A4.2 Liquid Crystal Displays
Many familiar electronic devices incorporate a liquid crystal display – the screens in calculators, phones, watches, TVs, laptops, some electronic readers etc.
Figure 4.2 An electronic screen showing a liquid crystal display
  • They only require a tiny current, so they are very energy efficient.
  • They are easily damaged and stop working at extreme temperatures since the molecular arrangement can break down – once the temperature moderates and the arrangement is re-established, they usually work again.
  • An LC display consists of a layer of liquid crystal molecules sandwiched between two transparent electrodes and two polarising filters.
  • The two polarising filters are set at right angles and, without the liquid crystal between them; no light would pass through the two polarising filter layers.
  • The way in which the molecules affect polarised light depends on their orientation. Orientation can be controlled by applying a voltage to each pixel of the display.
  • So, by controlling the voltage applied across the liquid crystal layer in each pixel, the light passing through the two layers will be observed as either a grey or black pixel. This is what builds the image.
Brief explanation: A small voltage turns the pixel grey but if the applied voltage is large enough, the orientation of the liquid crystal molecules in the centre of the layer are affected differently. The light passing through the layer will now be mainly polarised at right angles to the second polarising filter, so it is blocked and the pixel will appear black.
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