How Does a Photocopier Lens Work?

The Machine

Photocopiers are the latest in a long line of office machines designed to make small numbers of copies. The photocopier was the brainchild of Chester Carlson who used the new concept of "photoconductivity"---the ability of light to "excite" electrons to develop a kind of printing process. An image of the colored part of the picture was shot through a lens to a plate where negatively charged ions attracted a medium comprised of fungus spore. The lens focused the bright light, exciting a layer of photo-reactive sulfur on the plate. The negative image then was attracted by a positively-charged "corona wire" to a piece of paper, producing a positive copy.

Copier "bed" with scanning arm (center) traveled by light, mirror and lens--control cable is at top.
The Haloid Corporation of Rochester, New York, adopted Carlson and his invention. Together, they replaced the sulfur plate coating with selenium and developed a rolling drum to replace the copier's flat platen. The inky mold spore was replaced with media composed of iron powder (for color) and ammonium chloride salt (for clarity). They also suspended the "toner" in a plastic binder that melted from the heat of the process and fused to the paper. The plastic could carry color, making color printing possible. The modern photocopier eventually made Carlson and the corporation that marketed the machine as the "Haloid Xerox" rich. The Xerox 813, released in 1963, revolutionized the copy business.

A traditional copier lens.
The "lens" in a photocopier is a single achromatic (color-corrected) objective lens with an f-ratio of between 4 and 6. Most lenses are between 1 inch and 1-1/2 inch in diameter. The lenses move on a scanning arm that carries a bright light, a halogen lamp of between 300 and 600 watts or comparable florescent light. Depending on the design of the body of the copier, one or more mirrors turn the image---or it may go directly through the lens. The lens focuses the light on a photo-reactive drum (platen) that is covered with a thin layer of selenium. As the light travels toward the drum, it hits a wire (called a "corona wire") that sets up a pattern of static electricity on the surface of the platen. The mirror transmits only the line of image through the lens that is illuminated as the scanning arm moves across the original.

Laser copiers use stationary lenses--drum platen is at top.
Lenses in laser copiers operate a bit differently. They stay stationary as the laser is directed by a rotating mirror for each line on the platen. The corona wire (which operated a bit like the "blanket" in an offset press) is not used in modern copiers; the lens focuses light directly on the photo-reactive drum. Although the laser copier works a bit differently, it uses the same quality photographic lens to focus the light-electron beam on a drum that transfers an image using static electricity. Similar to earlier photocopiers, laser printers also require heat to bond the toner to the image.