Printers can be divided into three major categories:
1) Impact printers (daisy wheel and dot matrix printers or pin printers)
2) Laser type printers (including thermal dye sublimation printers)
3) Ink jet printers
Impact printers are the oldest and easiest to understand. They operate like an old typewriter whose "A" key strikes an ink-impregnated cloth ribbon, the "A" shape squishing the ink out of the ribbon onto the paper. Daisy wheel printers and high-speed sheet-fed printers use hammers to strike keys in much the same way. Daisy wheel printers change character sets by changing wheels. Band printers use a fixed band that travels across the front of the paper.
Dot matrix printers do the same thing except that the only shape the pin makes is a dot. The older dot-matrix printers have nine pins in the print head and are called nine-pin printers. The pins are set in a vertical line in the print head. It moves acros the page and the pins strike the ribbon, leaving their dots.
The distance moved is very small and constant and each pin hits the paper in an exact place. All dots hitting makes a vertical line. Moving that line nine dot-widths makes a small square. Properly used, the dots form an "A".
Dot matrix printers have these shapes in memory. Some will let you choose between shape sets, called fonts. Draft usually leaves spaces so that the printer can work faster and use up less of the ink in the ribbon. A good technique is to use Draft mode on the printer until a paper is final, then shift printing mode to Near Letter Quality or some font.
To print the letter "A" then, the computer sends a command to the printer port and across the printer cable to the printer. The computer then obeys the command by printing the letter "A". Simple. Now comes WYSIWYG (What You See Is What You Get), pronounced "wizzy-wig".
Software can put a picture or oddball character into a document. The poor printer doesn't know how to draw a cue ball! What the software then does is breaks down the picture into shades of grey, say, and decides how many dots in a given space make that shade. Then it breaks down the picture into horizontal bands nine dots high. Now the computer tells the printer that it will command by pins, not by character. The printer relaxes, has a margarita, and lets the picture happen.
Look close at your television. Very close. See those red, green and blue dots? They make every color from simmering pea green to construction helmet grey. Printers use yellow, magenta and cyan (yellowish, reddish and blueish) and the white of the paper to print all colors. Its a physics difference. Anyway, just like a rainbo, a picture's colors are separated out by some process and then printed just like black and white, only one at a time to make a COLOR picture.
Ink jet printers don't use pins. Instead, they use squirt guns. Very tiny, electric squirt guns. Unlike dot matrix printers, ink jet printers mostly use inks that are water soluble. Yes, those run when they get wet. Ink cartridges, unlike printer ribbons, are also the print head. Each time you buy a new cartridge you get what amounts to a new printer. Otherwise, they work exactly the same way a dot matrix printer does.
Also, while a dot matrix printer prints lighter and lighter with ribbon age, an old ink jet cartridge dries up and stops printing. First blank lines may appear across characters and then printing stops entirely. Read manufacturer's instructions to check the cartridge and clean the print head before you trash it. Three-color ink cartridge/head units will always run out of one color first, making pictures go off color, even if two of the three chambers are nearly full! However, ink jet printers make better pictures than ANY dot matrix color printer and are quieter.
Laser printers expose an electrostatic drum or belt in the pattern to be printed. This places a static charge on the electrostatic drum or belt. The belt picks up charged toner (a dry powder). Then the charged toner is transfered to the paper by attracting it to an oppositely charged wire. Then the toner is heated, making it stick. The image on the belt is erased by another charged wire and excess toner returns to the reservoir.
A laser beam is a very narrow beam of light. It is formed when a crystal is excited by heat or electricity. Mirrors are painted at either flat end of the crystal (assume a dowel shape), like opposite mirrors in a bathroom reflecting to infinity. Electrons are excited into higher orbits and then deexcite or fall into lower orbits. This releases a photon, which is a package of energy (light). All of the packages of light are exactly the same size. Then, the light lines up, stacks up as it reflects from the ends of the crystal. Each photon has just enough energy to excite another electron which deexcites. All of the light comes out lock-step, coherent light.
What makes this useful is that there is a tiny flaw in the mirror at one end and light escaping comes out absolutely straight. It can be aimed across the room or across a football field and spread very little, unlike a flashlight. Finally, the laser printer uses the same dot-matrix method of the pin printer to scan across the drum.
There are color laser printers. They are very expensive.
There are other printers that work similar to the laser printer but use light generated by light emitting diodes.
While I don't know exactly how thermal wax or thermal dye sublimation printing works, I got a color 3x5" picture scanned and printed as an 8x10" color picture that didn't show any dots. Very impressive. Very expensive.
Actully, solids changing physical state directly to gas and skipping the liquid state is sublimation, I just haven't gutted that type of printer yet.
Updated January 24, 1998, 10:52pm. firstname.lastname@example.org