Refinement of an experimental technique for measuring birefringence by the polarization-dependent transmission method

We discuss a modified polarization-dependent transmission method to measure birefringence in a nematic liquid crystal (NLC) cell of moderate thickness. Determining birefringence accurately by monitoring the transmitted intensity in a thick NLC cell (and hence a large path difference) is challenging for the following two reasons: (a) a large drop in transmitted light intensity due to scattering losses near the clearing temperature and (b) a sharp jump in the birefringence over a narrow temperature window close to the clearing point. To detect the transmitted light intensity profile precisely, we converted the incident laser into a modulated signal using an optical chopper and measured the transmitted light intensity using a sensitive photodetector coupled to a lock-in amplifier (LIA). Furthermore, the transmitted intensity was normalized against a reference beam to account for any fluctuations in the laser source. The birefringence and the critical exponents (β and α) calculated using this method agree with reported data within experimental error.