Measurement of the magnitude and axis of corneal polarization with scanning laser polarimetry.

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Abstract
RETINAL SCANNING laser polarimetry evaluates the thickness of the retinal nerve fiber layer (RNFL) by assessing the change in polarization of an illuminating laser beam that is reflected from the retina.1,2 This technique is based on the substantial birefringent (birefractive) properties of the RNFL. Light refracted from the RNFL (an anisotropic structure) is polarized, resulting in 2 refracted rays. One of the rays (the ordinary ray) travels with the same velocity as the illuminating beam along the optical axis of the tissue (fast axis) while the other ray (the extraordinary ray) travels with a velocity that is dependent on the propagation direction within the tissue (slow axis). The distance of separation (retardance) between the 2 rays increases with greater tissue thickness. However, the RNFL is not the only birefringent structure of the eye; the Henle fiber layer in the macula is also birefringent. This layer, consisting of elongated photoreceptor fibers extending radially from the fovea, is structurally similar to the RNFL and exhibits significant birefringence. The cornea and, to a much lesser extent, the lens exhibit birefringence as well.