Corneal birefringence mapped by scanning laser polarimetry.

Corneal birefringence affects polarization-sensitive optical measurements of the eye. Recent literature supports the idea that corneal birefringence is biaxial, although with some disagreement among reports and without considering corneas with very low values of central retardance. This study measured corneal retardation in eyes with a wide range of central corneal retardance by means of scanning laser polarimetry (GDx-VCC, Carl Zeiss Meditec, Inc.), which computes the retardance and slow axis of the cornea from images of the bow tie pattern formed by the radial birefringence of the macula. Measurements were obtained at many points on the cornea by translating the instrument. Data were compared to calculations of the retardation produced by a curved biaxial material between two spherical surfaces. Most corneas showed one or two small areas of zero retardance where the refracted ray within the cornea aligned with an optical axis of the material. The retardation patterns in these corneas could be mimicked, but not accurately described, by the biaxial model. Two corneas with large areas of low retardance more closely resembled a uniaxial model. We conclude that the cornea, in general, behaves as a biaxial material with its fastest axis perpendicular to its surface. Some locations in a few corneas can be uniaxial with the optical axis perpendicular to the surface. Importantly, corneal birefringence varies greatly among people and, within a single cornea, significantly with position.