the printed image.EPis also used to represent electrostatic photocopying,
which is another name for this same process.
Hewlett-Packard (HP) process The HP process is essentially the same as
the EP process; it differs from the EP process only in some minor operating
procedures. It’s similar enough to be considered the same process, yet
different enough to get its own name.
Light-Emitting Diode (LED) process LED printers are not technically laser
printers, but outwardly you can’t really tell an LED printer from a laser printer.
An LED printer uses an array of around 2,500 light-emitting diodes (like very
small light bulbs) in place of a laser as the light source used to condition the
photosensitive drum.
Liquid Crystal Display (LCD) process LCD printers use light shone through
an LCD panel in place of the laser to condition the photosensitive drum. See
Chapter 16 for more information on how liquid crystal works, but in general,
liquid crystals are used to block and permit light to pass through onto the
drum creating a pattern of pixels or dots on the drum. LCD printers are also
called LCD shutter printers.Electrophotographic Process (EP)
Theelectrophotographicprocess(EP)usedinthelaserprinterhasitsrootsinthedrypho-
tocopy method called xerography. There are actually two methods used in photocopiers,
wet and dry. The wet method uses liquid inks and the dry method uses a dry, granular
ink powder, more commonly known as toner.
Closely aligned with the Xerox Corporation, xerography roughly translates to dry
writing and is the name for a photocopying process used in nearly all laser printers and
all dry photocopiers. Xerography is ideal for the laser printer because it requires no liquid
inks or special paper (e.g., those used with a thermal copier). This process relies on the
fact that some substances become electrically charged when exposed to a light source.
In the EP process, as xerography is called when applied to a laser printer, the printer’s
drum, which is made from selenium or another photosensitive (light sensitive) material,
is charged. The print image file generated on the computer by an application is used to
create a logical image of the document to be printed. This image is then used to guide the
laser and mirrors to shine light on the drum in the areas where no part of the image,
whether text or graphic, is to appear on the finished document. Where light strikes the
drum, it loses its charge.
Next, negatively charged toner is sprayed on the drum and adheres to the parts of the
drum that still has a positive charge. This creates a mirror image of the document on the
drum. A sheet of paper is fed from the paper supply, positively charged, and fed closely
past the drum and toner. The positive charge on the paper attracts the toner onto the pa-
per, and with only the positive charge holding the toner in place, the sheet is fed through
a set of heated rollers that literally melt the toner onto the paper to create a permanent
document. If multiple copies of the same document are being printed, then additional
Chapter 17: Printers^425