Photocopying

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Photocopying

Xerography

Addition of toner and fusing

Color copying

Electrostatic copying

Thermography

Diazo copying

Resources

Photocopying is the process by which light is used to make copies of book pages and other paper documents. Bulgarian physicist Georgi Nadjakov (1896– 1981) discovered that some dielectrics (materials that are unable to directly conduct electric current) became permanently polarized when placed into an electric field and exposed to light. Nadjakov’s work with the photoelectric effect led to the invention of the photocopier in the 1930s by American physicist and inventor Chester Carlson (1906–1968), who was also a New York patent attorney. Today the most widely used form of photocopying is xerography (dry writing), which was invented by Carlson. Indeed, the name of the company founded to develop Carlson’s invention, Xerox Corporation, has become synonymous with the process of photocopying. However, a number of other forms of photocopying pre-dated the Carlson invention and are still used for special applications. Among these other forms of photocopying are thermography, diazo processes, and electrostatic copying.

Xerography

Many different models of xerographic copying machines are available today, but they all operate on some common principles. The core of such machines is a photoconducting surface to which is added a negative charge of about 600 volts. The surface could be a selenium-coated drum or an endless moving belt mounted on rollers, for example. The charged placed on the photoconducting surface is usually obtained from a corona bar, a thin wire that runs just above the surface of the photoconducting surface. When the wire is charged negatively, a strong electrical field is produced that causes ionization of air molecules in the vicinity of the wire. The negative ions thus produced are repelled by the negatively charged wire and attach themselves to the photoconducting surface.

In another part of the machine, the original document to be copied is exposed to light. The light reflected off that document is then reflected off a series of mirrors until it reaches the negatively charged photoconducting surface. When light strikes the photoconducting surface, it erases the negative charges there.

Notice the way the image on the original document is transferred to the photoconducting surface. Dark regions on the original document (such as printed letters) do not reflect any light to the photo-conducting surface. Therefore, those portions of the photoconducting surface retain their negative charge. Light regions on the original document (such as blank spaces) do reflect light to the photoconducting surface, causing the loss of negative charge in these regions. A letter a, for example, on the original document becomes an a-shaped region of negative electrical charge on the photoconducting surface. Similarly, areas of gray in the original document are also matched on the photoconducting surface because greater or lesser amounts of light are reflected off the document, causing greater or lesser loss of negative charge on the photoconducting surface.

Addition of toner and fusing

The next step in copying involves the addition of a toner to the photoconducting surface. A toner is a positively charged material that is added to the photoconducting surface. Since it carries an electrical charge opposite that of the negatively charged photo-conducting surface, the toner sticks to the surface. The photoconducting surface now carries toner on its surface that matches regions of negative electrical charge that, in turn, matches dark regions on the original document, such as the a mentioned above.

Finally, paper carrying a negative electrical charge is brought into contact with the photoconducting surface. The negative charge on the paper is made great enough to pull the positively-charge toner away from the photo-conducting surface and onto itself. The letter a formed by toner on the photoconducting surface, for example, has now been transferred to the paper. The paper passes through a pair of rollers that fuses (squeezes) the toner into the paper, forming a positive image that exactly corresponds to the image on the original document.

As the final copy is delivered to a tray outside the machine, the photoconducting surface continues on its way. Any remaining electrical charge is removed and the surface is cleaned. It then passes on to the charger, where the whole cycle is ready to be repeated.

Many kinds of toners have been developed for use in this process. As an example, one kind of toner consists of a thermoplastic resin (one that melts when it is heated) mixed with finely divided carbon. When the copy paper is passed through the rollers at the end of the copying process, the resin melts and then forms a permanent mixture with the carbon when it re-cools. Another kind of toner consists of finely divided carbon suspended in a petroleum-like liquid. The toner is sprayed on the photoconducting surface and, when the liquid evaporates, the carbon is left behind.

Color copying

The general principle in color copying is the same as it is for black-and-white copying. The main difference is that the light reflected off the original document must be passed through three filters—one green, one blue, and one red—before it is transmitted to the photoconducting surface. Then, toner particles of three distinct colors— yellow, magenta, and cyan—must be available to correspond to each of the three document colors. The toners are added separately in three separate and sequential operations. These operations must be overlaid very carefully (kept in register) so that the three images correspond with each other exactly to give a copy that faithfully corresponds to the original document.

