A laser printer is a type of computer printer that uses a laser beam to produce an image which eventually appears as printed output. The image is created as an array of very fine dots. The use of a laser in this way allows the creation of very fine, detailed images, either text or pictures, with a print quality similar to the offset printing used to produce books and magazines. This chapter outlines the key features of laser printers, and introduces the basic concepts used to describe a page.
The key features of laser printers are their speed, resolution (fineness of print), printer language, and paper handling. As they use a non-impact printing technology laser printers are very quiet, and many users regard the lack of noise as an important buying criteria for office use. Some laser printers are simplex (print one side of the paper only), others can print duplex (both sides of the paper). A few laser printers can print colour images, but most are monochrome devices.
Laser printers are available in a wide range of speeds. Speed in laser printers is measured in pages per minute (p.p.m) or images per minute (i.p.m) (an image is one side of a sheet). These are the same on a simplex (single-sided) printer, but a duplex (double-sided) printer can have two images on each sheet of paper. As a duplex printer is basically a single-sided printer with the ability to turn the paper over to print on the other side, the duplex speed in p.p.m is about half the simplex speed.
The speed quoted by printer manufacturers is a maximum speed, usually the fastest speed at which the print engine (the mechanical printer mechanism which moves the paper) can operate. When printing simple pages (pages which do not have much text or complicated graphics) most printers will achieve the speed claimed by the manufacturer, however complex pages, with large quantities of text or complicated graphics, may require so much processing by the printer controller (the dedicated computer in a printer which formats data into a printable image) that the processing time is too long for the printer to run at full speed. This can have a dramatic effect, it is quite common for printers to run at only 10% of their full speed when printing complicated pages.
Colour laser printers are a special case when measuring speed, as each colour requires a separate pass through the printing mechanism. Most Colour laser printers use four colours, Cyan, Magenta, Yellow and Black, and can print in black and white or colour. When printing in colour they run at a quarter of their black and white printing speed, so a colour printer which prints 30 p.p.m black and white would run at about 7.5 p.p.m in colour mode.
Laser printers are normally classed by their speed, there are Personal Printers, which print around 4 - 5 p.p.m, "Office" or Desktop Printers, which normally fall in the range 8 - 12 p.p.m, Workgroup Printers, which typically output around 15 - 30 p.p.m, and Production Printers which are very large printers running at 50 p.p.m and over. The fastest Production Printers available for printing on cut-sheet stationery (individual sheets of paper) run at 135 p.p.m, but some specialised printers using continuous stationery (fan-fold paper) can produce over 200 p.p.m.
The resolution of a laser printer is the number of individual dots it can print within a specified area. Laser printers make their images using an array of dots called a "bitmap image". Most modern laser printers print at 300 x 300 dots per square inch, i.e.: 90,000 dots. As most printers have the same resolution horizontally and vertically, this measurement is usually abbreviated to "dots per inch" (d.p.i), which refers to the resolution in either the horizontal or the vertical axis. Some of the latest laser printers have a resolution of 600 d.p.i, or 360,000 dots per square inch. Obviously the greater the resolution, the finer and more detailed image a printer can produce.
Some older printers used with IBM mainframe computers print at 240 d.p.i, which was the standard resolution used by I.B.M. printers for many years. There are other printers with resolutions of 400 d.p.i, and 800 x 400 d.p.i (800 dots horizontally, 400 vertically), and some specialist printers used in the publishing industry can produce 1200 x 600 d.p.i.
At 300 d.p.i. and coarser resolutions it is possible for the human eye to see the jagged edge caused by making an image from an array of dots, so with the launch of its' LaserJet III desktop printer Hewlett-Packard introduced a technique called "Resolution Enhancement Technology" (RET) which automatically inserts smaller dots at the edges of lines and characters to smooth out the edge and reduce the jagged appearance. This technique does not increase the resolution of a printer, but it can improve the quality of printed image by eliminating rough edges. The resolution enhancement technique has been copied by many other printers and is generally referred to as "edge enhancement". RET is a trademark of Hewlett-Packard Corporation.
Picture of the letter "A" as an array of dots (left). In reality the dots overlap (right), as they are round or oval this is necessary to avoid small gaps between dots.
