The Twisted Nematic Cell

What It Is And How It Works

Liquid crystal displays require some sort of external light source in order to properly display a desired image. In notebooks this light source is the backlight. The liquid crystal layer has a certain electro-optical attribute which modulates the external light (backlight) to form the desired image in conjunction of the data signal information sent to the pixels on the LCD display.

Twisted nematic cells consist of two glass plates that contain conductive electrodes on the inner surfaces of each glass plate. An extremely thin layer of liquid crystals sit in between the transparent conductive electrodes

When light enters the panel, it first travels though a polarizer film that is attached to the backside of the glass plate. The conductive electrode's surface contains alignment layers to properly align the liquid crystal molecules in a 90-degree twist.

In the off state, or relaxed state, there is no voltage applied and the 90-degree twist of the liquid crystal director causes a 90-degree rotation of the polarization direction in the liquid crystal cell, in a wave-guiding manner

The second glass substrate has a linear polarizer that is perpendicular to the other polarizer and will transmit the light. This is referred to as normally on or normally white mode because there is no voltage applied to the cell when light is transmitted.

However, there is a threshold in which if the voltage gets too high, the twist is removed and the liquid crystal will no longer rotate the polarization direction. This will cause the polarizer closest to the display will block any light that is traveling.

Unlike other displays based on technology like dynamic scattering or light emitting diodes, the twisted nematic operates with very low voltage, consuming less power.

The twisted nematic field effect was developed at the Central Research Laboratories of the Swiss company Hoffmann-La Roche.

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