The effects of positive and negative lenses on the accommodative-convergence/accommodation ratio
- Authors: Gillan, W.D.H.
- Date: 2014-02-11
- Subjects: Eye - Accommodation and refraction , Lenses - Optical properties
- Type: Thesis
- Identifier: uj:3899 , http://hdl.handle.net/10210/9264
- Description: 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.
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Keratometric variation during pregnancy and postpartum
- Authors: Klaassen, Donald Gregory Istvan
- Date: 2012-08-27
- Subjects: Eye - Accommodation and refraction , Eye - Accommodation and refraction - Statistical methods , Eye - Accommodation and refraction - Data processing , Pregnancy
- Type: Thesis
- Identifier: uj:3234 , http://hdl.handle.net/10210/6644
- Description: 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.
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Short-term variation of refractive behaviour in human eyes
- Authors: Rubin, Alan
- Date: 2014-04-14
- Subjects: Eye - Accommodation and refraction , Optical properties
- Type: Thesis
- Identifier: uj:10593 , http://hdl.handle.net/10210/10117
- Description: M.Phil. (Optometry) , An investigation of the nature of variability or variation of refractive behaviour (in a sample of universi ty students studying optometry) is described. Measurements of refractive behaviour were obtained by means of autorefraction. This study was based upon multivariate methods of statistical analysis which have only recently become available in optometric science. Variation is examined using both quali tative and quanti tative methods including stereo-pair scatter plots, confidence and distribution ellipsoids, trajectories of change of dioptric power, meridional profiles, testing of hypotheses on means and variance-covariance, and graphs which represent the type of uniform variation in a 2-dimensional plane known as the i)-plane. These methods are of great assistance in developing an understanding of the nature of the variation shown, as well as, in developing an awareness of the distribution or spread of the population from which the sample was drawn. Analyses of variability of refractive behaviour on both an artificial, or test eye, and on several individual human eyes are also described. The significance of some important aspects of variabili ty of refractive behaviour involving normality and departures therefrom (such as results from outliers) are discussed and illustrated by means of examples. Distributions were found in which more than one mode was present (polymodal or multimodal behaviour). Distributions were also observed to vary from having an almost spherical spread of measurements (of refractive behaviour) to having a spindle or rod-like spread of measurements instead...
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Multivariate analysis in symmetric dioptric power space of refractive state at two different distances, with and without cycloplegia
- Authors: Unterhorst, Holly Anne
- Date: 2016
- Subjects: Eye - Refractive errors , Eye - Accommodation and refraction , Ophthalmic drugs
- Language: English
- Type: Masters (Thesis)
- Identifier: http://hdl.handle.net/10210/91206 , uj:20080
- Description: Abstract: Refractive error (an imperfection of ocular image formation) is one of the most common reasons that patients seek out optometrists. Uncompensated refractive error, which is related to second order wavefront aberrations, causes unwanted blurring of retinal images. Distance refractive error can be affected by the presence of active ocular accommodation. Young patients, in particular, have very active accommodative systems and when active during the measurement of refractive error, can lead to erroneous results. Accommodation theoretically should only be active when a patient regards a proximal or near object; however this is not always the case and therefore pharmaceuticals (cycloplegics) have been employed by ophthalmologists and optometrists to temporarily paralyze the accommodative system in order for refractive error to be evaluated at any distance more accurately. The primary aim of this study is to use multivariate methods of analysis to evaluate and study the changes in refractive error occurring at distance (a stimulated six metre target) and at near (40 centimetres), as well as exploring the effects of accommodation and cycloplegia on the measured refractive errors. The instruments used included the NIDEK ARK-700 autorefractor and the NIDEK OPD-Scan ARK 10000 wavefront aberrometer. Ten subjects were recruited for this particular study and eight samples were taken per individual under four different experimental conditions (four samples per instrument per subject). These samples were so named; the Distance Samples Without Cycloplegia, the Distance Samples With Cycloplegia, the Near Samples Without Cycloplegia, and the Near Samples With Cycloplegia and are referred to as such throughout his dissertation. These samples were collected with both the autorefractor and the wavefront aberrometer mentioned above and converted, using matrix methods, into scalar coefficients of power and therefore the stigmatic and antistigmatic components are represented as FI (the stigmatic coefficient), FJ (the ortho-antistigmatic coefficient) and FK (the oblique-antistigmatic coefficient). The sample size for each experimental condition was 25 measurements. The samples without cycloplegia were obtained first on both the autorefractor and wavefront aberrometer, and once these samples had been collected, one drop of 1% cyclopentolate hydrochloride was added to each of the subject’s right eyes and once the accommodative system was seemingly paralyzed the distance and near samples under the influence of cycloplegia were taken once again with each instrument... , M.Phil. (Optometry)
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