A laser beam projects an image of the page to be printed onto an
electrically charged rotating drum coated with selenium or, more common
in modern printers, organic photoconductors. Photoconductivity allows
charge to leak away from the areas exposed to light. Dry ink (toner)
particles are then electrostatically picked up by the drum's charged
areas, which have not been exposed to light. The drum then prints the
image onto paper by direct contact and heat, which fuses the ink to the
paper.
Laser printer speed can vary widely, and depends on many
factors, including the graphic intensity of the job being processed. The
fastest models can print over 200 monochrome pages per minute (12,000
pages per hour). The fastest color laser printers can print over 100
pages per minute (6000 pages per hour). Very high-speed laser printers
are used for mass mailings of personalized documents, such as credit
card or utility bills, and are competing with lithography in some
commercial applications.[1]
The cost of this technology depends on a
combination of factors, including the cost of paper, toner, and
infrequent drum replacement, as well as the replacement of other
consumables such as the fuser assembly and transfer assembly. Often
printers with soft plastic drums can have a very high cost of ownership
that does not become apparent until the drum requires replacement.
Duplex
printing (printing on both sides of the paper) can halve paper costs
and reduce filing volumes. Formerly only available on high-end printers,
duplexers are now common on mid-range office printers, though not all
printers can accommodate a duplexing unit. Duplexing can also give a
slower page-printing speed, because of the longer paper path.
In
comparison with the laser printer, most inkjet printers and dot-matrix
printers simply take an incoming stream of data and directly imprint it
in a slow lurching process that may include pauses as the printer waits
for more data. A laser printer is unable to work this way because such a
large amount of data needs to output to the printing device in a rapid,
continuous process. The printer cannot stop the mechanism precisely
enough to wait until more data arrives without creating a visible gap or
misalignment of the dots on the printed page.
