Influence of surface reflectivities on measurement of the torsional anchoring strength of nematic liquid crystals

The polarisation dependent normal incidence transmissivity of a multilayered system encompassing within it an optically uniaxial layer has been analysed in detail using multilayer optics modelling. It is shown that even for this simple case of normal incidence the orientation of the uniaxial sample and two polarisers needed to give a transmission minimum is significantly influenced by the optical phase shifts caused by reflections at the isotropic boundary layers. We highlight the failure of the simplified approach to this problem which uses the Jones matrix, ignoring multiple reflections within the system. Consequently we show that the standard technique of quantifying the boundary twist offset angles by determining with polarisers the minimum points in the normal incidence transmission for a thin nematic liquid crystal cell may well lead to seriously wrong values for the torsional anchoring strength of aligning surfaces.