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What is a VDT?

By N.M. Shanley
Updated May 16, 2024
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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.

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Discussion Comments
By everetra — On Oct 16, 2011

@Mammmood - The term VDT is not one that I hear often nowadays. I only hear it in situations where there are “dummy” terminals.

I used to work at a company that had a bunch of computer terminals. Each computer was a VDT workstation. They all performed the same function, which was to run a simple data entry program.

They were hooked up to a mainframe, which was the real brain of the operation. I don’t know that the computers had their own brains – or chips – so to speak. I don’t know exactly how that works, but at best, they were stripped down versions of real computers.

By Mammmood — On Oct 16, 2011

@hamje32 - What’s interesting about the VDT screen technology is the blurring of distinctions between television technology and computer technology. Put simply, they are the same.

Your computer flat panel and your flat panel television use the exact same technology. This is a far cry from the old vacuum tubes that powered old television sets, which were a different technology than what was used in early desktop computers.

You can even have a plasma computer monitor, if you want to fork up the money. The sameness of the technology is what makes it possible for you to watch movies on your computer.

By hamje32 — On Oct 15, 2011

I well remember the early days of the personal computer revolution. One of my computers was a small Compaq, and the article is not kidding, this thing had an orange screen.

It was a computer, for sure, but it had limitations when it came to its visual display. Still, even in those days and with such low resolutions, there were games that were being played on it.

As long as the game was good and its storyline was engrossing enough, you forgave the orange screen. It was at least an improvement over my first computer, the TRS-80 Model III, which had no color whatsoever. It was white on black, monochrome.

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