Picture of the small dots inserted by the Edge Enhancement process (left), and the finished effect showing much smoother edges (right).
A few specialist production printers, such as the Xerox 4650, use a technique called interpolation. The 4650 prints at 600 d.p.i., but for speed it can process images at 300 d.p.i and automatically scale or "interpolate" them to 600 d.p.i. when printing. This is useful on high speed production printers because a 600 x 600 d.p.i image contains four times as much data as a 300 x 300 d.p.i image, and thus need four times the processing power to format the image in a given time.
The language used by a printer is the set of commands it obeys to format data sent from a computer. These commands are embedded in the data by the computer, and interpreted by the printer. There are many printer languages, some of these are explicitly designed for laser printers, some are designed for older, simpler printers, but can be interpreted by some laser printers for compatibility with old computer software.
The choice of printer language is very important, as most computer applications only support a subset of the printer languages in use. IBM mainframe and minicomputer software usually only supports printer languages proprietary to I.B.M. In the Macintosh environment most applications output Adobe PostScript, an industry standard language for describing complex pages. In the P.C. world applications normally support a range of printer languages including all the "industry standard" or commonly used languages, and a handful of older languages designed for dot-matrix or daisywheel printers.
Printer languages generally fall into two categories, Page Description Languages (PDLs) and Escape Code languages. Page Description Languages are generally more versatile and sophisticated allowing more complex pages and graphics to be created, and are suitable for typographically advanced documents such as presentation materials, technical manuals, catalogues, advertising brochures etc.. Page Description Languages are normally used by software packages which produce sophisticated, highly formatted output, including graphics design programs, advanced word-processing packages, spreadsheets with extensive charting capabilities, and desktop publishing packages.
Escape Code languages are generally characterised by the structure of the commands they use, each command is prefixed by a special code (normally the Escape code, (1B hexadecimal, 27 decimal) hence the name) to signify that the following characters are a command, and not data to be printed. Escape Code languages do not offer the flexibility of PDLs, and are suited to simple documents such as letters, database printouts, and documents containing simple graphics. Most Escape Code languages do not offer sophisticated typographic features, so while they may be able to use a range of different fonts, most Escape Code languages can only print text in a limited number of sizes, and cannot handle special effects such as printing text at an angle or on a curved line.
The main disadvantage with Page Description Languages is that they require a lot of processing power, with the result that PDL printers format data more slowly than printers using an Escape Code language. The only way of overcoming this is to make the printer controller more powerful, with the result that most PDL printers are more expensive than Escape Code Printers. Another disadvantage of PDLs is that some older applications cannot use PDL printers, so most PDL printers have an Escape Code language built into them as a fallback contingency.
Laser printers are very attractive to anyone working with graphics because of their print quality. As a result of this, some laser printers have the ability to emulate other graphics output devices such as pen plotters. The majority of pen plotters use a language called HPGL (Hewlett-Packard Graphics Language), and it is common for a laser printer to offer the ability to use HPGL. As most laser printers are monochrome, and only handle small paper sizes, whereas most pen plotters are used with coloured pens and very large paper sizes, the laser printer is normally used as a draft output facility. This is still very valuable, as a complex drawing can easily take 30 minutes to draw on a plotter, while the draft output from a laser printer is produced in a few seconds. In some specialist graphics applications, such as printed circuit board design, laser printers are the first choice for printer output because only monochrome images are required and the print sizes are relatively small.
Laser printers normally use cut-sheet stationery (there are a very few high speed printers using continuous stationery). The page size used in most laser printers is fixed by the size of the paper input tray (the paper container which inserts into the paper feeder mechanism). Most laser printers are supplied with single-size paper trays for the standard business stationery, which in North America is "Letter" size (8.5" x 11"), and in the rest of the world is A4 (210mm x 297mm). Other paper sizes can be handled using different paper trays, which are normally optional extras, or via a "manual feed" facility, which allows individual sheets of odd-sized paper to be used. The manual feed process is cumbersome, as each sheet of paper has to be fed by hand into the printer. A few models of printer provide an adjustable paper tray capable of taking a range of paper sizes, either as standard or as an optional extra.
