Chapter 7 Digital Prepress 133
Ink-jet printers
Ink-jet printers form images by using a print
head that projects tiny droplets of ink onto the paper
surface and provide a resolution of 300 dpi or more.
Positioning the droplet is carefully controlled. Ink-
jet printers are often used to make color proofs of
graphics and page proofs to show a client, as a color-
accurate representation of the fi nal printed product,
Figure 7-9.
Laser printers
Laser printers operate much like a photocopy
machine. A photocopy machine uses refl ected light
to create an image on a drum, while a laser printer
uses a laser beam to create an image on the drum,
Figure 7-10. The laser printer has a print engine that
translates the output of the computer into a bitmapped
image for printing. A laser transfers the page image
to a light-sensitive drum that has a positive electrical
charge. As the laser light moves across the rotating
drum, it emits the image drawn from printer memory.
The polarity of the drum changes in the areas where
the laser has transferred the image to be printed.
Toner is a positively charged powder that is attracted
only to the negatively-charged areas on a page to
create an image. The paper with toner applied
passes between heated rollers that fuse the powder
onto the paper and produce a permanent image.
Laser printers can produce images of 300 dpi and
higher, and are typically available in both color and
black-only models.
Imagesetter and Computer-to-Plate Devices
When high-resolution images must be output for
commercial printing, an imagesetter or computer-
to-plate (CTP) device is used, Figure 7-11. A digital
Figure 7-9. An ink-jet printer can be used to produce
a high-quality proof copy of graphics or page layouts.
Output is relatively slow when compared to laser
printers. (Epson America, Inc.)
Manual paper
feed
Image charged on
rotating drum
Image
output
Heated rollers
melt toner powder
onto paper
Toner or
powder clings to
charged paper
Cassette
paper feed
Figure 7-10. A laser printer produces an image by
a scanning technique combined with electrostatic
principles. Rollers feed paper through the printer at a
constant speed. The paper source may be a cassette (tray)
for multiple sheets, or a manual feed for single sheets.
WYSIWYG: A monitor display method used by word
processing and page layout programs, in which the monitor
displays a RGB representation of the printed output.
dot pitch: A measurement of the vertical distance between
rows of pixels on a monitor, stated in decimal fractions of a
millimeter.
additive color formation: Theory based on mixing red,
green, and blue light in various combinations to create a
color reproduction or image.
subtractive color formation: The combination of cyan,
magenta, yellow, and black inks to produce a printed image.
color management system (CMS): An electronic prepress
tool that provides a way to correlate the color-rendering
capabilities of input devices, color monitors, and output
devices to produce predictable, consistent color.
WYSIWYP: A monitor display method used by word
processing and page layout programs that uses color
management software to produce a CMYK representation
of the printed output.
print engine: A small computer component inside a laser
printer that translates the output of the computer into a
bitmapped image for printing.
toner: Positively charged powder that is attracted to negatively
charged image dots to make up the printed image on a page.