electrophotographic

The printing technique used in copy machines, laser and LED printers. It uses electrostatic charges, dry ink (toner) and light. A selenium-coated, photoconductive drum is positively charged. Using a laser or LEDs, a negative of the image is beamed onto the drum, cancelling the charge and leaving a positively charged replica of the original image.

A negatively charged toner is attracted to the positive image on the drum. The toner is then attracted to the paper, which is also positively charged. The final stage is fusing, which uses heat and pressure, pressure alone or light to cause the toner to permanently adhere to the paper.

Back to the Late 1930s
Electrophotography was invented by Chester F. Carlson in his Queens, New York laboratory in 1938. His first of 28 patents on the subject was issued in 1940. By 1947, the Haloid Corporation in conjunction with Batelle Development Corporation were working with Carlson on his invention, and xerography was officially announced in 1948. Xerography replaced the messy liquid ink of the duplicating machines of the day with a dry, granular ink, which is how the name came about. In Greek, xerography means "dry writing." Xero means "dry," and graphy means "write."

Electrophotographic Process

Electrostatic charges are used to create a charged light image on the drum. The toner is attracted to the drum and then to the paper.

Laser and LED Printers

The difference between a laser and LED printer is how the light image is "painted" onto the drum. The laser uses a single light source that is directed by moving mirrors. The LED printer uses an array of hundreds of LEDs that are selectively beamed onto the drum. In a real laser printer, there are many more lenses and parts than in this simple diagram.

The Model A

The first electrophotographic copier was sold in 1950. Although manually operated, it provided the experience and revenue to develop automatic xerographic machines. In 1959, Xerox's 914 copier became a huge success. (Image courtesy of Xerox Corporation.)

Chester Carlson

In this 1965 photo, Carlson enacts his 1938 experiment in which he wrote "10-22-38 ASTORIA" with india ink on a glass slide. The room was darkened and a zinc plate, covered with sulphur, was rubbed vigorously with a handkerchief to apply an electrostatic charge. He put the slide on the plate, exposed it to light for a few seconds, removed the slide and sprinkled lycopodium powder on the plate. He gently blew off the loose powder and what remained was the first electrophotographic copy. After Xerox became very successful, Carlson was showered with honors and wealth. In 1968, he died of a stroke on a street in New York. (Image courtesy of Xerox Corporation.)