Desktop laser printers generally do not cater for sizes larger than A4 or Letter, as larger sizes of paper preclude the small footprint which characterises a desktop printer, but some larger printers will print on A3 or 17" x 11" at half-speed.
In addition to paper, most laser printers will print on other materials including lightweight card, adhesive labels, and overhead projection transparencies. Individual models of printer each have their limitations in handling these materials, depending on the design of the printing mechanism, these limitations are normally specified by the printer manufacturer in the user manual. Most laser printers will also print on envelopes using the manual feed facility, and a few printers are available with an optional envelope feeder which is capable of holding and feeding a stack of envelopes.
The number and capacity of paper input feeders on a printer varies enormously, depending on the size and type of printer. The majority of personal printers provide one input feeder holding about 100 sheets of standard paper, and a manual feed facility. Paper feeders on larger printers tend to be correspondingly larger, office printers typically hold 250 sheets per tray, and often allow two trays to be loaded at once. Workgroup printers sometimes have special "high capacity feeders", motor-driven paper feeders holding around 1,000 sheets. Large production printers may have several motor-driven feeders holding up to 2,500 sheets each (a 90 p.p.m production printer can use 5,000 sheets in an hour).
The majority of desktop printers are simplex, i.e. they print on one side of the printer only. A few of the larger desktop printers are available with an optional duplex unit, which turns each sheet of paper over so that the other side can be printed on. The majority of floor-standing printers are large and expensive enough to justify the inclusion of a built-in duplex unit. Duplex printing is essential for production applications where finished professional documents are being produced, and for short-run prints of technical manuals etc., but is relatively unimportant for most office printing needs. Obviously duplex printing reduces paper consumption, and is therefore useful for large reports and environmentally conscious users, but the majority of users producing office correspondence, short reports and presentation materials do not benefit significantly from duplex printing.
The way that duplex printers operate is to print one side of the paper, turn the paper over, and then print on the other side, i.e. one duplex sheet requires two printing operations. Because of this most duplex printers run at half speed, or slightly less, when printing duplex.
The majority of office users do not need, and cannot justify, a duplex printer, but some occasionally require a duplex printout. If this is required, most printers will tolerate "manual duplex operation", whereby one side of the document is printed, and then the user reloads the paper the other way up and prints the second side. Manual duplex is a complicated procedure on multi-page documents, as it requires application software which will print alternate pages, e.g. for a 20 page document, pages 1,3,5 through to 19 are printed first, then the stack of printed paper is reloaded into the printer the other way up, and pages 20, 18, 16 through to 2 are printed on the other side. Some desktop publishing packages make this easier by allowing the user choose to print only odd or only even pages, and to print from the beginning of the document to the end, or from the end of the document to the beginning.
Duplex printers are usually less reliable than simplex printers, as the paper turning mechanism is mechanically complicated, and the paper is more likely to jam when one side has already been printed. Printer mechanisms also vary in their consistency and accuracy when feeding the paper, and do not always feed exactly square, so that the edge of the printed area is not parallel with the edge of the paper. This fault, called "skew", is particularly obvious when printing duplex as the printing on the two sides of the paper will not align properly.
In order to understand laser printers it is necessary to have some concept of the way they work and the data they use. The next chapter describes the basic mechanisms of a laser printer and how it works to create and print a page, but to understand this a little knowledge of the concepts used to describe a page is needed.
The image on a printed page comprises two elements, text and graphics. These two elements may be described in several ways, and different models of laser printer use different methods to describe and construct the image on a page.
The term "text" describes the letters and other symbols on a page, which form words in a human-readable language. Anything printed on a page which is not a picture or graphic illustration is text. Text is made up of pre-defined letters and symbols which are normally stored in the printer as fonts. Fonts are complex structures which have several main characteristics, including character set, typeface, typestyle, size and orientation. These characteristics can be divided further, but they are sufficient for a basic understanding of fonts.
