City Research Online

Abstract

An original method of measuring the radius of curvature of soft lenses is described. It is based on the use of the sagitta principle and used ultrasound to measure the sagitta itself. The time taken for sound to travel from the transducer to the lens apex and back to the transducer (Round Trip Time [RTT]) was measured. The sagitta itself was calculated from the RTT and the velocity of sound at the specified temperature but it was found to be more accurate to use a series of rigid test pieces to calibrate the system. This enabled a RTT to be converted directly to a radius value. Whilst the methowas kept constant this was an indirect advantage as the dimensions of hydrogel lenses do vary with temperature. The method described has been acknowledged in an International Standard (ISO 10388; 1996) related to the measurement of contact lenses. The method remains the only one in general use to measure sagitta without any mechanical interference with the lens.

The apparatus evolved to produce equipment that was compact and had integral temperature control of the saline. Measurement of the Back Optic Zone Radius and Front Optic Zone Radius was investigated in detail. The variance in a series of readings for a single lens on one day was very small ( 0.02mm [ modal value ] ) but this variance increased to 0.12mm when mean values were compared on three different days. However, even this was still adequate when radius fitting steps are usually 0.3mm apart. Measurement repeatability was more dependent on material than lens dimensions.

It was possible to evaluate the change in radius with temperature especially in the band 20° to 30°C. The change in radius in this band was linear in relation to the water content of the lens. A 85% hydrogel changed 0.39mm for this 10°C change in temperature.

Other uses of an ultrasound system are described viz determination of water content, measurement of centre thickness, estimation of lens power and evaluation of the surface topography of soft lenses.

Publication Type:	Thesis (Doctoral)
Subjects:	R Medicine > R Medicine (General)
Departments:	School of Health & Psychological Sciences > Optometry & Visual Sciences School of Health & Psychological Sciences > School of Health & Psychological Sciences Doctoral Theses Doctoral Theses

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