How does LCD TVs work?


Nowadays, LCD (liquid crystal display) and Plasma technologies are the most widely used technologies in HD TVs. In fact, LCD technology is used all around us (Laptop screens, digital clocks, CD players …). In this article, we will explain briefly the key points of that technology and we will answer the question: how does an LCD TV work?

Let’s start with “liquid crystals” physically are. We learned in school that there are three states of matter: gases, liquids, and solids (crystals). So, the name "liquid crystal" sounds a little bit like a contradiction. In fact, as shown in figure1, molecules in Solids always maintain their orientation and stay in the same position with respect to one another. In liquids, molecules can change their orientation and move anywhere in the liquid. But there are some substances that can exist in an intermediate state where their molecules tend to maintain their orientation, like the molecules in a solid, but also move around to different positions, like the molecules in a liquid. This means that liquid crystals are neither a solid nor a liquid. That's how they ended up with their seemingly contradictory name.

Ok, let’s go back to TVs now. A TV screen is formed of hundreds of thousands of pixels [elementary small dots] (2 million pixels for the new 1080p HDTVs). Each pixel consists of 3 sub-pixels, each having at his end a Red, green, or blue filter. These sub-pixels are so small and close to each others that their colors will mix in human eyes.

LCD technology
LCD technology


In LCD TVs, a white light is provided at the back of the screen. If liquid crystals were not there , the white light will go through all pixel filters and will mix again at the output in each pixel  creating a white light (white screen). The role of the liquid crystals is “surprisingly” not to create light!!! But to remove undesirable parts of the light. For example, in a specific pixel, we want a purple color; the liquid crystal will block the light in the green sup-pixel, letting the blue and red going through. At the output, red and blue will mix creating a purple pixel.

We are almost set, the only thing left is: how does liquid crystals block the light? And the answer is quite easy. One of the optical proprieties of LQ is that they let through the light when they are aligned, and block it when they are not. So suppose 2 layers of liquid crystals and that one of them is fixed. By rotating the orientation of the second layer, we control the amount of light going through the layer.

Now, we have all the ingredients of the working process, let’s summarize and assemble all the parts together:

  •  A white light is provided on the back on the screen.
  • This white light is the input of all pixels and the sub-pixels they contain.
  • A built-in processor converts the video signal to a grid of data associating to every sub-pixel the amount of light to block.
  • This numerical data is converted to an electrical signal changing the orientation of the liquid crystal layer.
  • The 3 colors of light assemble out the pixel and mix to give a colorful pixel.
  • The full image is the created. 

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