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.