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: xerography, a 1938 invention by Chester Carlson, which revolutionized the way of reproducing documents. In simple terms, a photocopier creates a perfect copy of an image or text by using the electrical charges and light sensitivity of particular materials. It all starts with an electrostatic charge on a surface called a photoreceptor, continues with the projection of the original image, the adhesion of toner powder to the charged areas and the transfer of 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). Understanding 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 in 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 1938, when Chester Carlson, an attorney in the New York patent office, was looking for a way to reproduce documents without using liquid chemicals or having to retype them by hand. The basic principle on which the operation of the photocopier is based is xerography, from the Greek xeros (dry) e graphé (writing), i.e. “dry writing”. The name was chosen at the end of the 1940s by a small American company, Haloid, which later became the famous Xerox Corporation. Xerox itself explains how Carlson developed the technique on which the operation of photocopiers is based:
Carlson initially called the process “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 Xerox refers to. At the heart of every copier is a component called a photoreceptor, a light-sensitive surface that can be made as a rigid drum or a flexible belt. It is covered with a thin layer of photoconductive material, i.e. a compound that acts as an insulator when in the dark but becomes conductive when exposed to light.
- Charging: 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.
- Exposure: the document image 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 a latent electrostatic image, invisible but ready to be developed.
- Development, where the protagonist is toner powder. These are tiny particles (typically between 5 and 10 micrometers in diameter) made up of a mixture of resins, pigments and plastic additives. The toner is mixed with small metal spheres that charge it with triboelectricity, a physical effect due to the 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 gamut.
- 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.
- Fusing: 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.
- 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
Over the decades, photocopiers have evolved and, in the 1990s, they became 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.
