Specifications

Figure 2 The stages of laser imaging with a typical laser printer are shown here.
The laser assembly in a laser printer, sometimes called a raster output scanner (ROS), is used to create
an electrostatic pattern of dots on a photosensitive drum (called the photoreceptor) that corresponds
to the image stored in the page buffer. The laser assembly consists of the laser, a rotating mirror, and a
lens. The laser always remains stationary. To create the pattern of dots across the horizontal width of
the drum, the mirror rotates laterally, and the lens adjusts to focus the beam so the dots on the outer
edges of the drum are not distorted by having been farther from the light source. The vertical motion
is provided by the slow and steady turning of the drum.
Caution
Because the drum is sensitive to any form of light, it should not be exposed to room light or daylight for extended periods
of time. Some printers have a protective mechanism that shields the drum from exposure to light whenever you open the
printer’s service compartment. Even when this is the case, however, you should leave the compartment open only long
enough to service the printer or change the toner cartridge.
The photoreceptor drum, which in some printers might actually be a belt, is coated with a smooth
material that holds an electrostatic charge that can be discharged on specific areas of its surface by
exposure to light. The initial charge over the entire surface of the drum can be applied by either a
charger corotron or conditioning rollers. A corotron is a wire carrying a very high voltage that causes the
air immediately around it to ionize. This ionization charges the drum’s surface and also produces
ozone, the source of the smell that is characteristic of laser printers. Most recent laser printers use
charged rollers instead of corotrons specifically to avoid the production of ozone.
Note
Some laser printer manufacturers, such as HP, refer to coronas instead of corotrons. They perform the same functions.
ABC
Drum
Paper
Toner
Developer (spreads toner
across drum)
Charger corotron (corona wire)
(applies charge to drum)
Discharge lamp (erases drum surface)
Fuser rollers (melts toner to paper)
Detrac
corotron
(cancels charge
on paper)
Transfer corotron
(transfer corona)
(applies charge to paper)
Laser beam
Rotating mirror
Laser
Laser beam (writes page to drum
by discharging portions of drum surface)
ABC