Chapter 17: Printers^431
very common. Heavy-duty workgrou plaser printers can offer u pto 2400d pi, but these are
normally outside the price range of most home or small office users. A 600dpi laser printer
offeringstandardpaperwidths(8.5inches)usesover5,000dotsineachrowonthedrum.
A color laser printer must image each of its colors separately. This is why color laser
printers have two page-per-minute (ppm) ratings: one for monochrome and one for
color. The color ppm rating will always be the slower of the two. A laser printer may have
a 16ppm rating for monochrome but only 3ppm for printing color documents.
Most color printers use the four CMYK colors (cyan, magenta, yellow, and black) to
create its color palette, and for each color used in a document, a complete print cycle must
be completed. That is, for each color, the drum is written, the controller directs the correct
color toner to be applied, the partial image is transferred to the paper, and the excess
toner is removed. The paper actually makes as many as four passes around the drum to
collect each color layer of the image. The fusing process is performed only once on the
page, after all of the colors have been applied.
Hewlett Packard uses what it calls aone-passsystem. In this system, each layer of
toner is applied to the drum before the full-color buildup is transferred to the paper. For
each color in the image, the drum completes a complete cycle (except that there is only
one conditioning phase). After all of the colors to be used are added to the drum, the paper
passes the drum for a single transfer phase.One-transferwould be a much more descrip-
tive name for this technique.
TheadvantageoftheHPone-passprocessisthatcolorregistrationissuesarevirtually
eliminated. As each layer of the color is applied, the paper must be kept in exact registra-
tion to the drum for each pass so that the dots that must be adjacent to one another actu-
ally are. Should the paper become even slightly misaligned as it is passed around the
drum multiple times, the color layers may be overlaid, produce the wrong color or shade,
or distort the image.
High-end color laser printers use a belt to which the toner from each color layer is
transferred. After all of the colors are on the belt, the toner is transferred to the paper and
fused. The use of a belt ensures that any paper registration problems are eliminated.
Building Up the Image
The light source (whether laser, LED, or LCD) of a “laser” printer can create millions of
dots on the print drum. These dots are then coated with toner, and the toner is transferred
and fused to a sheet of paper. The challenge of color laser printing is creating millions of
colors and shades using only four CMYK colors.
Two main color printing technologies are used in color laser printing:
Bi-level This basic color technology provides no control of the intensity of a
color. Each color dot is either on or off. The color is either there or it’s not; there
is no in-between shading. Dithering, explained earlier in the chapter, extends
the bi-level process to create transitions between colors and place color in adjacent
or neighboring dots to create color visuals. (See the “Color Halftoning” discussion
in the “Inkjet Printing Process” section of this chapter.)