Selective Reflection by Cholesteric Liquid Crystals

Selective light reflection at the plane structure of a cholesteric liquid crystal is treated by the theory of de Vries(1) for normal incidence only. A more general approach is outlined for arbitrary angles of incidence. Conditions for the borders of the reflection bands are given. Also the cases of ordinary total reflection are included. Using the derived formula, the two principal dielectric constants are determined from the angles of total reflection measured with a refrectometer. Their values are used to calculate ther eflection spectrum of a layer of finite thickness at normal incidence which is compared with an experimental spectrum. In agreement with the experimental curve the calculated spectrum shows a central band with a nearly rectangular shape flanked by a series of maxima and minima due to interferences, the positions of which depend strongly on the dielectric constants.