Chap 2 Related Works
2.2 LC Operation theory
In this section, we will stand on electro-optical convert principle to describe LC operation, TFT-LCD module architecture, VESA standard timing specification, and column and row driver.
2.2.1 Twisted-Nematic Cell
The electro-optical effect in twisted nematic (TN) liquid-crystal cell is now widely used in active-matrix liquid-crystal displays. As shown in Figure 3, a thin layer of nematic liquid-crystal is placed between two glass plates which provided a transparent and conductive coating.
Figure 3. Functional of the TN cell [7]
The orientation of liquid-crystal molecules or director on both plates is aligned to be nearly parallel to the surface of the glass plate (planer or homogeneous alignment), and as shown schematically in Figure 4, this orientation on each plate twisted 900 with respect to that of the other plate, so that the orientation is continuously twisted from the bottom to the top by900. The homogeneous alignment of liquid-crystal molecules on each plate is produced by rubbing the surface of a polyimide thin file in the proper direction with a fabric.
Figure 4. Molecular alignment in TN cell.
With a twist angle by 900 and a cell thickness by 5um, the resulting pitch of cholesteric equivalence is 20um. Since this is much greater than the wavelength of light, the polarization plane of linearly polarized light traveling normal to the glass plates rotates with the liquid-crystal axis; the 900 twist of the director should lead to a 900 rotation of the linearly polarized light. Therefore, in this normally white (NW) mode of operation where the two polarizers are set perpendicular to each other, the light is transmitted through the cell when there is no applied voltage.
2.2.2 Light Valve Characteristics
When voltage applied to the TN cell, it modified the director orientation. When applied to the cell modifies the director orientation. When the applied voltage is below a threshold level, there is no charge of the orientation, but at the threshold the molecular orientation begins to be aligned to the electric field and tends to be perpendicular to the glass plate. At voltage well above the threshold the alignment of the molecules is completely parallel to the field except the regions adjacent to the surface of glass. There are two kinds of models in TN cell. One is normal black (NB) and the other is normal white (NW). These two models respectively correspond to the parallel and the perpendicular polarizer schemes. See Figure 5.
Figure 5. Normal black (NB) and normal white (NW) models of operation.
The TN cell becomes optically inactive and linearly polarized high travels through the cell without any rotation of the polarization then the light polarization becomes perpendicular to the output polarizer resulting in no transmission of light.
The originally proposed TN cell used a parallel scheme: the two polarizer where parallel to each other. In the NB (normal black) configuration, the light is not transmitted in a field-free state and is transmitted when the applied voltage is above the threshold. When there is no bias voltage. However, transmission is suppressed to zero only for monochromatic light of wavelength . Therefore in a practical display in which a broad-band illumination is used. In the NB mode this leakage reduce a contrast ration which is the ratio between the “on” and
“off” transmission.
In the normally white mode, if operation has little reduction in the contrast ratio since this ration is governed by an “off” transmission corresponding to a high bias voltage. When a sufficiently high voltage is applied, the liquid-crystal molecules are aligned parallel to the electric field or perpendicular to the glass substrate and there is no rotation of electric vector of the polarized light. Transmission is suppressed completely regardless of the wavelength of the light. The transmission in the “on” state is wavelength dependent. But this transmission does not have a critical effect on the contrast ratio. A contrast ratio exceeding 100 can be readily in the normally white mode of operation. This is why normally white mode is preferred in the practical TFT-LCD display.
2.2.3 Threshold Voltage of TN Cell
Liquid-crystal modules are easily affected by an external field. The TN cell is sandwiched between the two glass plates and the director orientation at the surfaces of these plates differs from each other by an angle.
In Equations (1) and (2) the time constant is proportional to the square of cell gap.
Reducing the cell gap is therefore an effective way to shorten the cell response time. Where threshold voltage on TN cell is V ,th r is rise time constant, d is delay time when threshold voltage turn off on TN cell [12].
For a display like an LCD as well as CRT, the rise and fall times are defined for convenience as the times when the brightness reaches, after the turn on and turn off the drive voltage, 90% and 10% respectively of full brightness, see Figure 6. These turn on and turn off times (ton and toff ) are given by
Figure 6. Response time of a twisted-nematic liquid-crystal cell.
An example of the temperature dependence of ton and toff is shown in Figure 7. The turn on time is usually shorter than turn off time, and room temperature these times are about 10ms [12]. The response time is reasonable for such application TV and computer terminals.
Figure 7. Temperature dependence of the response time of the TN cell.