Friday, August 19, 2011
Inkjet Printer
Inkjet Printer
Although inkjets were available in the 1980s, it was only in the 1990s that prices dropped enough to bring the technology to the high street. Canon claims to have invented what it terms 'bubble jet' technology in 1977, when a researcher accidentally touched an ink-filled syringe with a hot soldering iron. The heat forced a drop of ink out of the needle and so began the development of a new printing method.
Inkjet printers have made rapid technological advances in recent years. The three-colour printer has been around for several years now and has succeeded in making colour inkjet printing an affordable option; but as the superior four-colour model became cheaper to produce, the swappable cartridge model was gradually phased out.
Inkjets have one massive attraction over laser printers; they produce colour, and that is what makes them so popular with home users. The down side is that although inkjets are generally cheaper to buy than lasers, they are more expensive to maintain. Cartridges need to be changed more frequently and the special coated paper required to produce high-quality output is very expensive. When it comes to comparing the cost per page, inkjets work out about ten times more expensive than laser printers.
Since the invention of the inkjet, colour printing has become immensely popular. Research in inkjet technology is making continual advances, with each new product on the market showing improvements in performance, usability, and output quality. As the process of refinement continues, so the price of an inkjet printers continue to fall.
Operation
Inkjet printing, like laser printing, is a non-impact method. Ink is emitted from nozzles as they pass over a variety of possible media, and the operation of an inkjet printer is easy to visualise: liquid ink in various colours being squirted at the paper to build up an image. A print head scans the page in horizontal strips, using a motor assembly to move it from left to right and back, as another motor assembly rolls the paper in vertical steps. A strip of the image is printed, then the paper moves on, ready for the next strip. To speed things up, the print head doesn’t print just a single row of pixels in each pass, but a vertical row of pixels at a time.
On ordinary inkjets, the print head takes about half a second to print a strip across a page. Since A4 paper is about 8.5in wide and inkjets operate at a minimum of 300dpi, this means there are at least 2,475 dots across the page. The print head has, therefore, about 1/5000th of a second to respond as to whether or not a dot needs printing. In the future, fabrication advances will allow bigger print-heads with more nozzles firing at faster frequencies, delivering native resolutions of up to 1200dpi and print speeds approaching those of current colour laser printers (3 to 4ppm in colour, 12 to 14ppm in monochrome).
There are several types of inkjet technology but the most common is ‘drop on demand’ (DOD). This works by squirting small droplets of ink onto paper, through tiny nozzles: like turning a hosepipe on and off 5,000 times a second. The amount of ink propelled onto the page is determined by the driver software that dictates which nozzles shoot droplets, and when.
The nozzles used in inkjet printers are hair fine and on early models they became easily clogged. On modern inkjet printers this is rarely a problem, but changing cartridges can still be messy on some machines. Another problem with inkjet technology is a tendency for the ink to smudge immediately after printing, but this, too, has improved drastically during the past few years with the development of new ink compositions.
Thermal technology
Most inkjets use thermal technology, whereby heat is used to fire ink onto the paper. There are three main stages with this method. The squirt is initiated by heating the ink to create a bubble until the pressure forces it to burst and hit the paper. The bubble then collapses as the element cools, and the resulting vacuum draws ink from the reservoir to replace the ink that was ejected. This is the method favoured by Canon and Hewlett-Packard.
Thermal technology imposes certain limitations on the printing process in that whatever type of ink is used, it must be resistant to heat because the firing process is heat-based. The use of heat in thermal printers creates a need for a cooling process as well, which levies a small time overhead on the printing process.
Tiny heating elements are used to eject ink droplets from the print-head's nozzles. Today's thermal inkjets have print heads containing between 300 and 600 nozzles in total, each about the diameter of a human hair (approx. 70 microns). These deliver drop volumes of around 8 - 10 picolitres (a picolitre is a million millionth of a litre), and dot sizes of between 50 and 60 microns in diameter. By comparison, the smallest dot size visible to the naked eye is around 30 microns. Dye-based cyan, magenta and yellow inks are normally delivered via a combined CMY print-head. Several small colour ink drops - typically between four and eight - can be combined to deliver a variable dot size, a bigger palette of non-halftoned colours and smoother halftones. Black ink, which is generally based on bigger pigment molecules, is delivered from a separate print-head in larger drop volumes of around 35pl.
Nozzle density, corresponding to the printer's native resolution, varies between 300 and 600dpi, with enhanced resolutions of 1200dpi increasingly available. Print speed is chiefly a function of the frequency with which the nozzles can be made to fire ink drops and the width of the swath printed by the print-head. Typically this is around 12MHz and half an inch respectively, giving print speeds of between 4 to 8ppm (pages per minute) for monochrome text and 2 to 4ppm for colour text and graphics.
