Accurate Measurement of the Helical Twisting Power of Chiral Dopants

Abstract 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 determined from the separation of laser beams reflected from the windows of the test cell. Here we propose to use an optical fiber based spectrometer to accurately measure the call thickness instead. Knowing the cell thickness at the positions of the disclination lines allows evaluation of the HTP. We show that this extension of the Cano-wedge method greatly increases the accuracy with which the HTP is determined. We have applied this technique to determine the HTP of ZLI811 in a variety of hosts with negative dielectric anisotropy.