Significance of Becke Line Refractive Indices in Synthetic Fibers

Abstract

Microscopy literature states that the Becke line gives the refractive index of the fiber surface In this work, interferometric measurement of the refractive index across the fiber showed that Becke refractive indices are not confined to the fiber surface, but may occur at any point on the fiber radius. Further, the Becke indices need not occur in the same axial fiber element. When either refractive index varies along a fiber radius, Becke n111 corresponds to the maximum n11 found by interferometric measurement, and Bocke n1 occurs at the minimum interferometric n11. In some cases, two Becke n11 values appear, the second corresponding to an inflection or a plateau in the plot of interferometric refractive index vs. fiber radius. Birefringence obtained by Berek or Soleil compensator techniques (Δ nC) is the same as that obtained from Becke or interferometric data (Δ n B) when no refractive index gradients exist across the fiber. If gradients are present, Δ nC is always greater than Δ nB by an amount related to the gradient magnitude.