Functioning of Laser printer

The above diagram shows different functional blocks of a laser printer.

The basic approach of laser printer is to, first, form an image of the page that is to be printed on a photosensitive drum in the machine. Powdered ink or "toner" is then applied to the image on the drum. Next, the image is electrostatically transferred from the drum to a sheet of paper. Finally, the inked image on the paper is "fused" with heat.

In case of color laser printers, this process occurs four times, one each for the Cyan, Magenta, Yellow and Black components of the image.

Laser : It is a device through which a laser beam is emitted.

Mirror : Mirror acts as an acoustic-optical deflector which is inserted between the laser and the mirror system used to switch ON and OFF the light beam

Light beam modulator : It is inserted between the laser and the mirror system used to modulate the light beam. It is controlled by the end generator.

Rotating drum with multiple mirrors : This drum is rotated in synchronization with the photosensitive drum so that the light beam is able to scan the whole surface of the drum.

Photosensitive drum : This drum is used to create an image of light beam which is then created to ink image by spreading the toner on the surface of the drum.

Working mechanism : Laser is used for creating image on a photosensitive drum. This photosensitive drum is initially charged by charging electrodes before being exposed to a coherent monochromatic light beam from laser. The light beam scans parallel to the axis on the photosensitive drum using a rotating polygonal mirror. As the beam hits one face of the mirror, the movement of the mirror causes one scan across the drum. The drum and mirror system are continuous revolving in synchronism so that the beam scans the whole surface of the drum. The light beam is switched ON and OFF by an acoustic-optical deflector which is inserted between the laser and the mirror system. The light beam selectively discharges area on the drum and then toner material is spread over the surface to form an ink image. This is then transferred to a roll of paper and finally made permanent by heat.
Depending on the area to which toner is attracted, laser printers are classified into 2 types :

Write white : In write white laser, the laser writes a negative image to the drum and the toner is attracted to the areas untouched by the laser beam.

Write black : In write black laser, the toner is attracted to the areas touched by
the laser beam.

Advantages : 

(a) Letter quality is excellent. 

(b) Speed is high. 

(c) Supports text and graphics. 

(d) No mechanical motion is involved so it is quite in operation.

Disadvantages :

(a) Cost is more. 

(b) Maintenance should be done by an expert, preferably an engineer.