limits of applicability of the Mie theory to ellipsoidal particles. Experimental measurements of the scattering matrix elements for spherical particles (latex spheres) and ellipsoidal particles (Bacillus subtilis) were used to test the validity of our analytical approach.
    Proc SPIE 01/1994;
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    ABSTRACT: The polarization state of a beam of light is fully described by the four elements of the Stokes vector. How the Stokes vector is transformed upon scattering is described by the sixteen element Mueller scattering matrix. Polarized light scattering provides a useful tool to study macromolecular structure. In particular, measurements of Mueller matrix elements have been used to study chromosome structure; changes in these measured quantities can be used to monitor structure as a function of cell cycle. Such measurements done on helical structures can be compared to theoretical computations to determine the geometrical parameters describing the helix. The matrix elements that are most sensitive to the chiral parameters of the helix are largest when the wavelength of light is of the order of the pitch of the helix. Therefore, polarized light scattering measurements made on DNA plectonemic helices would provide the most information in the far ultraviolet (UV) and x-ray region. The Mueller matrix elements are calculated using the coupled dipole approximation in the orientation average at wavelengths in the visible, ultraviolet, and x-ray regions. Each base-pair of the idealized plectonemic helix is represented by a single dipole. A complex polarizability tensor is assigned to each dipole. Calculations are sensitive to the writhe and polarizability of the DNA molecule.
    The Journal of Chemical Physics 01/1994; 101(5):4214-4221. · 3.12 Impact Factor
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    ABSTRACT: Comparisons are made between a refined model using coupled-dipole theory and the first Born approximation for light scattering from helices. The use of the first Born approximation to model polarized light scattering from a thin wire helix is further developed in order to include all sixteen Mueller scattering matrix elements. (The Mueller matrix fully describes how a structure alters the polarization state of light upon scattering). Comparisons of predicted Mueller matrices between the two theories show that, in some cases, good agreement is obtained. The predicted Mueller matrix for an ensemble of randomly oriented helices using the first Born approximation is calculated. The models based on the first Born approximation and on coupled-dipole theory were also compared to data taken from octopus sperm. We conclude that the first Born approximation may be useful for predicting some elements such as S14 (the matrix element that describes the depolarization of incident circularly polarized light), but for other elements the coupled-dipole theory is better suited.
    Proc SPIE 12/1992;
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    ABSTRACT: Striking light-scattering behavior was observed from a marine dinoflagellate, Prorocentrum micans. Measurements of the angular dependence of the 16 Mueller matrix elements were performed on single cells with a polarization-modulation nephelometer by using a new method for cell immobilization. First the dinoflagellate cells were immobilized in a transparent silica gel containing alcohol, and then a second liquid was diffused into the gel to match the index of refraction of the gel network, thereby producing a transparent support medium that scatters less than one tenth the amount of light scattered by a single cell at 90 degrees . Measurements of scattering by a single cell revealed that all 16 matrix elements were significantly nonzero and different from each other. All matrix elements have an extremely rich, reproducible structure that is highly dependent on cell orientation. The matrix elements symmetrically across the diagonal were not equivalent. Striking features of the measurements are the large peak values of S(13), S(14), and other off-diagonal block elements. We believe that this is the first report of such scattering signals by single, suspended marine microorganisms.
    Applied Optics 05/1992; 31(15):2924-31. · 1.69 Impact Factor
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    ABSTRACT: The polarization state of light in the ocean can be used to enhance visibility. The consequences of scattering from nonspherically-symmetric particles on light propagation and visibility in the ocean was investigated. To calculate scattering from nonspherical marine microorganisms, it is usually necessary to resort to approximate methods. One promising approximation is the coupled-dipole approach in which an arbitrarily-shaped object is divided into a number of identical elements arranged on a cubic lattice. Each element is treated as a spherical, dipolar oscillator with its polarizability specified by the real and imaginary parts of the index of refraction. Interactions between dipoles are included by determining the field at a particular dipole due to the incident field and the fields induced by the other dipole oscillators. The scattered field is then the sum of the fields due to each oscillator. The coupled-dipole method is promising because, in principle, an organism of any shape can be modeled, and all 16 elements of the scattering matrix calculated. This approach has been applied to calculate scattering from spherical particles to verify the limits of the approximation, and from other shapes to investigate the effects of nonsphericity and chirality on scattering. In particular, all 16 Mueller matrix elements for the scattering were calculated from a finite cylinder, a single- strand helix, 14-strand helix, and ensembles of these particles. The effects of pitch, size, wavelength, and complex index of refraction were investigated. The results provide insights into the magnitude and type of depolarization effects associated with various marine microorganisms containing these structures.
    Proc SPIE 01/1991;
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    ABSTRACT: Light scattering measurements were performed on single, immobilized dinoflagellates as well as on suspensions of the species Prorocentrum micans. The normalized Mueller scattering matrix element S14, which indicates an ability to depolarize circularly polarized light, is reported for both cases. The measurements involving single cells were performed on Prorocentrum micans, Gonyaulax polyhedra, and Crypthecodinium cohnii. The results show that the previously reported large S14 signal is not peculiar to P. micans. Time-dependent measurements of live cultures of P. micans show a large, high-frequency S14 signal. This signal is a diurnal function of the time of day, with a maximum at midnight. Investigations of the relationship between single cell and suspension measurements reveal that the large angle- dependent S14 peaks from immobilized single dinoflagellates are responsible for a large time-dependent S14 signal at 90 degree(s) in suspension measurements. The results of these experiments provide further evidence for the hypothesis that the chromosomes of the dinoflagellates are responsible for the large observed S14 signals. The unusual depolarization properties of dinoflagellates should be considered when using polarized light to enhance image contrast in underwater imaging.
    Proc SPIE 01/1991;
  • Proc SPIE 09/1990;
  • Proc SPIE 01/1990;