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R.L. McCally, J.A. Bonney-Ray; Experimental Investigation of Corneal Birefringence Model Predictions . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3670.
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Purpose: Polarized light is a powerful tool for investigating corneal lamellar structure. In addition it is used to probe the retinal nerve fiber layer; however retardation introduced by corneal birefringence can influence the results and interpretation of such measurements. Computer calculations of polarized light propagation through model corneas suggest that retardation depends both on the angle of incidence, θ, and the plane of incidence, and that the minimum value of retardation generally occurs at non-perpendicular incidence. The purpose of this study is to investigate the validity of the model predictions. Methods: Corneas bathed in silicone oil were mounted in a goniometric holder that allows for rotations about a vertical axis through the cornea apex (to vary θ) and for azimuthal rotations, ϕ, about an axis perpendicular to the cornea surface at the apex (to vary the angle between the plane of incidence and the component of the effective optic axis parallel to the cornea surface). Cross-polarized (I+) and parallel (I||) transmission were measured as the incident polarization direction was rotated through 360° at each value of θ and ϕ. Data were analyzed to obtain the phase retardation of the equivalent optical system as functions of θ and ϕ. Results: Retardation at a fixed angle of incidence varies periodically with ϕ, with maxima and minima occurring at ~180° intervals. As θ is increased from 0° (i.e., perpendicular incidence) at the azimuthal position of a retardation minima, the retardation decreases to a value near zero and then increases as θ is increased further. These observations are in accordance with the model predictions. Conclusions: Experimental verification of theoretical model predictions suggests that the model can be used to guide and interpret further experiments aimed at gaining a better understanding of the cornea’s lamellar architecture. It may ultimately be possible to determine an incidence configuration that would eliminate the confounding effects of corneal birefringence on retinal polarimetry measurements.
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