Estimation of order parameter of a liquid crystal variable retarder using Haller's approximation.

We use a liquid crystal variable retarder (LCVR) for polarization modulation of the input beam in a polarimeter intended for solar observations. It is known that the retardance of LCVR depends on the voltage and temperature. Voltage at a constant temperature is used for fast modulation. However, fluctuations in the temperature reduce the accuracy in the polarimetric measurements. In order to understand these, we have performed calibration of the LCVR with respect to temperature and estimated the different parameters, critical exponent (β), maximum retardance (δ0 ), and order parameter (S) of the liquid crystal using Haller's approximation. We also study the dependence of these parameters with voltage. It is observed that the change in order parameter with change in temperature varies linearly with voltage in the range of 1-7 V.