Light transmission in the human cornea as a function of position across the ocular surface: theoretical and experimental aspects

Abstract

This article investigates the theoretical basis for differences in visible light transmission through the human cornea as a function of distance from the center. Experimentally, transmission decreases approximately linearly up to 3 mm from the central axis, then quadratically beyond this. It is known that collagen fibril number density and collagen fibril radii change from the central region to the corneal periphery. We modeled, using the direct-summation-of-scattered-fields method, the effects these ultrastructural changes would be expected to have on light transmission, accounting for the increase in corneal thickness from center to edge. Fibril positions for the modeling were obtained from electron micrographs of human cornea. Theoretically, transmission remains fairly constant across the central cornea; then, as the fibril diameter increases, the predicted scattering increases. Interfibrillar spacing changes alter the refractive index ratio between matrix and fibril; this was modeled in our theoretical deductions. Fibril number density had a minimal effect on light propagation. Our theoretical deductions were in broad agreement with our experimental data. It is concluded that the reduced transparency in the peripheral stroma is primarily caused by changes in fibril radius and an increase in refractive index ratio between the fibril and the interfibrillar substance.

FIGURE 1

Transmission electron micrograph of human cornea (courtesy of R. D. Young).

FIGURE 2

Translational transparency for human corneas (n = 10) near physiological hydration at 500 nm.

FIGURE 3

Representative three-dimensional plot of translational transparency versus wavelength across a meridian of the human cornea at physiological hydration (this cornea is represented in Fig. 2 by gray square points).

FIGURE 4

Transparency at 500 nm as a function of radial distance from the optical center of the cornea (n = 10 for positions 0–4 mm, n = 3 for 5 mm, error bars are standard deviation, and fit line is a quadratic function).

FIGURE 5

Theoretical deduction compared with experimental measurement of corneal transparency across the corneal surface at 500 nm. Error is ±SD; fit lines are quadratic functions.