Study On Size Retrieval Of Weakly-scattering Particles From Light Scattering

Light scattering analysis is a non-invasive and non-destructive measuring technique, has the advantages of high measurement speed and great repeatability. It has been widely used for particle characterization test in in various domains. Marine phytoplankton, for example, is often studied by light scattering analysis. In this thesis, models of typical weakly-scattering objects are established, and the scattering characteristics are analyzed; the inverse scattering problem is studied, and a method to determine the size and the shape of particles from angular resolved scattered field is proposed; a possible experimental system is designed, and numerical experiments are taken as a demonstration of this method; based on the theoretical studies, an on-line measurement system of three-dimensional volume scattering function (VSF) of particles in water is designed, and an measuring experiment is taken. This thesis offers a new approach to study marine microbes and other weakly-scattering particles.The scattered light field of some typical marine phytoplankton is calculated according to the morphology and size using the Discrete Dipole Approximation (DDA) method. The results show that for weakly-scattering objects with sizes in the order of wavelength, the polarization of incident light has appreciable impact on the scattering light distribution; and the distribution is different in each scattering plane under the incident of linear polarized light. The polarized light scattering characteristics are sensitive to the orientation, shape, size and nuclei size of the nucleated cell. The study on polarized light scattering can be used in classification and research of marine microbes and other cells.The existing particle size analysis methods mainly aim at spherical particles. Non-spherical particles are always taken as approximate spheres. Based on optical diffraction tomography, an inverse scattering approach able to determine the size and the shape of particles from angular resolved scattered field is proposed in this thesis. Algorithm optimization is made for spherical and non-spherical particles, such as ellipsoids and cylinders, respectively. Theoretically computed scattered field is used as the input data of the inverse scattering program. Numerical simulation results demonstrate that the size and shape of weakly-scattering particle of wavelength order can be estimated accurately in the presence of moderate noise. Both the amplitude and phase of scattered field has to be known in the diffraction tomography algorithm. However, the phase information is difficult to measure. In this thesis, a method to inverse particles from scattered light intensity is proposed. Error-reduction algorithm, a kind of phase retrieval algorithm is brought in. The phase information of the scattered field, the shape and size of the scattering particle is retrieved after many times of iterations. Numerical simulations are taken using weakly-scattering spherical particles. The retrieved results are accurate.An on-line system is designed to measure the VSF of particles suspending in water. A parabolic mirror is used to collect scattered light in the three-dimensional space and to map the light into a two-dimensional plane. The light is received by an ICCD device. A photoelectric detector is used to detect the particle and a signal generator is used to trigger the ICCD. The measurement data can be used for particle retrieval. A simplified measuring device is built and experiments are taken using standard particle samples. The processed experiment data yields good fits to theoretical predictions.