D.Phil. (Optometry) / The measurement of Contrast Visual Acuities (contrast VA) is recognised in various studies as an important psychophysical measure of visual function, and contrast VA is often used to understand visual limitations or functional disability. Contrast VA is also useful for monitoring the effects of disease modifying therapies. High and medium contrast levels are generally used in studies to evaluate contact lens performance, the outcomes of surgical procedures and for assessing activities of daily living. Measurement of stimuli with low contrast levels are also sometimes applied in diagnosing, monitoring and evaluating disease processes and their management, especially where high contrast visual acuity remains intact. This is believed to be the first study that comprehensively investigates the reliability (or repeatability) of four contrast levels using the computerized Thomson Test Chart 2000 XPert. (A similar study with four contrast levels and both univariate and multivariate analysis as applied in this thesis has not been performed elsewhere). Although the main emphasis of this study was to explore various issues relating to short-term repeatability of contrast VA, both within and across individuals, both univariate and multivariate statistical analyses were also used in this thesis to investigate age and gender related changes in measurements of contrast VA. Together the results from this thesis provide test and re-test contrast VA reliability measures and some basic or preliminary statistical normative contrast VA values, which should aid clinicians to confidently detect abnormal measurements which, in turn, promotes good clinical practice. For this thesis and within a clinical environment to investigate inter-subject variation in contrast VA, two measurements of contrast VA at four specified contrast levels (100%, 10%, 5% and 2.5%) at a 6 metre distance were obtained for the right eyes only of 155 healthy participants. Thus, at each contrast level 155 test and re-test contrast VA measurements were determined using the computerized Thomson Test Chart 2000 XPert. All measurements were determined through the optimal refractive compensation for each right eye of the 155 participants concerned. In a subset of ten subjects or participants, samples of thirty consecutive measurements of contrast VA at each of the four contrast levels were also obtained to explore short-term intra-subject variation in contrast VA. A simple questionnaire was administered to all subjects to obtain biographical, general and ocular health histories. Visual assessment included II subjective clinical refraction, stereopsis, colour vision, direct ophthalmoscopy and biomicroscopy to understand the eyes of the participants and exclude possible factors that could cause ocular or neurological changes in the retina or in vision thereby influencing contrast VA in a detrimental fashion...

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Thesis (M.S.)--Ohio State University, 2009. / Title from first page of PDF file. Includes vita. Includes bibliographical references (p. 57-59).

Several studies have been conducted in the past which determined that some near-infrared (NIR) sources are capable of inducing a thermal lens within ocular media of rhesus and, potentially, human subjects. Typically, the role of thermal lensing in the eye was explored in terms of its influence on damage thresholds for these NIR lasers entering the eye. However, the effect of a thermal lens on visible wavefronts entering the eye has yet to be explored. In recent years military and law enforcement agencies in the United States and elsewhere have devoted considerable resources to the area of "non-lethal weapons." Devices such as tasers, spike strips and ocular interruption (OI) devices provide the user with an escalation of force while minimizing casualties and collateral damage. One particular form of OI device, the laser dazzler, employs a visible laser capable of saturating retinal receptors causing a temporary flash blindness effect. While these visible devices have proven safe and effective in the field, an inherent risk exists when any light source is used to saturate retinal tissue. By adding the use of a thermal lens, these OI devices would create significant distortions in the visible wavefront to alter vision and/or increase the diameter of a focused visible dazzler at the retina to both improve safety and effectiveness of the visible device. This dissertation describes experiments involving artificial eye, human subject, and computational modeling which were conducted to quantify the impact of thermal lensing on visual acuity. / text

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A dissertation submitted to the Faculty of Health Sciences, University of the Witwatersrand, in the fulfillment of the requirements for the degree of Master of Medicine in Ophthalmology. Johannesburg, February 2017 / Purpose: To evaluate if corneal collagen crosslinking carried out on patients with keratoconus, slows down or halts the progression of keratoconus. To determine which group of keratoconus patients benefited most from the procedure. Methods: A retrospective record review of 41 eyes of 29 patients. Visual acuity and keratometry measurements were recorded for the involved eye pre-crosslinking and at 3 months and 6 months post-crosslinking. A comparison of these variables pre-crosslinking and at 6 months post-crosslinking was made to determine if there was a flattening of corneal curvature (keratometry readings) and an improvement in visual acuity. Patients were further divided into 3 groups of keratoconus, based on their keratometry readings (measured in diopters): mild keratoconus (≤47 diopters), moderate keratoconus (48 – 54 diopters) and advanced keratoconus (≥55 diopters), to determine which group of keratoconus had the best keratometry reduction readings. Results: After crosslinking took place on 41 eyes, the UnVA of 16(39%) eyes showed an improvement at 6 months, 17(41%) eyes showed no change and 8(20%) eyes showed a decrease in UnVA at 6 months, compared to pre-CXL values. For BCVA, 12(29%) eyes showed an improvement at 6 months, 18(44%) eyes showed no change and 11(27%) eyes showed a decrease in BCVA at 6 months, compared to pre-CXL values. Keratometry readings however showed that 23(56%) eyes had an average flattening of corneal curvature readings of 0.7 D and the remaining 18(44%) eyes showed more steepening (worsening) of the corneal curvature readings of 0.9 D after 6 months post-CXL. 30(73%) eyes had mild keratoconus, 7(17%) had moderate keratoconus and 4(10%) had advanced keratoconus. 19 of the 30 eyes in the mild keratoconus group (73%) showed an average flattening of corneal curvature of 0.6 D. 4 of the 7 eyes in the moderate keratoconus group (17%) showed an average flattening of corneal curvature of 0.7 D. All 4 patients in the advanced group (10%) had steepening (worsening) of their corneal curvatures with an average of 1.2 D. Conclusion: Corneal collagen crosslinking performed on keratoconus patients at least halts the progress of keratoconus. 6 months after CXL most patients showed minimal change from pre-CXL to 6 months in both visual acuity and keratometry. However a longer follow up period and larger sample size is needed to determine if vision and keratometry readings can improve significantly. / MT2017