Engineering topics

Lock, James A.

Publications and source records attributed to Lock, James A..

At least 19 records

NASA NTRS2016

First-Principles Definition and Measurement of Planetary Electromagnetic-Energy Budget

The imperative to quantify the Earths electromagnetic-energy budget with an extremely high accuracy has been widely recognized but has never been formulated in the framework of fundamental physics. In this paper we give a first-principles definition of the planetary electromagnetic-energy budget using the Poynting- vector formalism and discuss how it can, in principle, be measured. Our derivation is based on an absolute minimum of theoretical assumptions, is free of outdated notions of phenomenological radiometry, and naturally leads to the conceptual formulation of an instrument called the double hemispherical cavity radiometer (DHCR). The practical measurement of the planetary energy budget would require flying a constellation of several dozen planet-orbiting satellites hosting identical well-calibrated DHCRs.

phenomenology↗

NASA NTRS2015

A Persistent Feature of Multiple Scattering of Waves in the Time-Domain: A Tutorial

The equations for frequency-domain multiple scattering are derived for a scalar or electromagnetic plane wave incident on a collection of particles at known positions, and in the time-domain for a plane wave pulse incident on the same collection of particles. The calculation is carried out for five different combinations of wave types and particle types of increasing geometrical complexity. The results are used to illustrate and discuss a number of physical and mathematical characteristics of multiple scattering in the frequency- and time-domains. We argue that frequency-domain multiple scattering is a purely mathematical construct since there is no temporal sequencing information in the frequency-domain equations and since the multi-particle path information can be dispelled by writing the equations in another mathematical form. However, multiple scattering becomes a definite physical phenomenon in the time-domain when the collection of particles is illuminated by an appropriately short localized pulse.

particles↗

NASA NTRS2006

Scattering of a Tightly Focused Beam by an Optically Trapped Particle

Near-forward scattering of an optically trapped 5 m radius polystyrene latex sphere by the trapping beam was examined both theoretically and experimentally. Since the trapping beam is tightly focused, the beam fields superpose and interfere with the scattered fields in the forward hemisphere. The observed light intensity consists of a series of concentric bright and dark fringes centered about the forward scattering direction. Both the number of fringes and their contrast depend on the position of the trapping beam focal waist with respect to the sphere. The fringes are caused by diffraction due to the truncation of the tail of the trapping beam as the beam is transmitted through the sphere.

Lock, James A.↗

NASA NTRS2003

A Theoretical Light Scattering Model of Nanoparticle Laser Tweezers

Accomplishments this reporting period include: 1. derived, programmed, checked, and tested the Mie light scattering theory formulas for the radiation trapping force for both the on-axis and off-axis geometry of the trapping beam plus trapped spherical particle; 2. verified that the computed radiation trapping force for a freely propagating focused Gaussian laser beam incident on a spherical particle agrees with previous published calculations; 3. compared the small particle size and large particle size limits of the Mie calculation with the results of Rayleigh scattering theory and ray scattering theory, respectively and verified that the comparison is correct for Rayleigh scattering theory but found that ray theory omits an important light scattering effect included in the Mie theory treatment; 4. generalized the calculation of the radiation trapping force on a spherical particle in the on-axis geometry from a freely propagating focused Gaussian laser beam to the realistic situation of a Gaussian beam truncated and focused by a high numerical aperture oil-immersion microscope objective lens and aberrated by the interface between the microscope cover slip and the liquid-filled sample volume; and 5. compared the calculated radiation trapping force for this geometry with the results of previously published experiments and found that the agreement is better than when using previously developed theories.

Lock, James A.↗

NASA NTRS2002

Detection, Evaluation, and Optimization of Optical Signals Generated by Fiber Optic Bragg Gratings Under Dynamic Excitations

The dynamic response of a fiber optic Bragg grating to mechanical vibrations is examined both theoretically and experimentally. The theoretical expressions describing the consequences of changes in the grating's reflection spectrum are derived for partially coherent beams in an interferometer. The analysis is given in terms of the dominant wavelength, optical bandwidth, and optical path difference of the interfering signals. Changes in the reflection spectrum caused by a periodic stretching and compression of the grating were experimentally measured using an unbalanced Michelson interferometer, a Michelson interferometer with a non-zero optical path difference. The interferometer's sensitivity to changes in dominant wavelength of the interfering beams was measured as a function of interferometer unbalance and was compared to theoretical predictions. The theoretical analysis enables the user to determine the optimum performance for an unbalanced interferometer.

Adamovsky, Grigory↗

NASA NTRS2002

Optical Scattering Characterization for the Glennan Microsystems Microscale Particulate Classifier

Small sensors that are tolerant to mechanically and thermally harsh environments present the possibility for in-situ particle characterization in propulsion, industrial, and planetary science applications. Under a continuing grant from the Glennan Microsystems Initiative to the Microgravity Fluids Physics Branch of the NASA-Glenn Research Center, a Microscale Particle Classifier (MiPAC) instrument is being developed. The MiPAC instrument will be capable of determining the size distribution of airborne particles from about 1 nm to 30 micrometers, and will provide partial information as to the concentration, charge state, shape, and structure of the particles, while being an order of magnitude smaller in size and lighter in weight than presently commercially available instruments. The portion of the instrument that will characterize the nm-range particles will employ electrical mobility techniques and is being developed under a separate grant to Prof. David Pui of the University of Minnesota. The portion of the instrument that will characterize the micrometer-size particles such as dirt, pollens, spores, molds, soot, and combustion aerosols will use light scattering techniques. The development of data analysis techniques to be employed in the light scattering portion of the instrument is covered by this grant.

