IT212, How Computers Work Printers

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IT-212, How Computers WorkPrinters

• Electrical and Computer Engineering
• Spring 2002

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Evolution of Computer Printing

• Monospace Text, Courier 10
• Speed, 10 Characters/sec to Pages/minute
• Proportional Spacing, Elaborate Fonts
• Graphics
• Color

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Printing Terminology

• Typeface
• Style of letter formation (e.g. Courier, Times-
  Roman, Arial)
• Font
• Specific combination of typeface (Times Roman),
  size (28 Point), and attribute (Bold).
• Point
• Unit of measure, 1/72 of an inch

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Typefaces
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Printing Terminology

• Nominal Page Size
• 80 characters/line
• 60 lines/page
• 4800 characters/page
• Print Quality
• Draft Readable
• Near-letter Blacker, fast printing
• Letter quality Highest quality, adds serifs

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Printing Terminology

• Kerning
• Spacing between letters for proportional spaced
  typefaces
• e.g., tw closer together than op because it
  looks better
• Descenders
• Characters which have parts below the normal
  printed line, e.g., y and g

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Printing Terminology, HSI

• Chromaticity diagram
• Any color characterized by its H, S, and I
• Mathematically convertible to other
  representations, e.g., RGB, CYMK
• Hue
• Commonly referred to as color
• Saturation
• How red the color red is
• Intensity
• How bright the color is

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Printing Terminology

• Resolution Measured in Dots Per Inch, dpi
• 75 minimal
• 300 LaserJet
• 1440 Professional print quality

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Creating Color

• Two Methods for Creating a Particular Hue
• Multiple layers require transparent inks
• Color also depends on medium which it is applied
  to
• Adjacent
• Different colored dots closer together than the
  eye can resolve appear to be single color
• Dithering

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Additive Color

• Mechanism used on TV, color monitors
• Red, Green, and Blue
• All colors added together produce white

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Subtractive Color

• Control what color is reflected
• White all colors reflected
• Red only red is reflected, all other colors
  absorbed (subtracted)
• Black no colors are reflected

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Printing Color, CYMK

• Color printing uses 4 pigments
• Cyan (blue-green)
• Yellow
• Magenta (purple-red)
• blacK
• Fixed dot size
• Lighter shades are produced by leaving more dots
  white (unprinted)

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Printing Color
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Dithering

• To Produce Colors Beyond Simple Mix of Primary
  Colors (8 Colors), Specific Combinations of
  Yellow, Cyan, and Magenta Dots Are Used
• Dots Are Not Placed in Same Spot, but Close
  Together (Dithering)
• Eye Blends Discrete, Separately Colored Dots Into
  Single Color

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Color Dithering
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Paper Characteristics

• Uncoated paper
• Rough surface, scatters light
• Blurs image
• Inks bleed into paper
• Coated paper
• Smoother surface reflects light more precisely
• Coating prevents color bleed
• Thickness measured by weight in pounds

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Printer Classification

• Line Printers
• Character based
• One line of characters sent at a time
• May be printed by one row of dots at a time
• May have graphics mode
• Page Printers
• Entire page sent to printer
• Printer rasterizes page in its memory
• Page printed one line at a time

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Font Formation

• Bitmapped Fonts
• Bit pattern stored in ROM or downloaded from host
  computer into printer
• One pattern for each character of each typeface
  of each attribute of each sizelot of storage
• Outline (vector) Fonts
• Mathematically described
• Scalable
• Bold facing, italics
• Easier conformance to various print densities

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Bitmapped Fonts

• For each letter/symbol define exactly which dots
  within a defined matrix (40x40 grid) are used
  (dark)
• Most printers include some bitmapped fonts (e.g.,
  Courier and Line Printer) in ROM
• Adding fonts
• Plug-in Cartridge w/Fonts in ROM
• RAM within printer for font map download

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Bitmapped Fonts

• PC Notifies Printer Which of Bitmapped Tables
  Should Be Used to Represent Letters to Be
  Displayed
• PC Sends Text Encoded in ASCII
• A (65)10 041H
• Bitmapped Table Defines Exactly How to Form Dots
  to Create Letter

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Laser Printer (1)

• Laser beam forms bit pattern on rotating drum by
  causing charge to leak off where illuminated
• Exact position on drum is set by spinning mirror
  which redirects beam along horizontal line.
• As exposed portions of drum rotate further, they
  contact black powder (toner).
• Electrically charged toner particles are
  attracted to dots on drum written by laser

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Laser Printer (2)

• Drum contacts paper and electrical charge on
  paper transfers toner to paper
• Drum passes corona wire to returns entire
  surface of drum to negative electrical charge
• Paper passes to fuser.
• Fuser uses heat and pressure to melt and press
  wax within toner into paper

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Laser Printer (3)
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Color Laser

• Single-pass, Multi-issue
• Solid-ink
• Dye-sublimation
• Multi-pass
• Thermal-wax
• Heat transfers wax to wide ribbon then to paper
• 4-pass color laser
• 4 printer engines, each applies own toner to
  cartridge

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Color Laser Printing

• Laser Beam Writes Pattern on Electrically Charged
  Drum
• CYM or K Is Picked up From Cartridge on Each
  Pass and Deposited on Transfer Belt
• When All 4 Toners Have Been Placed on Transfer
  Belt, It Is Passed Over Paper
• Fuser Melts/presses Toner Into Paper

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Tandem Color Laser

• Alternative to CYMK Cartridge
• Use Separate Lasers and Drums for Each Color
  Which Allows All Colors to Be Applied to
  Secondary Belt at Once

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Color Ink-Jet

• Inexpensive
• Slow
• Simultaneous Issue of 4 Colors
• Colors can run if paper gets wet

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Color Ink-Jet Printers(1)

• Print head, including Ink filled cartridge moves
  horizontally across paper surface
• Each of 4 cartridges (CYMK) has 50 ink-filled
  firing chambers

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Color Ink-Jet Printers (2)

• Ink Flows Into Firing Chamber
• Quickly Heated Ink Vaporizes Creating Bubble
• Vapor Bubble Pushes Ink Out Nozzle
• Droplet Is Propelled Toward Paper by Bursting
  Bubble
• Removal of Heat Collapses Bubble and Pulls More
  Ink Into Firing Chamber.

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Color Ink-Jet Printers (3)
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Solid-Ink Color Printers (1)

• Example Tektronix Phaser 350
• Ink Solid at Room Temperature Distributed in 4,
  Wax-like Uniquely Shaped Blocks
• Printer Melts Ink Into Reservoirs in Print Head
• Base of Nozzle Contains Piezoelectric Membrane
• To Fill Nozzle, Piezoelectric Membrane Wall
  Extends Out, Forcing Ink in
• Amount of Ink Controlled by Amount of Flexure

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Solid-Ink Color Printers (2)