PhD Defense: Yanqiao (Joyce) Huang
Human Lens Modeling and Biometric Measurement Technique
This dissertation conducts theoretical and instrumental aspects of research aiming at extending knowledge and understanding of the optical design of the human eye. The first part of the thesis describes a newly constructed dynamic eye model that includes a gradient index (GRIN) lens to simulate eye accommodation. The GRIN profile of the crystalline lens is defined by a single continuous GRIN equation with optical power variability. In describing the lens accommodation process, different expansion coefficients are given to the lens nucleus and cortex to mimic lens dynamics. A relaxed state eye, a 4-D accommodated eye and a 10-D accommodated eye are simulated on computer for studying and analyzing the first order and third order properties. This eye model can be further improved if giving accurate biometric measurement data on accommodating eyes. The second part of the thesis proposes an original interferometric technique that has potentials for non-invasive ocular biometric measurements. This technique, termed spatial coherence interferometry, utilizes spatially incoherent monochromatic light as the illumination source, and employs the principle of low coherence interferometry to perform optical sectioning. Generalized coherence function for multi-layer sample is derived and theoretical axial longitudinal resolution is formulated. A spatial coherence interferometer with tunable coherence length is built, and detailed instrumental design and specifications are illustrated. Factors affecting system longitudinal resolution are examined. The instrument is first tested on plane mirrors for characterizing the longitudinal resolution. Various experiments are conducted including target searching, curved surface profiling and multi-layer sample sectioning. Finally en face surface profiling is performed on a pair of life size model eyes, and full field interferograms from various ocular surfaces are generated sequentially due to optical sectioning. In future research, biometric measurement data on accommodating eyes obtained from the spatial coherence interferometer can be used to cross-validate and improve the theoretical eye model and the measurement accuracy of this technique.