Output Devices

Output Devices

Output devices work in one of two main ways: raster or vector.

In a raster device the output is scanned across the screen many times a second and it either leaves a glowing dot or it doesn't (see the animation). The dots join up to form the picture that we see on the screen. This type of output device is relatively cheap to make - but sometimes the lines drawn on them can look a bit jagged.
A more expensive output device is the vector device. This joins perfectly straight lines between dots. These devices are particularly useful for people who need to draw accurate lines - such as designers.


1 Visual Display Units (VDUs)

If you are working on a computer you need a device that will show you the results of any caluclations or other processes that the computer may have been doing on your input data. One of the most common forms of output device is the Visual Display Unit (VDU) or monitor.

The type of monitor shown here is based around a Cathode-Ray Tube (CRT), it is a raster device and has a similar build to a television. The picture is made up of many millions of glowing dots on the surface of the tube.

Many laptops and portably computers use Liquid Crystal Displays (LCD) as they use much less power than a CRT display and will drain the laptop's batteries much more slowly than a CRT would. The problem is that LCD screens are much more expensive.

Answer these questions:

  1. What output method joins straight lines between fixed points ?
  2. What output method scans across a screen or page ?
  3. What does LCD stand for ?
  4. What does CRT stand for ?
  5. Why are LCD screens used in laptops ?

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2 Hard-copy Devices

inkjet printer

Inkjet printers are very common for most home users of computers as they are cheap to buy and can print in both colour and black and white.

An inkjet printer works by accurately moving a print-head over a piece of paper. As the print-head moves of the paper tiny jets of coloured ink are sprayed onto the surface of the paper: this is the 'ink jet' which gives the printer its name.

The main problem with inkjet printers is that they are quite slow and they a fairly noisy.

Laser printers have always been the best quality printers for most general printing jobs. They are fast, accurate and quiet. However they are also much more expensive than inkjet printers.

A laser printer works by the print head putting a very small electrical charge onto a metal drum. The drum is then coated in a coloured powder (called toner) which only sticks where the electrical charge was. As the paper passes under the drum the toner comes off and onto the paper.
Once the paper has been heated the toner sticks to the paper.

Both inkjet printers and laser printers are raster devices: they scan across the paper as the print. This means that they can sometimes have problems drawing thin lines accurately.

laser printer

If you really need to draw a lot of lines with a great deal of accuracy then you need to use a plotter like the one shown here. Plotters are vector devices and they work by using special pens which are used to draw lines from one point to another.

This type of plotter works by moving both the pen and the paper. It is able to handle very large pieces of paper and is ideal for Computer Aided Design (CAD) applications.


Answer these questions:

  1. What sort of device is an inkjet printer ?
  2. What sort of device is a plotter ?
  3. What sort of printer would a designer use ?
  4. What does CAD stand for ?
  5. Which printer is best suited for home use ?

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