What is FSTN LCD (Film Compensated Super Twisted Nematic)


The FSTN means Film Compensated Super-Twisted Nematic. In order to understand FSTN technology better, we need to start from TN and STN.

How TN display Works?

TN LCD Display means Twisted Nematic Liquid Crystal Display. It is the first commercial success LCD technology which is still widely used in low cost and low power consumption applications. Of course, we mean it passive TN LCDs.

If TN technology combined with TFT (Thin Film Transistor), it becomes active matrix display which is widely available for full color display market. Because the name of TN is very technical, when we hear TFT display which we know it is for short of TN type TFT LCD displays. How TN display technology works, showing in Fig.1.

Fig. 1 How TN LCD works

When the voltage is off, we see from Fig.1, the unpolarized light from light source goes through the top polarizer to become linear light. When it meets with liquid crystal molecular, it twists with the liquid crystal twist layer. After twisting 90o, it passes through the bottom glass and arrives to the bottom polarizer.

As the linear light is in parallel position with the bottom polarizer, it goes through polarizer, and we can see the light. When the voltage is off, the liquid crystal molecular will be in perpendicular to the glass surface and lose its twist.

When the linear light meets the liquid crystal layer, it keeps the original transmissive without twisting. It will not pass through the bottom polarizer, as the linear light and bottom polarizer are in perpendicular position. We do see the light.

From the above, we understand:

  • – why we name TN as the twist display, as it twists 90o
  • – we get a positive display which is black characters on white background

 

 

What is STN LCD?

STN LCD means Super Twist Nematic Liquid Crystal Display. TN LCD displays have the advantage of low cost and low power consumption, but it also has its intrinsic disadvantages, such as narrow viewing angle, poor contrast and limit to be used in high multiplex (duty) application.

They can only be used for some alphanumeric displays. Scientists and engineers invented STN LCD to overcome the limitation of TN LCD to make high duty graphic LCD display possible.

Compared with TN LCD’s 90o twist degree, STN LCD increases the twist to 180o to 270o . With this change, STN display’s electro-optical performance changes greatly. See Fig. 2.

Fig. 2 Electro-optical Performance of Different Twist Angles

 

We can see STN display has much improved multiplexing driving capability. Actually, STN technology can drive up to 1/240 duty dot matrix displays.

But STN LCDs also have their intrinsic issues. The main issue is that their nature color is either yellow, green or blue.

Fig. 3 Yellow Green STN LCD

 

Fig. 4 Blue STN LCD

 

 

The problems of the intrinsic colors are:

  • Human eyes are used to see black and white displays,
  • It is impossible to produce full color displays by adding RGB (Red, Green, Blue) color filter.

 

These problems are the reasons why FSTN LCD Display was invented.

Japanese scientists and engineers used a layer of retardation film to compensate the color to make STN color changing to Black and White. Monochrome black and white FSTN makes CSTN (Color STN) possible.

Fig. 5 FSTN Polarizer Structure

 

Compared with TN displays, FSTN displays have much higher contrast and wider viewing angles. But FSTN technology are only limited to passive matrix application. They can’t be used together with TFT technology like TN technology to become active matrix displays.

 

Types of FSTN Displays

FSTN displays, the same as other types of LCD, have positive and negative modes.

Fig.6 Positive FSTN LCD

 

Fig.7 Negative FSTN LCD

 

In order to get better contrast negative FSTN, two films are used. We call it FFSTN (Double Film FSTN).

 

Fig.8 Negative FSTN vs FFSTN

 

FSTN: Advantages and Disadvantages

Advantages:

  • Sharp contrast and easier to read with better looking of the products.
  • Super wide viewing angles to enhance the visibility even with very small pixel display with less power.
  • Built with reflective and transflective polarizer to make FSTN LCDs sunlight readable.
  • As FSTN LCDs can be made high density together with COG (Chip on Glass) technology, the FSTN display modules can be made high compact to make thin and light possible.

 

Disadvantages:

  • More expensive material used and equipment to manufacture make FSTN LCDs more expensive.
  • Slower response time because of larger twist angles. The normal operating temperature range is -20 oC to +70 oC. The color is also changed with temperature, the improvement way is to use DSTN (double STN cells) which can compensate for the color changes and achieve high contrast ratio at low and high temperature.
  • The same as other LCDs, FSTN LCDs can’t emit the light themselves. They have to assembled with backlight to make them LCD display modules to be able to use in applications. But backlight will drive a lot of power.