Technical Info. - LCD Technologies

LCD Basic

 

TN

Twisted Nematic (TN) LCD consists of a layer of liquid crystal material supported by two glass plates. The liquid crystal material is a mixture of long, cylindrically shaped molecules with different electrical and optical properties, depending on direction

On the inner surfaces of the glass plates are transparent electrodes, which are patterned to form the desired visual image. The inner surfaces are coated with a polymer, which is rubbed so that the liquid crystal material at one surface lies perpendicular to the other. Across the film of liquid crystal, the molecules form a 90° twist.
 

On the outer surface of the glass plates, polarizers are placed so they are parallel to the liquid crystal orientation and perpendicular to each other. in the "off" state, light entering the first polarizer is guided by the liquid crystal layer twist to the second polarizer, through which it is transmitted. When the cell is energized, the LC material is aligned with the electric field; light transmitted through the first polarizer is blocked by the second polarizer, forming a dark image. The effect may be reversed if the polarizers are placed parallel to each other, and a light image on a dark background is formed

The TN technology comes in a single coloration, it is Black characters on a gray background.  It is the least expensive, but has the lowest visual quality, primarily in viewing angle.

STN

Super Twisted Nematic LCD's have a twist that is greater than 90 but less than 360 degrees. Currently most STN displays are made with a twist between 180 and 270 degrees. The higher twist angles cause steeper threshold curves which put the on and off voltages closer together. The steeper thresholds allow multiplex rates greater than 32 to be achieved.

In addition to normal supertwist displays, there also exist double supertwist and triple supertwist displays. In general, the more twists, the higher the contrast.

Supertwist displays are also known as supertwist nematic (STN) displays.

FSTN

The most recent advance has been the introduction of Film compensated Super Twisted Nematic (FSTN) displays. This adds a retardation film to the STN display that compensates for the color added by the birefringence effect. This allows a black and white display to be produced and provides for a higher contrast and wider viewing angle.

The FSTN technology comes in a single coloration, Black characters on a White / Gray background. Of the three technologies listed here, it is the most expensive, but it has better viewing angles and contrast that the STN technology listed above.

 CSTN

Short for color super-twist nematic, an LCD technology developed by Sharp Electronics Corporation. Unlike TFT , CSTN is based on a passive-matrix , which is less expensive to produce. The original CSTN displays developed in the early 90's suffered from slow response times and ghosting. Recent advances in the technology, however, have made CSTN a viable alternative to active-matrix displays. New CSTN displays offer 100 ms response times, a 140 degree viewing angle, and high-quality color rivaling TFT displays - all at about half the cost.

DSTN

Short for double-layer supertwist nematic, a passive-matrix LCD technology that uses two display layers to counteract the color shifting that occurs with conventional supertwist displays.

TFT

Short for thin film transistor, a type of LCD flat-panel display screen, in which each pixel is controlled by from one to four transistors. The TFT technology provides the best resolution of all the flat-panel techniques, but it is also the most expensive. TFT screens are sometimes called active-matrix LCDs.