Fig. 11. Nerve fiber layer birefringence. Birefringence in the nerve fiber layer arises from the parallel arrangement of microtubules within the retinal nerve fiber layer (RNFL). Light polarized in one plane travels more slowly through the birefringent RNFL than light polarized perpendicularly to it. This difference in speed causes a phase shift (retardation) between the perpendicular light beams.