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A new approach is proposed for measuring the cell thickness, twist angle, pretilt angle, and azimuth angle of twisted-nematic liquid crystal (TNLC) cells utilizing the Stokes parameter method and a genetic algorithm (GA). In the proposed approach, the Stokes parameters are measured for linearly polarized probe lights with polarization angles of 0deg and 45deg, respectively, with the azimuth angle of the test cell set to 0deg. The TNLC is then rotated through 45deg and the Stokes parameters are remeasured. The four sets of Stokes parameters are processed by GA for the data fitting to inversely derive the cell parameters, and especially the pretilt angle is successfully obtained with slight tilt of the cell. Also, the proposed method enables the four parameters of the TNLC cell to be measured over the full azimuth angle range and involves a single wavelength only. Unlike existing methods that need continuously rotating the polarizer for achieving a curve fitting in Stokes parameters or using multiple wavelengths, this technique first proposed in this study supplies a simpler and cheaper methodology to extract the four parameters of TNLC cell. The experimental results show that the maximum standard deviations of the cell thickness, twist angle, pretilt angle, and azimuth angle are Deltad = 0.002 mum, Deltaphi = 0.09deg, Deltathetasp = 0.28deg, and Deltaalpha = 0.08deg, respectively. Also, the azimuth angle alpha can be extracted in the full range in this study.
Date of Publication: Sept.15, 2009