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Improved particle position accuracy from off-axis holograms using a Chebyshev model
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0003-0398-1919
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0003-4879-8261
2018 (English)In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 57, no 1, p. A157-A163Article in journal (Refereed) Published
Abstract [en]

Side scattered light from micrometer-sized particles is recorded using an off-axis digital holographic setup. From holograms, a volume is reconstructed with information about both intensity and phase. Finding particle positions is non-trivial, since poor axial resolution elongates particles in the reconstruction. To overcome this problem, the reconstructed wavefront around a particle is used to find the axial position. The method is based on the change in the sign of the curvature around the true particle position plane. The wavefront curvature is directly linked to the phase response in the reconstruction. In this paper we propose a new method of estimating the curvature based on a parametric model. The model is based on Chebyshev polynomials and is fit to the phase anomaly and compared to a plane wave in the reconstructed volume. From the model coefficients, it is possible to find particle locations. Simulated results show increased performance in the presence of noise, compared to the use of finite difference methods. The standard deviation is decreased from 3–39 μm to 6–10 μm for varying noise levels. Experimental results show a corresponding improvement where the standard deviation is decreased from 18 μm to 13 μm.

Place, publisher, year, edition, pages
Optical Society of America, 2018. Vol. 57, no 1, p. A157-A163
Keyword [en]
Digital holography, Scattering, particles, Mie theory, Image processing
National Category
Atom and Molecular Physics and Optics Applied Mechanics
Research subject
Experimental Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-66958DOI: 10.1364/AO.57.00A157ISI: 000418614400020Scopus ID: 2-s2.0-85039793350OAI: oai:DiVA.org:ltu-66958DiVA, id: diva2:1164505
Funder
Swedish Research Council, 621-2014-4906
Note

Validerad;2018;Nivå 2;2017-12-15 (andbra)

Available from: 2017-12-11 Created: 2017-12-11 Last updated: 2018-04-16Bibliographically approved
In thesis
1. 3D localization in digital holography from scattered light from micrometer-sized particles
Open this publication in new window or tab >>3D localization in digital holography from scattered light from micrometer-sized particles
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

When a particle is illuminated by a beam of light it will scatter and redistribute the light in all directions. How it scatters depends on the size, shape and refractive index of the particle. Additionally, it depends on the wavelength and polarization of the illuminating beam. The direction and distance to the observer relative the particle also needs to be considered.  A digital holographic imaging system is used to collect parts of the scattered light from micrometer-sized particles. By utilizing digital holography a three-dimensional reconstruction of the imaged scene is possible. Traditionally, particles are localized based on the intensity in the holographic reconstructions. In this licentiate thesis, the phase response of the scattered light is investigated and utilized. An alternative method for locating spherical particles is presented. The method locate particles based on a simple feature of a propagating wave, namely the fact that the wavefront curvature changes from converging to diverging at the axial location of the particle. The wavefront curvature is estimated using two different methods. The first method estimates the lateral phase-gradients using a finite-difference method. The second method uses a three-dimensional parametric model based on a Chebyshev polynomial expansion. The methods are demonstrated using both simulations and experimental measurements. The simulations are based on the Lorenz-Mie scattering theory for spherical particles and are combined with an imaging system model. Experiments are performed using an off-axis polarization sensitive digital holographic system with a coherent Nd:YAG laser. Measurements of stationary particles are made to validate and evaluate the proposed method. It is found that these methods estimate the true axial position and does not have the offset that is associated with intensity-based methods. Additionally, it is possible to exclude noise that shows up as false particles since noise does not have the same phase response as a real particle. The second method, that uses a parametric model, also improves the standard deviation in the positioning.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2018
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keyword
Digital Holography, Polarization, Particle Scattering, Metrology
National Category
Applied Mechanics
Research subject
Experimental Mechanics
Identifiers
urn:nbn:se:ltu:diva-68374 (URN)978-91-7790-114-3 (ISBN)978-91-7790-115-0 (ISBN)
Presentation
2018-06-14, E243, Luleå Tekniska Universitet, Luleå, 09:00 (Swedish)
Opponent
Supervisors
Funder
Swedish Research Council, 621-2014-4906
Available from: 2018-04-17 Created: 2018-04-16 Last updated: 2018-04-17Bibliographically approved

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