Lock, James A.↗

NASA NTRS1999

Laser Light Scattering with Multiple Scattering Suppression Used to Measure Particle Sizes

Laser light scattering is the technique of choice for noninvasively sizing particles in a fluid. The members of the Advanced Technology Development (ATD) project in laser light scattering at the NASA Lewis Research Center have invented, tested, and recently enhanced a simple and elegant way to extend the concentration range of this standard laboratory particle-sizing technique by several orders of magnitude. With this technique, particles from 3 nm to 3 mm can be measured in a solution. Recently, laser light scattering evolved to successfully size particles in both clear solutions and concentrated milky-white solutions. The enhanced technique uses the property of light that causes it to form tall interference patterns at right angles to the scattering plane (perpendicular to the laser beam) when it is scattered from a narrow laser beam. Such multiple-scattered light forms a broad fuzzy halo around the focused beam, which, in turn, forms short interference patterns. By placing two fiber optics on top of each other and perpendicular to the laser beam (see the drawing), and then cross-correlating the signals they produce, only the tall interference patterns formed by singly scattered light are detected. To restate this, unless the two fiber optics see the same interference pattern, the scattered light is not incorporated into the signal. With this technique, only singly scattered light is seen (multiple-scattered light is rejected) because only singly scattered light has an interference pattern tall enough to span both of the fiber-optic pickups. This technique is simple to use, easy to align, and works at any angle. Placing a vertical slit in front of the signal collection fibers enhanced this approach. The slit serves as an optical mask, and it significantly shortens the time needed to collect good data by selectively masking out much of the unwanted light before cross-correlation is applied.

Meyer, William V.↗

NASA NTRS1996

Ray Scattering by an Arbitrarily Oriented Spheroid: 2. Transmission and Cross-polarization Effects

Transmission of an arbitrarily polarized plane wave by an arbitrarily oriented spheroid in the short-wavelength limit is considered in the context of ray theory. The transmitted electric field is added to the diffracted plus reflected ray-theory electric field that was previously derived to obtain an approximation to the far-zone scattered intensity in the forward hemisphere. Two different types of cross-polarization effects are found. These are: (a) a rotation of the polarization state of the transmitted rays from when they are referenced with respect to their entrance into the spheroid to when they are referenced with respect to their exit from it and (b) a rotation of the polarization state of the transmitted rays when they are referenced with respect to the polarization state of the diffracted plus reflected rays.

Lock, James A.↗

NASA NTRS1995

Interpretation of Extinction in Gaussian-Beam Scattering

The extinction efficiency for the interaction of a plane wave with a large nonabsorbing spherical particle is approximately 2.0. When a Gaussian beam of half-width w(sub 0) is incident upon a spherical particle of radius a with w(sub 0)/a less than 1, the extinction efficiency attains unexpectedly high or low values, contrary to intuitive expectations. The reason for this is associated with the so-called compensating term in the scattered field, which cancels the field of the Gaussian beam behind the particle, thereby producing the particle's shadow. I introduce a decomposition of the total exterior field into incoming and outgoing portions that are free of compensating terms. It is then shown that a suitably defined interaction efficiency has the intuitively expected asymptotic values of 2.0 for w(sub 0)/a much greater than 1 and 1.0 for w(sub 0)/a much less than 1.

Lock, James A.↗

NASA NTRS1995

Partial-Wave Representations of Laser Beams for Use in Light-Scattering Calculations

In the framework of generalized Lorenz-Mie theory, laser beams are described by sets of beam-shape coefficients. The modified localized approximation to evaluate these coefficients for a focused Gaussian beam is presented. A new description of Gaussian beams, called standard beams, is introduced. A comparison is made between the values of the beam-shape coefficients in the framework of the localized approximation and the beam-shape coefficients of standard beams. This comparison leads to new insights concerning the electromagnetic description of laser beams. The relevance of our discussion is enhanced by a demonstration that the localized approximation provides a very satisfactory description of top-hat beams as well.

Gouesbet, Gerard↗

NASA NTRS1995

Improved Gaussian Beam-Scattering Algorithm

The localized model of the beam-shape coefficients for Gaussian beam-scattering theory by a spherical particle provides a great simplification in the numerical implementation of the theory. We derive an alternative form for the localized coefficients that is more convenient for computer computations and that provides physical insight into the details of the scattering process. We construct a FORTRAN program for Gaussian beam scattering with the localized model and compare its computer run time on a personal computer with that of a traditional Mie scattering program and with three other published methods for computing Gaussian beam scattering. We show that the analytical form of the beam-shape coefficients makes evident the fact that the excitation rate of morphology-dependent resonances is greatly enhanced for far off-axis incidence of the Gaussian beam.

