Reference Series Table of Contents For This Issue

How Computers Work, Part I
August 2001• Vol.5 Issue 3
Page(s) 78-83 in print issue

Shedding Light On Laser Printers
Static Electricity, Plastic Dust & Lasers Create Stunning Images
There is really very little mystery in what happens under the plastic shell of a laser printer. In fact, once you understand the basis of their crisp images, the purposes of their physical components start to make sense.

The laser “draws” the printable image on a special light-sensitive drum, which then collects powdered ink ( toner) using an electrostatic charge and rolls it on to the page. A hot roller melts the toner to the paper, and you have a printout. With this concept in mind, follow along with this in-depth study of a typical laser printer. (NOTE: The following descriptions are general and may differ from specific models of laser printers.)

  The Printing Process. When you print an image or document from an application, the printer’s driver software translates the data from an on-screen format (red-green-blue, or RGB) to a printable format (monochrome [black] or CMYK [cyan-magenta-yellow-black] for color printers). The driver sends the entire page’s information to the laser printer via a parallel cable, network connection, or USB(Universal Serial Bus) cable.

In the printer, the raster image processor (RIP) translates the page’s information into a printable image. The RIP uses its own microprocessor and memory to convert the image into a map of dots (called a raster or bit map). The rasterized data will be fed to the laser to be re-created one horizontal line at a time. Meanwhile, rubberized rollers pick up a clean sheet of paper from the input tray and feed it into the printer.

The laser inside a laser printer is aimed at a spinning six- or eight-sided mirror. Each face of this mirror guides the laser’s beam through prisms toward the image drum, and its spinning motion is timed to precisely reflect the laser from one end of the drum to the other. By the time the next face of the mirror catches the laser, the image drum will have rotated enough for the next line of information to be “written” to it.

Remember that each horizontal line of data sent to the laser consists of “dots” or “bits.” Every dot is a signal to turn the laser on or off, at a rate of up to 30,000 times per second or more. (NOTE: In some printers, the laser is constantly on, but liquid crystals act as tiny electronic “shutters” to either veil or reveal the light.) As the blinking laser plays across the rotating image drum, it “draws” the image to the drum dot by dot.

The laser can write a document’s image to the image drum because the drum is coated with an OPC (optical photoconducting) substance and sealed with acrylic or polycarbonate. An OPC, which is usually organic or selenium based, will conduct an electrostatic charge unless exposed to light, such as a laser. You’ll see the terms “image drum” and “OPC drum” used interchangeably.

The first commercial desktop laser printer was HP's original LaserJet (1984).
The image drum’s OPC surface gets a negative charge from a charge roller or corona wire (a high-voltage wire that emits a strong magnetic field; found mostly in color printers) as the drum spins inside the laser printer. Next, the spinning mirror scans the blinking laser across the OPC drum’s surface, causing each point of the drum it hits to lose its electrostatic charge. The other parts that remain charged attract particles of toner from the developing roller (which spins inside the toner-filled cartridge) to form a negative of the image on the drum. This process, in which the laser defines the white sections of the page, is called the white write process.

The more common black write process (used by Hewlett-Packard, Canon, and others) harnesses the laser to draw the actual text or graphics. With the black write process, the printer negatively charges the toner before the Developer roller brings the toner near the OPC drum. The laser-struck, neutral areas of the OPC drum (the text or graphics) attract the charged toner while the negatively charged areas of the drum repel it.

The developing roller may carry a charge to temporarily attract the toner to itself. A doctor blade, set at a precise gap from the developer roller, makes sure just the right amount of toner sticks to the roller.

The clean piece of paper now rolls between the OPC drum (coated with toner in the pattern of the image) and a transfer roller. (NOTE: In certain printers, such as the Canon imageCLASS C2100, an intermediate drum or belt takes the toner from the OPC and deposits it on the paper.) Color lasers and some monochromes use a separate printer corona wire that keeps the transfer roller positively charged, attracting the toner from the OPC and to the paper.

The dry, powdery toner must now be melted to become a permanent image on the page. To do this, the paper rolls between a hot fuser roller and an unheated pressure roller (or, in a color printer, two hot fuser rollers). A halogen lamp tucked inside the fuser roller heats it up to temperatures of about 300 degrees Fahrenheit.

Many laser printers, especially color models, require vials of silicon-based fuser oil. The printer automatically applies a thin coating of this oil to each fuser roller to prevent the toner from sticking to it. A fuser wand with a felt lining cleans the fuser roller(s) as it spins.

