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 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.
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. 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. 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.