Laser printers as well as
LED printers rely on one and the same technology used at first in photocopying machines. This process is known as
electrophotography and was invented in 1938 and developed by
Xerox and
Canon in the later 1980s.
Electrophotographic process in laser printers, involves six basic steps:
A
photosensitive surface (
photoconductor) is uniformly
charged with static electricity by a
corona discharge. Then the charged photoconductor is
exposed to an optical image
through light to discharge it selectively and forms a
latent or invisible
image.
Development is done by spreading
toner, a fine powder, over the surface, which adheres only to the charged areas, thereby making the latent image visible. At the next step an electrostatic field
transfers the developed image from the photosensitive surface to a sheet of paper. Then the transferred image is fixed permanently to the paper, by
fusing the toner with pressure and heat. The last step is
cleaning of all excess toner and electrostatic charges from the photoconductor to make it ready for next cycle.
Laser printers as well as
LED printers offer the
best print quality i.e. the highest
resolution. The unique difference is the method of exposition or formation of the latent image.
Laser printer scanning assembly:
Laser printers rely on a
laser beam and
scanner assembly to form a latent image on the
photo-conductor bit by bit. The
scanning process is similar to electron beam scanning used in CRT.
The laser beam
modulated by electrical signals from the printer's controller is directed through a
collimator lens onto a rotating
polygon mirror (
scanner), which reflects the laser beam. Then reflected from the scanner laser beam pass through a
scanning lens system, which makes a number of corrections to it and scans on the photoconductor.
This technology is the major key for ensuring high precision in laser spot at the focal plane, accurate dot generation at a uniform pitch and therefore better printer's resolution.
Laser printer scanning process: the latent image creation
LED printers,developed by
Oki and
Panasonic, use an
array of small
Light
Emitting
Devices to form the latent image, hence no scanner is required. Thus LED technology offers some potential advantages over laser one. However, several problems are related to this method. A main problem is the integration of the
2400 diodes (A4 format with 300 dpi) into a staggered or single row, along with
lens arrays for focusing the LED emission onto the photosensitive surface.
The image scanning process is similar to that in
line dot matrix printers as shown bellow.
The LED printer scanning process
Laser printers as well as LED printers are the right solutions for applications demanding high quality with high-speed output of images and text data, such as desktop publishing.
