How the photocopier works, the invention that revolutionized offices around the world

When you press the “Start” button on a copier, something surprisingly complex happens within seconds. Inside the machine, a physical and chemical process is activated based on the interaction between light, electric charges and matter: the xerographyinvention of 1938 Of Chester Carlsonwhich revolutionized the way documents are reproduced. In simple terms, a photocopier creates a perfect copy of an image or text using the electric charges and the sensitivity to light of particular materials. It all starts with an electrostatic charge on a surface called webcontinues by projecting the original image, adhering toner powder to the charged areas, and transferring this image onto a sheet of paper which is finally heated to permanently fuse the toner. In a few moments, the document is ready.

This technology, seemingly trivial in everyday use, has replaced slow and laborious processes such as carbon paper or the mimeograph, paving the way for the rapid and low-cost production of copies of any document. Today, modern digital copiers integrate scanners, laser printers and microprocessors that allow you to connect to computer networks, send faxes and even convert printed text into editable digital files via optical character recognition, the so-called OCR (Optical Character Recognition). Comprehend how a photocopier works will allow you to better appreciate this tool and take a peek inside one of the most elegant applications of electrostatic physics at the service of everyday life.

What is xerography: history and origin of the technique developed by Chester Carlson

The history of the photocopier dates back to 1938when Chester Carlsonan attorney in the New York patent office, was looking for a way to reproduce documents without using liquid chemicals or having to rewrite them by hand. The basic principle on which the operation of the copier it’s there xerographyfrom Greek xeros (dry) e graphé (writing), that is “dry writing”. The name was chosen in the late 1940s by a small American company, the Haloidwhich later became famous Xerox Corporation. Xerox itself explains how Carlson developed the technique on which the operation of photocopiers is based:

Carlson initially called the trial “electrophotography”. The process is based on two natural phenomena: the attraction of materials with opposite electrical charges and the improved ability of some materials to conduct electricity when exposed to light. Carlson devised a six-step process for transferring an image from one surface to another using these phenomena.

The six stages of the photocopying process

Let’s take a closer look at the six-step process referenced by Xerox. The heart of every copier is a component called weba light-sensitive surface that can be made as a rigid drum or as a flexible belt. It is covered with a thin layer of material photoconductivethat is, a compound that acts as an insulator when in the dark but becomes a conductor when exposed to light.

1. Loading: In the dark, a very high electrical voltage is applied near thin metal wires or a roller, generating an electric field that causes the ionization of air molecules. The ions, i.e. the electrically charged particles, are deposited on the surface of the photoreceptor, generating an electric field.
2. Exposure: the image of the document is “written” on the photoreceptor. In modern digital copiers this occurs via a modulated laser beam or a system of LEDs that project the image precisely. In analog cameras, however, it was the light reflected from the document that hit directly on the photoreceptor. The illuminated areas of the photoreceptor lose part of their electrical charge, while the areas left in shadow retain it: the result is alatent electrostatic imageinvisible but ready to be developed.
3. Developmentwhere the protagonist is the dust of toner. These are tiny particles (usually between 5 and 10 micrometers in diameter) formed from a mixture of resins, pigments and plastic additives. The toner is mixed with small metal balls that charge it for triboelectricitya physical effect due to rubbing between different materials, the same one that generates static electricity. The toner particles, now electrically charged, are attracted to the areas of the photoreceptor that have remained charged and adhere to them, recreating the image of the document in visible form. In color copiers this step occurs four times, once for each base color – cyan, magenta, yellow And black – which combine to produce the final color range.
4. Transfer: The powdered image passes from the photoreceptor to the paper. This occurs by placing the sheet in contact with the photoreceptor and applying an electric charge of the opposite sign to that of the toner. The electrostatic attraction causes the dust to detach from the photoreceptor and adhere to the sheet, reproducing the image. Immediately afterwards, another controlled discharge neutralizes the paper, which is separated from the photoreceptor without damage and continues its journey inside the machine.
5. Fusion: the toner (which until then had only rested on the surface of the paper) is made permanent. The sheet passes through two rollers: one heated and one pressure. The hot roller loosens the toner particles, while the second roller presses them firmly against the paper fibers. The result is a sharp and resistant image, which is in effect fused with the fibers of the sheet and, therefore, can no longer be removed.
6. Cleaning: After each print, there is always a small amount of residual toner left on the drum. To prevent these particles from interfering with the next copy, cleaning is performed, usually using a cleaning roller.

Naturally, the process just described takes place in a few seconds and is completely invisible to the user who puts the photocopier into operation.

The evolution of photocopiers

As the decades passed photocopiers have evolved and, in the 1990s, they went digital. In the latter, in fact, an optical scanner acquires the image of the document and stores it in digital format. From that moment, the laser no longer directly reproduces the reflected light of the document, but writes the image on the drum pixel by pixel, as if it were “printing” the copy from a file. This system allows you to obtain multiple copies from a single scan, automatically improve image quality and connect the machine to a network.

Today, multifunction copiers are real workstations: they can print, scan, send documents via fax or e-mail and archive copies in PDF format. In offices and schools they are often equipped with ADF (Automatic Document Feeder), an automatic sheet feeder that handles double-sided copying and automatic collation, sometimes with stapling or binding of documents. The domestic versions, decidedly more compact, adopt the same logic but simpler mechanics.