A visual display terminal, or video display terminal (VDT) is a computer screen. The VDT displays the text and images generated by a computer. VDTs can be part of a computer, or separate hardware devices tethered to a computer with a cable. These separate devices are sometimes called computer monitors.
When personal computers were first mass-marketed in the mid-1980s, the first VDTs displayed a single color, usually green or orange. Today's VDTs can reproduce millions of colors, all created by mixing the primary colors red, green, and blue.
Displays reproduce text and images using pixels. A pixel is the smallest unit of color reproduced by a VDT. The size of the pixel depends on the screen's resolution. The higher the resolution, the smaller the pixel, and the sharper the image. VDTs project pixels on the screen using different kinds of technology.
These include cathode-ray tube (CRT), liquid crystal display (LCD), and gas plasma technologies. A CRT is a vacuum tube similar to the picture tube in a television. Electricity is moved back and forth across the screen, hitting phosphor dots on the inside of the tube. Like televisions, VDTs that use these tubes are bulky and heavy.
The advent of newer technologies, that followed CRTs, allowed for the creation of much lighter and thinner VDTs, known as flat panel displays. Flat panel displays can be a few inches wide, compared with CRT displays that can be more than a foot wide. LCD and gas plasma are two common types of flat panel displays.
LCD screens have been used since the 1980s in digital watches and calculators. LCD screens are now commonly found in laptops and other small computers. These screens are back-lit using either a fluorescent light or light-emitting diode (LED). The back light is always on and images are reproduced on the VDT by blocking portions of the light.
Electricity passed through liquid crystal inside the screen changes the polarity of the crystal. The polarized crystal blocks the light. A filter positioned in front of the back light changes the color of the light that is not blocked, and is projected on the VDT.
Plasma screens work using gas instead of liquid crystal. They have small chambers on the screen filled with plasma gas. Color is added to the gas by mixing phosphor and electricity, in a way similar to how color is projected on a CRT screen. Originally, plasma screens used more energy than LCD screens. Upgraded technology has since improved the plasma VDT energy efficiency.