9. Applications of Liquid Crystals
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Twisted Nematic Cell
The development of the principles of operation of the twisted nematic (TN) liquid crystal display device made a crucial impact on the development of the liquid crystal displays technology of the present days. The TN device consists of the twisted nematic liquid crystal confined between two glass substrates with homogeneous planar orientation: the orientation of the director n on one substrate is rotated by 90o with respect to n on another substrate. Such a design induces 90o twist of the nematic liquid crystal. The cell with twisted nematic LC is placed between two polarizers: the polarizer P on the input end and the analyzer A on the output end of the TN device (Figs.39,40). The director n on the input end is parallel to the transmission axis of the polarizer. Two operation modes of the TN device are possible depending on the orientation of the analyzer transmission axis. If the analyzer is parallel to the polarizer it is called a normally black (NB) mode (Fig.39). If the analyzer is perpendicular to the polarizer it is called a normally white (NW) mode (Fig.40).
TN display device in normally black mode When no voltage is applied, the polarized light after polarizer enters the TN cell and, following the twisted orientation of the LC molecules, change its polarization by 90o (Fig.39). On the TN cell output the light polarization is perpendicular to the analyzer transmission axis; therefore, the light is blocked on the device exit by the analyzer. Figure 39. Twisted nematic display device in normally black mode: black arrows show transmission axes of polarizers; violet stripes represent polarized light When voltage is applied, the LC molecules align along the electric field and the polarization of the light passing through the TN cell does not change. Thus, light passes through the device because its polarization is parallel to the analyzer transmission axis.
TN display device in normally white mode When no voltage is applied to the TN device, the polarized light after the polarizer enters the TN cell and, following the twisted orientation of the LC molecules, change its polarization by 90o (Fig.40). On the exit from the TN cell, the light polarization is parallel to the analyzer transmission axis, therefore, the light passes through the system. Figure 40. Twisted nematic display device in normally white mode: black arrows show transmission axes of polarizers; violet stripes represent polarized light When voltage is applied, the LC molecules align along the electric field and the polarization of the light passing through the TN cell does not change and is perpendicular to the analyzer transmission axis. Thus, light is blocked by the analyzer on the device output.
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