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The optical and visual performance of refractive and diffractive bifocal contact lenses

Woods, R.L. (1992). The optical and visual performance of refractive and diffractive bifocal contact lenses. (Unpublished Doctoral thesis, City, University of London)

Abstract

The effects of changes in the optical design of simultaneous-vision bifocal contact lenses (BCL) upon optical and visual performance were investigated. In this context, the effects of age upon contact lens wear and theoretical aspects of BCL and previous studies of BCL were reviewed.

The surface profile of both rigid and soft diffractive BCL was investigated with an interferometric technique (Nomarski).

The optical performance (Modulation Transfer Function) was measured with a solid state EROS.

In preliminary studies (a) a range of procedures for the measurement of contrast sensitivity (CS) with a monitor display were determined; and (b) the sensitivity of a monitor-based measure of CS, the Vistech chart, the Pelli-Robson chart the Melbourne Edge Test and low and high contrast visual acuity (VA), all measured at two luminance levels, to changes in the design of rigid diffractive BCL was investigated to determine an optimal routine for the assessment of visual performance.

The main study used the monitor-based CS, the Pelli-Robson chart and low and high contrast VA to measure the visual performance of small groups of subjects wearing the BCL.
The reliability of the optical and the visual performance measures was poor, and related to the poor image quality typical of BCL.

The effect of variations in the central optic zone diameter, pupil size, decentration and BCL design (centre-distance or centre-near) of concentric-design refractive BCL were investigated. The complex relationship between the measured variables and changes with spatial frequency with refractive BCL were demonstrated, leading to predictions for optimal designs.

The effect of changes in wavelength, pupil size, decentration, diffractive zone junction (DZJ) height and DZJ shape of rigid diffractive BCL were investigated. The effect of changes in the manipulated variables, the surface quality and other aspects of manufacture of diffractive BCL were examined, leading to suggestions for improvements in design.

The effect of changes in DZJ height, DZJ shape and manufacture technique (lathe or mould) of soft diffractive BCL were investigated. In addition a "reverse" add soft diffractive BCL was investigated.

The interferometric measurements of the diffractive BCL indicated that small variations in the parameters of interest affect both optical and visual performance.

Changes in visual performance, generally, were matched by similar changes in optical performance. Models to describe visual performance based upon optical performance measures of the same BCL were demonstrated. The utility of these empirically derived equations in the development of future BCLs is discussed.

Publication Type: Thesis (Doctoral)
Subjects: R Medicine > RZ Other systems of medicine
Departments: School of Health & Psychological Sciences > Optometry & Visual Sciences
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