LCD INTRODUCTION

In this section, we will explain everything ranging from the properties of liquid crystal molecules to the basic principle of display technology by using TN type liquid crystals as an example. The parallel arrangement of liquid crystal molecules along grooves, when coming into contact with grooved surface in a fixed direction, liquid crystal molecules line up parallelly along the grooves.

Nature State

1.Molecules are arranged in a loosely ordered fashion with their long axes parallel.

2.When coming into contact with a finely grooved surface (alignment layer).

3.Molecules line up parallel along grooves.

Light travels through the spacing of the molecular arrangement. The light also “twists” as it passes through the twisted liquid crystals. Light passes through liquid crystals, following the direction in which the molecules are arranged. When the molecule arrangement is twisted 90 degrees as shown in the figure, the light also twists 90 degrees as it passes through the liquid crystals.

Light bends 90 degrees as it follows the twist of the molecules. Molecules rearrange themselves when voltage is applied when voltage is applied to the liquid crystal structure, the twisted light passes straight through. The molecules in liquid crystals are easily rearranged by applying voltage or another external force.  When  voltage  is  applied, molecules rearrange themselves vertically (along with the electric field) and light passes straight through along the arrangement of molecules.

Blocking light with two polarizing filters. When voltage is applied to a combination of two polarizing filters and twisted liquid crystal, it becomes a LCD display.Light passes when two polarizing filters are arranged with polarizing axes as shown at left figure.Light is blocked when two polarizing filters are arranged with polarizing axes as shown at right figure.

TN type LCDs

A combination of polarizing filters and twisted liquid crystal creates a liquid crystal display.

When two polarizing filters are arranged along perpendicular
polarizing axes, light entering from above is re- directed 90 degrees along the helix arrangement of the liquid crystal molecules so that it passes through the lower filter.

When voltage is applied, the liquid crystal molecules
straighten out of their helix pattern and stop redirecting the angle of the light, thereby preventing light from passing
through the lower filter.

This figure depicts the principle behind typical twisted nematic (TN) liquid crystal displays. In a TN type LCD, liquid crystals in which the molecules form a 90-degree twisted helix, are sandwiched between two polarizing filters. When no voltage is applied, light passes; when voltage is applied, light is blocked and the screen appears black. In other words, the voltage acts as a trigger causing the liquid crystals to function like the shutter of a camera.