M.Phil. / Keratometric readings on three subjects were taken both during pregnancy and postpartum. One subject was visually non-compensated and did not require refractive correction, one was a contact lens wearer and one had undergone radial keratotomy. Twenty readings were taken by means of an automatic keratometer on each eye, morning and afternoon, every fortnight. The recent matrix method of optometric statistical analysis was employed and the results graphically compared and analysed. Findings indicate diurnal variations including variation in corneal curvature and variance through the course of normal pregnancy. Most evident was an increase in keratometric variation in all three subjects at the time of birth and a substantial decrease in corneal refractive power in the subject who had before undergone radial keratotomy. This result may have far-reaching implications on the long term prognosis of refractive surgery especially for females of child bearing age. Outliers representing transient increases in curvature were most common in the vertical meridian (indicating possible lid interaction), while the presence of bimodal distributions suggests a sensitivity of the automatic keratometer to changes in head posture.

M.Phil. (Optometry) / The accommodative-convergence accommodation (AC/A) ratio is a commonly used relationship in the practice of optometry. Many practitioners make use of the AC/A ratio as an aid to diagnostic and prognostic decisions. It is not perfectly clear what effects positive and negative lenses have on the AC/A ratio. A number of questions remain relating to linearity, symptomatology and equality of lens effects on the AC/A ratio. This experiment was designed and conducted in an attempt to answer some of these unanswered questions and forms the foundation of this thesis. A literature review of available knowledge related to this study is detailed. An instrument is described which was constructed to measure the necessary accommodation and vergence changes induced by various stimuli. A sample of first year students at the Rand Afrikaans University department of Optometry was selected according to various acceptance criteria. A total of 109 students were screened , of which a group of 26 students was subjected to the experimental investigation. The data were then subjected to a statistical analysis in an attempt to reveal correlations, linearity and group formations.

M.Phil. / The primary purpose of this study was to investigate in different age groups the various relationships between autorefraction and subjective refraction, between autorefraction and retinoscopy and between retinoscopy and subjective refraction. Multivariate statistical methods are used to explore the nature of the relationships between these three different methods of evaluating refractive state. This is the first occasion in which these statistical approaches have been used to address these relationships within a sample of black South Africans. The short-term variation of measurements of the autorefractor used in this study was evaluated by means of an artificial or model eye. Here samples of fifty measurements each were obtained every two hours over a period of one day. Scatter plots, meridional profiles and hypothesis tests of equality of variance-covariance and of equality of mean autorefraction are used for analysis of the measurements obtained. Such analysis suggests that the variability in autorefraction across the day was of small magnitude and less than that typically found with human eyes. The implication is that many other factors besides the instrument itself are responsible for the variation found when using an autorefractor on a human eye. These factors include changes in accommodation, ocular fixation, the blinking process and attention. The environment within which the autorefractor is placed is important and changes in, for example, temperatur may have subtle but significant influences. The major part of this dissertation describes a study performed over a period of about seven months during which a sample of 240 male and female subjects were chosen randomly from a larger group of about 2800 patients who came to have their eyes examined in a clinical practice. A questionnaire was completed by each subject and retinoscopy was performed followed by subjective refraction and then autorefraction. Ocular health was assessed and visual acuities were measured. The study subjects were further divided into six smaller subgroups according to their ages and the analysis of results obtained with retinoscopy, subjective refraction and autorefraction included both qualitative and quantitative multivariate methods such as stereo-pair scatter plots, trajectories of change of dioptric power, meridional profiles, confidence and distribution ellipsoids, and testing of hypotheses of equality of variance-covariance and of equality of means. All of these methods help in understanding the nature of the various relationships between the different refractive methods, and they also are important when considering concepts such as mean refractive state and variation of refraction in human eyes. They also are used to develop an understanding of the distribution or spread of the population of refractive states from which the 'sample was obtained. For the three refractive methods . (retinoscopy, subjective refraction and autorefraction) the spread or distribution of measurements in the six age groups suggests that generally the refractive behaviour of right and left eyes was similar. A possible shift towards hyperopia and more astigmatism with increasing age is apparent. For example, mean autorefraction for the right and left eyes in the youngest and oldest age groups (Groups 1 and 6 respectively) is -0.41/-0.06 x 148 and -0.32/-0.07 x 177 and 0.41/-0.57 x 102 and 0.57/-0.32 x 75. Similarly mean subjective refraction for the right and left eyes of Group 1 is 0.10/-0.14 x 97 and 0.30/-0.17 x 81 and is 0.78/-0.70 x 95 and 0.89/-0.47 x 82 for the right and left eyes of Group 6 respectively. Again mean retinoscopy for the iv right and left eyes of Group 1 and Group 6 respectively is 0.69/-0.05 x 91 and 0.65/-0.21 x 82 and 1.72/-0.88 x 89

