electrophotographic

The printing technique used in laser and LED printers and most copy machines. 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, and the toner is then attracted to the paper, 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.

Xerography Was Introduced in 1948
In 1938, electrophotography was invented by Chester Carlson in Queens, New York, and his first of 28 patents was issued in 1940. By 1947, Haloid Corporation and Batelle Development Corporation were working with Carlson, and xerography became official in 1948. A huge breakthrough, it replaced the messy liquid ink of the duplicating machines of the day with a dry, granular ink. In Greek, 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.

"Painting" the Drum

The laser printer uses a single light directed by moving mirrors, but there are more lenses and parts than are shown here. The LED printer has a stationary array of thousands of tiny LEDs that are selectively beamed onto the drum.

The Model A - 1949

The first Xerox copier was manually operated, but it provided the experience and revenue to develop the automatic, floor-standing Xerox 914 ten years later. The 914 was a huge success. (Image courtesy of Xerox Corporation.)

The 1938 Experiment Replicated in 1965

With india ink, Carlson wrote "10-22-38 ASTORIA" on a glass slide and rubbed a sulphur-covered zinc plate with a handkerchief to apply an electrostatic charge. He put the slide on the plate, exposed it to light with the room dark, 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 successful, Carlson was showered with honors and wealth. At age 62 in 1968, he died of a stroke on a New York street. (Image courtesy of Xerox Corporation.)