In the 1980s, dot-matrix and laser printers were pre-dominant, with inkjet technology not emerging in any significant way until the 1990s. The laser printer was introduced by Hewlett-Packard in 1984, based on technology developed by Canon. It worked in a similar way to a photocopier, the difference being the light source. With a photocopier a page is scanned with a bright light, while with a laser printer the light source is, not surprisingly, a laser. After that the process is much the same, with the light creating an electrostatic image of the page onto a charged photoreceptor, which in turn attracts toner in the shape of an electrostatic charge.

Laser printers quickly became popular due to the high quality of their print and their relatively low running costs. As the market for lasers has developed, competition between manufacturers has become increasingly fierce, especially in the production of budget models. Prices have gone down and down as manufacturers have found new ways of cutting costs. Output quality has improved, with 600dpi resolution becoming more standard, and build has become smaller, making them more suited to home use.

Laser printers have a number of advantages over the rival inkjet technology. They produce much better quality black text documents than inkjets, and they tend to be designed more for the long haul - that is, they turn out more pages per month at a lower cost per page than inkjets. So, if it’s an office workhorse that’s required, the laser printer may be the best option. Another factor of importance to both the home and business user is the handling of envelopes, card and other non-regular media, where lasers once again have the edge over inkjets.

Considering what goes into a laser printer, it is amazing they can be produced for so little money. In many ways, the components which make up a laser printer are far more sophisticated than those in a computer. The RIP (raster image processor) might use an advanced RISC processor; the engineering which goes into the bearings for the mirrors is very advanced; and the choice of chemicals for the drum and toner, while often environmentally unsound, is fascinating. Getting the image from the PC’s screen to paper requires an interesting mix of coding, electronics, optics, mechanics and chemistry.

Communication
A laser printer needs to have all the information about a page in its memory before it can start printing. How an image is communicated from the PC’s memory to a laser printer depends on the type of printer being used. The crudest arrangement is the transfer of a bitmap image. In this case there is not much the computer can do to improve on the quality, so sending a dot for a dot is all it can do.

However, if the system knows more about the image than it can display on the screen there are better ways to communicate the data. A standard A4 sheet is 8.5in across and 11in deep. At 300dpi, that is more than eight million dots compared with the eight hundred thousand pixels on a 1024 by 768 screen. There is obviously scope for a much sharper image on paper - even more so at 600dpi, where a page can have 33 million dots.

The major way quality can be improved is by sending a page description consisting of outline/vector information and allowing the printer to make the best possible use of it. If the printer is told to draw a line from one point to another, it can use the basic geometric principle that a line has length but not width, and draw that line one dot wide. The same holds for curves, which can be as fine as the resolution of the printer allows. The idea is that one single page description may be sent to any suitable device, which would subsequently print it to the best of its ability - hence the much-touted term, device independent.

Text characters are made up of lines and curves so can be handled in the same way, but a better solution is to use a pre-described font shape, such as TrueType or Type-1 formats. Along with precise placement, the page description language (PDL) may take a font shape and scale it, rotate it, or generally manipulate it to its heart’s content. There’s the added advantage of only requiring one file per font as opposed to one file for each point size. Having predefined outlines for fonts allows the computer to send a tiny amount of information - one byte per character - and produce text in any of many different font styles and many different font sizes.

Operation
Where the image to be printed is communicated to it via a page description language, the printer’s first job is to convert the instructions into a bitmap. This is done by the printer’s internal processor, and the result is an image (in memory) of which every dot will be placed on the paper. Models designated ‘Windows printers’ don’t have their own processors, so the host PC creates the bitmap, writing it directly to the printer’s memory.

At the heart of the laser printer is a small rotating drum - the organic photo-conducting cartridge (OPC) - with a coating that allows it to hold an electrostatic charge. A laser beam scans across the surface of the drum, selectively imparting points of positive charge onto the drum's surface that will ultimately represent the output image. The area of the drum is the same as that of the paper onto which the image will eventually appear, every point on the drum corresponding to a point on the sheet of paper. In the meantime, the paper is passed through an electrically charged wire which deposits a negative charge onto it.

 

12laser.gif (15097 bytes)

On true laser printers, the selective charging is done by turning the laser on and off as it scans the rotating drum, using a complex arrangement of spinning mirrors and lenses. The principle is the same as that of a disco mirror ball. The lights bounce off the ball onto the floor, track across the floor and disappear as the ball revolves. In a laser printer, the mirror drum spins incredibly quickly and is synchronised with the laser switching on and off. A typical laser printer will perform millions of switches, on and off, every second.

Inside the printer, the drum rotates to build one horizontal line at a time. Clearly, this has to be done very accurately. The smaller the rotation, the higher the resolution down the page - the step rotation on a modern laser printer is typically 1/600th of an inch, giving a 600dpi vertical resolution rating. Similarly, the faster the laser beam is switched on and off, the higher the resolution across the page.

As the drum rotates to present the next area for laser treatment, the written-on area moves into the laser toner. Toner is very fine black powder, negatively charged so as to cause it to be attracted to the points of positive charges on the drum surface. Thus, after a full rotation the drum's surface contains the whole of the required black image.

A sheet of paper now comes into contact with the drum, fed in by a set of rubber rollers. As it completes it's rotation it lifts the toner from the drum by virtue of its magnetic attraction, thereby transferring the image to the paper. Negatively charged areas of the drum don't attract toner and result in white areas on the paper.

Toner is specially designed to melt very quickly and a fusing system now applies heat and pressure to the imaged paper in order to adhere the toner permanently. Wax is the ingredient in the toner which makes it more amenable to the fusion process, while it's the fusing rollers that cause the paper to emerge from a laser printer warm to the touch.

The final stage is to clean the drum of any remnants of toner, ready for the cycle to start again.

There are two forms of cleaning, physical and electrical. With the first, the toner which was not transferred to the paper is mechanically scraped off the drum and the waste toner collected in a bin. Electrical cleaning takes the form of covering the drum with an even electrical charge so the laser can write on it again. This is done by an electrical element called the corona wire. Both the felt pad which cleans the drum and the corona wire need to be changed regularly.

Many so-called laser printers are actually LED printers. LED printers are a cheaper alternative to conventional lasers, where the single laser and directing mirror are replaced by a fixed line of LEDs. A 300dpi LED printer will have 300 LEDs per inch, over the required page width. The advantage is that a row of LEDs is cheaper to make than a laser and mirror with lots of moving parts. The disadvantage is that the horizontal resolution is absolutely fixed, and while you some resolution enhancements can be applied, none of them will be as good as the possible resolution upgrades offered by true lasers. LCD printers work on a similar principle, using a liquid crystal panel as a light source in place of a matrix of LEDs.

Colour lasers
12colour.gif (4178 bytes)Laser printers are usually monochrome devices, but as with most mono technologies, laser printing can be adapted to colour. Any colour can be made from a combination of cyan, magenta and yellow and a colour laser produces colour output by performing four passes through the electro-photographic process, generally placing toners on the page one at a time or building up the four-colour image on an intermediate transfer surface.

Most modern laser printers have a native resolution of 600 or 1200dpi. Lower resolution models can often vary the intensity of their laser/LED spots, but deliver coarser multi-level toner dots resulting in mixed 'contone' and halftone printing, rather than continuous tone output. Rated print speeds vary between 3 and 5ppm in colour and 12 to 14ppm in monochrome. A key area of development, pioneered by Lexmark's 12ppm LED printer launched in the autumn of 1998, is to boost colour print speed up to the same level as mono with simultaneous processing of the four toners and one-pass printing.

The Lexmark Optra Colour 1200N achieves this by having completely separate processes for each colour. The compactness which results from use of LED arrays instead of the bulky focusing paraphernalia associated with a laser imaging unit allows the colour engine to be built with four complete printheads arranged. The CMY and K toner cartridges are laid out in-line down the paper path and each unit has its own photo-conductive drum. Above each unit in the printer's lid are four LED arrays - again, one for each colour. Data can be sent to all four heads simultaneously. The process starts with magenta and passes through cyan and yellow, with black laid down last.

Apart from their speed, one of the main advantages of colour lasers is the durability of their output - a function of the chemically inert toners that are fused onto the paper's surface rather than absorbed into it, as with most inkjets. This allows colour lasers to print well on a variety of media, without the problems of smudging and fading that beset many inkjets. Furthermore, by controlling the amount of heat and pressure in the fusing process, output can be given a user-controllable 'finish', from matte through to gloss.

