How a Laser Copier Works
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Making Copies
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The laser copier is an improvement on the original photocopier introduced by the Xerox Corporation in the 1960s. The key to both machines is the use of photoconductive material, like selenium, to produce a negative image made entirely of charged particles. Deep inside the machine, an aluminum drum coated with the photoconductor is shielded from any outside light. As the copying process is initiated by a user, an electrical current is passed through the drum, which charges the surface of the photoconductor. The first copy is produced by a digital scanner that acts somewhat like a fax machine. The scanner produces a digital image of the document, stored internally. This digital image is then used to guide a laser, which fires a tiny beam of light at the photoconductor in the pattern of the original document. Everywhere the original was blank white, the laser carves out a space on the photoconductor, which loses its charge when struck by light. When the laser has finished its task, a pattern of charged particles remains that is an exact negative image of the original document.The scanner is very efficient because it allows multiple pages to be stored digitally for producing collated copies instead of having to manually rescan or sort them as would be necessary with an analog photocopier.
Toner
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The crucial ingredient between the charge negative and the final copy is the ink toner. A very finely powdered ink, toner is attracted to the charged regions of the photoconductor. It is sprayed over the photoconductor and sticks to the charged parts of the image through static electricity, just like hair sticks to the surface of a balloon if they're rubbed together long enough to produce an electric charge. Since the charged parts of the negative represented the dark portions of the original document, the image in toner exactly matches the original.
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Getting it on Paper
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Once the toner image has been produced, all that's left is to transfer the toner image to a sheet of paper. Obviously, powdered ink won't stick to paper the way it does to the charged regions of a photoconductor. But, when heated and exposed to pressure, the granules of toner melt just enough to produce a continuous liquid ink. Just as this occurs, a blank sheet of paper is brought to the toner image by a series of belts and pressed against the toner, producing the final copy. The paper is ejected from the machine as a new sheet is brought to the toner, which is heated again to produce the transfer. Eventually, when a new copy is ready to be made, the laser comes alive again and sprays the entire surface of the photoconductor, discharging any image that remained and causing any stray toner to fall away.
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