Meanwhile, a rubber blade scrapes excess toner from the OPC drum. The printer exposes the drum to a light source to deplete the old magnetic pattern on its surface, and the charge roller or cartridge corona wire gives the drum a fresh electrostatic charge. The OPC drum is now ready to receive a new image from the laser.

Finally, the paper rolls into the output tray. The melted toner particles cool rapidly, making an image far more durable than that of an inkjet. This entire procedure may sound complicated, but it can take less than two seconds once the printer has rasterized the image. Some workgroup-level laser printers can print a respectable 50ppm (pages per minute).

  Components. Color laser printers generally are more expensive than monochrome lasers, and they require more maintenance. Instead of one toner color (black), color lasers may have three or four (cyan, yellow, magenta, and preferably black as well). Because the laser cannot imitate different colors, the information for each color is written to the OPC drum on a separate rotation. Thus, the cyan parts of the image may be written first and the cyan toner collected, then the yellow, and so on. This means the OPC drum must rotate three or four times for one color image, making typical color printers slower than a similar monochrome unit.

Most color laser printers today use a one-pass or single-pass design. This printing method uses four lasers or LED arrays (light-emitting diodes; miniature solid-state lamps) and four OPC drums, before transferring the toners to the page (or an intermediate belt or drum) simultaneously. One advantage of the single-pass method is that it's easier to align the colors on the drum than on a loose sheet of paper. The other is that it brings color laser speeds much closer to monochrome speeds.

LEDs replaced lasers in OkiData's print heads and many Xerox units. Rather than a spinning mirror directing a laser, an LED print head has a row (array) of tiny lights that spans the width of the OPC drum. LED print heads have no moving parts, so they're more reliable than laser heads.

The Xerox 9700, which debuted in 1977, had the ability to print as many as 120 pages per minute on letter-sized paper.
Many laser printers perform image processing on the print data between the RIP and the laser to improve the output. For example, Hewlett-Packard's REt (Resolution Enhancement technology) adjusts the size and position of dots on the edge of text or graphics to avoid a jagged appearance. It does this by altering the duration and timing of the laser's on/off pulses.

Toner usually is made of powdered black plastic. Some manufacturers combine developer (iron oxide; also known as rust) with the toner. Developer helps the developer roller attract and hold the toner until the OPC drum needs it. Toner also may include silica sand (to keep the toner flowing), wax (for better dispersion after melting; may also eliminate the need for fuser oil), and/or a charge dye (to regulate the toner's electrostatic charge). Toner particles generally are five to 15 microns (millionths of a meter) wide.

Color toners are difficult to produce because their color dyes may change their electrostatic properties. Each color toner requires its own developer supply.

Paper selection is important. Proper laser printer-compatible paper doesn't have to be expensive, but it should have the brightness and conductive properties necessary for good prints. It also will be less likely to jam in the tight paper path between the various rollers.

PDLs (Page description languages) are specialized languages that applications use to send complex page data to a laser printer. Hewlett-Packard's PCL (Printer Control Language) and Adobe's PostScript are the most common. PostScript provides a better match between the on-screen image and its printed counterpart, but it requires increased processing power and adds to a printer's price. Both languages are widely cloned by various manufacturers.

Resolution refers to the number of dots per square inch (dpi) a printer can produce. The more dpi, the finer the detail, which requires more memory and time to print. The industry standard presently is 1,200 x 1,200 dpi for desktop lasers, and 600 x 600 dpi for high-volume production. High resolutions are only possible with extremely fine toner.

Laser printers are bilevel; that is, they either print a dot or they don't. They have no control over the intensity of their dots. However, by dithering, or printing larger "spots" composed of a controlled number of dots, the printer can produce different shades (called halftones). Lighter areas of a graphic are printed with spots containing fewer dots, and vice versa. Because a halftone graphic consists of larger spots, its resolution will be lower than a solid black image that can be made of dots.

Finally, some corona wires' electrostatic discharge creates ozone gas, which can be harmful in quantity. To counteract this, the printer includes an ozone filter, which is filled with activated carbon. It changes the ozone back into harmless oxygen. Newer corona wires use a reversed charge and don't produce ozone.

The end result of all this is a quiet, productive printing device for businesses and home offices. Now that you know how these devices work, it's interesting to chart their development. We spoke with representatives from Xerox, Hewlett-Packard, and Canon to get a clearer picture of laser printers' past, present, and future.

 History. In 1968, Canon introduced the New Process system of electrophotography. It incorporated light, static electricity, and toner powder to create images on paper. This laid the technological foundation for future laser printers.

