TNWhen two polarizing filters are arranged along perpendicular axes, as in the first illustration, light passes through the lead filter and follows the helix arrangement of the liquid crystal molecules. The light is twisted 90 degrees, thus allowing it to pass through the lower filter. When voltage is applied, however, the liquid crystal molecules straighten out of their helix pattern. Light is blocked by lower filter and the screen a ppears black because of there being no twisting effect.
The multiplex rate is equal to the number of rows that can be displayed simultaneously. For example, a display with a multiplex rate of 16 can display 16 rows of information simultaneously.
TN FEATURES:
HTN
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.
HTN FEATURES:
STN
STN (Super Twisted Nematic) is a high information content LCD that uses adjustable interference of two optical modes to achieve a large number of multiplexed
lines, higher contrast and higher level of gray scale. It results in the birefringence mode. An improved twisted nematic fluid (200' twist or greater) which
has better contrast and optimum viewing range than standard twisted nematic.
The graph shows a comparison of a voltage versus transmitted light curve of a typical TN and a supertwist nematic (STN) display. (Generally, a greater twist angle equates to higher multiplexibility.) The V90 and V10 points on the graph indicate the voltages that yield 90 percent and 10 percent light transmission, respectively.
STN displays have a steeper curve than TN displays, which allows higher multiplex levels for STN displays. (STNs were developed primarily to overcome the difficulties encountered in multiplexing TN displays.)
The multiplex rate is equal to the number of rows that can be displayed simultaneously. For example, a display with a multiplex rate of 400 can display 400 rows of information simultaneously.
STN FEATURES:
FSTN
FSTN (Film Compensation Super Twisted Nematic) is an LCD with an extra film added to the outside of the cell to compensate the colour shift of blue on green to black
on white. The film is made of a polymer with double refraction to remove the interference of colours. It results in retardation compensation.
The film (the top layer in the figure) is placed in the display, either under or over the top polarizer. Some film compensation systems use two films, one on the rear that serves as a collimator, and one on the front that serves as a dispersion film to broaden the viewing cone. Film compensation improves the viewing angle, but does not affect switching time. FSTN is all standard STN displays with a polymer film applied to the glass as a compensation layer instead of the second cell as in the case of the DSTN. This simpler and more importantly cost effective method provides the preferred black on white image for this display technology.
FSTN FEATURES:
DSTN
DSTN (Double Super Twisted Nematic) 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 FEATURES:
ECB colour (FSTN)
Electronically Controlled Birefringence (ECB) or Vertically Aligned Nematic (VAN) is a electro-optical effect that was first described in 1971. Through this effect a number of
distinct colours can be displayed in a LCD.
The ECB mode colour displays are realized by controlling both the birefringence of liquid crystals and a pair of optimizing polarizers with retardation films, which are brought from operating voltage rate impressed to liquid crystal layer and inclination of liquid crystal molecules. Thus, the wavelength of light exciting a pixel is determined by the voltage applied to that pixel.
Almost every other LCD manufacturer offers this technology today.
Types of ECB Product
STN: multiple colours on green background
FSTN: multiple colours on pale background
ECB Features:
Multi colour TN
Multi-colour TN display is based on the negative mode. The colour coatings and the black matrix (mask) are inside the LCD cell. The black mask allows the light go through characters and icons only, not the background.
The total number of colours ranges from one to three in addition to the black matrix. Customers can select the colour for each segment. No bleaching of colours is observed even when operating at 80 C, ideal for automotive
and other outdoor applications. Displays from static to 1/8 multiplex are available.
Black mask is used to mask off non- switching areas in alpha-numeric segment displays. The purpose is to improve contrast and readability. This way, a stronger backlight can be used.
Advantages: