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| 2. |
- Kristensson, Elias, et al.
(författare)
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Modeling of the SLIPI technique with the large scatterer approximation of the RTE
- 2017
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Ingår i: 2017 32nd General Assembly and Scientific Symposium of the International Union of Radio Science.
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Konferensbidrag (refereegranskat)abstract
- To overcome the blur when visualizing of the interior of a turbid media by means light-based methods, a technique called Structured Laser Illumination Planar Imaging (SLIPI) is employed. The method is based on a 'light coding' strategy to distinguish between directly and multiply scattered light, allowing the intensity from the latter to be suppressed by post-processing data. In this paper, we explain the origin of the deviations between SLIPI measurements and the results obtained by the Bouguer-Beer law. The explanation employs the large scatterer approximation of the Radiative Transfer Equation. Our results indicate that forward-scattering can lead to deviations between experiments and theoretical predictions, especially when probing relatively large particles.
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| 3. |
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| 4. |
- Kristensson, Elias, et al.
(författare)
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Physical explanation of the SLIPI technique by the large scatterer approximation of the RTE
- 2017
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Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073. ; 189, s. 112-125
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Tidskriftsartikel (refereegranskat)abstract
- Visualizing the interior of a turbid scattering media by means light-based methods is not a straightforward task because of multiple light scattering, which generates image blur. To overcome this issue, a technique called Structured Laser Illumination Planar Imaging (SLIPI) was developed within the field of spray imaging. The method is based on a ‘light coding’ strategy to distinguish between directly and multiply scattered light, allowing the intensity from the latter to be suppressed by means of data post-processing. Recently, the performance of the SLIPI technique was investigated, during which deviations from theoretical predictions were discovered. In this paper, we aim to explain the origin of these deviations, and to achieve this end, we have performed several SLIPI measurements under well-controlled conditions. Our experimental results are compared with a theoretical model that is based on the large scatter approximation of the Radiative Transfer Equation but modified according to certain constraints. Specifically, our model is designed to (1) ignore all off-axis intensity contributions, (2) to treat unperturbed- and forward-scattered light equally and (3) to accept light to scatter within a narrow forward-cone as we believe these to the rules governing the SLIPI technique. The comparison conclusively shows that optical measurements based on scattering and/or attenuation in turbid media can be subject to significant errors if not all aspects of light-matter interactions are considered. Our results indicate, as were expected, that forward-scattering can lead to deviations between experiments and theoretical predictions, especially when probing relatively large particles. Yet the model also suggests that the spatial frequency of the superimposed ‘light code’ as well as the spreading of the light-probe are important factors one also needs to consider. The observed deviations from theoretical predictions could, however, potentially be exploited to assess particle size
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| 5. |
- Gustavsson, Magnus, et al.
(författare)
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Multiple scattering by a collection of randomly located obstacles - numerical implementation of the coherent fields
- 2016
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Ingår i: Journal of Quantitative Spectroscopy & Radiative Transfer. - : Elsevier BV. - 0022-4073. ; 185, s. 95-100
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Tidskriftsartikel (refereegranskat)abstract
- A numerical implementation of a method to analyze scattering by randomly located obstacles in a slab geometry is presented. In general, the obstacles can be of arbitrary shape, but, in this first implementation, the obstacles are dielectric spheres. The coherent part of the reflected and transmitted intensity at normal incidence is treated. Excellent agreement with numerical results found in the literature of the effective wave number is obtained. Moreover, comparisons with the results of the Bouguer–Beer (B–B) law are made. The present theory also gives a small reflected coherent field, which is not predicted by the Bouguer–Beer law, and these results are discussed in some detail.
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| 6. |
- Karlsson, Anders, et al.
(författare)
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Microwave theory
- 2015
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Bok (övrigt vetenskapligt/konstnärligt)
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| 7. |
- Kristensson, Gerhard
(författare)
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Coherent scattering by a collection of randomly located obstacles --- an alternative integral equation formulation
- 2015
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Ingår i: Journal of Quantitative Spectroscopy & Radiative Transfer. - : Elsevier BV. - 0022-4073. ; 164, s. 97-108
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Tidskriftsartikel (refereegranskat)abstract
- Scattering of electromagnetic waves by discrete, randomly distributed objects is addressed. In general, the non-intersecting scattering objects can be of arbitrary form, material and shape. The main aim of this paper is to calculate the coherent reflection and transmission characteristics of a slab containing discrete, randomly distributed scatterers. The integral representation of the solution of the deterministic problem constitutes the underlying framework of the stochastic problem. Conditional averaging and the employment of the Quasi Crystalline Approximation lead to a system of integral equations in the unknown expansion coefficients. Of special interest is the slab geometry, which implies a system of integral equations in the depth variable. Explicit solutions for tenuous media and low frequency approximations can be obtained for spherical obstacles.
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| 8. |
- Kristensson, Gerhard
(författare)
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Electromagnetic scattering by layered cylinders --- low frequency reduction of RCS
- 2015
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In this paper we present two different ways of reducing the radar cross section (RCS) of thin metallic wires at low frequencies. This reduction is achieved by adding a dielectric coating on the wire. The entire cylindrical structure is finally protected by a thin exterior non-magnetic dielectric layer (eagle skin). The contribution to the RCS has either an electric or an magnetic origin. To reduce the RCS it is possible to chose material parameters so that both the electric and the magnetic contributions vanish. This leads, however, to non-physical material parameter values. The underlying principle in the second method is to chose material parameters in the dielectric coating such that the electric and magnetic contributions annihilate each other. Fortunately, this annihilation can be obtained with physically realistic material parameters. Explicit numerical examples illustrate both ideas.
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| 9. |
- Kristensson, Gerhard
(författare)
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Evaluation of some integrals relevant to multiple scattering by randomly distributed obstacles
- 2015
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Ingår i: Journal of Mathematical Analysis and Applications. - : Elsevier BV. - 0022-247X. ; 432:1, s. 324-337
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Tidskriftsartikel (refereegranskat)abstract
- This paper analyzes and solves an integral and its indefinite Fourier transform of importance in multiple scattering problems of randomly distributed scatterers. The integrand contains a radiating spherical wave, and the two-dimensional domain of integration excludes a circular region of varying size. A solution of the integral in terms of radiating spherical waves is demonstrated. The method employs the Erdelyi operators, which leads to a recursion relation. This recursion relation is solved in terms of a finite sum of radiating spherical waves. The solution of the indefinite Fourier transform of the integral contains the indefinite Fourier transforms of the Legendre polynomials, which are solved by a closed formula.
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