Lock, James A.↗

NASA NTRS1994

Rigorous Justification of the Localized Approximation to the Beam-Shape Coefficients in Generalized Lorenz-Mie Theory: On-Axis Beams - Part 1

Generalized Lorenz-Mie theory describes electromagnetic scattering of an arbitrary light beam by a spherical particle. The computationally most expensive feature of the theory is the evaluation of the beam-shape coefficients, which give the decomposition of the incident light beam into partial waves. The so-called localized approximation to these coefficients for a focused Gaussian beam is an analytical function whose use greatly simplifies Gaussian-beam scattering calculations. A mathematical justification and physical interpretation of the localized approximation is presented for on-axis beams.

Lock, James A.↗

NASA NTRS1994

Rainbow Scattering by a Coated Sphere

We examine the behavior of the first-order rainbow for a coated sphere by using both ray theory and Aden-Kerker wave theory as the radius of the core alpha(sub 12) and the thickness of the coating beta are varied. As the ratio beta/alpha(sub 12) increases from 10(sup -4) to 0.33, we find three classes of rainbow phenomena that cannot occur for a homogeneous-sphere rainbow. For beta/alpha(sub 12) approx less than 10(sup -3), the rainbow intensity is an oscillatory function of the coating thickness, for beta/alpha(sub 12) approx. 10(sup -2), the first-order rainbow breaks into a pair of twin rainbows, and for beta/alpha(sub 12) approx. 0.33, various rainbow-extinction transitions occur. Each of these effects is analyzed, and their physical interpretations are given. A Debye series decomposition of coated-sphere partial-wave scattering amplitudes is also performed and aids in the analysis.

Lock, James A.↗

NASA NTRS1993

Calibration of the forward-scattering spectrometer probe - Modeling scattering from a multimode laser beam

Scattering calculations using a detailed model of the multimode laser beam in the forward-scattering spectrometer probe (FSSP) were carried out using a recently developed extension to Mie scattering theory. From this model, new calibration curves for the FSSP were calculated. The difference between the old calibration curves and the new ones is small for droplet diameters less than 10 microns, but the difference increases to approximately 10 percent at diameters of 50 microns. When using glass beads to calibrate the FSSP, calibration errors can be minimized by using glass beads of many different diameters, over the entire range of the FSSP. If the FSSP is calibrated using one-diameter glass beads, then the new formalism is necessary to extrapolate the calibration over the entire range.

Hovenac, Edward A.↗

NASA NTRS1993

Calibration of the Forward-scattering Spectrometer Probe: Modeling Scattering from a Multimode Laser Beam

Scattering calculations using a more detailed model of the multimode laser beam in the forward-scattering spectrometer probe (FSSP) were carried out by using a recently developed extension to Mie scattering theory. From this model, new calibration curves for the FSSP were calculated. The difference between the old calibration curves and the new ones is small for droplet diameters less than 10 micrometers, but the difference increases to approximately 10% at diameters of 50 micrometers. When using glass beads to calibrate the FSSP, calibration errors can be minimized, by using glass beads of many different diameters, over the entire range of the FSSP. If the FSSP is calibrated using one-diameter glass beads, then the new formalism is necessary to extrapolate the calibration over the entire range.

Hovenac, Edward A.↗

NASA NTRS1993

Diffraction of a Gaussian Beam by a Spherical Obstacle

The Kirchhoff integral for diffraction in the near-forward direction is derived from the exact solution of the electromagnetic boundary value problem of a focused Gaussian laser beam incident on a spherical particle. The diffracted intensity in the vicinity of the particle is computed and the way in which the features of the diffraction pattern depend on the width of the Gaussian beam is commented on.

Lock, James A.↗

NASA NTRS1993

Contribution of High-Order Rainbows to the Scattering of a Gaussian Laser Beam by a Spherical Particle

I review the theory of the scattering of a Gaussian laser beam by a dielectric spherical particle and give the details for constructing a computer program to implement the theory. Computational results indicate that if the width of the laser beam is much less than the diameter of the particle and if the axis of the beam is incident near the edge of the particle, the fifth-, sixth-, and ninth-order rainbows should be evident in the far-field scattered intensity. I performed an experiment that yielded tentative evidence for the presence of the sixth- order rainbow.

Lock, James A.↗

NASA NTRS1992

Experimental testing of four correction algorithms for the forward scattering spectrometer probe

Three number density correction algorithms and one size distribution correction algorithm for the Forward Scattering Spectrometer Probe (FSSP) were compared with data taken by the Phase Doppler Particle Analyzer (PDPA) and an optical number density measuring instrument (NDMI). Of the three number density correction algorithms, the one that compared best to the PDPA and NDMI data was the algorithm developed by Baumgardner, Strapp, and Dye (1985). The algorithm that corrects sizing errors in the FSSP that was developed by Lock and Hovenac (1989) was shown to be within 25 percent of the Phase Doppler measurements at number densities as high as 3000/cc.

Hovenac, Edward A.↗