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Twisted Nematic (TN), Super Twisted Nematic (STN), Film Compensated STN (FSTN),
and Color STN (CSTN) are the terms used to describe four types of Liquid
Crystal Displays, each twisting the orientation of the light passing through
the Liquid Crystal Display structure differently to effect contrast and
coloration. We also compare coloration, viewing angles, and costs between
the technologies.
Twisted Nematic (TN) LCDs
TN displays have a twist (the rotation of the molecules from one plane of
the display to the other) of 90 degrees or less. All passive direct drive,
active matrix, and most passive low level (x2 to x32) multiplexed LCD's have
a 90 degree twist.
The basic 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.
High Twisted Nematic (HTN) LCDs
HTN (High Twisted Nematic) displays are
based on a higher molecular twist (usually 110°) than TN (90°) and therefore
offer wider viewing angles and improved contrast. In fact, these HTN
products offer viewing characteristics close to those of STN technology. As
low operating voltage as 2.5V and marginal extra cost over TN means that the
products are well suited to hand-held applications.
Super Twisted Nematic (STN) LCDs
Although Twisted Nematic LCDs may be driven in a time multiplexed fashion to
increase the amount of information displayed, they are restricted in terms
of reduced contrast and limited viewing angle. To achieve more highly
multiplexed displays, super twist technology is employed.
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 this type of display, the LC material undergoes a greater than 90° twist
from plate to plate; typical values range from 180 to 270°. The polarizers
in this case are not mounted parallel to the LC at the surface but rather at
some angle. The cell, therefore, does not work on a light "guiding"
principle, as in Twisted Nematic LCDs, but instead on a birefringence
principle. The position of the polarizers, the cell thickness, and the
birefringence of the LC are carefully chosen to result in a particular color
in the "off" state. Usually, this is a yellow-green to maximize the contrast
ratio. The LC in the cell is "super twisted" that will give it the ability to
use a high multiplex rate. As the twist is increased, the LC molecules in
the middle of the layer are aligned with the applied electric field by
smaller changes in voltage. This gives rise to a very steep transmission vs.
voltage curve, allowing up to 240-line multiplexing.
The STN technology comes in two colorations, Green STN and Silver STN. The
STN-Green has Dark Violet / Black characters on a Green background. The STN-Silver
has Dark Blue / Black characters on a Silver background. It is in the middle
of the road as far as cost, but has very good visual quality. The contrast
is similar to TN technology.
Film Compensated Super Twisted Nematic (FSTN) LCDs
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.
Double Super Twisted Nematic (DSTN)
DSTN was
the first commercial black and white conversion of the STN display. DSTN
displays are actually two distinct STN filled glass cells glued together.
The first is a LCD display, the second is a glass cell without electrodes or
polarizers filled with LC material for use as a compensator which increases
contrast and gives the black on white appearance.
DSTN provides better contrast than STN
and FSTN, and offers automatic contrast compensation with temperature. Its
response time is significantly enhanced. DSTN reduces the tendency of a
screen to be slightly red, green or blue. Since its polarizer mode is
negative, DSTN LCDs need backlighting, which is provided by either LED or
CCFL only. It provides a resolution up to 122 x 32 dots. DSTN is suitable
for use in/with automobiles, gasoline pumps, etc.
Color Super Twisted Nematic (CSTN) LCDs
Color STN Technology is actually STN technology that uses a white backlight
and color filters to produce the hues required for a color display. Each
visual pixel of a CSTN display is actually physically 3 separate pixels
using a colored filter of Red. Green, and Blue. Each of those colors are
controlled individually by the graphic controller chip. So in actually; a
320 by 240 pixel CSTN display actually contains 960 by 240 individually
colored pixels. |
