Spatially resolved birefringence of the retinal nerve fiber layer assessed with a retinal laser ellipsometer

A retinal laser ellipsometer has been developed by coupling a Fourier ellipsometer to a laser scanning system. The instrument has been used to assess the origin and the amount of change in the state of polarization of a laser beam that has double passed the retina around the optic nerve head of postmortemhuman eyes. Eight eyes with no history of glaucoma were studied. At 200 points around the optic nerve head of each eye the Mueller matrices of the retina were examined for the amount of retardation, the orientation of the optic axis, and the amount of dichroism. The degree of polarization preservation of the detected light varied between 50% and 87%. Little dichroism was found, and there was no obvious correlation to the physical arrangement of any retinal structure. However, there was a substantial amount of linear uniaxial birefringence with the optic axis perpendicular to the incident laser beam. Furthermore the calculated optic axis direction showed a strong correlation with the physical orientation of the radial symmetrically arranged retinal nerve fiber axons around the optic nerve head. The local distribution of the corresponding retardation values showed two maxima that coincided with the areas of the thickest retinal nerve fiber layer. These results support the hypothesis that the thickness of the form birefringent retinal nerve fiber layer can be assessed by ellipsometric methods.