FAQ - Basis of LCDs

• Twisted Nematic LCDs
• Supertwisted Nematic LCDs
• Positive and Negative mode
• Reflective, Transflective and Transmissive
• Connecting to LCD
• Temperature Range
  
• LED Backlight
• EL Backlight
• CCFL Backlight
• Zebra(Elastomer)Connector
• Heat Seal Connector
• FPC Connector
• Pin(Clip Terminal)Connector
• Bezel
• Holographic Reflector

Twisted Nematic LCDs:

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 þn (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 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. 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 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.

Supertwisted Nematic LCDs:

The biggest problem with early multiplexed LCDs was the reduction in contrast ratio with number of addressed lines. This problem was essentially eliminated with the invention of the supertwisted nematic (STN) LCD in the early 1980s. It was found that if the twist angle was increased to 270 degrees, the slope of the brightness-voltage curve approached infinity; under this condition, a large number of lines could be multiplexed. This higher twist angle was achieved by adding higher concentrations of cholesteric liquid crystal to the nematic mix and by increasing the tilt angle at the glass surface.

The first successful STN LCDs used a birefringence mode to create a "yellow mode" and a "blue mode." Although the result was not optimum for general display use, it was possible to demonstrate 200:1 multiplexing with greater than 5:1 contrast ratio. For the first time, LCDs could be seriously considered for use in portable computers.

The next advance was the development of compensated STN LCDs to produce true black-and-white images. Using either a second STN LCD with opposite twist or a retardation film, several manufacturers were able to produce black- and-white LCDs with high contrast and multiplexibility. Today, the film- compensated STN (FSTN) is preferred because of its thin profile and low weight compared to the double STN (DSTN) type. FSTN LCDs with multiplexing ratios as high as 480:1 have been demonstrated in both black and white and full color. Full color is achieved in the same manner as in active matrix LCDs; that is, RGB filters are patterned on one of the glass plates to control the color of the light transmitted through the LCD.

Positive and Negative mode:

Positive mode is darker characters on whiter background,
Negative mode is whiter characters on darker background.

Positive mode	Negative mode

Reflective, Transflective and Transmissive:

LCDs are offered in three basic light transmission modes: reflective, transflective and transmissive.

Reflective LCD In the reflective mode, ambient light is used to illuminate the display. This is achieved by combining a reflector with the rear polarizer. It works best in an outdoor or well-lighted office environment.
	Reflector bonded to the rear polarizer reflects the incoming ambient light. Low power consumption.
Transflective LCD Transflective LCDs are a mixture of the reflective and transmissive types, with the rear polarizer having partial reflectivity. They are combined with a backlight for use in all types of lighting conditions. The backlight can be left off where there is sufficient outside lighting, conserving power. In darker environments, the backlight is turned on to provide a bright display. Transflective LCDs will not "wash out" when operated in direct sunlight
	Transflector bonded to the rear polarizer reflects light from front as well as enabling lights to pass through the back. Used with backlight off in bright light and with it on in low light to reduce power consumption.
Transmissive LCD Transmissive LCDs have a transparent rear polarizer and do not reflect ambient light. They require a backlight to be visible. They work best in low light conditions with the backlight on continuously
	Without reflector or transflector bonded to the rear polarizer. Backlight required. Most common is transmissive negative image.

Connecting to LCD:

Rubber Connector Structure Connecting Method Pitch
Pin Connector Structure Connecting Method Pitch
Heat Seal Connector Structure Connecting Method Pitch
TAB (Tape Automatic Bonding) Structure Connecting Method Pitch
COG (Chip On Glass) Structure Connecting Method Pitch

Temperature Range: