A numerical test of the normal incidence uniaxial model of corneal birefringence.

The suggestion that the central cornea can be modeled as a uniaxial birefringent material with its optic axis normal to the surface is explicitly tested by numerical calculations. A theoretical framework is presented to model the corneal stroma as a series of stacked, uniaxial birefringent layers (lamellae). Calculations are then made of the transmission of normally incident, linearly polarized light through model systems having various azimuthal orientations of the layers, motivated by the suggestion of an overall "random" organization of the stromal lamellae. It is concluded that the uniaxial description, and the assumptions upon which that description is based, do not hold for the cornea. In particular, the calculations are in agreement with recent experiments in which one always observes a non-zero cross-polarized transmission (hence birefringence) at normal incidence.

Numerical modeling of the cornea's lamellar structure and birefringence properties.

A model of the cornea's lamellar structure is proposed that is capable of explaining experimental results obtained for the transmission of normal-incidence polarized light through rabbit and bovine cornea. The model consists of a large number of planar lamellae, each approximated as a uniaxial birefringent layer, stacked one upon another with various angular orientations. Polarized light transmission through the composite system is modeled theoretically by use of the Jones matrix formalism. The light transmission is calculated numerically for a large number of model lamellae arrangements, each generated from a statistical description, and histograms are constructed of various properties of the light transmission, including the minimum and maximum cross-polarized output intensities. It is demonstrated that various structural and optical parameters of the lamellae arrangements of actual corneas may be estimated by comparison of the calculations with detailed experimental data. Certain characteristics of the histograms are identified that permit a clear distinction between random and partially ordered systems. Comparisons with previously published experimental data provide strong evidence that the lamellae orientations are not entirely random, but rather a significant fraction are oriented in a fixed, preferred direction.