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Photocopier

Photocopier


Have you ever noticed that when you place a picture on a photocopier turned one way the copy comes out flipped upside down? Have you ever tried to enlarge a copy of an object to fit into an special frame, and had to enlarge it several times before you got the size "just right?" Have you ever made a photocopy of a book cover or of your hands? Well, if you have witnessed or experienced any of these things, then you have been involved in some of the everyday mathematics that surrounds us.

Congruency, Similarity, and Nonsimilarity

The photocopier can produce copies that have virtually the same size as the original item copied. The two objectsoriginal and its copyare said to be mathematically congruent to each other. Two 2-dimensional objects are congruent if the objects have the same shape and the same size. On the photocopier, the copy comes out as a reflection of the original (flipped upside down), so you may have to flip the copy to fit it on top of the original to see the perfect match in shape and size.

The photocopier also can produce copies of objects that are smaller (reduction) or larger (magnification) than the original object. Mathematically, we refer to the process of making this different-sized copy as a dilation. The copy has the same shape as the original, but not the same size. Certain corresponding measurements (such as the left side on an original and the left side on a copy) are multiples or fractions of the original. Objects that fit this description are referred to as mathematically similar shapes. The word "similar" is being used here in a more restricted way than you probably use it in everyday conversations.

In the illustration below are four pairs of similar shapes. For example, A is the copy of A. Do these shapesA and A, B and B, C and C, and D and Dseem similar?

In contrast, the illustration at the top of the next page shows two pairs of shapes that are almost but not quite similar. Shape E is almost similar to E and F is almost similar to F. Yet the ratios of width to height are different in the two pairs, and cause the shapes to fail the test of being similar.

Examples of Similarity

A few examples of similarity on a photocopier illustrate the mathematics behind reductions and enlargements.

Copy of a Triangle. Imagine your original shape is a triangle with base length that is 2 inches and height that is 1 inch. (See the single small triangle on the left in the table.) If you enlarge the triangle with a photocopier and measure the base length and height of the copy, you can compare those new lengths to the original values. You will find that you can either multiply or divide by one specific number to get the measurement of the copy based upon the measurement of the original.

In this example, the length of the base of the copy of the triangle is 4 inches, yet the original base length was 2 inches. What number might you multiply times 2 to get 4? So, if you multiply the same number, 2, times the height measure of the original triangle (1 inch), you will get 2 inches (2 × 1 inch), the height of the similar copy. Since multiplying the measurements of the original triangle by 2 will yield the measurements of the new triangle, we say we have used a scale factor of 2.

There are other ways of referring to this same scale factor of 2. We might say that the ratio of side lengths of the similar copy to the side lengths of the original is 2:1, or 2-to1. When we use a photocopier, we use percentages to refer to the scale factor. In this example, the scale factor on the photocopier would be represented as 200%:100%, or we would simply choose the enlargement (or magnification) factor to be 200%.

COMPARISON OF AN ORIGINAL AND ITS COPY
Original Triangle Formula Copy of Triangle
(scale factor of 2)
2 inches Base length (b) 4 inches
1 inch Height (h) 2 inches
0.5(2)(1) = 1 sq. in. Area = 0.5bh 0.5(4)(2) = 4 sq. in.

Determining the relationship between the area of the original shape and its similar copy is more challenging. You will need to give thought to the overall size of the photocopy, because you should make sure that the size of paper in the photocopier is large enough to hold the copy of the original shape. So, if you have doubled the base length and doubled the height of the original triangle, do you think the area will also be doubled?

There are at least two ways to find the answer, as shown in the table. Calculate the area with a formula or discover the area by tessellating copies of the smaller triangle until you cover the similar copy of the original triangle. ("Tessellating" means to assemble the smaller triangles so they adjoin one another with no gaps in between.)

Reducing a Photograph. Suppose one of your friends has given you a 5- by-7-inch photograph. Although you plan to place the photo into a frame, you would like to have a wallet-sized copy of this photo, too. The photo is not copyrighted to prohibit copying, so you find a color photocopier to make your copy. Your wallet will hold a 2.5-by-3.5-inch photo. Which setting will you use on the photocopier to make your copy?

