printers

							•   Impact printers
         – First printers are just like typewriters
         – Two categories
                   • Character printers – daisy wheel printers, line printers
                   • Dot matrix impact printers
         –
•   Xerographic printers
         – Laser, LCS(Liquid crystal shutter), LED(Lighting emitting diodes)
         – Three physical facts underlie xerographic printing
                   • Photoconductive materials
                   • Material that have opposite electrostatic characteristics are attracted to one another.
                   • You bond particles of one material to another material by melting and re-freezing some
                       bonding agent.
         – Drum or belt: either in the form of a rigid cylinder of metal with a thin coating of the
              photoconductor on its outer surface, or in the form of an electrically conducting closed-loop belt
              with similar coating on one side.
•   Working principle
         1. The photoconductive surface (which is kept in a darkened space) is cleaned of any residual toner
              and then charged to a high electrostatic potential.
         2. This charge is selectively drained off by shining light onto only selected regions of the
              photoconductive surface.
         3. The surface is flooded with toner particles. These are mixture of tiny specks of some highly
              colored material and equally tiny chunks of plastic bonding material. The toner particles stick only
              to the portions of the photoconductor that are still carrying electrostatic charges.
         4. Any oppositely charged surface (paper or something) is pressed against the toner-coated
              photoconductive surface (drum).
         5. The paper is then separated from the drum. The paper holds toner particles on its selected regions
              by the electrostatic attraction.
         6. The toner-ladden paper is heated, thus fusing the ink particles to the paper.
         7. Computer printers builds the image one pel at a time, just as a computer display shows graphic
              images by building them up one pel at a time on the monitor’s screen.
         8. Pel – minimum size dot that you can print.
   Ink-jet printers
         o Sprays a liquid ink onto the page. Most of them are similar to dot matrix printers
   Hot wax (crayon) printers
         o Solid ink that is melted before it enters the ink-jets. Sometimes uses ribbon waxed with color.
   Dye-sublimation printers

   Ways to describe images to be printed

        o   Bitmapped page image
        o   Pure ASCII text page
        o   Plotter language
        o   Page description languages
                  PCL – from hewltt-packard
                  PostScript – from Apple computers
   Smart page printers – more RAM is necessary inside the printers
   Dumb page printers

   Color printing

        o    Color model: 4 –color printers, cyan (C), yellow (Y), magenta(M), black (K).