Piezo-electric technology
Epson's proprietary inkjet technology uses a piezo crystal at the back of the ink reservoir. This is rather like a loudspeaker cone - it flexes when an electric current flows through it. So, whenever a dot is required, a current is applied to the piezo element, the element flexes and in so doing forces a drop of ink out of the nozzle.
There are several advantages to the piezo method. The process allows more control over the shape and size of ink droplet release. The tiny fluctuations in the crystal allow for smaller droplet sizes and hence higher nozzle density. Also, unlike with thermal technology, the ink does not have to be heated and cooled between each cycle. This saves time, and the ink itself is tailored more for its absorption properties than its ability to withstand high temperatures. This allows more freedom for developing new chemical properties in inks.
Epson's latest mainstream inkjets have black print-heads with 128 nozzles and colour (CMY) print-heads with 192 nozzles (64 for each colour), addressing a native resolution of 720 by 720dpi. Because the piezo process can deliver small and perfectly formed dots with high accuracy, Epson is able to offer an enhanced resolution of 1440 by 720dpi - although this is achieved by the print-head making two passes, with a consequent reduction in print speed. The tailored inks Epson has developed for use with its piezo technology are solvent-based and extremely quick-drying. They penetrate the paper and maintain their shape rather than spreading out on the surface and causing dots to interact with one another. The result is extremely good print quality, especially on coated or glossy paper.
Colour perception
Visible light falls between 380nm (violet) and 780nm (red) on the electromagnetic spectrum, sandwiched between ultraviolet and infrared. White light comprises approximately equal proportions of all the visible wavelengths, and when this shine on or through an object, some wavelengths are absorbed and others are reflected or transmitted. It's the reflected or transmitted light that gives the object its perceived colour. Leaves, for example, are their familiar colour because chlorophyll absorbs light at the blue and red ends of the spectrum and reflects the green part in the middle.
The 'temperature' of the light source, measured in Kelvin (K), affects an object's perceived colour. White light, as emitted by the fluorescent lamps in a viewing box or by a photographer's flashlight, has an even distribution of wavelengths, corresponding to a temperature of around 6,000K, and doesn't distort colours. Standard light bulbs, however, emit less light from the blue end of the spectrum, corresponding to a temperature of around 3,000K, and cause objects to appear more yellow.
Humans perceive colour via a layer of light-sensitive cells on the back of the eye called the retina. The key retinal cells are the cones that contain photo-pigments that render them sensitive to red, green or blue light (the other light-sensitive cells, the rods, are only activated in dim light). Light passing through the eye is regulated by the iris and focused by the lens onto the retina, where cones are stimulated by the relevant wavelengths. Signals from the millions of cones are passed via the optic nerve to the brain, which assembles them into a colour image.
Creating colour
Creating colour accurately on paper has been one of the major areas of research in colour printing. Like monitors, printers closely position different amounts of key primary colours which, from a distance, merge to form any colour; this process is known as dithering.
Monitors and printers do this slightly differently however because monitors are light sources, whereas the output from printers reflects light. So, monitors mix the light from phosphors made of the primary additive colours: red, green and blue (RGB), while printers use inks made of the primary subtractive colours: cyan, magenta and yellow (CMY). White light is absorbed by the coloured inks, reflecting the desired colour. In each case, the basic primary colours are dithered to form the entire spectrum. Dithering breaks a colour pixel into an array of dots so that each dot is made up of one of the basic colours or left blank.
The reproduction of colour from the monitor to the printer output is also a major area of research known as colour-matching. Colours vary from monitor to monitor and the colours on the printed page do not always match up with what is displayed on-screen. The colour generated on the printed page is dependent on the colour system used and the particular printer model; not by the colours shown on the monitor. Printer manufacturers have put lots of money into the research of accurate monitor/printer colour-matching.
Modern inkjets are able to print in colour and black and white, but the way they switch between the two modes varies between different models. The basic design is determined by the number of inks in the machine. Printers containing four colours - cyan, yellow, magenta, and black (CMYK) - can switch between black and white text and colour images all on the same page with no problem. Printers equipped with only three colours, can’t.
