What is FSTN LCD?
FSTN stands for Film Compensated Super Stranded Nematic. In order to understand FSTN technology better, we have to start from TN and STN.
How does the TN screen work?
LCD display TN stands for twisted nematic liquid crystal display. It is the first commercially successful LCD technology that is still widely used in low-cost, low-power applications. Of course, we mean passive TN LCD displays.
If TN technology is combined with TFT (Thin Film Transistor), it becomes an active matrix display that is widely available for the full color display market. Because the name of TN is very technical, when we hear TFT display, we know that it is short for TN type TFT LCD Displays . How the TN display technology works, shown in Fig.1.
When the voltage is off, we see in Fig. 1 that unpolarized light from the light source passes through the top polarizer to become linear light. When it meets the liquid crystal molecule, it is twisted with the liquid crystal twist layer. After turning 90 o , it passes through the lower crystal and reaches the lower polarizer.
Since linear light is parallel to the bottom polarizer, it goes through the polarizer and we can see the light. When the voltage is off, the liquid crystal molecule will be perpendicular to the glass surface and will lose its twist.
When the linear light meets the liquid crystal layer, it keeps the original transmission without twisting. It will not pass through the lower polarizer, since linear light and the lower polarizer are perpendicular. We do see the light.
From the above we understand:
– why we call the rotating screen TN, since it rotates 90 o
– we obtain a positive screen that is black characters on a white background
What is STNLCD?
STN LCD stands for Super Twist Nematic Liquid Crystal Display. TN LCD displays have the advantages of low cost and low power consumption, but they also have their inherent disadvantages, such as narrow viewing angle, poor contrast, and limit to use in high multiplex (duty) applications.
They can only be used for some alphanumeric displays. Scientists and engineers invented the STN LCD to overcome the limitation of the TN LCD and make a high-performance graphic LCD possible.
Compared with TN LCD 90 o degree of rotation, STN LCD increases the rotation to 180 o to 270 o . With this change, the electro-optical performance of the STN display changes a lot. See figure 2.
We can see that the STN screen has much improved multiplexing driving ability. Actually, STN technology can drive dot matrix displays up to 1/240 service.
But STN LCD displays also have their inherent problems. The main problem is that its natural color is yellow, green or blue.
The problems of intrinsic colors are:
Human eyes are used to see black and white screens,
It is impossible to produce full color displays by adding an 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 delay film to compensate for color and make the STN color change to black and white. monochrome black and white FSTN makes CSTN (Color STN) possible.
Compared to TN displays, FSTN displays have much higher contrast and wider viewing angles. But FSTN technology are only limited to passive matrix application. They cannot be used in conjunction with TFT technology such as TN technology to become active matrix displays.
FSTN: advantages and disadvantages
Advantages:
Sharp contrast and easier to read with better product appearance.
Super wide viewing angles to improve visibility even with very small pixel displays with less power.
Constructed with reflective and transflective polarizer to make FSTN LCD sunlight readable.
As FSTN LCDs can be made high density along with COG (Chip on Glass) technology, FSTN display modules can be made very compact to be thin and light.
Disadvantages:
More expensive material used and more expensive FSTN LCD manufacturing equipment .
Slower response time due to larger turning angles. The normal operating temperature range is -20 o C to +70 o C. The color also changes with temperature, the way of improvement is to use DSTN (double STN cells) which can compensate color changes and achieve high ratio contrast at low and high temperature.
Like other LCD displays, FSTN LCD displays cannot emit light on their own. They have to be assembled with backlight to make them LCD Display Modules to be able to use in the applications. But the backlight will drive a lot of power.