Output devices are pieces of equipment that are used to get information or any other response out of a computer.

CRT Monitor -  The commonest visual display unit (VDU) used by desktop computers today. It is similar to a TV but has no UHF receiver. For the scientists amongst you - it uses a cathode ray tube (CRT) to fire electrons at a coating of very tiny phosphor dots on the inside of the screen. This causes the dots to glow. A red, green and blue (RGB) dot makes up a pixel of visible light. By varying the intensity of each of these primary colours, the whole pixel will appear to the human eye as any colour desired.

This is one RGB pixel of light R + B = M (magenta) B + G = C (cyan) G + R = Y (yellow) R + G + B = W (white)

The more dense the pixels the greater is the clarity, or resolution, of the screen image.

• VGA (Video Graphics Array) = 640 x 480 pixels
• SVGA (Super Video Graphics Array) = 800 x 600 pixels
• XGA (Extended Graphics Array) = 1024 x 768 pixels

  LCD Monitor - smaller, lighter and using much less power than a normal CRT monitor makes them ideal for portable lap-top computers. Also used in watches and calculators. For the scientists amongst you this is all to do with the polarisation of light by an electromagnetic field applied to a crystal which produces a liquid crystal display (LCD). These screens are rapidly replacing CRT both for computers and for home television.

Daisy-Wheel Printer - like a typewriter but with the preformed letters on the ends of spokes to form a wheel. The letters strike an inked ribbon onto paper. Good (typewriter quality) resolution. Very slow - ½ ppm (page per minute). Very noisy. Can only print the characters provided on the wheel and no graphics (line drawings, pictures). One colour (monochrome) only. PROBABLY NO LONGER IN USE.

Dot-Matrix Printer - a set of steel pins strike an inked ribbon onto paper producing any desired sequence of dots. Low resolution - 72 dpi (dots per inch) - can just see the dots. Quite fast - 1 ppm. Slightly noisy. Can print any shape of character (font) stored in the computer memory and any graphics, all on the same page. PROBABLY NO LONGER IN USE.

Ink-Jet Printer - fires a jet of liquid ink through tiny holes. High resolution - 300 to 600 dpi for almost professional quality, sharp printing. Fast - 3 ppm. Very quiet. Cost around £100. Full black and colour print on same page. Thousands of colours are created by mixing tiny dots of cyan, magenta and yellow (CMY) ink on the paper. Replacement ink cartridges for each colour cost around £10-25. Better printers have separate black, cyan, magenta and yellow cartridges for more economical replacement when empty.

This is one CMY pixel of ink C + M = B (blue) M + Y = R (red) Y + C = G (green) C + M + Y = B (black)

Photographic quality printers can have even more colours to mix for greater accuracy.

Why not use red, green and blue ink?

Laser Printer - works like a photocopier, powdered ink is fused onto paper by heat and pressure. Very high resolution - 600 to 1200 dpi for full professional quality. Very fast: 6 -16 ppm for multiple copies. Almost silent. Cost around £100. Replacement black 'toner' cartridges cost around £50 but last a long time and much more economical than ink-jet printing. Colour laser printers are more expensive at around £250 with £50 for each of 4 replacement cartridges but still cheaper to run than colour inkjet printers.

Braille Printer - by converting text into the Braille code, this printer produces patterns of raised dots on paper for use by the blind.

Graphics Plotter - uses high precision motors controlled by the computer to draw on paper with coloured ink pens. Used for drawings where a high degree of accuracy is required such as building plans, printed circuit boards and machine parts.

Speaker - for music or speech from programs, CD-ROMs and musical instruments.

Light-Emitting Diode (LED) - small low power devices which emit light. Used to indicate various events such as power on or hard disk in operation and to monitor other control applications.

Relay switches & motors - a computer can be programmed very easily to turn switches on and off at the required times. For example to control traffic lights or electric motors in a robot arm. Used in the automobile industry to spray body shells or to assemble and weld parts together; or to assemble delicate electronic components on a printed-circuit board for computers, radios and almost anything else you can think of.

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This is for the scientifically minded only:

Question: Why doesn't the printer use the three primary colours: red, green and blue?

Answer: Green ink appears green because when white light (W = R+G+B) is shining on it, it absorbs the red and blue, and reflects the remaining green light to our eyes (W-B-R = G). Similarly, red ink absorbs the green and blue, and reflects red light (W-B-G = R). Therefore, if we mix red and green inks then all three colours are absorbed by the mixture, no light is reflected and it will appear black: R+G = W-B-G + W-B-R = W-B-G-R = W-W (black). The same will happen with any combination of these three primary colours, so no ink colours can be made with them at all.

Question: Why does the printer use the three secondary colours: cyan, magenta and yellow?

Answer: Yellow ink appears yellow because when white light (W = R+G+B) is shining on it, it absorbs the blue and reflects red and green light which we see as yellow (Y = W-B). Cyan ink absorbs red and reflects blue and green which we see as cyan (C = W-R). Therefore, if we mix yellow and cyan inks then both red and blue light are absorbed by the mixture, and green is reflected - it will appear green: Y+C = W-B + W-R = W-B-R = G (green). Any combination of the three secondary colours (CMY) will produce one of the primary colours (RGB). Varying the amount of each coloured ink will produce all the colours possible.

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