What is Xerography?

Xerography is a photocopying technology that uses light and electric charge to copy images onto another sheet of paper. When it was invented in 1937, the process, then known as electrophotography, took a lot of manual attention to operate and generated very little interest. Today, however, xerography is easy to use and found in printers, fax machines, and the photocopiers that can be found in almost any office.

Modern photocopiers are automated so that the user simply has to insert the image he wants copied, close the lid, press "start," and wait for a copy to be made. Inside, however, there is a complex process taking place. Most xerography machines have a plate, band, or cylindrical drum that is coated with a photoconductive material such as amorphous selenium. Photoconductors are substances that become more electrically charged when exposed to light. A mechanism in the photocopier called the Corona Unit evenly applies a positive or negative charge to the drum.

A beam of light is then shone onto the image that is being copied. The image reflects the light onto the drum, projecting more light from the whiter parts of the image and no light through the darker parts. This creates a reverse picture on the drum, with the whiter parts composed of excited particles due to the light exposure and the darker parts composed of less excited particles. This is called a latent image.

A mixture of toner and carrier is then applied to the drum. The carrier particles have an opposite charge to the excited particles, or the light exposed area, in the latent image. The toner, a powder that provides the color in the xerography machine, sticks to the carrier particles, where the dark parts of the original image would be. A paper is then placed between the drum and a charged plate called a transfer corona. Using a heat roller and the electrical attraction of the transfer corona, the toner is lifted off the drum and pressed into the paper, creating the photocopied image.

Xerography was first invented by Chester Carlson, a patent attorney living with his family in Queens, NY. He and his assistant, Otto Kornei, developed the first, rudimentary xerography machine by manually applying the electrostatic charge, the light, the powder, and the pressure needed to make a photocopy. When they finally succeeded, however, it took Carlson another ten years to garner corporate interest. In 1944, Carlson won an agreement with Haloid, now known as Xerox, who was able to turn xerography into useful, commercial products. The product met with wild success after it was released in the form of an automated office photocopier in 1959.

Today, many other companies and products use xerography. The technology is used in both laser and LED printers as well as facsimile machines, more commonly known as fax machines. The product can have a band or plate instead of a drum and often uses organic photoconductors (OPCs) instead of the amorphous selenium originally used by Xerox. Depending on the age and make of the machine, it may only be useful for black and white printed writing and basic images. Newer machines, however, can create high quality, complex images in black and white or color.