Many of the cheaper inkjet models have room for only one cartridge. You can set them up with a black ink cartridge for monochrome printing, or a three-colour cartridge (CMY) for colour printing, but you can’t set them up for both at the same time. This makes a big difference to the operation of the printer. Each time you want to change from black and white to colour, you must physically swap over the cartridges. When you use black on a colour page, it will be made up from the three colours, which tends to result in an unsatisfactory dark green or grey colour usually referred to as composite black. However, the composite black produced by current inkjet printers is much better than it was a few years ago due to the continual advancements in ink chemistry.
Print quality
The two main determinants of colour print quality are resolution, measured in dots per inch (dpi), and the number of levels or graduations that can be printed per dot. Generally speaking, the higher the resolution and the more levels per dot, the better the overall print quality.
In practice, most printers make a trade-off, some opting for higher resolution and others settling for more levels per dot, the best solution depending on the printer's intended use. Graphic arts professionals, for example, are interested in maximising the number of levels per dot to deliver 'photographic' image quality, while general business users will require reasonably high resolution so as to achieve good text quality as well as good image quality.
The simplest type of colour printer is a binary device in which the cyan, magenta, yellow and black dots are either 'on' (printed) or 'off' (not printed), with no intermediate levels possible. If ink (or toner) dots can be mixed together to make intermediate colours, then a binary CMYK printer can only print eight 'solid' colours (cyan, magenta, yellow, red, green and blue, plus black and white). Clearly this isn't a big enough palette to deliver good colour print quality, which is where halftoning comes in.
Halftoning algorithms divide a printer's native dot resolution into a grid of halftone cells and then turn on varying numbers of dots within these cells in order to mimic a variable dot size. By carefully combining cells containing different proportions of CMYK dots, a halftoning printer can 'fool' the human eye into seeing a palette of millions of colours rather than just a few.
In continuous tone printing there's an unlimited palette of solid colours. In practice, 'unlimited' means 16.7 million colours, which is more than the human eye can distinguish. To achieve this, the printer must be able to create and overlay 256 shades per dot per colour, which obviously requires precise control over dot creation and placement. Continuous tone printing is largely the province of dye sublimation printers. However, all of the mainstream printing technologies can produce multiple shades (usually between 4 and 16) per dot, allowing them to deliver a richer palette of solid colours and smoother halftones. Such devices are referred to as 'contone' printers.
Recently, 'six-colour' inkjet printers have appeared on the market, specifically targeted at delivering 'photographic-quality' output. These devices add two further inks - light cyan and light magenta - to make up for current inkjet technology's inability to create very small (and therefore light) dots. Six-colour inkjets produce more subtle flesh tones and finer colour graduations than standard CMYK devices, but are likely to become unnecessary in the future, when ink drop volumes are expected to shrink to around 2 to 4 picolitres. Smaller drop sizes will also reduce the amount of halftoning required, as a wider range of tiny drops can be combined to create a bigger palette of solid colours.
Long-time market leader Hewlett-Packard has consistently espoused the advantages of improving colour print quality by increasing the number of colours that can be printed on an individual dot rather than simply increasing dpi, arguing that the latter approach both sacrifices speed and causes problems arising from excess ink - especially on plain paper. HP manufactured the first inkjet printer to print more than eight colours (or two drops of ink) on a dot in 1996, it's DeskJet 850C being capable of printing up to four drops of ink on a dot. Over the years it has progressively refined its PhotoREt colour layering technology to the point where, by late 1999, it was capable of producing an extremely small 5pl drop size and up to 29 ink drops per dot - sufficient to represent over 3,500 printable colours per dot.
Colour management
The human eye can distinguish around a million colours, the precise number depending on the individual observer and viewing conditions. Colour devices create colours in different ways, resulting in different colour gamuts.
Colour can be described conceptually by a three-dimensional HSB model:
Hue (H) refers to the basic colour in terms of one or two dominant primary colours (red, or blue-green, for example); it is measured as a position on the standard colour wheel, and is described as an angle in degrees, between 0 to 360.
Saturation (S), also referred to as chroma, refers to the intensity of the dominant colours; it is measured as a percentage from 0 to 100 percent - at 0% the colour would contain no hue, and would be grey, at 100%, the colour is fully saturated.
Brightness (B) refers to the colour's proximity to white or black, which is a function of the amplitude of the light that stimulates the eye's receptors; it is also measured as a percentage - if any hue has a brightness of 0%, it becomes black, with 100% it becomes fully light.
RGB (Red, Green, Blue) and CMYK (Cyan, Magenta, Yellow, Black) are other common colour models. CRT monitors use the former, creating colour by causing red, green, and blue phosphors to glow; this system is called additive colour. Mixing different amounts of each of the red, green or blue, creates different colours, and each can be measured from 0 to 255. If all red, green and blue are set to 0, the colour is black, is all are set to 255, the colour is white.
