Types of Visual Display Units

Liquid crystals are liquid materials that have some of the optical properties of crystals. A display made of liquid crystals acts like an array of tiny shutters that transmit or block light. A bright light source called a backlight, situated behind the LCD screen, shines through the LCD, creating thousands of tiny dots of red, blue and green that form a color image. Because the backlight is sealed inside the display, you normally never see it directly, only its light filtered through the LCD panel.

Some LCD displays use a fluorescent lamp as the white backlight. The lamps are thin, lightweight, inexpensive and produce a bright white light. On the downside, fluorescents contain small amounts of mercury vapor. Although the mercury does not pose a serious problem in household and office settings, the environmental consequences of heavy metals from monitor e-waste are significant.

The LED backlight is a newer technology for LCD displays that uses light-emitting diodes instead of a fluorescent lamp. The LED produces white light, but uses no mercury.

Although the OLED screen is superficially similar to LCD technology, OLEDs require no backlight; they produce their own light. Because of this advantage, OLED displays can be much thinner than an LCD equivalent. And because a backlight consumes significant amounts of energy, OLEDs help improve battery life in mobile devices. Although the image quality of OLED displays is very good, their working lifetime is currently not as good as LCDs.

Before the 1990s, nearly all computer displays, television sets and video monitors used cathode-ray tube technology. A CRT is a thick glass vacuum tube, one end of which is a flattened screen with a phosphor coating on the inside. In the vacuum, a beam of electrons from a hot metal filament at the end opposite the screen strikes the phosphors, producing a glow. An electronic steering mechanism bends the beam, causing it to scan across and down the screen, "painting" a series of visible images on it. Although CRTs produce high-quality pictures, LCD and other new technologies are much lighter and safer, and have led to the cathode-ray tube's obsolescence.

A plasma display screen consists of tiny gas capsules arranged in a grid; when stimulated by electricity, the gas glows much in the same manner as a neon sign. Some aspects of image quality, such as the darkness of blacks and the vividness of colors, can be better in plasma screens than LCDs. However, LCDs are more energy-efficient than plasmas; due to battery life concerns, virtually all laptop computers have LCD screens and not plasma technology. Most plasma screens currently sold tend to be in the 40-inch to 60-inch size range where image quality helps justify the greater energy consumption.