Supertwisted Nematic (STN) LCD Display
The biggest problem with early multiplexed TN (Twisted Nematic) LCDs was the reduction in contrast ratio with number of addressed lines. This problem was essentially resolved with the invention of the supertwisted nematic (STN) LCD in the early 1980s.
A super-twisted nematic display (STN) is a type of monochrome passive-matrix liquid crystal display (LCD). This type of LCD was invented at the Brown Boveri Research Center, Baden, Switzerland, in 1983. Twisted Nematic (TN) LCDs with a 90 degrees twisted structure of the molecules have a contrast vs. voltage characteristic unfavorable for passive-matrix addressing. STN displays, with the molecules twisted from 180 to 270 degrees, have superior characteristics. The main advantage of STN LCDs is their more pronounced electro-optical threshold allowing for passive-matrix addressing with many more lines and columns.
|Fig.1 Supertwisted Nematic (STN) LCD Display||Fig.2 STN vs. TN LCD|
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 to the glass surface.
Fig.3 STN LCD with different twist angles
But twist angle of 270 degrees proved very challenge in manufacturing. The yield is very low. Most of the LCD manufacturers today use around 240 degrees twist.
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.
Different type STN LCD displays
Yellow Green STN (Yellow Green Super-Twisted Nematic, Natural Color for Positive Mode)
Yellow Green Background with Dark Blue Character Color
Fig.4 Yellow Green STN LCD
Blue STN (Blue Super-Twisted Nematic, Natural Color for Negative Mode)
Blue Background with White Character Color (Most popular)
Fig.5 Blue STN LCD
Gray STN (Gray Super-Twisted Nematic)
Gray Background with Blue Character Color achieved by applying a violet color polarizer to offset the nature yellow green color of STN LCD.
Fig.6 Gray STN LCD
FSTN (Film Compensated Super-Twisted Nematic)
Gray Background with Black Character Color (Positive Mode)
Fig.7 Positive FSTN LCD
Or Dark Background with White (or other colors) Character Color (Negative Mode)
Fig 8. Negative FSTN LCD with White Backlight
Fig.9 Negative FSTN LCD with Blue Backlight
Fig.10 Negative FSTN LCD with Green Backlight
Fig.11 Negative FSTN LCD with Red Backlight
Fig.12 Negative FSTN LCD with Amber Backlight
FFSTN or DFSTN (Double Layer Compensation Film Super-Twisted Nematic)
In order to get higher contrast, another film can be added. FFSTN has much better contrast compared with FSTN. As FFSTN is more expensive, normally it is used for negative mode.
Fig.13 FSTN LCD vs FFSTN LCD Blue Backlight
Fig.14 FSTN LCD vs FFSTN LCD White Backlight
DSTN (Double Layer Super-Twisted Nematic)
An earlier passive matrix LCD technology that used an extra compensating layer to provide a sharper image which was mostly used for automotive because of its high cost. During to the TFT LCD Progress, DSTN is very rare to use anymore.
Fig.15 DSTN LCD
ASTN (Advanced or Automotive Super-Twisted Nematic)
An extra special EP (Elliptical Polarizing Film) are elliptical polarizing films for STN cell color compensation. EP is used as a compensator which can increases contrast and gives the black on white appearance especially during the temperature change.
Fig.16 ASTN ISO Contrast Diagram
Fig.17 ASTN LCD Display
ESTN (Enhanced Super Twisted Nematic)
ESTN is used in negative mode displays and can be optimized to the backlight color to increase the contrast ratio. It is an ideal display for the automotive and industrial sectors where wide operating temperature range is crucial.
CSTN (Color Coded Super Twist Nematic)
An LCD capable of displaying a limited range of colors. With the progress of TFT LCD technology, CSTN is gradually phasing out the market. Only a few manufacturers in the world still produce CSTN for low power application.