A character set is a collection of symbols, the letter "e" is a character, as is the symbol "¼". There are many thousands of characters used by different languages around the world, and storing them all would require a large amount of memory. To alleviate this problem characters are stored in useful collections or sets, which contain the letters and symbols needed for everyday use in a particular language or group of languages. There are character sets for use with specific languages (French, German, Swedish etc.), and character sets containing less frequently used characters (mathematical symbols, decorative symbols). In this way the massive portfolio of characters in use throughout the world is broken down into manageable collections which are small enough to allow rapid access to the character required, and which are logically arranged so that frequent swapping between character sets is avoided. In this way each font contains only a subset of possible characters, called the character set.
A typeface is a particular visual design of characters. There are many different typefaces used for different purposes. Some typefaces are very clear and easy to read, others are highly ornate and decorative, there are typefaces designed to pack the maximum amount of information onto a page, and others which are designed to be read by machines as well as people. Examples of common typefaces include "Times" and "Courier".
A typeface is normally available in a variety of different combinations of weight (boldness) and slant (italicisation). These factors combined are the typestyle. A typestyle in which the characters are made of very fine lines is referred to as "light", and one where the lines are thick is called "heavy" or "bold". If the characters are upright the typestyle is described as "Roman", and where they are slanted the typestyle is normally referred to as "Italic". There are many possible combinations of slant and weight, but the common ones used in laser printing are "Roman", "Italic", "Bold" and "Bold-Italic".
When a character is printed on the page, it is at a specific size. Characters are normally measured by height, as this determines the spacing interval at which each line of text is placed. The unit of measurement for character height comes from the printing industry, and is the "point", abbreviated to "pt". One point is one seventy-second of an inch, i.e. there are 72 points to an inch. The size of a typeface is measured as being the distance from the apex of the highest character to the bottom of the lowest descender (the portion of a character which descends below the baseline of the text). In some printers fonts are stored at a particular size, so the printer may contain a 10pt font, a 12pt font etc. In other printers the font may be stored in a generic form which can be scaled to any size on demand.
When a character is printed on the page, it is printed at a specific angle. Normally a line of text is printed parallel to the short edge of a sheet of paper, characters designed to be printed this way are referred to as "portrait". Characters which are designed to be used when a line of text is printed parallel to the long edge of the paper are referred to as "landscape". Most printers can automatically rotate text between portrait and landscape orientations, but some older models cannot, and have to store the two orientations in different fonts. Some advanced printers are able to rotate characters to any angle, allowing them to print diagonal text, text which follows a curve, and other special effects.
The term "graphics" describes any items on a page which are not text characters, thus a horizontal or vertical line is a graphic item, as is an illustration or a picture. Graphics may be described to a printer as either an array of dots (bitmap graphics), or as a collection of lines (vector graphics).
Bitmap graphics are graphic images (illustrations, pictures etc.) which are sent to the printer as an array of dots. Each dot is normally either black or white, and the pattern made by the dots forms a visual image. As the dots on a laser printer are very fine (a 300 d.p.i. printer uses 90,000 dots per square inch), a bitmap graphic contains a lot of information. This information has to be sent to the printer by a computer, and held in the printer, so bitmap graphics often take a long time for the computer to send to the printer, and require a lot of memory in the printer. A laser printer works by forming a page as an array of dots, so it is a simple process for the printer to take the array of dots that is a bitmap graphic, and add it to the array of dots which makes the page being printed, because of this bitmap graphics are commonly used on cheap printers with little processing power, and on high speed printers that do not have long in which to assemble a page.
Many graphic images (illustrations) can be defined as a set of lines or curves. These are referred to as vector graphics, because each line is described as a mathematical vector defining the path between two points (co-ordinates) on the page. A vector requires relatively little information to describe it, irrespective of size. Most vectors can be described by start and end co-ordinates, line thickness, and line colour (in monochrome laser printers the colour is the shade of grey required). Curves may be described as a series of vectors, or as a mathematical equation describing the curve. In this way a large and complicated picture can be drawn using relatively little information, so vector graphics can be sent very quickly from a computer to a printer, and only require a little storage space on the printer. When the printer creates a page using vector graphics it has to convert the vectors into an array of dots (a bitmap) for printing. This process requires a lot of processing power, so vector graphics are normally only found on the more expensive, high specification models of printer. Some types of picture, such as photographs, cannot be described using vector graphics, so all printers have the ability to use bitmap graphics which can be used to describe any picture.