From ECDL.web

A laser printer is a common type of computer printer that rapidly produces high quality text and graphics on plain paper. As with digital photocopiers and multifunction printers (MFPs), laser printers employ a xerographic printing process.

The laser printer was invented at Xerox in 1969, the prototype was built by modifying an existing xerographic copier. The first commercial implementation of a laser printer was the IBM model 3800 in 1975, used for high-volume printing of documents such as invoices and mailing labels. It is often cited as "taking up a whole room," implying that it was a primitive version of the later familiar device used with a personal computer. The first laser printer designed for use in an office setting was released with the Xerox Star 8010 in 1981. After personal computers became more widespread, the first laser printer intended for a mass market was the HP LaserJet 8ppm, released in 1984. The HP LaserJet printer was quickly followed by laser printers from Brother Industries, IBM and others.

Laser printers brought fast, high quality text printing with multiple fonts on a page to the business and consumer markets. No other commonly available printer could offer this combination of features. As with most electronic devices, the cost of laser printers has fallen markedly over the years. In 1984, the HP LaserJet sold for $3500, had trouble with even small, low resolution graphics, and weighed 71 pounds (32 kg). In contrast, as of 2008 low end monochrome laser printers often sell for less than $75 and still outperform the LaserJet Classic in nearly all situations.

[edit] How it works

Each horizontal strip of dots across the page is known as a raster line. The built-in Raster Image Processor generates a bitmap of the final page from the source. In a color printer, each of the four (cyan, magenta, yellow and black) toner layers is stored as a separate bitmap, and all four layers are typically preprocessed before printing begins.

A corona wire positioned parallel to the drum, or in more recent printers, a primary charge roller, projects an electrostatic charge onto a revolving photosensitive drum or belt, which is capable of holding an electrostatic charge on its surface while it is in the dark. The laser is aimed at a rotating polygonal mirror, which directs the laser beam through a system of lenses and mirrors onto the photoreceptor. The laser beam neutralizes (or reverses) the charge on the black parts of the image, leaving a static electric negative image on the photoreceptor surface to lift the toner particles.

The surface with the latent image is exposed to toner, fine particles of dry plastic powder mixed with carbon black or colouring agents. The charged toner particles are given a negative charge, and are electrostatically attracted to the photoreceptor's latent image, the areas touched by the laser. Because like charges repel, the negatively charged toner will not touch the drum where the negative charge remains.

The photoreceptor is pressed or rolled over paper, transferring the image. The paper passes through rollers in the fuser assembly where heat (up to 200 Celsius) and pressure bond the plastic powder to the paper. One roller is usually a hollow tube (heat roller) and the other is a rubber backing roller (pressure roller).

When the print is complete, an electrically neutral soft plastic blade cleans any excess toner from the photoreceptor and deposits it into a waste reservoir, and a discharge lamp removes the remaining charge from the photoreceptor.

[edit] Color laser printers

Color laser printers use colored toner (dry ink), typically cyan, magenta, yellow, and black (CMYK). While monochrome printers only use one laser scanner assembly, color printers often have two or more scanner assemblies.

Color printing adds complexity to the printing process because very slight misalignments known as registration errors can occur between printing each color, causing unintended color fringing, blurring, or light/dark streaking along the edges of colored regions. To permit a high registration accuracy, some color laser printers use a large rotating belt called a "transfer belt". The transfer belt passes in front of all the toner cartridges and each of the toner layers are precisely applied to the belt. The combined layers are then applied to the paper in a uniform single step.

Color printers usually have a higher cost per page production cost than monochrome printers.

[edit] Anti-counterfeiting marks

Many modern colour laser printers mark printouts by a nearly invisible dot raster, for the purpose of identification. The dots are yellow and about 0.1 mm in size, with a raster of about 1 mm. This is purportedly the result of a deal between the U.S. government and printer manufacturers to help track counterfeiters. The dots encode data such as printing date, time, and printer serial number in binary-coded decimal on every sheet of paper printed, which allows pieces of paper to be traced by the manufacturer to identify the place of purchase, and sometimes the buyer. Digital rights advocacy groups are concerned about this erosion of the privacy and anonymity of those who print.

[edit] Safety hazards, health risks, and precautions

• Shock hazards. Although modern printers include many safety interlocks and protection circuits, it is possible for a high voltage or a residual voltage to be present on the various rollers, wires, and metal contacts inside a laser printer.
• Toner particles are designed to have electrostatic properties and can develop static-electric charges when they rub against other particles, objects, or the interiors of transport systems and vacuum hoses. Because of this and its small particle size, toner should not be vacuumed with a conventional home vacuum cleaner. Static discharge from charged toner particles can ignite dust in the vacuum cleaner bag or create a small explosion if sufficient toner is airborne. Toner particles melt (or fuse) when warmed. Small toner spills can be wiped up with a cold, damp cloth.
• Ozone hazards. As a natural part of the printing process, the high voltages inside the printer can produce a corona discharge that generates a small amount of ionized oxygen and nitrogen, forming ozone and nitrogen oxides. In larger commercial printers and copiers, a carbon filter in the air exhaust stream breaks down these oxides to prevent pollution of the office environment. However, some ozone escapes the filtering process in commercial printers, and ozone filters are not used in many smaller consumer printers. When a laser printer or copier is operated for a long period of time in a small, poorly ventilated space, these gases can build up to levels at which the odor of ozone or irritation may be noticed. A potential for creating a health hazard is theoretically possible in extreme cases.
• Respiratory health risks. Some printers emit sub-micrometre particles which may be associated with respiratory diseases.