Kossel diagram of an aligned cholesteric phase

The selective reflection of light by the cholesteric phase has been examine d using the Kossel technique. The Kossel diagram observed for an aligned mo nodomain sample, viewed down the helicoidal axis, using crossed polarizers, consists of a single bright annulus. Within this annulus two features are visible-a bright central ring and a pattern of four spiral arcs. When the a nalyzer is rotated from the crossed to the uncrossed position, one diagonal ly opposite pair of arcs remains stationary and the other pair rotates arou nd the ring until it becomes superimposed on the stationary pair. The hande dness of the spiral pattern is related to the chirality of the phase and of fers a direct way of experimentally determining the twist sense-a cholester ic phase with a right-handed twist giving a clockwise spiral pattern. The p resence of the more intense central ring and the spiral "extinction" patter n are in accordance with the analytical modeling of the selective reflectio n of cholesteric phases by Oldano et al. [S1063-651X(99)02802-0].