Abstract
HE optical performance of a contact lens is limited by its design. One or more of several parameters (e.g., the optical transfer function or thirdorder aberrations) can be used to describe the optical performance of a given lens. Using a particular parameter, the lens designer can produce the 'perfect' lens, which is free of that parameter under specific test conditions. In general rigid contact lenses suffer from high degrees of spherical aberration 1 and it has been claimed that the reduced visual performance in the hydrogel lens wearer is partially attributable to this factor. 2 A major flaw in this area of investigation is the assumption that the spherical aberration of the lens in air is a true representation of the spherical aberration of the lens on the eye. Cox 3 calculated the theoretical spherical aberration of hydrogel and rigid lenses when placed on an aspheric cornea. For the hydrogel lenses, Cox assumed the lens back surface to conform to the shape of the aspheric cornea. His results and conclusions are correct in relation to his modelling technique. However, a major flaw in all attempts to model the spherical aberration of the contact lens-eye system, is the fact that the remaining optics of the eye are not considered. The known optical and shape characteristics of both corneal surfaces, together with those of the intra-ocular lens, must be considered in order to produce a comprehensive spherical aberration profile of the contact lenseye complex. Utilising the information gained by such theoretical excursions will produce a clearer insight into the ideal on-eye asphericities of a soft lens which should, in most cases, produce an optimal optical performance on the criterion of spherical aberration reduction. In order to produce such lenses and to test their on-eye asphericities, we require an appropriate model eye and a keratoscope that measures on-eye lens front surface shape within the limits of the pupil. The obvious question which comes to the fore is one of relevance: What is the point of producing a lens that reduces the spherical aberration of the~ eye-lens system? For almost two decades the tradi
Published Version
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