How Does a Laser Printer Work

A laser printer is one of the most ingeniously designed electronic devices, whose working is based on the xerography or electrophotography. If you have always wondered how do laser printers work to provide neatly printed pages, this article will be an interesting read. Here I provide an explanation of the working of a laser printer in a nutshell.
Techspirited Staff
Laser printers can print out pages faster than older dot matrix and inkjet printers. Besides outclassing other printer types in terms of speed, it also beats most in terms of printing accuracy, with each word and graphic precisely printed. One wonders how a laser light, which is a monochromatic beam of light facilitates printing in these machines. In this Techspirited article, I have demystified the working of a laser printer for you. By the time you are through this article, you will appreciate this marvelous electronic contraption even more.
About Laser Printers
Gary Starkweather invented the laser printer in 1969, while working at Xerox. He used the principle of xenographic printing to develop this high speed printer which is rapidly capturing the market today. The first commercially functional laser printer was IBM model 3800, which was as big as a room. As technology improved, the laser printer kept evolving to become considerably smaller in size, more accurate and faster in printing pages. The technology that once cost thousands of dollars is available for less than a $100 today, with portable laser printers being the premier printing choice in most business establishments. Let us see how laser printers can print as many as 200 pages per minute accurately.
How Do Laser Printers Work?
The only scientific principle you need to remember, in order to understand the working of a laser printer is 'Like charges repel each other and unlike charges attract each other'. Let us understand the process of monochrome laser printing stepwise in the following lines. You load blank pages in a laser printer and give the print command on your computer, to find an accurately printed (warm) page emerging from the other side of the printer within seconds. Here's what happens inside the printer in those few seconds.
Laser Printers Work
Step 1: Printer Controller Receives Page Data & Creates Raster Image
As you provide the print command, the computer encodes the data using 'Printer Command Language', 'Adobe Postscript' language or 'XML Page Specification' to describe the entire page in terms of vector graphics. This information is received by the 'Printer Controller'. It reads the data, arranges the page according to specifications and then orders the 'Raster Image Processor' embedded within the printer to convert it into a bitmap or raster image. This image is temporarily stored in the printer memory, after which actual printing process begins.
Step 2: Rolling Photoreceptor Drum is Positively Charged
Central to the working of the laser printer is a rolling photoconductive drum, which can hold charge on its surface until exposed to light, which makes it discharge. Light photons incident on the drum's surface improve the conductivity in that region to make it locally discharge in that area. You may interpret this as photons erasing the stored charge in incident region. Using a high voltage corona wire, the drum is either positively or negatively charged. In the course of this discussion, let us assume that the drum is positively charged.
Step 3: Laser Draws an Electrostatic Image of Page on Photosensitive Surface
This rolling drum is then exposed to light output of the laser. Using a complex system made up of mirrors and lenses, the laser 'draws' the bitmap image on the surface of the drum. According to the data fed to the raster processor, the laser photon stream hits the moving surface of the photosensitive drum. The region where photons hit the drum gets discharged creating a net negative charge on the surface of the drum. Part by part, the entire bitmap or raster image is etched onto the drum in the form of a negative electrostatic image! Imagine a glass windows surface coated with dust. Just as you can 'draw' on the window by wiping off dust from the glass surface with a finger, so does a laser draw an image on the drum by wiping off positive charge. Once the entire image is drawn, the drum rolls further.
Step 4: Positively Charged Toner Particles Are Embedded in Negative Regions
Along the drum path, a toner developer is placed, which subjects the surface to come in contact with 'positively charged' toner particles. The toner is a dry powder made up of pigment and a plastic polymer. Since unlike charges attract and like charges repel, the positively charged surface of the drum doesn't pull any toner particles. Only the negatively charged (discharged) region on the drum, which makes up the page image attracts or pulls toner particles to the surface. Thus toner particles get embedded on the drum, right over the embedded electrostatic image of the page!
Step 5: Paper Passes Over Drum to Print Image
Now this toner embedded drum surface comes in contact with negatively charged paper. As the paper surface comes in contact with the drum, only the positively charged toner particles stick to the paper, creating an exact image of the page (That's right, because unlike charges attract)! Now the paper rolling out has toner particles attached to it.
Step 6: Heated Rollers Fuse Toner on Paper
This page is then passed through hot Teflon coated rollers which melt plastic in the toners, to make it stick on paper, providing us with a printed paper that is the exact physical copy of the soft copy on computer! It then rolls out, ready to be picked up and used for whatever purpose it is printed.
Thus, using xenographic printing technique, employing a laser that etches an 'electrostatic image' of a page on a positively charged photoreceptor drum and the precise attachment of charged toner particles in the discharged regions, a laser printer provides crisp printed pages at a phenomenal speed! As a laser printer vs. inkjet printer comparison will reveal, a laser printer is substantially fast compared to its competitor, due to the inherent advantages of inbuilt technology. While inkjet printer must spray ink, a laser printer lets toner get attached only in positively charged regions of the paper, making printing much easier and accurate. Hope this short write-up explaining the working of a laser printer was as interesting to read, as it was to write about! A laser printer's working is an excellent demonstration of what a combination of simple scientific principles applied together can do for you.