Xerox’s major advancement in the field came in 1973, according to Cathy Lewis, now vice president of marketing for the Color Solutions Business Unit. Its EARS (Electronic Alto Research Character Generator Scanned Laser Output Terminal) was a laser printer prototype that used continuous-sheet (fanfold) paper. It printed four million impressions in its four-year life span. Two years later Canon developed a laser beam printer prototype, the LBP-4000.

In 1977, Xerox unveiled the 9700, a high-volume laser printer capable of 120ppm at 300dpi. It printed on cut sheets instead of fanfold paper, introducing a new level of usefulness for the office.

Canon developed the first self-contained laser toner cartridge for the LBP-CX print engine in 1984, basing it upon an earlier cartridge for the PC-10 personal copier, which debuted in 1982.

Hewlett-Packard released the first commercial desktop laser printer in May 1984, which was the same time the IBM PC was sweeping the country. The LaserJet, which was based upon a Canon CX print engine, offered 300dpi at 8ppm. The printer cost $3,495 when it debuted in 1984.

Hewlett-Packard introduced REt in March 1990 with the LaserJet III and inspired similar image processing by other vendors. PCL 5, a concurrent development in page description languages, added the ability to scale fonts to different sizes without the need for a separate bitmap for each size. The availability of scalable fonts, along with the introduction of the sub-$1,000 LaserJet IIP in September 1990, helped popularize desktop publishing.

Other HP achievements in the early 1990s included the first laser printer designed for Ethernet or Token Ring local-area networks (the LaserJet IIISi in March 1991), and the first laser with graphical remote management software (JetAdmin, introduced with the LaserJet 4Si in April 1993.)

About this time, Lewis says, Xerox and other manufacturers shifted from helium-based lasers to diode lasers for better reliability, smaller size, and lower cost. (A diode is an electronic component through which current can pass only one way). Modern lasers are generally red and solid-state, compared to earlier blue lasers that had gaseous components.

A radically different connection method, the infrared port, allowed users to quickly print files from an infrared-capable handheld PC without a cable. In March 1995, HP’s LaserJet 5P became the world’s first printer to support infrared file transfers.

Recent laser printer advancements have involved combining several related functions into one device. One example is the advent of "send once, print many" features, which combine the traditional roles of printers and copiers. HP's LaserJet 5Si Mopier (introduced in November 1996) received and printed data like a laser printer, but also could produce multiple original pages ("mopies") like a photocopier. The LaserJet 5Si also could photocopy, collate, and staple documents. This foreshadowed other MFDs (multifunction devices) which can offer scanning, copying, printing, and faxing in one unit.

Duplexing, or printing on both sides of the paper, has long been a problem for laser printers. This is because the toner from the first printing can melt when run through the fuser again for the second image. Several monochrome laser printers could duplex automatically by the late 1990s, but HP's 4500 and 8500 (October 1998) were the first color laser printers to do so.

 Today’s Lasers. Today, Canon produces about 80% of the world's laser print engines, and is a leader in research and development. It also has a longstanding partnership with HP. In fact, Thiel says, most of HP's laser printers are hybrids. Canon designs and manufactures the hardware print engines and laser components, while HP engineers the printed circuit boards, firmware, and printing software. Xerox printers, on the other hand, may use Fuji Xerox, Sharp, or Samsung print engines.

The laser strikes a spinning multisided mirror, which continuously reflects the laser from side to side. The light then travels through prisms, off another mirror, and on to the OPCdrum.
Both Rogers and Thiel say that Web-based printer management is one of the most important features in today's laser printers. Software such as Xerox's CentreWare IS or HP's WebJetAdmin lets an administrator control or troubleshoot a specific printer from any Web access point in the world. Thiel also sees a trend toward communication between devices without a host network or even user input. Today's focus is the ability to print over the Web, Thiel says, not just over a specific network.

Thiel says HP's Digital Sending is "essentially color fax at a fraction of the cost." An HP multifunction device such as the LaserJet 3200 can scan a color document and then e-mail it to another printer as an Adobe .PDF file.

Newer HP printers such as the LaserJet 4100 can also run Java applets (programs) using what HP calls an EVM (embedded virtual machine). "The printer effectively becomes a programming environment," Thiel said. "Imagine sending a Java applet to a printer and instructing the printer to search for keywords on designated websites, pull the information and have it printed so that it awaits your 8 a.m. arrival at the office." HP also says EVM technology could e-mail someone when the printer is out of toner, for example.

Other HP technologies include a "private printing" function for secure printing of sensitive documents; "proof and hold," which lets the user check a sample printout before printing the rest of the job; and ImageREt, which blends varying amounts of color toner in the space of a single pixel for smoother color images.