If history is anything to go by, the future for laser and LED colour printing looks bright. Within four years of the first appearance of colour lasers in 1994 prices approximately halved. With the market continuing to be stimulated, both by falling prices and improved technology, it looks inevitable that the laser or LED colour laser will become as commonplace and as indispensable as the photocopier.

Consumables
Most lasers use cartridge technology based on an organic photoconductive (OPC) drum, coated in light-sensitive material. During the lifetime of the printer, the drum needs to be periodically replaced as its surface wears out and print quality deteriorates. The cartridge is the other big consumable item in a laser printer. Its lifetime depending on the quantity of toner it contains. When the toner runs out, the cartridge is replaced. Sometimes the toner cartridge and the OPC drum are housed separately, but in the worst case, the drum is located inside the cartridge. This means that when the toner runs out, the whole drum containing the OPC cartridge needs to be replaced, which adds considerably to the running costs of the printer and produces large amounts of waste.

The situation is even worse with a colour laser - which can actually have up to nine separate consumables items (four colour toners, an OPC belt or drum, a developer unit, a fuser unit, fuser oil and a waste toner bottle). Many of these must be fitted when the printer is set up, and all expire after varying pages counts, depending on the manufacturer and usage. This high component count is a major reason for the cost and general lack of usability and manageability of colour lasers, and its reduction is a major focus for laser printer manufacturers. 

Some have tried to improve this situation by making drums more durable and eliminating all consumables except for toner. Kyocera, for instance, was the first manufacturer to produce a 'cartridge-free' printer which uses an amorphous silicon drum. The drum uses a robust coating which lasts for the lifetime of the printer, so the only item requiring regular replacement is the toner and even this comes in a package made from a non-toxic plastic, designed to be incinerated without releasing harmful gases.

Environmental issues
Unfortunately, the technology used in laser printers makes ozone an inherent by-product of the printing process. The level of emission depends on where and how a printer is kept. Areas with large concentrations of dust, small enclosed offices or poorly ventilated rooms can cause high ozone intensity. Some printers contain filters to limit ozone concentration to levels below standards which have been established by various bodies - the American Conference of Governmental Industrial Hygienists, for example. After a certain number of pages have passed through a printer (usually about 150,000) the filter should be replaced by an authorised service engineer.

Power-saving abilities are also becoming important in laser printer design. The Environmental Protection Agency (EPA) has stipulated that for a printer to gain Energy Star Compliance, it must dramatically reduce its power consumption when not being used. The power saver usually works by warming up the printer only when it is sent a job. If the printer is left idle for a certain period of time, the printer’s power consumption is reduced. Usually this period of time can be altered by the user and, if preferred, the power saver can be turned off altogether.

Page description languages
Communication between a computer and a printer is very different today to what it was several years ago. Text was sent in ASCII along with simple character codes instructing bold, italic, condensed or enlarged type. Fonts consisted of those built into the printer, distinguished more often than not by a switch selecting serif or sans serif. Graphics were produced line by line, slowly and streakily. The one big advantage of ASCII-described text is that its transmission happens quickly and easily: if the electronic document contains a letter A, the ASCII code for an A is sent and the printer, recognising the code, prints an A. The big problem was that without careful planning, the printed letter rarely ended up in the same position it held on the screen. Worse, the entire process was device-dependent, and so unpredictable, with different printers offering different font shapes and sizes.

PostScript
The situation changed dramatically in 1985 with Adobe’s announcement of PostScript Level 1, based on Forth and arguably the first standard multi-platform device-independent page description language. PostScript describes pages in outline, vector form which is sent to the display or printing device to be converted into dots (rasterised) at the device’s best ability. A monitor could manage 75dpi, a laser 300dpi and an image-setter up to 2400dpi. Each one produced more faithful representations of the PostScript description than the last, but all had the sizes and positions of the shapes in common. Hence device independence and the birth of the acronym, WYSIWYG - What You See Is What You Get.

