A unified dielectric-tensor model of macular form birefringence and dichroism

RESUMO

Optical anisotropy of the Henle fiber layer (HFL) underlies two macular polarization phenomena, form birefringence (the macular cross) and dichroism (Haidinger's brushes), which have conventionally been treated as unrelated. We model the HFL as a radial diattenuating retarder and derive a single complex dielectric tensor: effective-medium theory for parallel cylinders gives the real part, and cylindrical averaging of the measured transmembrane xanthophyll tilt gives the imaginary part. The birefringent slow axis is radial and the absorption axis tangential, perpendicular by structural necessity. The model reproduces measured dichroic ratios of 1.04--1.14 for a single calibrated oriented-pigment fraction, and supplies the input parameters for full electromagnetic treatments.