LCD Basics


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 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:

STN Panels
Normal Range Operating Storage
0 - +50 -20 - +60
Wide Range -20 - +70 -30 - +80
STN Modules
Normal Range Operating Storage
0 - +50 -20 - +60
Wide Range -20 - +70 -30 - +80
TN Panels
Normal Range Operating Storage
0 - +50 -20 - +60
Wide Range -10 - +60 -20 - +70
-20 - +70 -30 - +80
-30 - +80 -40 - +90
TN Modules
Normal Range Operating Storage
0 - +50 -20 - +60
Wide Range -20 - +70 -30 - +80
-30 - +85 -40 - +90

This table reflects a variety of operating and storage temperature ranges offered with Orient Display's liquid crystal displays. Temperature options available with all standard products of Orient Display are indicated under TN and STN Modules