Abstract
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 0$^{\circ}$ and 45$^{\circ}$, respectively, with the azimuth
angle of the test cell set to 0$^{\circ}$. The TNLC is then rotated through 45$^{\circ}$ 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 $\Delta
d =0.002~\mu{\hbox {m}}$, $\Delta \phi =0.09^{\circ}$, $\Delta \theta_{p} =0.28^{\circ}$, and $\Delta \alpha =0.08^{\circ}$, respectively. Also, the azimuth angle $\alpha $ can be extracted in the full
range in this study.
© 2009 IEEE
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