What is a Liquid Crystal Display?

Jeremy Laukkonen

Liquid crystal displays (LCD) are a type of flat monitor device that renders an image by using sandwiched layers of glass, electrodes, and liquid crystals (LCs). These LCs are physically liquid, though they also display a number of crystal-like properties in the way the molecules are arranged. The unique properties of LCs allow them to change alignment in a predictable way when an electrical current is introduced. This can be used in a liquid crystal display to activate discrete, monochrome patterns like in a clock radio, or tiny pixels in a high resolution computer monitor or television. Liquid crystal displays are found in items ranging from watches to cellular phones and laptop computers.

A desktop computer with a TFT LCD monitor.
A desktop computer with a TFT LCD monitor.

Each liquid crystal display can consist of a number of different components. There is typically a layer of LCs that is sandwiched between the electronics necessary to activate them, one or more layers of glass, and polarizers or reflectors. LCDs that rely on outside light will typically have a reflector as the back layer so that light can pass through the LCs and return to the eyes of a viewer. Other liquid crystal displays use a backlight instead of a reflector so that the device requires no external light source. Polarizing layers are typically necessary due to the way that LCs rotate to block light or allow it through.

A LCD monitor.
A LCD monitor.

The simplest form of liquid crystal display uses a type of passive matrix. These displays have individual electrical connections for each discrete section of LCs, which is effective when dealing with the few elements found in devices such as clock radios, calculators, and watches. More complex LCDs can also use passive matrix displays, though the technology becomes less efficient as more elements are added in. Early laptop screens made use of passive matrix LCDs, though active matrix technology took over in the mid 1990s.

A liquid crystal display that uses an active matrix can employ one of about five different technologies to create a visible image. Each of these different active matrix technologies use thin film transistors (TFTs) to energize each individual pixel in the display. Some active matrix LCDs also use diodes and other components, though they all use TFTs to create a sharper, more responsive image. Since the active matrix technology allows for faster refresh rates, sharper images, and better colors, this type of liquid crystal display is found in a variety of applications including flat screen televisions, laptops, and cellular phones. Many TFT LCDs also incorporate a specialized layer that can turn them into touchscreen displays.

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