Call number: LD2668 .T4 1937 A805 / Master of Science

published_or_final_version / Medical Sciences / Master / Master of Medical Sciences

D.Phil. / Visual acuity and refractive state probably are the two most basic or fundamental quantities that concern optometry, ophthalmology and vision science. Both of these variables are complicated and their mathematical and statistical use in research and other activities has been poorly understood. During particularly the last decade, modern statistical multivariate methods have become available to optometry and ophthalmology and this has assisted with the understanding of concepts such as refractive state (and its underlying nature, namely, dioptric power). It is now possible to transform measurements of refractive state from the conventional notation that is commonly used in the fields of optometry and ophthalmology to an orthodox mathematical form that can be understood by scientists and mathematicians. With this matrix form of refractive state it then is possible to use appropriate methods of linear algebra and multivariate statistics. Other theoretical approaches and analytical procedures or methods also have become accessible or have been developed recently as a consequence of this significant shift in thought as regards the fundamental nature of dioptric power. On the other hand, the scientific understanding of visual acuity (that is, the measurement of the resolving ability of the eye) perhaps has been somewhat neglected. Certainly there has been an abundance of research involving visual acuity and there also has been discussion concerning some of the difficulties that become apparent when measuring or attempting to interpret results from studies involving visual acuity. Visual acuity, unlike refractive state, can be represented by means of a single number and thus univariate, rather than multivariate, statistical methods are appropriate. And, of course, univariate statistics is less complicated than the multivariate form. But there are various difficulties with the understanding and researching of visual acuity. Some of these difficulties are relatively simple and thus can be solved more easily. For example, visual acuity measurements can be obtained from charts designed according to a logarithmic scale, or measurements from other charts can be transformed to a logarithmic scale. And thereby the ordinal nature of the basic or more common visual acuity scale is avoided and certain statistical methods become available that otherwise would not be possible. But more fundamentally visual acuity probably cannot truly be considered without the subject from which the measurement is obtained and, more particularly, the refractive state of the eye concerned. So the visual acuity and refractive state of an eye, perhaps, should be more appropriately regarded as a unitary concept that ideally should not be separated into two distinct parts. Thus to truly understand the relationship between visual acuity and refractive state we need to understand the 4-dimensional (mathematical) nature of the particular relationship involved. It follows then that the relationship between visual acuity and refractive state is a multivariate problem and that multivariate methods are best suited to its consideration. If we then begin to take into account other variables such as age or the ocular health, or say, iris aperture diameter of the eye then the complicated multivariate nature of the situation becomes even more obvious. In this dissertation an attempt is made to consider the possibilities of a modern multivariate approach to studies involving visual acuity, refractive state and other variables. The methodology used in this dissertation differs from those used in previous studies involving visual acuity and refractive state and other related variables. For example, here Jackson crossed cylinders are used extensively to produce dioptric blur or defocus in experimental subjects (positive and negative spheres also are used to a more limited extent). In previous published studies spherical or, less commonly, cylindrical lenses were used instead. Another difference between this dissertation and previous research studies is that the visual stimulus that the subjects observed, generally but not always, was a meridionally-independent or non-directional letter 0. The reasons for this choice is explained in the dissertation but in other research an enormous variety of visual stimuli have been used depending upon the interests of the researchers. But even more essentially this dissertation differs significantly from that of previous studies in terms of the manner in which the various experimental and other results (for instance, that from earlier researchers such as HB Peters) are presented. Entirely new, and largely unpublished, methods are used in many parts of this dissertation that probably represent a paradigmatic transition in understanding of visual acuity and its relation to refractive state. New terms such as decompensation and accompensation surfaces of constant visual acuity and antistigmatic ellipses are defined herein. (Briefly, one imagines starting from a state of compensation (of the refractive state of, say, an eye viewing a stimulus).

