TN LCD vs FSTN LCD Technology

TN LCD Technology Introduction

TN LCD (Twist Nematic Liquid Crystal Display) is the oldest type of LCD display technologies commercially available in the market. It achieved widely successful and attracted the worldwide interests in the development of LCD display technology. It is still in big scale production today, most for digit (segment) display. Please see the link below for more depth introduction of TN LCD.

OD-607A (LCD Glass Panel)

Fig. 1 An Example of TN LCD Glass Panel

TN LCD can be used as display modes of positive or negative display . Positive mode is black characters on grey background, while negative mode is grey characters on black background

It is low cost, but viewing angle and contrast are poor. It can’t drive high multiplex application. TN LCDs are mostly used for application below ¼ duty. Some strict customers complain about the “ghosting” of TN display at ¼ duty, therefore, a slight improved HTN LCD (Highly Twist Nematic) is used. To some very cost sensitive applications, TN LCD can be used up to 1/16 duty. But most of the applications over 1/8 multiplex, the customers choose STN or FSTN LCDs.

 

FSTN LCD Technology Introduction

FSTN LCDs (Film Compensated Super-Twisted Nematic) is a kind of  STN (Super-Twisted Nematic) LCDs FSTN LCD contain a retardation film with polarizer applied to the STN display. The retardation film is used for compensate the intrinsic yellow green color of STN LCD displays to produce a black and white display, which is the must to produce color displays. It is also a monochrome LCD display but it has a higher contrast and wider viewing angle than TN display. FSTN displays have higher contrast and wider viewing angle.  They are good enough to be used in up to 1/240 duty graphic LCD display applications. Because it is black/while, they can be used for CSTN (Color STN) after applying a RGB color filter. But it is more expensive than TN LCD.  FSTN LCD is also kind of monochrome display which have both positive and negative mode. With fine manufacturing, the pixel size can be as small as 0.08mm.

Fig. 2 Positive TN LCD

Fig. 3 Negative TN LCD

 

 

Difference between FSTN and TN LCD Technologies 

 

  • – Structure:  TN LCD twists 90 degree, FSTN LCD twist 180 to 270 degree.

 

  • – Liquid crystal material: Both use nematic LC materials but FSTN fluid has more cholesteric doping materials in it in order to help its molecular to twist higher degree.

 

  • – Orientation layer: TN LCD can use normal polyimide while FSTN LCD has to use special high pretilt polyimide.

 

  • – Contrast Ratio: FSTN LCD has much better sharp image than TN LCD especially at higher multiplexing application.

 

  • – Viewing Angle: FSTN LCD has much better wide viewing angle than TN LCD especially at higher multiplexing application. TN LCD has a weak viewing direction, that is the reason we have to define 6 or 12 o’clock viewing angle. But FSTN LCD can be viewed clearly from all the viewing directions.

 

  • – Response time:  Between FSTN LCD has higher twist angle, it takes more time to make a change. It has longer response time or slower response speed than TN LCD.

 

  • – Operation Temperature Range:  Because of extremely slow response and background color change for FSTN LCDs at low temperature, FSTN LCDs can only be used within -20oC to +70 oC, but TN LCDs can be used with -40 oC to +90 oC which can be easily used for outdoor applications. FSTN LCD response speed in extremely cold environment can be improved by adding a heater. FSTN LCD color change at low and high temperature can be solved by adding a reversed optical compensation cell as DSTN (Double STN), but the cost will be more than double.

 

  • – Power Consumption: Theoretically, FSTN and TN LCDs should consume the same level of power. But FSTN LCDs are normally used for high multiplex application, they use higher frequency drive and their LCD controllers and drivers normally consume more power than TN LCDs. While in general opinions, TN uses much less power than STN or FSTN LCD displays.

 

  • – Cost: FSTN displays are about 50% or 100% more expensive than TN Displays.

 

  • – Assembly (Display Module):  Because most of TN displays have fewer contacts, it can use pins, zebra, FPC (Flex Printed Circuit) to make the connections. When assembled in the LCD modules, normally COB (Chip on Board) is used. While FSTN displays have more contacts, high density and more reliable assembly ways have to be used. COG (Chip on Glass) excels because of its low cost and compatible for high volume production.

 

  • – Backlight: Both TN LCDs and FSTN LCDs can’t emit light themselves. They need backlight in order to be visible in dim environment.

 

  • – Sunlight Readable: It is a big advantage for both TN and FSTN LCD. By applying a reflective or transflective polarizer at the back of the LCD, the high contrast display can be achieved under sunlight.

 

  • – Competition in the market:  FSTN technology is facing the strong competition from other display technologies, especially from TFT (Thin Film Transistor) LCD Technology  because its sharp cost drop, superior color saturation, good contrast ratio and wide viewing angle. TFT LCDs belongs to active matrix LCD displays. Now, COB type graphic dot matrix FSTN LCD market also disappeared. Around 2000, most of the mobile phones were made by FSTN displays. The market was dominated by Nokia with 85% market share. The market was totally replaced by color TFT displays. Thanks for low cost and low power consumption COG assembly technology, FSTN displays take the market of small size and lower than 1/160 multiplex applications.  As for TN displays, they face a little challenge from passive OLED (Organic Light Emitting Diode).  TN technology can also be used for active TFT LCDs which are facing increasing challenging from AMOLED (Active Matrix OLED). Now, micro LED displays are also emerging in the market. TN type active matrix TFT LCD displays are facing pressure in large size display market.