Phase-space representation of digital holographic and light field imaging with application to two-phase flows

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 125-133).In this thesis, two computational imaging techniques used for underwater research, in particular, two-phase flows measurements, are presented. The techniques under study, digital holographic imaging and light field imaging, are targeted at different flow conditions. In low-density flows, particles and air bubbles in water can be imaged by a digital holographic imaging system to provide 3D flow information. In the high density case, both occlusions and scattering become significant, imaging through these partial occlusions to achieve object detection is possible by integrating views from multiple perspectives, which is the principle of light field imaging. The analyses on the digital holographic and light field imaging systems are carried out under the framework of phase-space optics. In the holographic imaging system, it is seen that, by tracking the Space bandwidth transfer, the information transformation through a digital holographic imaging system can be traced. The inverse source problem of holography can be solved in certain cases by posing proper priori constraints. As is in the application to two-phase flows, 3D positions of bubbles can be computed by well tuned focus metrics. Size statistical distribution of the bubbles can also be obtained from the reconstructed images.(cont.) Light field is related to the Wigner distribution through the generalized radiance function. One practical way to sample the Wigner distribution is to take intensity measurements behind an aperture which is moving laterally in the field. Two types of imaging systems, the light field imaging and the integral imaging, realize this Wigner sampling scheme. In the light field imaging, the aperture function is a rect function; while a sinc aperture function in the integral imaging. Axial ranging through the object space can be realized by digital refocusing. In addition, imaging through partial occlusion is possible by integrating properly selected Wigner samples.by Lei Tian.S.M

Compressive phase retrieval

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 129-138).Recovering a full description of a wave from limited intensity measurements remains a central problem in optics. Optical waves oscillate too fast for detectors to measure anything but time{averaged intensities. This is unfortunate since the phase can reveal important information about the object. When the light is partially coherent, a complete description of the phase requires knowledge about the statistical correlations for each pair of points in space. Recovery of the correlation function is a much more challenging problem since the number of pairs grows much more rapidly than the number of points. In this thesis, quantitative phase imaging techniques that works for partially coherent illuminations are investigated. In order to recover the phase information with few measurements, the sparsity in each underly problem and ecient inversion methods are explored under the framework of compressed sensing. In each phase retrieval technique under study, diffraction during spatial propagation is exploited as an effective and convenient mechanism to uniformly distribute the information about the unknown signal into the measurement space. Holography is useful to record the scattered field from a sparse distribution of particles; the ability of localizing each particles using compressive reconstruction method is studied. When a thin sample is illuminated with partially coherent waves, the transport of intensity phase retrieval method is shown to be eective to recover the optical path length of the sample and remove the eect of the illumination. This technique is particularly suitable for X-ray phase imaging since it does not require a coherent source or any optical components. Compressive tomographic reconstruction, which makes full use of the priors that the sample consists of piecewise constant refractive indices, are demonstrated to make up missing data. The third technique, known as the phase space tomography (PST), addresses the correlation function recovery problem. Implementing the PST involves measuring many intensity images under spatial propagation. Experimental demonstration of a compressive reconstruction method, which finds the sparse solution by decomposing the correlation function into a few mutually uncorrelated coherent modes, is presented to produce accurate reconstruction even when the measurement suers from the 'missing cone' problem in the Fourier domain.by Lei Tian.Ph.D