Analytical approach for unique determination of cell parameters and equivalent birefringent parameters of a generally twisted nematic liquid crystal device

In the measurement of a twisted nematic liquid crystal device (TNLCD) by an optical apparatus, the cell parameters of the TNLCD may result in multiple solutions in the measurement that all agree with the measured data; hence manufacturers cannot find a set of correct solutions from among the ambiguous ones. With the help of the optical equivalence theorem of a unitary optical system, the ambiguity of the measured parameters of a TNLCD, including cell parameters and equivalent birefringent parameters, can be simultaneously removed by an analytical approach using a single-wavelength polarimeter. The procedure for unique determination of the cell parameters is performed using a self-consistent condition to select a set of the correct solutions from all the possible solutions. The proposed method can be applied to characterize a generally TNLCD for which the twisted angle is close to 270° and the liquid crystal phase retardation is over 2π.