You probably already know that the acronym LCD stands for Liquid Crystal Display, but what is a liquid crystal, and how does it help the display work?

Most crystals, such as quartz, are solid minerals. The liquid crystal in an LCD display however, is a liquid chemical. The molecules in liquid crystal are attached together in millions of little thread shapes, which can pass light through. These threads naturally arrange themselves into precise patterns, which can act as a lens. These patterns can be changed with the application of heat, pressure, or electricity. This property is used to make the crystal pass light through, or block it out as needed.

Each pixel of an LCD screen is made up of 3 sub-pixels, red, green and blue. By lighting up individual sub-pixels, any colour of light can be made. Each sub-pixel of the LCD contains a little pocket of liquid crystal, sandwiched between 2 layers of glass. The inside surfaces of these pieces of glass both contain very fine microscopic grooves running along them.

The orientation of these grooves are at 90 degrees to each other. The threads of liquid crystal try to align themselves along both sets of grooves, producing a gradual 90 degree twist in the pattern of threads. When light passes through, it is also twisted through 90 degrees.

The left diagram shows the light passing through, the right shows the light being blocked off by the polarizing filters.

On either side of the glass, there are polarising filters. These are like microscopic Venetian blinds, blocking out all light that isn't travelling at the correct angle. As the diagram above shows, the filters are arranged so that light can only pass through the whole sandwich of filters and liquid crystal when the light follows the 90-degree twist of the crystals.

When a small voltage is applied to the liquid crystal, the threads change orientation, lining themselves up with the electric current. This means the light no longer twists by 90 degrees and is prevented from making its way through both filters. By varying the voltage, you can vary the amount of light passing through. To make this into a display, all that is needed is coloured filters in front of the sub-pixels, a strong backlight behind, and some driving electronics.

So what does "TFT" mean?

You may have heard the term TFT used to describe flat screen monitors. TFT stands for Thin Film Transistor, and is a technology used in almost every LCD monitor. Each sub-pixel of a TFT monitor is controlled by a transistor on the glass itself. This gives the monitor very precise control over the screen, allowing it to display true 16-bit colour.

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