The Faraday rotation of a linearly polarized electromagnetic wave probing a helical-axis-stellarator plasma is a useful electron density diagnostic. The twisted magnetic geometry results in a large magnetic held component in the vertical plane that is assumed to contain the probe beam(s). As a result, significant polarization rotations result for far-infrared beams traversing plasmas of modest densities and magnetic fields. Linear-birefringence-induced beam elliptization is small and the sensitivity to secondary (bootstrap) currents is weak. A new technique for measurement of the polarization state and its application to densitometry of plasma confined by the H-1 Heliac is discussed and modelled using the three-dimensional, free-boundary MHD equilibrium code, VMEC.