טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentCarmon Yuval
SubjectWave Front Detection of Human Ocular Aberrations
DepartmentDepartment of Physics
Supervisor Dr. Erez Ribak
Full Thesis textFull thesis text - English Version


Abstract

High order optical aberrations in the human eye impose a major physical limit vision quality. Introduction of adaptive optics and the Hartmann Shack sensor (HSS) into the field of ocular optics has produced many important results over the past decade. A HSS measures the local slopes of a wave front. It is constructed of a two dimensional lattice of lenslets. The wave front is dissected by the lenslet's subapertures, and focused onto a common plane. The image spot from each lenslet is shifted by a value proportional to the wave front's gradient.

In ocular wave front measurements, a laser beam enters the eye creating a spot on the retina. The highly anisotropic retinal tissue creates a speckle field, which adversely affects the HSS's performance. An acousto-optic cell was used to reduce the speckle. The incoming beam is widened in real space and in k space, averaging out speckles formed on the retina.

The relationship between the optical aberrations of the cornea and the internal ocular optics were investigated. The full ocular aberrations were measured with a HSS. A system was constructed to measure the internal optics of the eye. It was composed of goggles and a custom lens system. When the goggles are filled with saline solution, the anterior corneal refraction was effectively nulled. It was found that not all higher order aberrations have the same compensation behavior. The ocular wave front pattern throughout the day was studied. Wave fronts were measured with a HSS every fifteen minutes, for eight hours. Both subjects exhibited aberrations that changed significantly over the day.

A method for retrieving the underlying wave front gradient from a Hartmann Shack sensor using Fourier techniques is developed. It is based on applying a digital demodulation on the Fourier transform of the Hartmann Shack image, and then applying a low pass filter. The result of these two steps is the Fourier transform of the phase gradient. The affect of amplitude changes of the incoming electromagnetic field on the centroiding method in a HSS was studied. The affect was shown to be strongly dependent on the relative size of the diffraction pattern in relation to the lenslet diameter. If the spot size relative to the lenslet diameter approaches zero - the centroid is unaffected. Otherwise, changes in the field amplitude can exacerbate distortion of the centroid results.