Numerical approximation of light scattering from tenuous cylindrical membranes at normal incidence

Light scattering and form birefringence of parallel cylindrical arrays that represent cellular organelles of the retinal nerve fiber layer

The retinal nerve fiber layer (RNFL) comprises bundles of unmyelinated axons that run across the surface of the retina. The cylindrical organelles of the RNFL (axonal membranes, microtubules, neurofilaments, and mitochondria) as seen by electron microscopy were modeled as parallel cylindrical arrays in order to gain insight into their optical properties. Arrays of thin fibrils were used to represent organelles that are thin relative to wavelength, and the model took into account interference effects that may arise from spatial order. Angular and spectral light-scattering functions were calculated for the backscattering hemisphere. Scattering was much larger from axonal membranes than from microtubules or neurofilaments. Spectra from 400 to 700 nm show that scattering increases at shorter wavelengths for both axonal membranes and microtubules. At 560 nm, scattering from mitochondria modeled as thick cylinders was approximately the same as that from microtubules but showed little wavelength dependence. The model reveals differences in backscattered polarization ratios that may permit experimental discrimination between microtubule and membrane mechanisms for the RNFL reflectance. Calculated backscattering exceeds measured values by at least 1 order of magnitude, but calculated form birefringence for microtubule arrays is approximately the same as measured birefringence.