Twisted Nematic LCDs

Liquid crystals were actually discovered over 100 years ago, but they did not find commercial applications until the invention of the twisted nematic (TN) LCD by Schadt and Helfrich in 1971 (Schadt and Helfrich, 1971). Nematic liquid crystals have a short-range order and have some of the properties of uniaxial crystals. In the natural state, the molecules have no long-range order and so scatter light. If the molecules are oriented, however, they can become transparent with crystalline optical properties. In a typical LCD, the molecules are aligned by mechanically rubbing polyimide layers on two pieces of glass. In the TN cell, the alignment is at right angles between the two inside surfaces on the glass. A small amount of cholesteric LC is usually added to encourage twisting in one direction only. The aligning layer usually causes a small tilt on the LC molecules at the surface, typically 1-3 degrees; this effect can be important in determining maximum contrast ratio or response time.

In a typical TN LCD, illustrated in  Figure 4.1, crossed polarizers are aligned parallel to the rubbing direction. Polarized light is transmitted and rotated by the liquid crystal molecules if the product of (birefringence) and cell spacing is much greater than half the wavelength of the incident light. For the condition of crossed polarizers, the light is transmitted through the second polarizer. If an electric field is applied to the transparent conductors, the molecules rotate and the light transmits through the cell without rotation. The second polarizer absorbs the incoming light and the cell appears dark. If the second polarizer is aligned parallel to the first, then the light is transmitted with an applied field.

The transmission of the LCD as a function of applied voltage is shown in  Figure 4.2. There is a threshold behavior for most LCDs and no change in transmission occurs until a threshold voltage, Vth, is reached. Transmission then decreases as the voltage increases until saturation is reached. The threshold voltage is typically 1.5-2.5 volts, and saturation occurs at about 4-5 volts. Much research has gone into both lowering the threshold voltage and increasing the sharpness of the transfer curve. It should be noted that the LCDs show an rms response because of the slow response of the LC and the fact that the LC molecules have a very weak dipole moment.


Figure 4.1. Typical Twisted Nematic LCD (Normally White Mode)


Figure 4.2. LCD Transmission (Brightness) As a Function of Applied Voltage

For direct-drive LCDs, such as are used in simple indicators, high contrast can be achieved by driving the LC into saturation. Contrast ratios in excess of 100:1 can be achieved in this mode. To address multiple lines, as is typical in computer or TV screens, multiplexed addressing is used. Information is applied to column electrodes one row at a time. The number of lines that can be multiplexed depends on the steepness of the transfer characteristic, as has been described by Alt and Pleshko (1974). The ratio of the voltage in the selected state, Vs, and the nonselected state, Vns, is given by  where N is the number of rows multiplexed. For example, if N = 200, the difference between on and off states is only 7%; to achieve a reasonable contrast ratio, a very steep electro-optic transfer characteristic is required. The limit for TN LCDs is about 64:1 multiplexing; supertwisted nematic LCDs have a much steeper characteristic and can be used with multiplexing ratios up to 480:1.






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