There are at least two ways to find the answer without guessing at the reduction. If the original has a width of 5 inches, and your copy requires a width of 2.5", the ratio is 5:2.5 which equals 2:1. Since you are making a reduction, you must reverse the order to 1:2. Hence, you will need to use 50% as the reduction factor on the photocopier.

The 50% setting works for the width of the photo, but will it work for the length of the photo as well? If we want a similar copy, we realize that the scale factor for the length of the photo must be the same as the scale factor of the width. Thus, the photocopier setting should be correct at 50%. Will a 50% reduction from 7 inches in length yield a new length of 3.5 inches? Yes, 50% of 7 is 3.5.

Enlarging a Photograph. Now suppose you decide to also enlarge the 5- by-7-inch photo to make it fit into an 8-by-10-inch frame. Can you figure out the setting for the photocopier?

First, try changing the 5-inch width to 8 inches. So the scale factor is eight-fifths, or 1.6. On the photocopier, you would choose 160%. This same scale factor should give the desired length of 10 inches on the photocopy.

Will a magnification of 160% from 7 inches in length yield a new length of 10 inches? Not quite, because 160% of 7 is 11.2 inches. That poses a problem if you are using 8.5-by-11-inch paper in the photocopieryour copy will be 0.2 inches too long! So what do you do? You could switch to a larger paper tray in the photocopier. However, your copy will be too large to fit into the 8-by-10 photo frame. You can also make a decision to cut off part of the length of the photo, or use a smaller width and try for a photo-copy that will be similar to the original. If you choose the latter option, you will decide on a slightly smaller magnification setting on the photocopier.

Remember there is more than one way to solve a problem, so the drawing below shows another approach you may try when you make measurements to determine a proper setting for a reduction or magnification on the photocopier.

see also Congruency, Equality, and Similarity; Percent; Scale Drawings and Models; Ratio, Rate, and Proportion.

Iris DeLoach Johnson

Bibliography

Bloomfield, Louis A. How Things Work: The Physics of Everyday Life. New York: John Wiley & Sons, Inc., 1997.

Garfunkel, Solomon, Godbold, Landy, and Pollock, Henry. Mathematics: Modeling our world. ARISE Course 3. Cincinnati: South-western Educational Publishing, 1999.

O'Daffer, Phares G., and Clemens, Stanley R. Geometry: An Investigative Approach, 2nd ed. Reading, MA: Addison-Wesley, 1992.

Serra, Michael. Discovering Geometry: An Inductive Approach. Berkeley: Key Curriculum Press, 1997.

Walton, Stewart, & Walton, Sally. Creative Photocopying: Using the Photocopier for Crafts, Design, and Interior Decorations. New York: Watson-Guptill Publications, 1997.

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Photocopying

Photocopying

Photocopying is the process of photographically reproducing a document of text, illustrations, or other graphic matter. The most common photocopying method used today is called xerography (from the Greek words for "dry" and "writing").

The process of photocopying

The mechanics of photocopying is based on the principle of photoconductivity (when certain substances allow an electric current to flow through them when light is applied). For example, when light is absorbed by some of the electrons (particles that have a negative charge) that make up selenium (a nonmetallic chemical element that is used in the photocopying process), the electrons are able to pass from one atom to another when voltage is applied. When the light source is taken away, the electrons lose their mobility or ability to move.

Words to Know

Electrostatics: Relating to painting with a spray that utilizes electrically charged particles to ensure a complete coating.

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 method of copying that uses dry powder, electric charge, and light to fuse an image onto paper.

During the process of photocopying, the round drum (usually made of aluminum) inside the copier is coated with a layer of selenium that is given a positive electrical charge. After placing the document to be copied on the glass-topped surface of the copier, a light exposes the image of the

document onto the drum. This causes the positive charge on the selenium-coated drum to fade except from the area to be copied. This area remains charged.

