Overview
The lens in the eye focuses light onto the back of the eye so that objects can be seen clearly. But most people will develop cataracts with age, meaning that the lens turns from clear to cloudy over time.
Cataract is usually corrected with surgery to replace the natural lens with an artificial one. Artificial lenses make vision clearer but can sometimes lead to other problems, such as distorted vision, glare, flashes and halos around bright lights.
Light passing through a natural or artificial lens is bent by an amount (its ‘refractive index’) that is linked to how ‘solid’ the material it’s made of is (its density).
The refractive index of natural/biological lenses varies across the lens. It gradually increases from the outside of the lens to the centre, and this gradient helps reduce distortions and other problems. However, artificial lenses are currently made from materials that have the same density throughout.
Lens manufacturers are interested in developing gradient index implants to reduce these unwanted effects on vision in lenses that can focus over varying distances.
The aim of this study is to measure the refractive index gradient in human lenses of varying ages and to investigate changes in focus for different lens shapes and for different colours of light. The study will also look at the factors that obstruct light, how they change with age and what effect they have on image quality. This information is essential for creating a new generation of implants that behave more like natural lenses.
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Scientific summary
The contribution of refractive index to optical quality of the eye: age-adjusted gradient index eye models for novel implant design
The eye lens is a major element in the optical function of the eye. It has a sophisticated gradient index design that is responsible for maintaining, over decades, the superior degree of image quality, required by the eye in order to meet dynamic visual demands. With age, optical properties alter and the major threat to their degradation is the formation of cataract, which has both ocular and systemic causes.
As age is the major risk factor for cataract and the average population age is rising, cataract incidence will increase. Coupled with the greater demand for life quality from a more informed populace, there is a need for superior implant designs that will more closely mimic the gradient index optics of the eye lens.
Refractive index of the eye lens is difficult to measure with precision without using advanced optical methods. A recently developed instrument, the X-ray Talbot interferometer, can measure the refractive index in different planes and detect subtle discontinuities in the gradient profile. It will be used to determine the refractive index in lenses of varying ages. In addition, refractive index for three visible wavelengths will be measured using ray-tracing to determine dispersion.
This information, together with measures of age-related changes in attenuation factors and shape-related effects on aberrations, will be used to construct eye models in order to determine the optimal features in the eye lens that act to maintain image quality with age and that can be used to design gradient index implants adjusted for age.