Electrostatic copying

A process somewhat similar to that used in xerography is electrostatic copying. The major difference between these two processes is that in electrostatic copying, the endless photoconducting surface is omitted from the machine and the copy paper is specially treated to pick up the toner.

The paper used in electrostatic copying is treated with a material consisting of zinc oxide combined with a thermoplastic resin. When that paper is fed into the copy machine, it is first passed through a corona charging bar, similar to the one used in xerography. Within the charging bar, the zinc oxide coating picks up a negative electrical charge.

In the next section of the copy machine, the original document is exposed to light, which reflects off the white portions of the document (as in a xerographic machine). Dark portions of the document, such as the letter a in the document, do not reflect light. Light reflected off the original document is then reflected by a series of mirrors to the treated copy paper, which has been passed into this section of the machine. Light striking the copy paper removes negative charges placed by the charged bar, leaving charged sections that correspond to the absence of light; that is, the dark places on the original document. In this respect, the copying process is exactly like that which occurs in xerography.

Next, the exposed copy paper is passed through a toner bath, where positively charged toner attaches itself to negatively-charged areas on the copy paper. When the paper is passed through a pair of rollers, the toner is pressed into the copy paper, forming a permanent positive image that corresponds to the image on the original document.

The electrostatic copy process became less popular when xerographic processes were improved. The main drawback of the electrostatic process was the special paper that was needed, a kind of paper that felt different from ordinary paper and was more expensive to produce and to mail.

Thermography

Thermography (heat writing) is a method of copying that is based on the fact that dark regions of a document absorb heat more readily than do light spaces. If heat is applied to this page, for example, it will be absorbed more readily by the letters on the page than by the white spaces between the letters. As with electrostatic copying, themographic copying requires the use of specially treated paper. The paper used in thermography is coated with ferric [iron(III)] compounds in an acidic environment. When the paper is exposed to heat, a chemical reaction occurs that produces a dark image.

In use, a thermographic copy machine requires that the original document and the copy paper be placed

KEY TERMS

Copy paper— Plain or treated paper on which the image of an original document is produced in a copy machine.

Corona bar— A device used to add an electrical charge to a surface, given that name because a pale blue light (a corona) often surrounds the device.

Diazo copying— A copying process that makes use of changes in certain chemical compounds (diazonium compounds) when heat is added to them.

Electrostatic copying— A copying process similar to xerography, but somewhat simpler in its procedure and requiring a specially-treated copy paper.

Photoconducting surface— Any kind of surface on which a copy of a document can be made using light as the copying medium.

Thermography— A type of photocopying in which portions of specially treated copy paper darken as a result of being exposed to heat.

Toner— A material that carries an electrical charge opposite to that of a photoconducting surface that is added to that surface in a copy machine.

Xerography— A type of photocopying that makes use of an endless photocopying surface to record light and dark areas in an original document as charged or uncharged areas on a photoconducting surface.

into the machine in contact with each other. Some machines also use a transfer sheet placed in contact with the copy paper on the opposite side of the original document. A beam of infrared light is then shined through the document-copy paper (or document-copy paper-transfer sheet) combination. The infrared light heats dark spaces on the original document more strongly than light spaces. These heated areas—the places where text occurs on the document, for exam-ple—then cause darkening on the copy paper, producing a positive image copy of the original document.

Diazo copying

Diazo copying gets its name from the fact that it makes use of copy paper that has been treated with a type of chemical known as diazonium compounds. As with the thermographic process described above, diazonium compounds change color when exposed to heat. In diazo copying, the original document and the diazo-treated copy paper are placed in contact with each other in a light box. Then, they are exposed to a strong source of ultraviolet light. Dark regions on the original document become warm, causing corresponding areas on the diazo paper to darken. The color in these regions is brought about by exposing the copy paper to a developing agent such as ammonia gas. Blue-printing and brown-printing are specialized kinds of diazo copying.