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					Pixels, PPI, DPI, and LPI
  for Scanning, Printing,
     and Web Publishing
         Digital Media
         Spring 2003
A Pixel
 A pixel on your computer monitor is
  a point of light
 What is the maximum resolution of
  your monitor?
       Probably 1024 X 768
   How many pixels per inch is this?
       ~ 91 ppi
   What is standard for computer
A Pixel
   A pixel in an image file is a piece of data.
   It could be represented in 1 bit, 1 byte, or
    3 bytes
   How many colors can you represent if a
    pixel is
       1 bit
         • 21 = 2
       1 byte
         • 28 = 256
       3 bytes
         • 224 = ~16 million
 Resolution of your computer screen
  is measured in pixels per inch (ppi).
 Resolution of an image file is
  measured in pixels per inch.
 What’s the relationship?
       There’s a one-to-one relationship between pixels in the
        image file and pixels displayed on the computer screen.
        However, this does NOT mean that the picture, when it
        is printed out, will be the same size as the picture
        displayed on the computer screen.
A Dot
   Resolution of a scanner is measured in
    dots per inch (dpi). This is essentially the
    same as pixels per inch (ppi) in the sense
    that it is pieces of information per inch.
   If you scan at a resolution of 72 dpi (or
    ppi) you’re getting 72 pixels of
    information about the picture in each
    direction for each inch of the picture.
    (5184 pixels per inch2)
A Dot
   The resolution of a printer is measured in
    dots per inch (dpi).
   Each dot is a spot of color output by the
   The dot is the smallest dot of color the
    printer creates.
   Your printer is probably 300 dpi, 600 dpi,
    or 1200 dpi. Note that this is really dots
    per inch in both the x and y directions.
Inkjet Printers
   The dot of ink sprayed out by an inkjet
    printer is wet. One dot blends into
   In general, when you’re scanning a
    picture to be printed out on an inkjet
    printer, a resolution of no more than 1/3
    the resolution of the printer is sufficient.
   Thus, for a 600 dpi inkjet printer, 200 dpi
    resolution should be enough.
Laser Printers, Halftone
Dots, and LPI
 A dot is the smallest printable unit
  for a laser printer.
 A line is created from a group of
  dots. The line is a series of half-
  tone dots. (p. 59, Avoiding the Scanning Blues, p. 87, The Non-
    Designer’s Scan and Print Book.)

   Thus, laser printers are
    characterized by the lpi at which
    they print as well as the dpi.
 Half-Tone Dots

Each line printed out
by a laser printer is a
series of half-tone
dots. For a grayscale
picture, a half-tone
dot looks more or less
gray depending on
how many primitive
dots it is composed of.

From p. 59, Avoiding the Scanning
Blues by Taz Tally, Prentice-Hall
Publishers, 2001.
Laser vs. Inkjet Printers
   Laser printers use AM screening
    (amplitude modulation). That is, the size
    of the halftone dot changes to change the
   Inkjet printers use FM screening
    (frequency modulation). That is, all dots
    are the same size, but the number of dots
    in the picture changes.
(p. 44, Avoiding the Scanning Blues)
Common DPI and LPI for
Laser Printers
   300 dpi    60 lpi (5 pixels2 per
               halftone dot)

   600 dpi    85 lpi (~7 pixels2
               per halftone dot)

   1200 dpi   110 lpi (~11 pixels2
               per halftone dot)
Resolution When Printing
From a Laser Printer
Experience shows that a good rule of
 thumb is to scan at a resolution that
 is 1.5 to 2 times the lpi of the laser
 printer you’ll be printing on.

   e.g., for a 600 dpi laser printer, scan
    your picture at about 130-170 ppi

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