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Principle TFT LCD

TFT LCD principle and operation


The display points – pixels (Picture Elements) are controlled by a matrix of TF transistors (TFT – Thin Film Transistor). To generate a graphic image, two components are needed – light and colour. The light component is provided for by the background CCFLs (CCFL – Cold Cathode Fluorescent Lamp), powerful sources of primary white light. The final image is derived from the white light using the LCD (Liquid Crystal Display) technology. Any colour can be created from three basic colour constituent parts: red, green and blue. For each such part of each pixel (a colour dot) there is a transistor controlling the associated liquid crystal.

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Liquid crystals are technological materials whose molecular structure changes with the voltage applied. This allows for control of the amount of light passing through the crystal. Each colour dot is bordered by two polarised filters, a colour filter (red, green or blue) and two equalising layers, all supported by thin glass panels. The associated transistor controls the voltage applied to the equalising layers. The resulting electric field causes changes in the liquid crystal structure (orientation of the crystal elements)

The voltage control makes possible scores or even hundreds of different LC conditions and therefore intensities of the respective colour tones. And as a single pixel includes three basic colour dots, the overall visual effect can be hundreds of thousands of different colours.

The following pictures describe one of the TFT LCD technology variants, namely that known as “Twisted Nematic TFT”:

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Fig. 2 – The liquid crystal status with no voltage applied

With no voltage applied to the colour dot unit, the passing light rays are rotated so that they may pass unhindered through the second (output) polarised filter. Consequently, the full background light generated by the CCFLs passes through the colour dot and the visual effect is white light.

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Fig. 3 – The liquid crystal with full control voltage applied

If full control voltage is applied to the colour dot, the light rays pass through the liquid crystal unaffected and are consequently blocked by the second polarised filter. The resulting visual effect is black light.

As shown above, each pixel consists of three colour dots (sub-pixels). These are arranged horizontally so that in the case of a display with resolution 1,600 x 1,200 pixels there in fact are 4,800 sub-pixels on each line. The pixel width is very small, typically 0.24 to 0.29mm, on most advanced displays 0.12mm. Another important factor is the distance between neighbouring pixels. For practical reasons, small displays are rarely made as high-resolution devices.