The negatively charged toner (ink) is then sprayed onto the drum, which forms an exact duplicate copy of the document. After that process is completed, a sheet of copy paper is passed by the drum at the same time that a positive electric charge is passed under the paper. The positive charge attracts the negatively charged toner image on the drum and the toner sticks in the same pattern onto the paper. Heat is quickly applied

to the copied image on the paper and it adheres the toner permanently to the paper.

Over the years, many improvements in the photocopying machine have taken place. Some enhancements include sorting and collating (arranging in order), enlarging or reducing the copied material, printing on both sides of the paper, and reproducing in color.

Inventor of photocopying

Xerography was invented by American physicist Chester F. Carlson (19061968) in 1938. After earning his physics degree from the California Institute of Technology in 1930, Carlson accepted a job working for the P.R. Mallory Company, an electronics business in New York. Working in the patent department, Carlson was frustrated by the difficulty of obtaining copies of patent drawings and specifications. He decided to use his time away from work to find a solution to the problem.

Focusing on the concept of electrostatics, Carlson spent four years before succeeding in production his first "dry-copy." The first successful copy was a notation of the date and location that read "10.22.38 Astoria." (Carlson lived in Astoria, Queens, New York at the time.) In 1940, Carlson obtained the first of many patents for his xerographic process. Wanting to find a company that would help him develop and market his idea, Carlson began showing his solution to many organizations. After more than twenty firms turned down his invention, Carlson finally reached an agreement in 1944 with the Battelle Memorial Institute, a nonprofit research organization. Three years later, the Haloid Company (later the Xerox Corporation) became a partner in the development of the xerography technology. Finally, after years of development, the first office copierthe Xerox 914was introduced in 1959.

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photocopying

photocopying, process whereby written or printed matter is directly copied by photographic techniques. Generally, photocopying is practical when just a few copies of an original are needed. When many copies are required, printing processes are more economical. However, when a printing process is used, the master or stencil required can sometimes be produced by photocopying. Principal photocopying processes include silver halide, transfer, plan, thermographic (see thermography), and electrostatic (e.g., xerography, which has become so widespread that the process is popularly almost synonymous with photocopying). Two well-known silver halide processes, photostating (see photostat) and microfilming, use cameras to make photographic copies of an original. Microfilming generates copies that are from 1/12 to as little as 1/100 the size of the originals, allowing great economy in space and materials when long-term storage is necessary. Microfilms are read by either projecting them or photographically printing them as enlargements. In transfer processes the original is placed in contact with negative paper and exposed to light. In the diffusion transfer the negative is developed while in contact with the positive. During development the chemicals forming the image in the negative diffuse to the positive, producing an image there. In gelatin transfer the negative is developed and then pressed against positive paper. A dyed gelatin on the negative is picked up by the positive, producing an image on it. Transfer methods are less expensive than silver halide processes, but the image produced by the former deteriorates with time. Plan copying is used to copy materials such as architects' drawings and engineers' plans. In one variety of plan copying known as the whiteprint process, an original is made on translucent paper. The paper is placed over a sheet coated with a diazo compound and exposed to a source of ultraviolet light. The compound covering the area that is exposed to the light decomposes. The compound shielded from the ultraviolet rays by the dark areas of the original can be developed to form a positive image. The blueprint process, another method of plan copying, has been largely superseded by whiteprints, which are of better quality and cost approximately the same.

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photocopying

photocopying Reproduction of words, drawings, or photographs by machine. In a photocopying machine, a light shines on the item to be copied, and an optical system forms an image of it. Various techniques may be used to reproduce this image on paper. In a modern plain-paper copier, the image is projected onto an electrically charged drum, coated with the light-sensitive element selenium. Light makes the selenium conduct electricity, so bright areas of the drum lose their charge. The dark areas, which usually correspond to image detail, retain their charge, and this attracts particles of a fine powder called toner. Electrically charged paper in contact with the drum picks up the pattern of toner powder. A heated roller fuses the powder so that it sticks to the paper and forms a permanent image.

