Download files
Abstract
Peripheral refractive error has assumed considerable importance with the discovery that it can influence eye growth. The link between the peripheral state of the eye and myopia development demands rapid and accurate measurements at individual and population levels. Currently, the use of conventional refraction techniques requires time-consuming sequential re-alignments.
The aims of this thesis were to identify and assess methodological limitations of current techniques, test new concepts and develop a method of obtaining more rapid and accurate peripheral refraction measurements.
At first, the impact of pupil misalignment was investigated using a conventional autorefractor. As visual field angle increased, tolerance to pupil misalignment decreased significantly, making peripheral measurements particularly susceptible to this measurement error. It was also shown that the peripheral entrance pupil shape is not elliptical as currently assumed, adding further potential for pupil misalignment. Based on these findings, means to rectify pupil alignment-related errors when using conventional instruments were established and validated.
Having ascertained limitations of current peripheral refractometry, a novel instrument concept was proposed, the EyeMapper. The EyeMapper was designed to perform a rapid peripheral (and central) refraction scan, from -50° to +50°, using 10 stationary deflecting prisms and a scanning mirror. Like most autorefractors, the operation was based on the ring-autorefraction principle. The optical design, consisting of 5 intertwined optical sub-systems was developed. Safety aspects and criteria for instrument components were assessed and the operation principle was verified experimentally. Experimental testing identified an obstacle relating to the ring-image analysis and it revealed that peripheral higher order aberrations have the potential to interfere with the sphero-cylindrical refraction readings obtained when applying this ring-autorefraction principle. A technique that segregates higher and lower order aberrations was thus deemed more suitable for measuring peripheral refraction. Hence, the EyeMapper design was updated to include wavefront measurements. The prototype instrument was then built and experimentally tested over a range of refractive errors. The EyeMapper uses an array of beam steering mirrors and a scanning mirror to perform a rapid peripheral refraction scan in one meridian. Three-dimensional power maps of the eye can be obtained by pivoting the instrument around its optical axis.