How Photocopiers and Laser Printers work
By Louis E. Frenzel
Everyone talks about how the personal computer has changed the way we do work in the office-place. While computers have indeed affected not only the way we work but our productivity, there is another electronic device that has had an even greater impact. That device is the photocopier. Before the photocopier, we had to use carbon paper to make copies of letters or other typed documents. Today carbon paper is rarely used except in multipart forms.
All typed documents are duplicated by putting them through a photocopy machine. And photocopiers allow the duplication of any printed material, making it possible to copy and distribute vast amounts of printed material with speed and ease.
The photocopier is also a part of the one of the most popular types of computer printers, the laser printer. The laser printer is basically a photocopier with a laser that traces out the material to be copied.
The basic photocopying process was developed by Chester Carlson in the 1930s. The technique is generally known as electro-photography or xerography, which is Greek for “dry writing.” This technique uses static electricity and photoconductive materials to transfer an image from a source to a blank sheet of paper.
Figure 1 shows a diagram of a basic photocopy machine. It consists of a drum made of aluminum whose surface is coated with a material such as selenium. The selenium is very light sensitive. The photocopy process begins by giving the drum a positive electrical charge. This is done by rotating the drum adjacent to a fine wire closely spaced from the drum’s surface. A high voltage of 6000 to 7000 volts is applied to the wire.
The wire, Called a corona, ionizes the air around it and thereby produces an electric charge which is transferred to the drum. A positive charge is evenly distributed across the drum surface. As long as the drum is kept in the dark, it acts as an insulator and therefore retains the positive charge.
However, if the drum should be exposed to light, it becomes a good conductor and the electrical charge is transferred to the aluminum base of the drum and conducted away, thereby neutralizing the surface. Once the drum is positively charged by the corona, the page being copied is scanned by a light and an optical system which focuses the reflected light on the drum as it rotates.
This distributes the information to be copied across the drum surface. The electrical charge on the drum is dissipated wherever light strikes it. The dark parts of the image, namely the black type, do not discharge the drum. That portion of the drum retains the positive charge. The charged areas of the drum accurately represent the type, graphics or other information being copied.
Next, a powdered dry ink called toner is applied to the drum. The toner is given a negative charge. Since opposite charges attract, the negative toner is picked up by the positively charged areas on the drum. The image to be copied therefore is now present in toner form on the drum. Plain white paper is pulled into the copier mechanism by rollers and given a positive charge by a corona.
The paper is moved past the drum where the toner is attracted to the paper. The paper then passes through heated rollers that melt the toner ink and fuse the image to the paper. A very high quality copy of the original emerges from the machine. The cycle can repeat. The drum is exposed to light to erase any residual image and is again precharged before being exposed to the next item to be copied.
The basic photocopy mechanism, which consists of the photo-sensitivedrum, corona, toner and, associated rollers and other mechanics is generally referred to as the photocopier “engine.” This engine is the heart of the popular laser printer. The light source and optical mechanism in the photocopier is replaced with a sophisticated laser raster scanning system that is used to expose the drum in the pattern to be printed.
The basic laser printer mechanism is shown in Figure 2. It consists of a semiconductor laser diode which is turned off or on by the electronics to produce a dot matrix image of the material to be printed. The laser beam is shined on a multi-surface or polygonal rotating mirror. The mirror, which rotates at high speed, causes the laser beam to be scanned across the drum as the drum rotates. The rotating mirror and drum move in Synchronism so that a raster scan is produced by the laser beam on the drum. The static charge on the drum is neutralized where the laser light strikes.
A laser printer is really a high density dot matrix printer. Because the laser beam can be focused to an extremely small pinpoint of light, the dot density is extremely high. Most common laser printers have a density of 300 dots per inch. All type fonts or graphics symbols to be printed are created with multiple dots. Because so many dots are used to form a character, it is difficult to distinguish it from a continuously formed letter quality character typical of that printed by a daisywheel printer or typewriter.
Figure 3 shows a block diagram of the electronics in a laser printer. Information to be printed is first sent to the printer by way of a serial or parallel I/O interface. The data is usually text in ASCII code format, this is stored in the printer RAM. A microprocessor controller inside the printer examines the data to be printed and builds a bit map from that information. A bit map is a dot pattern representing a character in RAM.
Essentially what happens is that the ASCII Codes are translated into bit maps of the characters they represent. As a result, the microprocessor builds up the entire page by assembling bit maps for the characters to be printed. Because the dot density is very high, a large RAM is required to store a Single page of information. Most laser printers Contain one to two megabits of RAM.
When the entire full page image has been stored in RAM, the individual bits are scanned out sequentially and fed to the laser beam, turning it off and on. The laser beam scans across the drum, discharging some areas but leaving other areas charged. The charged areas represent the image to be printed.
Once the entire page has been scanned and the drum appropriately discharged, the printing process becomes the same as that used in copying. Toner is applied to the drum and adheres to the charged areas. The blank paper is charged by the corona and passed adjacent to the drum where it picks up the toner image. The paper is then passed through heated rollers that fix the image to the paper. The result is an extremely high quality printed image.
The laser printer offers numerous advantages over the older and more common impact-type dot matrix and daisywheel printers. One of its best features is its silent operation. Impact printers make a considerable amount of noise, which is disturbing in an office environment. Laser printers, like photocopiers, are almost totally silent in their operation.
Figure 3: Block diagram of the electronics in a laser printer.
In addition, laser printers are extremely fast. The average laser printer can produce approximately 12 pages per minute and the fastest models can print over 200 monochrome pages per minute.
Finally, the printing quality is excellent. In fact, laser printers are commonly used in desktop publishing systems for creating the camera-ready copy of documents to be printed.
Perhaps the greatest disadvantages of the laser printer is its complexity and high cost. However, as the printer has become more popular and volume has increased, prices have dropped considerably. New technological advances are permitting even higher printing speeds as well as improved quality through greater dot density. Laser printers with a dot density of up to 1200 dots per inch are available for extremely high-resolution printing.
Louis E. Frenzel is president of Teknowledgy Sources, Inc., an Oakton, Virginia-based venture involved in the development of technical training materials in computer and electronics subjects.