טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentMeitav Nizan
SubjectHigh Resolution Imaging through Random Phase Aberrations
DepartmentDepartment of Physics
Supervisor Dr. Erez Ribak
Full Thesis textFull thesis text - English Version


Abstract

High-resolution optical imaging has a great importance both in microscopy and in imaging of remote objects. The resolution of an imaging setup is ideally limited by the diffraction properties of the light and the aperture of the setup. However in many cases inhomogeneities in the medium, or imperfections in the optics, degrade the resolution from its diffraction-limited value. Whenever phase aberrations vary spatially or temporally, suppressing these aberrations cannot be achieved by conventional optics and resolution is compromised.

To improve the resolution in those cases of optical aberrations, usually two general schemes are used. The first method is based on repeatedly measuring the distorted wave front and correcting it by a deformable mirror.  This method provides excellent results in astronomy, due to the nature of the atmospheric turbulence. In cases where the phase deformations vary significantly both spatially and temporally, adaptive optics cannot obtain a diffraction-limited resolution. The second scheme is based on the convolution relationship between the setup’s impulse response (called point spread function, PSF) and the object’s reflectance function. By evaluating the instantaneous PSF, the image resolution can be improved via a deconvolution process. However this method is usually very sensitive to the initial evaluation of the PSF and therefore is not widely spread.  In cases where deconvolution is used, it is usually implemented with other preliminary methods that reduce the optical aberrations.  

One important case of high resolution imaging is retinal imaging, since it enables functional and long-term research of the retinal layers in vivo, in addition to clinical benefits of early diagnosis of diseases.  To obtain maximal resolution, the pupil has to be dilated. This exposes larger area of the cornea and crystalline lens, which are less than perfect optically, resulting in increased ocular aberrations which are hardly corrected by the above-mentioned methods due to the rapid eye movements and tear film irregularities.

In this research three approaches to improve the resolving limit of optical setups with random phase deformations are presented. The first two methods are used for improving retinal imaging, by PSF estimation using prior knowledge on retinal cones, and by corneal index matching  to reduce its  aberrations. Using these approaches I was able to obtain a resolution of a single photoreceptor cell in the parafovea. The third method I developed provides a good PSF estimator by speckle pattern illumination. It is aimed for high-resolution dynamic imaging of fluorescent objects.