How Laser Printers Work

Laser printers ware invented at XEROX in 1969 by by researcher Gary Starkweather.Laser Printers are digital printing devices that are used to create high quality text and graphics on plain printer.

Working Principle
1)A laser beam projects an image of the page to be printed onto an electrically charged rotating Photo sensitive drum coated with selenium.
2) Photo conductivity allows charge to leak away from the areas which are exposed to light and the area gets positively charged.
3)Toner particles are then electrostatically picked up by the drum’s charged areas, which have  been exposed to light.
4)The drum then prints the image onto paper by direct contact and heat, which fuses the ink to the paper.


Working Mechanism
Conversion of image from PC to Printer involves following Procedures
1)Raster Image Processing
2)Charging of Photosensitive Drum
3)Exposing
4)Developing
5)Transferring
6)Fusing
7)Cleaning

Raster Image Processing

Creation of image to be printed is done by Raster Image Processor(RIP) which may be encoded in any number of special page description languages such as HP Printer Command Language (PCL), or Microsoft Open XML Paper Specification (OpenXPS). Raster image processor generates the bitmap of final page in raster memory.



Charging of Photosensitive Drum
A corona wire positioned parallel to the drum or primary charge roller, projects an electrostatic charge onto the  photosensitive drum, which is capable of holding an electrostatic charge on its surface while it is in the dark.An AC bias is applied to the primary charge roller to remove any residual charges left by previous images. The roller will also apply a DC bias on the drum surface to ensure a uniform negative potential.There are two main types of photosensitive drum; ‘OPC (Organic Photoconductor) Drum’ and ‘A-Si (Amorphous Silicon) Drum’ ,that are used in laser printers.



Exposing

A laser beam is projected at a rotating polygonal mirror,which directs the laser beam onto the optical drum.the laser beam converts blank points of image to positive charge.,leaving a static electric negative image on the photo sensitive drum surface to lift the toner particles.The stream of rasterized data held in memory turns the laser on and off to form the dots on the cylinder.



Development
The latent image surface is now bring in contact with toner,which contains fine particles of dry plastic powder mixed with carbon black.These particles are now given negative charge.when photosensitive drum come in contact with toner,the toner particle get struck on the drum to area which are exposed by laser beam and are positively charged.

TransferringThe Photosensitive drum is than rolled over the paper and the image is transferred to the Paper.This paper is than feeded to next assembly.
FusingThe Printed paper is passed through fuser assembly.This assembly passes the paper at 200 Degree Temp.The main purpose of this assembly is to dry up the ink and make it permanent on the paper.
CleaningOn Completion of Print, an electrically neutral soft plastic blade cleans  excess toner from the photo Sensitive drum and deposits it into a waste reservoir, and a discharge lamp removes the remaining charge from the photo sensitive drum.

This whole process of printing is shown below.