M.Phil. (Optometry) / Different representations of dioptric power and their relative coordinate systems are summarised. The transition matrices required to switch from one coordinate system to another are provided. Three sets of data are analysed; a sample of 205 refractions, a sample of 205 keratometric readings and a sample of 790 autorefractive excesses of 790 autorefractions over 790 subjective refractions. Brief mention is made of emmetropisation. In the event that such a driving force exists, the possible effects on the distributional characteristics of refractive error are noted. Normality and the assessment thereof are discussed qualitatively and quantitatively. The univariate marginal and multivariate joint distributions .of the samples are examined using the coordinate system introduced by Deal and Toop (1993): their vector is represented by d=(d 1 d 2 d3)~ Departure from normality is determined in three ways; assessment of the linearity of the chi-square probability plots, measures of skewness and measures of kurtosis. Marginal normal probability plots are included for completeness. The statistical procedures and some of the theory involved in the implementation of these techniques are described briefly to assist in the interpretation of the distribution analysis. Marginal transformations are employed to improve the normality of the marginal distributions in an attempt to reduce the multivariate departure from normality. Power transformations and shifted power transformations are described and applied to the data.

M.Sc. (Optometry) / The difference between objective automatic and subjective clinical refraction is studied using new statistical techniques. The question, by how much the two refraction techniques differ, is investigated by subtracting the subjective refraction finding from the auto refractory finding and then examining the distribution of the difference or excess as it will be called here. Computerized procedures were developed to automate mathematical and statistical methods of analysis. The methods were applied to two different samples: one of patients visiting an ordinary optometric practice (the clinical sample) and the other of children refracted in a screening program (the sample of school children) . The clinical sample, consisting of mainly older patients, is examined first. The difference between the autorefractor and subjective findings is studied and described. This difference could be used to compare different types of auto refractors assuming that the subjective refraction is correct. For the purpose of this study, however, the results of eight autorefractors are grouped together, combining left and right eyes, to serve as basis for studying the older population. The study shows that on the average there is no clinically significant excess. This means that there is on average no clinically significant difference between the automatic and clinical refraction for this population.

No description available.

Much concern has been expressed about the ability of the visual display terminal to provide adequate stimuli for accommodation. As a result it has been hypothesized that an observer may have to continually refocus to maintain accommodation on the display and this contributes to the visual fatigue experienced by VDT users. The increased use of multicolor CRTs in the workplace adds yet another factor, chromatic aberration, to affect the observers' accommodation to information presented on the CRT. Two experiments were run to determine the effect of viewing characters of one chromaticity and purity on a background of another chromaticity or purity or chromaticity and purity. The observer's accommodation response was continuously sampled throughout the presentation of each target/background combination. Mean accommodation response and the standard deviation of the mean accommodation response were then calculated to ascertain the effect the target background combination had on the observers' accommodation response. The observers' were also required to perform a visual performance task for each target/background combination. The data collected indicate that chromatic characters when observed on chromatic background do not provide a strong stimulus for accommodation. Mean changes in the observers accommodation response were all within the depth of field except when characters were viewed on blue backgrounds. The variability in the observers accommodation response was not found to be a good predictor of image quality where only color contrast exists between foreground and surround. Task performance was highly correlated with effective contrast between the target and background as quantified by uniform color space modeling. / Ph. D.