What is Twisted Nematic display technology?

In order to fully understand TN LCD (Twist Nematic Liquid Crystal Display) technology, we will understand what is nematic liquid crystal first.

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What is Nematic Liquid Crystal?

All of us know that water (chemical formula is H2O )has 3 states: Solid (ice), Liquid(water) and Gas(Vapor). Liiquid state is isotropic, which means that its properties are uniform in all directions— the result of H2O molecules being in constant random motion. Solid state is crystalline, all the H2O molecules have their fixed position, is anisotropic; optical- and other properties such as thermal and electrical conductivity vary with direction.

The Nematic liquid crystal state is a unique state not included in the above 3 states. It is a state between the crystalline (solid) and isotropic (liquid) states. Even in the state of liquid crystals, there are several types of liquid crystal states, as below.

Nematic Phase

The nematic liquid crystal phase is characterized by molecules maintain the general order of tending to point in the same direction. It has one dimensional order. See Fig.1

Smectic Phase

In smectic phase, molecules show two-dimensional order not present in the nematic. The molecules maintain the general orientationally of nematic, but also tend to align themselves in layers or planes. It is the state between nematic (one-dimensional order) and solid state (three-dimensional order). See Fig.1.

Cholesteric Phase

The cholesteric (or chiral nematic) liquid crystal phase is typically the molecules are directionally oriented and stacked in a helical pattern, with each layer rotated at a slight angle to the ones above and below it. See Fig.1.

Nematic Liquid Crystal

Fig.1. Nematic, Smectic and Cholesteric Phases

 

In LCD display technology, only nematic liquid crystal has practically use and cholesteric liquid crystal is doped with small amount to help nematic liquid crystal molecules to twist. That is the origin of the term “Twisted Nematic” or TN for Nematic Display in short.  

 

What is TN (Twisted Nematic) display?

How TN display Works

A most simple TN LCD panel contains two polarizers, two glass substrates with transparent electrodes, ITO (Indium Tin Oxide) and a thin liquid crystal material layer sandwiched between the electrodes. See Fig.2

Twisted nematic display

Fig.2 How TN Panels work

 

When the voltage is off, the natural light from the light source goes through the top polarizer to become polarized linear light. When it meets with nematic liquid crystal molecules, 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 polarized light is in parallel position with the bottom polarizer, it goes through polarizer, and we can see the light (white or grey pixel).  

 

When the voltage is on, the liquid crystal molecular will be in perpendicular to the glass surface and lose its twist under electric field. When the polarized light meets the nematic liquid crystal layer, it keeps the original transmissive without twisting. It will be blocked by the bottom polarizer as the linear light and bottom polarizer are in perpendicular position. We do see the light (black pixel)

From the above, we understand:

  • Why we name TN as the twist display, as it twists 90o,
  • We get a positive display above, which is black characters on white or grey background.

Glass panels have two modes. Positive mode (black characters on white/gray background) or Negative mode (white/gray characters on black background). The reason why the TN LCD can’t get paper white background is the fact that polarization cut out most of the light, only 20% to 40% light can go through a TN LCD

 

Advantages and Disadvantages of TN Display Technology

Advantages:

  • – Low cost: The materials used are low cost and the manufacturing process is simple.
  • – Low power consumption: TN screen driving is simple and refresh rates can be slow, which make it friendly for battery powered and handheld devices.
  • – Faster response time or speed: TN LCDs can even be used for light shutters, welding helmets, and gaming monitors.
  • – Wide operation temperature range: TN LCDs can be used for extreme environments with operating temperature range -40 oC to +90 oC.
  • – Sunlight Readable: TN LCDs can easily be built for sunlight readable applications by adding reflective or transflective polarizers.

 

Disadvantages:

  • – Low Contrast Ratio: TN display panels have poor contrast.
  • – Viewing Angle:  TN LCDs have poor viewing angle especially one direction of viewing angle is totally not usable. That’s why we have to define TN LCDs viewing angle in the application to be either 6 or 12 o’clock.
  • – Limited multiplexing ability for passive TN LCDs: That is the reason why TN LCDs are mostly used for low end and simple applications, such as low cost watches, calculators, utility meters etc.

 

TN technology combined with TFT technology which can be used for high-resolution application and have a great commercial success in market today. TN TFT LCDs, also called TFT LCDs in the market, are low cost and are mostly used for laptops, computer monitors, medical monitors and other general use displays etc. thanks to their low cost. But for high-end market, TN technology is facing increasing competition from IPS (In-Plane Switching) technology and VA(vertical alignment) display technology. IPS monitors have much better viewing angle, contrast, color saturation and color reproduction compared with TFT monitors.

 

TN TFT technology are also 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. But we believe TN display will be available in the market for a long time thanks for its low cost, low power consumption and easy to produce and use.

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