Technical Articles and Editorials

The Inner World of LCD TVs - Part I


Sub-Pixel Magic

Of course, a large flat panel that gets uniformly brighter and darker is hardly useful as a video display. So the liquid-crystal material is actually separated into tiny cells that can be controlled individually. Each cell is called a sub-pixel, and in most LCD TVs, three sub-pixels form one full-color pixel. In a TV with 1920x1080 resolution, there are 2,073,600 pixels, or 6,220,800 sub-pixels.

Fig. 8: Sharp adds a yellow sub-pixel in its Quattron LCD TVs. (Source: Sharp Electronics)

Each sub-pixel is either red, green, or blue, and each full pixel includes one of each sub-pixel, as shown on the right in Figure 8. (Sharp adds a fourth, yellow sub-pixel in its Quattron LCD TVs, as shown on the left in the diagram above, for a total of 8,294,400 sub-pixels in a 1080p set.) If the light from the backlight is white—which it is—how do the sub-pixels acquire their color? Each sub-pixel cell includes a tiny color filter, either red, green, or blue (or yellow in Quattron sets), located after the light passes through the second polarizer.

Fig. 9: Sub-pixels in different arrangements (Source: Sharp Electronics)

The sub-pixels can be arranged in various ways, such as a delta or stripe configuration as shown in Figure 9. The delta configuration is better for video, while the stripe arrangement is better for text. However, the stripe layout is easier and cheaper to manufacture, and the pixels of modern TVs are so small that the sub-pixel arrangement doesn't make a lot of difference in visual performance. Thus, the vast majority of LCD TVs today use the stripe configuration.

Now, you can probably see how a full-color image is formed on the screen. The amount of light from the backlight that passes through each sub-pixel is independently controlled by applying an electric field of varying strength, changing the mix of red, green, and blue that makes up each pixel.

For example, if no field is applied to any of the sub-pixels, the light passes through all of them at maximum intensity, and the result is a bright white pixel as all three colors combine equally. If a strong field is applied to all three sub-pixels, very little light passes through any of them, resulting in a black pixel. Changing the field applied to each sub-pixel independently lets more or less light through each one, controlling the corresponding pixel's color and brightness.