Accurate measurement of the helical twisting power of chiral dopants

we report on an accurate measurement of the helical twisting power (HTP) of chiral dopants. In the usual Cano-wedge method, the wedge angle is determi ned from the separation of laser beams reflected from the windows of the te st cell. Here we propose to use an optical fiber based spectrometer to accu rately measure the call thickness instead. Knowing the cell thickness at th e positions of the disclination lines allows evaluation of the HTP. We show that this extension of the Cano-wedge method greatly increases the accurac y with which the HTP is determined.