Optical properties of in situ eye lenses measured with X-ray Talbot interferometry: a novel measure of growth processes

Abstract

The lens, a major optical component of the eye, has a gradient refractive index, which is required to provide sufficient refractive power and image quality. The refractive index variations across the lens are dependent on the distributions and concentrations of the varying protein classes. In this study, we present the first measurements of the refractive index in the in situ eye lens from five species using a specially constructed X-ray Talbot grating interferometer. The measurements have been conducted in two planes: the one containing the optic axis (the sagittal plane) and the plane orthogonal to this (the equatorial plane). The results show previously undetected discontinuities and fluctuations in the refractive index profile that vary in different species. These may be linked to growth processes and may be the first optical evidence of discrete developmental stages.

Figure 1. Images of a) porcine; b) ranine; c) murine; d) newt; e) piscine eyes in the sagittal plane.

The position of the equatorial plane is marked with the blue arrow and the optic axis, along which the sagittal refractive index profiles were measured, is marked with a red arrow. The scale bars in the right hand lower corner are equal to a) 4 mm (porcine); b) 2 mm (ranine); c) 1 mm (murine); d) 0.5 mm (newt); e) 1 mm (piscine).

Figure 2. Refractive index profiles in two orthogonal sections of the equatorial plane of a) porcine; b) ranine; c) murine; d) newt; e) piscine lenses plotted against the distance across the lens in mm.

Refractive index profiles along the optic axis (sagittal plane) of f) porcine; g) ranine; h) murine; i) newt; j) piscine lenses plotted against the distance across the lens in mm, from the anterior (-ve x-axis values) to the posterior (+ve x-axis values) poles.

Figure 3. Refractive index profiles in the equatorial planes of three representative porcine (a),c) and e)) and three representative murine (b),d) and f)) lenses.

Arrows point to discontinuities in the profiles.

Figure 4. Central region of refractive index profiles along the optic axis (sagittal plane) in three porcine lenses plotted against the distance across the lens in mm, from the anterior (-ve x-axis values) to the posterior (+ve x-axis values) poles.

Figure 5. Pictorial representation of isoindicial contours of refractive index in a lens where the contours in the inner regions of the lens are wider in the sagittal than in the equatorial plane.

Figure 6. Diagrammatic representation of the X-ray Talbot interferometer showing the sample cell with sample suspended on a rotatable rod; the phase and absorption gratings (G1 and G2 respectively) and the beam monitor.