How a photocopier copies with light and static charge

Press a copier's start button and the glass flashes. A blank sheet enters the machine. Seconds later, black marks emerge in the same arrangement as the original. Yet the original page never touches ink, and the moving light does not burn letters directly onto the new paper.

The machine solves the problem by building an image that exists briefly as an electrical pattern. Light removes charge from selected places on a special surface. Powder sticks to the charge that remains. Paper takes the powder, and heated rollers make it permanent. The whole method is called xerography, from Greek words for “dry writing”, because it forms the image with dry toner rather than photographic liquid.

A surface that changes when light arrives

At the centre of the process is a drum or belt called a photoreceptor, the light-sensitive surface that temporarily holds the electrical image. Its coating is a photoconductor. This material behaves much more like an electrical insulator in the dark but becomes more conductive when light reaches it. The machine first gives the dark photoreceptor a nearly uniform surface charge. Because the coating resists charge movement in darkness, that prepared state can last long enough for the next step.

Now imagine copying a page bearing one black letter A on white paper. In a classic analogue copier, a lamp illuminates the page and lenses project its optical image onto the photoreceptor. White paper reflects much more light than black toner. Where reflected light reaches the coating, the material conducts and surface charge drains towards the conductive layer beneath it. Where the dark A reflected little light, more charge remains.

The result is a latent image, meaning a pattern that is present but not yet visible. It is not a photograph made of pigment. It is a map of how much charge remains at different places on the surface. Chester Carlson's early patent describes the same central move: charge a photoconductive insulating layer, expose it to a light image, then retain a charge pattern in the less illuminated areas. Read Carlson's electrophotography patent.

This example follows the classic analogue arrangement documented by Xerox's technical account of xerography. Machines can reverse charge polarities or deliberately develop the areas that light exposes. The essential idea is not that “black is always positive”. It is that controlled illumination changes a photoreceptor's charge pattern.

Making the invisible image visible

The machine next brings toner close to the photoreceptor. Toner is not liquid ink. It consists of tiny particles that contain colourant and a plastic-like resin. Handling and mixing give the particles a controlled electrical charge. An electric field is the region around electrical charges where another charge feels a push or pull. The field around the latent image pushes or pulls the toner, so particles collect where the machine intends the printed marks to be.

Return to the letter A. The charge pattern now acquires a powder coating shaped like the letter. This is why the development stage is more than dust settling at random. Charge relationships, particle properties and the distance between components must be controlled well enough for toner to prefer the image areas. OpenStax follows the simplified charge, exposure, toner and transfer sequence.

The image is visible now, but it is still on the photoreceptor. A sheet of paper passes against it while another controlled electric field pulls the charged toner away from the drum and onto the paper. At this moment the powder is only loosely held. If the machine stopped here, the image could smear.

The sheet therefore passes between fuser rollers. Heat softens the resin in the toner, while pressure helps bond it to the paper. Heat does not decide where the letter goes; the earlier light-and-charge pattern already did that. Fusing turns a correctly placed but fragile powder image into a durable print.

Transfer is not perfect. Residual toner must be cleaned from the photoreceptor, and the remaining electrical pattern must be erased before the surface begins another cycle. The apparently simple act of making a second copy is therefore a loop: prepare, expose, develop, transfer, fuse and reset.

The idea arrived long before the office machine

Chester Carlson was trained in physics and worked with patents, a job that made the difficulty of copying documents hard to ignore. On 22 October 1938, Carlson and Otto Kornei produced an early electrophotographic image in Astoria, New York. The surviving record reads “10-22-38 ASTORIA”. The Smithsonian preserves the story and the experimental objects.

An experiment was not yet a convenient copier. The process needed better materials, repeatable charging and exposure, reliable powder development, paper handling, cleaning and a way to perform the sequence automatically. Battelle Memorial Institute agreed to work with Carlson in 1944. In 1947 it reached an agreement with the Haloid Company, later Xerox, to develop the technology. Xerox's documented history traces those partnerships. More than two decades after the Astoria experiment, the Xerox 914 reached the market in 1959. The Smithsonian describes the 914 as the first widely successful plain-paper copier.

That delay matters. Carlson's insight joined two physical effects, but a useful machine required an engineered cycle. A drum that copies once in a laboratory is not the same achievement as a device that feeds sheets, controls charge and heat, cleans itself and repeats the result.

Modern copiers changed the writer, not the alphabet

Many modern multifunction copiers first scan the original into digital data. A laser or row of light-emitting diodes then writes a controlled light pattern onto the photoreceptor. A laser printer begins with computer data rather than a sheet on glass. Both can still use the same electrophotographic sequence after exposure: a charge image controls toner, toner transfers to paper, and a fuser fixes it. Canon's technical explanation follows one digital scanning and negative-development arrangement.

Older analogue copiers projected the page's optical image directly onto the photoreceptor, while a modern machine may separate digital scanning from printing. Toner and inkjet ink are also not interchangeable. Inkjet printers place liquid droplets through nozzles; xerographic machines place dry charged particles through an intermediate electrical image.

The flash is therefore only the visible entrance to the process. The copier converts contrast on one page into light, light into an electrical pattern, that pattern into a powder image, and a temporary powder image into a permanent copy. For a few moments, between the glass and the paper tray, the page exists as something you cannot see: an organised absence and survival of electric charge.