Simple and direct measurements of pretilt angles in hybrid-aligned nematic liquid-crystal cells

We have developed a general approach to perform direct measurements of the pretilt angles from 0° to 75° in hybrid-aligned nematic (HAN) liquid-crystal cells whose cell gaps can also be accurately determined with the help of known pretilt angles. In this paper, we have used a Zeeman laser system to measure the angular-dependence phase retardation of the HAN cells and MATLAB mathematical software to carry out theoretical calculations and fit the measured data to derive the pretilt angles. In general, pretilt angles adjacent to opposite substrates of a HAN cell are different. Our measured pretilt angles of the HAN cell were in good agreement with the measured pretilt angles of two accompanying homogenous cells whose alignment methods were the same as applied to opposite substrates of the HAN cell, respectively. The advantage of direct measurement is easily applicable to measure the pretilt angles of aged HAN cells.