Director Deformation of Twisted Nematic Cell

The twisted nematic (TN) display is made up of a nematic liquid crystal which has an optic axis that undergoes a 90° twist, in between two substrates. Each substrate is coated with a conductive transparent electrode, indium tin oxide (ITO), which is then coated by a surfactant that is rubbed parallel to the substrate to create groves for the liquid crystal to align in. The optic axis of the liquid crystal undergoes this twist because the two substrates are placed so the rub directions are at 90° angles to one another. In the normally white (NW) mode of operation the optic axes of the polarizers along each side of the substrate are crossed. It is called normally white because in the unactivated state the display is transparent. In the normally black mode of operation the optic axes of the polarizers are parallel. In the unactivated state the display is black.[1]

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Ref: [1] http://www.lci.kent.edu/boslab/projects/optical_compensation/index.html