PostScript Level 1 appealed to the high-end publishers thanks mostly to the fact that proofs made on a 300dpi laser would be laid out identically to those on 2400dpi image setters used to make film. Furthermore, it was possible to send the PostScript instructions from any platform. All that was required was a driver to turn the document information into PostScript which could then be understood by any PostScript printer. These features coupled with graphics snobbery, particularly amongst the Apple Macintosh community, and the fact that Adobe is the only official licenser, made PostScript-equipped devices ultimately desirable and consequently expensive.

PostScript Level 2, released a few years ago, offered device-independent colour, data compression for faster printing, and improved halftone algorithms, memory and resource management. PostScript Extreme (formerly called Supra) is Adobe's newest variant, aimed at the top level of high-volume, high-speed printing systems like digital presses.

PCL
Adobe’s approach left a gap in the market which Hewlett-Packard strove to fill with its own device independent-ish page description language based on its Printer Command Language, PCL, which first appeared in the 1970s.

HP’s marketing has been entirely different to Adobe’s, opting for the mass cloners rather than exclusive licensing. This strategy has resulted in a plethora of printers equipped with clones of PCL costing much less than their PostScript-licensed counterparts. The problem with having so many PCL clones around is that it’s not possible to guarantee 100% identical output on all printers. This is only a problem when the intention is to use high-resolution bureaux and where an exact proof is required before sending them the document files. Only PostScript can offer an absolute guarantee.

PCL was originally made for use with dot-matrix printers and is an escape code rather than a complete PDL. Its first widespread incarnation, version 3, only supported simple printing tasks. PCL 4 added better support for graphics and is still used in personal printers. It requires less processing power than PCL 5, or the latest version PCL 6.

PCL 5, developed for the LaserJet III, offered a similar feature set to PostScript, with scaleable fonts through the Intellifont system and vector descriptions giving WYSIWYG on the desktop. PCL 5 also utilised various forms of compression which speeded up printing times considerably compared to PostScript Level 1. PCL 5e brought bi-directional communication for status reporting, but no extra print quality enhancements, while PCL 5c added specific improvements for colour printers.

In 1996 HP announced PCL 6. First implemented on the LaserJet 5, 5N and 5M workgroup printers, PCL 6 is a complete rewrite. It's a flexible, object-orientated control language, tuned for fast processing of graphically-rich documents and offers better WYSIWYG facilities. This makes it ideal for handling Web pages. The more efficient code, combined with faster processors and dedicated hardware acceleration of the LaserJet 5 printers, results in time-to-first-page speed increases of up to 32% over the LaserJet 4(M)+ printers they replaced.

GDI
The alternative to laser printers which use languages such as PostScript and PCL are Windows GDI (Graphical Device Interface) bitmap printers. These use the PC to render pages before sending them as a bitmap for direct printing, using the printer just as a print engine. Consequently, there's no need for expensive processors or large amounts of on-board RAM, making the printer cheaper. However, sending the complete page in compressed bitmap form takes time, reducing printing speed and increasing the time taken to regain control of the PC. GDI printers are, therefore, generally confined to the personal printer market.

Some manufacturers elect to use the Windows Print System, a standard developed by Microsoft to create a universal architecture for GDI printers. The Windows Printing System works slightly differently to the pure GDI model. It enables the Windows GDI language to be converted to a bitmap while printing; the basic idea being to reduce the heavy dependence of the printer on the PC’s processor. Under this system, the image is actually being rendered during the printing process which greatly reduces the amount of processing power required from the PC. Other laser printer models use a combination of GDI technology and traditional architecture, allowing fast printing from Windows as well as support for native DOS applications.

Adobe PrintGear
An alternative for personal printers is Adobe's PrintGear - a complete hardware/software system based on an Adobe custom-designed processor designed specifically for the lucrative small and home office (SoHo) market. Adobe claims that 90% of typical SoHo documents can be described by a small number of basic objects. They have consequently designed a dedicated 50MHz image processor to specifically handle these RISC-like tasks, which is claimed to offer large speed increases over traditional printer processors and be cheaper, too. A printer equipped with Adobe PrintGear typically features the dedicated processor and a sophisticated software driver, and offers options including tiling, up to 16 thumbnail pages per single sheet, two-sided printing, booklet printing and watermarking.