The dynamics of the eyeball, most notably the changes in intraocular pressure, need to be stabilized optically to prevent noticeable changes in image quality. This control depends on the rheological properties of the eyeball coats and how the elasticity of the cornea, sclera, and limbus vary relative to one another. Nonlinear finite element modeling shows that image quality can be preserved over a range of elastic moduli. For intraocular pressure variations from 8 to 40 mm Hg, optical image stability is best for an elastic secant modulus of the cornea of 0.267 MPa. Optimal quality is achieved when the elastic moduli of the limbus and sclera are, respectively, 3.6 and 4 times that of the corneal modulus.
Corneal and scleral thickness and anterior chamber dimensions are required for understanding developmental and pathological processes. Parameters of the eyeball are also required to calculate optical and material properties. As the eyeball resembles a pressure vessel, it has been suggested that elasticity of the cornea and sclera could be calculated from the measurements of thickness. Baseline corneal and scleral thicknesses and anterior chamber dimensions and how these change with incremental increases of intraocular fluid are measured in fresh porcine eyes using the Visante OCT (optical coherence tomography). At baseline, corneal thickness is almost constant. Anterior scleral thickness is variable, decreasing from 0.91±0.07 mm near the limbus to a minimum of 0.58±0.13. Posterior scleral thickness is more constant with an average of 0.78±0.09 mm. Near the optic nerve the thickness increases to 1.00±0.09 mm. Average baseline anterior chamber angle, diameter, and depth were found to be 33.15±4.91 deg, 13.60±0.38 mm, and 2.13±0.22 mm, respectively. After fluid injections, maximum changes in corneal and scleral thicknesses were 9 to 10 and 1 to 3%, respectively. Anterior chamber angle and depth decreased slightly but significantly. Changes in the eyeball coats with fluid injections, indicate that the pressure vessel model can be applied to the eye to calculate corneal and scleral elasticities.
A technique has been devised for the measurement of surface refractive index of the cornea in human eyes. It has been shown to be effective in laboratory studies of refractive index in optical components of the eye. A modified approach is proposed, in which a much more accurate measurement may be achieved, with the added advantage of reduced patient discomfort. This is achieved by a non-contact method, and frequency-domain analysis of the optical signals used in the characterisation, which are optimised for best signal to noise ratio.
The cornea does not play any significant role in accommodation and is not expected to alter in curvature during the process. Nevertheless, some residual shape change may occur. In this paper we present preliminary results of a study in which we looked at the shape of the cornea for two separate accommodative states. Corneal power was measured with a Javal keratometer in tow states of the eye on one eye each from 12 emmetropic subjects aged between 20 and 28 years. These results suggest that there may be slight changes in central corneal curvature with accommodation.
This paper presents an overview of our understanding of the optics of the lens and the changes to lenticular function with age. It deals specifically with (1) presbyopia, describing the latest theory which suggests that presbyopia is a product of age-related change and continued growth, and (2) the lens paradox, a hypothesis for resolving it and the experimental findings in support of the latter.
KEYWORDS: Light scattering, Luminescence, Eye, Opacity, 3D image reconstruction, Refraction, Visualization, In vivo imaging, In vitro testing, Refractive index
The cornea and the lens are the optical elements of the eye, responsible for correctly focusing light rays on the retina. As the eye ages, both the cornea and the lens undergo structural changes, some of which affect function. This review deals with such age-related changes and any functional manifestations arising from them.
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