Copying machine is any of several types of devices that copy documents or illustrations. Other devices, called duplicators, also produce copies. But duplicators use a special form called a master or plate, while copying machines do not require a master.
Manufacturers produce a wide variety of copiers, from personal copiers that make 5 to 8 copies a minute to units that can produce 180 copies a minute. Special features include automatic document feeding and sorting, image reduction and enlargement, duplexing (two-sided copying), color copying, and stapling. Some color copiers produce copies in black and one other color. Others produce full-color copies.
The chief methods of copying are (1) electrostatic copying, (2) inkjet copying, (3) projection copying, and (4) contact copying. Most copying machines use the electrostatic method.
Electrostatic copying
was invented in 1938 by Chester F. Carlson, an American physicist. Unlike earlier methods, which required liquid developers, Carlson’s process was completely dry. It became known as xerography, a term that comes from two Greek words meaning dry and writing.
In xerography, a drum, belt, or plate coated with the element selenium or some other light-sensitive material is charged with static electricity. A light source forms a positively charged image on the light-sensitive surface corresponding to the dark areas of the original, the document or illustration to be copied. In analog xerography, light reflected from the original passes through a lens to strike the light-sensitive surface. In digital xerography, laser light charges the surface. The remainder of the surface loses its charge. Next, negatively charged toner (powdered ink) is dusted onto the surface. Because oppositely charged materials attract each other, toner sticks to the image. The inked image is then transferred to positively charged paper and heated for an instant. The toner melts, creating a permanent copy.
Digital xerography developed from tools common in desktop publishing, the use of computers to produce printed materials. Most digital copiers combine a scanner with a laser printer. The scanner uses light sensors and other devices, or a digital camera, to convert the image on the original to a digital (numeric) file. The laser printer uses a laser to charge a drum in a pattern that corresponds to the original. Many digital copiers can transmit digital files representing documents to other digital devices, such as fax machines and personal computers. Some also perform other functions, such as sending and receiving fax messages. See Fax machine; Laser printer.
Inkjet copying
is performed by digital copiers that use tiny nozzles to spray droplets of ink onto the printing surface in a pattern corresponding to the original. Inkjet copiers are generally used for low-volume desktop copying. Several people and companies contributed to the development of inkjet copying beginning in the mid-1900’s.
Projection copying
was developed in the mid-1800’s. Types of projection copiers include the copy camera and the photostat machine. A copy camera performs the steps used to develop and print ordinary photographs. First, it takes a photograph of the original. The film is then developed by a chemical solution, producing a negative. To make a positive copy, the image on the negative is projected onto positive paper. Finally, the paper is developed to create the copy. A photostat machine projects light reflected from the original directly onto light-sensitive paper. Developing the paper produces the copy. The photostat machine, like any other projection copier, can enlarge or reduce the size of the copy made from the original.
Contact copying
was first used in the mid-1800’s. In this method, the original is placed in contact with light-sensitive negative paper and exposed to light. Next, the negative paper is held against positive paper, and the two papers are fed into a contact-copying machine. There, they pass through a developer, such as ammonia vapor or water. The developer brings out the image on the negative and transfers it to the positive paper (the copy). Blueprints and similar types of duplicates are made using the contact-copying method (see Blueprint).
See also Library (Photocopying); Xerox Corporation.
