The Topography of Vision in Mammals of Contrasting Life Style: Comparative Optics and Retinal Organisation

A schematic eye for the rabbit

Refractive index distribution and optical properties of the isolated human lens measured using magnetic resonance imaging (MRI)

A model for the human eye is proposed, similar to Gullstrand’s well-known 4-radius model, however with the front surface of the cornea and the back surface of the crystalline lens taken to be rotationally symmetric aspherics. Whereas for the cornea a polynomial is used based on experimental data of Bonnet, a second-order parabola was tentatively adopted for the back surface of the lens. This model results in slight spherical undercorrection, in agreement with experimental findings. On the other hand, the sine condition is not well satisfied, probably due to neglect of the shell structure of the lens. By ray tracing, astigmatism and coma as well as the meridional and sagittal focal lengths were computed up to a visual angle of 90°. Calculations were also made for the same model preceded by a plano-concave contact lens (Goldmann 3-mirror contact glass), showing that this combination results in considerably reduced astigmatism.

Theoretical Eye Model with Aspherics

PURPOSE: Magnetic resonance imaging (MRI) was used to map the refractive index distribution in human eye lenses in vivo and to investigate changes with age and accommodation.---- METHODS: Whole-eye MR images were obtained for sagittal and transverse axial planes in one eye each of 15 young (19–29 years) and 15 older (60–70 years) subjects when viewing a far (6 m) target and at individual near points in the young subjects. Refractive index maps of the crystalline lens were calculated by using a procedure previously validated in vitro.---- RESULTS: A central high refractive index plateau region and sharp decline in refractive index at the periphery were seen in all three groups. The peripheral decline was steepest in the older lenses and least steep in the young accommodated lenses. Average lens thickness increased (+0.27 mm; P < 0.05) and equatorial diameter decreased (–0.35 mm; P < 0.05) with accommodation. Axial thickness (+0.96 mm; P < 0.05) and equatorial diameter (+0.28 mm; P < 0.05) increased with age. The central index (1.409 ± 0.008) did not differ between groups. The axial thickness of the central plateau increased with age (+0.83 mm; P < 0.05) but not significantly with accommodation. The equatorial diameter of the central plateau increased with age (+0.56 mm; P < 0.01) and decreased with accommodation (–0.43 mm; P < 0.05).---- CONCLUSIONS. The refractive index of the central plateau region does not change significantly with accommodation or ageing, but its size increases with age and the peripheral decline in refractive index becomes steeper in older lenses.

/pdf/in-vivo-study-of-changes-in-refractive-index-distribution-in-3xgj5ldiki.pdf

In vivo study of changes in refractive index distribution in the human crystalline lens with age and accommodation.

The distribution of refractive indices in the crystalline lenses of rabbits was measured with a newly developed optical system. The distribution was assumed to be represented by n=n0-[Z2/A2+(X2+Y2)/B2],where n0 is the index at the center, Z, X, and Y are coordinates, and A and B are constants. Longitudinal spherical aberrations were calculated by ray tracing on models defined by the above equation, and it was found that such a distribution of refractive indices in the crystalline lens plays a very important role in decreasing the spherical aberration and increasing the refractive power.

Shigeru Fujimoto

Papers

Model of refractive-index distribution in the rabbit crystalline lens.