Tom Castro

LCD HDTV Operation

How LCD technology and an active matrix structure is used for LCD HDTV operation to produce an image is explained.

Liquid crystal display (LCD) high definition televisions (HDTVs) are thin, flat panels that use technology based on the electronic control of polarized light to produce images. LCDs consist of two polarized glass plates separated by a thin layer of liquid crystal, any number of color pixels, and arrayed in front of a backlight (light source).

LCDs use either cold cathode fluorescent lamps (CCFLs) or light emitting diodes (LEDs) as backlights or edge lights to illuminate their screens. CCFLs limit color rendition, dim over time and need to be replaced, and contain mercury. LEDs provide a wider range of colors, don't need to be replaced, use less power, and do not contain mercury.

Thin film transistors (TFTs) are fabricated on the back glass plate, a color filter is formed on the front glass plate, and liquid crystals are injected in-between. Liquid crystals are used since, by default, will arrange into a tight helix (twisted) pattern to block the passage of light. Then, if a current is applied to the liquid crystals they will straighten (untwist) to pass light. Varying the amount of current affects the amount of light that will be passed. The glass is polarized to filter the light and allow it to pass through in the desired direction.

High resolution color displays such as LCD HDTVs use an active matrix structure. TFTs (switching transistors) are arranged in a matrix on the back glass plate (substrate). Each TFT controls an individual color pixel, allowing each column to access one pixel. To access
a specific pixel the proper row is activated and a charge is sent down the correct column to the designated pixel. With voltage applied to the TFT, the liquid crystals straighten, allowing it to pass light to form the pixel.

To get full color, individual pixels are divided into three sub pixels colored red, green, and blue, respectively. The sub pixels are created by the color filter that only allows certain wavelengths to pass while absorbing the rest. Each sub pixel controlled independently can produce thousands or millions of colors for each pixel. The combination of pixels in different colors forms the image on the screen.

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