Sometimes, it's the little perks that count. Rogers mentioned a trend toward adding paper-input trays that can hold a full ream (500 sheets) to lower-end printers. This is not only more convenient for users, but it also reduces jams from dogeared or humid paper left unwrapped in partial reams. Also, he described printers that can not only staple pages, but punch holes for three-ring binders, such as Xerox's DocuPrint N4525 and Phaser 3400. Xerox is also installing support libraries and users' manuals on the hard drives in some of their higher-end printers, such as the Phaser 750DX color laser.

 The Future. Past trends and current technologies all indicate that speed, color, and the convergence of functions will be the rage in laser printers in the near future.

Trends. Many observers agree that MFDs will become more popular, often at the expense of single-purpose laser printers and copiers. Manufacturers attempt to add as many features as possible, such as document management and network connections, without degrading the reliability of the machine.

Another trend is a rise in demand for color printers, whether based on laser, solid-ink, or inkjet technology. "There's going to be a major migration happening from monochrome-based printing to color," Thiel says. He cites dropping prices, better performance, and improved print quality as reasons for the trend. Speaking of better performance, Rogers says Xerox's Phaser 2135 can print an astounding 21 color prints per minute. He also predicts that solid-ink printers such as Xerox's Phaser 850 will catch on as simpler, less expensive means to create color business documents than laser printers.

Printing speed should continue to improve, along with the delay before the first page of a job prints. Rogers says a straighter paper path helps the time to first print, while new toners with a lower melting point allow higher printing speeds overall. The latter also means the fuser roller doesn't have to be quite so hot.

Rogers says demand for monochrome models in the United States has plateaued or declined, while color lasers and MFDs have both increased their market shares. "In the last two years, the percentage of laser printers which are color have gone from probably less than five to approaching ten percent," he said. "Most companies that have a need for a (monochrome) laser printer already own (one) at this point, and it's become essentially a replacement market." Rogers also cited the slowed PC market as a factor, as well as the fact that laser printers have longer useful lives than computers. "There are still lots of happy customers with printers that are five years old now," he said. However, Roberto Lebron, assistant director of public relations for Canon, says "Home office and corporate users are generating (the) greatest growth in laser printer demand," today.

In most laser printers, the laser defines the image by drawing it to the OPCdrum. Each point the laser hits will lose its electrostatic charge, attract toner, and transfer it to the paper.
Technologies. One of the high-end technologies at Xerox is ion-deposition, also known as EBI (electron-beam imaging). This process is much faster than laser printing (an astounding 1,300ppm with fanfold paper), but very similar. Instead of a beam of light aimed at an OPC drum, a screen electrode sends electrons (negatively charged particles) at a dielectric (nonconducting) print drum. An image is formed with a negative charge, attracting magnetic toner. The image is affixed to the paper in two steps using a cold pressure roller and a flash fuser (a xenon light bulb). Ion-deposition is chiefly used for high-speed, low-resolution applications such as bill processing, Rogers says.

Thiel predicts a growing number of infrared-compatible laser printers, perhaps assisted by the spread of handheld computers and PDAs (personal digital assistants). Users of handhelds and printers with infrared capability can take advantage of "walk-up printing" without cables. Some notebooks can use the 802.11b wireless networking standard, with the possibility of Bluetooth wireless gear always on the horizon. Both could improve walk-up printing over infrared's line-of-sight requirement.

However, Rogers questions if printers should have wireless capabilities built in, rather than using add-on dongles (here, antenna devices) and wireless network access points to handle this. Although the 802.11b wireless standard is well-established, he said, Bluetooth is still not standardized. "Anything you do today (with Bluetooth) is going to be obsolete almost immediately," Rogers said. "Embedding that cost in every printer doesn't make a lot of sense." Lebron said that Canon wouldn't comment on wireless printing just yet.

Rogers pointed out that there are already vendors making effective wireless hardware that can link mobile devices with several printers on a network. "Our goal there is to make sure that we're compatible with those solutions--whatever the customer may choose," he said. "People certainly are not willing to upgrade their printer just to get a wireless interface. I guarantee you that."

The only certainty in the imaging frontier is that the manufacturers have more motivation than ever to improve their products, and that benefits the entire market.  

by Marty Sems

View the graphics that accompany this article.
(NOTE: These pages are PDF (Portable Document Format) files. You will need Adobe Acrobat Reader to view these pages. Download Adobe Acrobat Reader)

Want more information about a topic you found of interest while reading this article? Type a word or phrase that identifies the topic and click "Search" to find relevant articles from within our editorial database.

© Copyright by Sandhills Publishing Company 2001. All rights reserved.