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What is TN LCD (Twist Nematic Liquid Crystal Display) ?

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

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.

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.  

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