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photocopy

pho·to·cop·y / ˈfōtəˌkäpē/ • n. (pl. -cop·ies) a photographic copy of printed or written material produced by a process involving the action of light on a specially prepared surface. • v. (-cop·ies, -cop·ied) [tr.] make a photocopy of. DERIVATIVES: pho·to·cop·i·a·ble / -ˌkäpēəbəl/ adj.

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photocopier

pho·to·cop·i·er / ˈfōtəˌkäpēər/ • n. a machine for making photocopies.

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photocopier

photocopier •Dampier •Napier, rapier, tapir •Shakespeare • sepia • Olympia •copier • compeer • photocopier •cornucopia, dystopia, Ethiopia, myopia, subtopia, Utopia

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photocopy

photocopycrappie, crappy, flappy, gappy, happi, happy, nappy, pappy, sappy, scrappy, slap-happy, snappy, strappy, tapis, yappy, zappy •campy, scampi, vampy •harpy, okapi, serape, sharpie •raspy •Giuseppe, peppy, preppy •kelpie •kempy, tempi •Gillespie •crêpey, kepi, scrapie •creepy, sleepy, tepee, weepy •chippy, clippie, dippy, drippy, grippy, hippy, Lippi, lippy, Mississippi, nippy, slippy, snippy, tippy, trippy, whippy, Xanthippe, zippy •chickpea •crimpy, gimpy, skimpy, wimpy •crispy, wispy •turnipy • recipe • praecipe • gossipy •pipy, stripy •choppy, copy, floppy, jalopy, moppy, poppy, sloppy, soppy, stroppy •Pompey, swampy •waspie, waspy •photocopy • cowpea •dopey, Hopi, Opie, ropy, soapy, topi

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Photocopying

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. Nadjakovs work with the photoelectric effect led to the invention of the photocopier in the 1930s by American physicist and inventor Chester Carlson (19061968), 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 Carlsons 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 filtersone green, one blue, and one redbefore it is transmitted to the photoconducting surface. Then, toner particles of three distinct colors yellow, magenta, and cyanmust 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 areasthe places where text occurs on the document, for exam-plethen 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.

Resources

BOOKS

Considine, Glenn D. Van Nostrands Scientific Encyclopedia. New York: Wiley-Interscience, 2002.

Macaulay, David. The New Way Things Work. Boston, MA: Houghton Mifflin Company, 1998.

Mort, J. The Anatomy of Xerography: Its Invention and Evolution. Jefferson, NC: McFarland, 1989.

Trefil, James, ed. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.

David E. Newton

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Photocopying

Photocopying

Photocopying is the process by which light is used to make copies of book pages and other paper documents. Today the most widely used form of photocopying is xerography ("dry writing"), invented by New York patent attorney Chester Carlson in the 1930s. 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 photo-conducting 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 which 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 photo-conducting 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 photoconducting 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 photo-conducting surface, causing the loss of negative charge in these regions. A letter "a" 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 photoconducting surface, the toner sticks to the surface. The photoconducting surface now carries toner on its surface that matches regions of negative electrical charge which, 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 photoconducting 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 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 example—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 diazotreated copy paper are placed in contact with each other in a light box and then 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.

Resources

books

Considine, Glenn D. Van Nostrand's Scientific Encyclopedia. New York: Wiley-Interscience, 2002.

Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin Company, 1998.

Mort, J. The Anatomy of Xerography: Its Invention and Evolution. Jefferson, NC: McFarland, 1989.

Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.


David E. Newton

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.

Cite this article
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  • MLA
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  • APA

"Photocopying." The Gale Encyclopedia of Science. . Encyclopedia.com. 13 Sep. 2018 <http://www.encyclopedia.com>.

"Photocopying." The Gale Encyclopedia of Science. . Encyclopedia.com. (September 13, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/photocopying-0

"Photocopying." The Gale Encyclopedia of Science. . Retrieved September 13, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/photocopying-0

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Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

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American Psychological Association

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Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.