Visual Output Devices

Visual Display Unit (VDU)

Virtually all computers use some type of screen as their primary output device. There are two categories of screen: cathode ray tube and LCD.

CRT screens: The cathode ray tube (CRT) type screen is usually called a monitor and makes use of the same technology as a television screen. A beam of electronics is fired from an electronic gun at the back of the tube. This strikes the front of the tube which is covered in a phosphorescent material which glows when struck by electrons. Between the electron gun and the screen the beam is modulated by a signal to produce the image you see on the screen.

With CRT type screens, an important measure is the refresh rate. Roughly speaking, this is the number of times the image is refreshed every second. A low refresh rate makes the image appear to flicker. You need a refresh rate of at least 72 Hz (72 times a second) to avoid the appearance of flicker.

Solid state screens: Solid state screens, also known as LCD or Liquid Crystal Displays, make use of tiny transistors to emit light and create an image. Originally, LCD screens were confined to laptops, but they are increasingly used with desktops. They are usually called flat screens when used as separate units with desktops.

Resolution: An important characteristic of all screens is their resolution. Each point of light on the screen is called a pixel. The resolution of a screen is the maximum number of pixels that the screen can display. This is given as the number of pixels across (horizontal resolution) by the number of pixels down (vertical resolution). For example, 800 x 600. The greater the resolution the better. Modem screens can display 1024 X 768 or better.