How Liquid Crystal Displays Work

In the present world, you will find them almost everywhere, right from your mobile phone to computer screens. Liquid Crystal Displays, or LCDs, have become so indispensable that it is impossible to imagine a life without them.

Researchers have proven that it is not reflected brightness, but rather, contrast that enhances greater readability. This fact has been successfully exploited in LCDs where the displays on the screen are highlighted not through illumination, but through dark segments on a bright background, just as is the text you are reading right now.

Internal Composition Of LCDs

To understand how a LCD flat panel display is made, let's first look at the inner composition of a panel in a step by step manner.

  • On examining the cross section of an LCD, we find that it is made up of two very fine plates of glass. Between them a specially designed liquid crystalline molecular structure is sandwiched.
  • The molecules in this liquid crystal have the tendency to change their orientation in the influence of an electric field. The molecules are so arranged that without the presence of an electric field they remain perfectly transparent.
  • The electric current is applied to a set of electrodes positioned on the opposite sides of the liquid crystal.
  • A wall of SiO2 is applied on the electrodes to insulate them from the liquid crystal and the polarizers.
  • Polarizers are in the form of vertical and horizontal filter discs. The basic function of these are to filter out the respectively polarized light rays from the display. They are stuck onto the liquid crystals so that they are right angles to each other.

How an LCD Functions

Rotation of Polarized Light Rays in a LCD

  • As shown in the figure to the left, normally- in the absence of an electric field- the light rays entering the unit pass through the vertical polarizer and are oriented according to their angle.
  • The light ray crosses the display segments or the characters (liquid crystals) and enters the horizontal polarizer by rotating itself at 90 degrees and reaches the reflector.
  • The polarized light rays are reflected by the reflector and are sent back through the same process as they had entered, i.e. by rotating at 90 degrees through the respective polarizers. Thus the light rays are not absorbed by any of the character segments (liquid crystal) and we see the display to be totally clear.
  • However when an electric source is applied to the electrodes of the particular segments, the liquid crystal molecules of these segments align themselves according to the electric field.
  • Therefore in this case the vertically polarized light rays are unable to cross the segments and are rather absorbed by them to form dark images.
  • These images are visible to us on the display as "characters."

Today's modern liquid crystal displays have their own built in light source and thus the use of background reflectors are avoided. Here the images (characters) can be clearly visualized over the background light even in complete darkness.

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Image credit: Marvin Raaijmakers/Cc-by-Sa 2.5, courtesy Wikimedia Commons