Birefringence properties of human and rabbit cornea: Polarization rotation component

Purpose. In investigations of polarized light propagation through cornea, the stromal lamellae can be considered as a stack of birefringent plates. Theory shows that such a stack is optically equivalent to two optical elements in series - a polarization rotator followed by a single birefringent plate.¹ Previous experimental investigations of corneal birefringence have not recognized the rotator component. The purpose of this study was to confirm the existence of the rotator component and measure its strength. Methods. Cross-polarized transmission (I₊ transmission) was measured in perfused eye bank and freshly excised rabbit corneas as the incident polarization direction, θ, was rotated through 360°. After locating θ_min, the angle at which I₊ transmission is minimum, the incident polarization and analyzer directions were systematically varied from their crossed configuration in the neighborhood of θ_min to locate the settings that produced the absolute minimum transmission. The angular deviation of these settings from 90°, ω, is the rotation induced by the rotator component. Results. The existence of a polarization rotator component was confirmed in both human and rabbit cornea. In human ω varied between -2.0 and +2.4° (n=5) and in rabbit ω varied between -1.6 and -0.1° (n=6). As predicted in Reference 1, the percentage difference between I_+min and the absolute minimum transmission is directly proportional to sin² ω (R=0.99 for both human and rabbit). Conclusions. These results provide further confirmation of the validity of the modeling methods introduced in Ref. 1. Such methods, when combined with polarization experiments, allow one to determine features of lamellar structure that are important to understanding mechanical properties of the cornea.