Printed material is created by applying inks or toner to white paper. The pigments in the ink absorb light selectively so that only parts of the spectrum are reflected back to the viewer's eye, hence the term subtractive colour. The basic printing ink colours are cyan, magenta, and yellow, and a fourth ink, black, is usually added to create purer, deeper shadows and a wider range of shades. By using varying amounts of these 'process colours' a large number of different colours can be produced. Here the level of ink is measured from 0% to 100%, with orange, for example being represented by 0% cyan, 50% magenta, 100% yellow and 0% black.
The CIE (Commission Internationale de l'Eclairage) was formed early in this century to develop standards for the specification of light and illumination and was responsible for the first colour space model. This defined colour as a combination of three axes: x, y, and z, with, in broad terms, x representing the amount of redness in a colour, y the amount of greenness and lightness (bright-to-dark), and z the amount of blueness. In 1931 this system was adopted as the CIE x*y*z model, and it's the basis for most other colour space models. The most familiar refinement is the Yxy model, in which the near triangular xy planes represent colours with the same lightness, with lightness varying along the Y-axis. Subsequent developments, such as the L*a*b and L*u*v models released in 1978, map the distances between colour co-ordinates more accurately to the human colour perception system.
For colour is to be an effective tool, it must be possible to create and enforce consistent, predictable colour in a production chain: scanners, software, monitors, desktop printers, external PostScript output devices, prepress service bureaux, and printing presses. The dilemma is that different devices just can't create the same range of colours. It is in the field of colour management that all of this colour modelling effort comes into its own. This uses the device-independent CIE colour space to mediate between the colour gamuts of the various different devices. Colour management systems are based on generic profiles of different colour devices, which describe their imaging technologies, gamuts and operational methods. These profiles are then fine-tuned by calibrating actual devices to measure and correct any deviations from ideal performance. Finally, colours are translated from one device to another, with mapping algorithms choosing the optimal replacements for out-of-gamut colours that can't be handled.
Until Apple introduced ColorSync as a part of its System 7.x operating system in 1992, colour management was left to specific applications. These high-end systems have produced impressive results, but they are computationally intensive and mutually incompatible. Recognising the problems of cross-platform colour, the ICC (International Colour Consortium, although originally named the ColorSync Profile Consortium) was formed in March 1994 to establish a common device profile format. The founding companies included Adobe, Agfa, Apple, Kodak, Microsoft, Silicon Graphics, Sun Microsystems, and Taligent.
The goal of the ICC is to provide true portable colour that will work in all hardware and software environments, and it published its first standard - version 3 of the ICC Profile Format - in June 1994. There are two parts to the ICC profile; the contains information about the profile itself, such as what device created the profile and when and the second is colourmetric device characterisation, which explains how the device renders colour. The following year Windows 95 became the first Microsoft operating environment to include colour management and support for ICC-compliant profiles, via the ICM (Image Colour Management) system.
Ink
Whatever technology is applied to printer hardware, the final product consists of ink on paper, so these two elements are vitally important when it comes to producing quality results. The quality of output from inkjet printers ranges from poor, with dull colours and visible banding, to excellent, near-photographic quality.
Two entirely different types of ink are used in inkjet printers: one is slow and penetrating and takes about ten seconds to dry, and the other is fast-drying ink which dries at about 100 times this speed. The former is generally better suited to straightforward monochrome printing, while the latter is used for colour. With colour printing, because different inks are mixed, they need to dry as quickly as possible to avoid blurring. If slow-drying ink is used for colour printing, the colours tend to bleed into one another before they’ve dried.
The ink used in inkjet technology is water-based and this poses other problems. The results from some of the earlier inkjet printers were prone to smudging and running, but over the past few years there have been enormous improvements in ink chemistry. Oil-based ink is not really a solution to the problem because it would impose a far higher maintenance cost on the hardware. Printer manufacturers are making continual progress in the development of water-resistant inks, but the results from inkjet printers are still weak compared to lasers.
One of the major goals of inkjet manufacturers is to develop the ability to print on almost any media. The secret to this is ink chemistry, and most inkjet manufacturers will jealously protect their own formulas. Companies like Hewlett-Packard, Canon and Epson invest large sums of money in research to make continual advancements in ink pigments, qualities of lightfastness and waterfastness, and suitability for printing on a wide variety of media.
Today's inkjets use dyes, based on small molecules (<50nm), for the cyan, magenta and yellow inks. These have high brilliance and wide colour gamut, but aren't light-fast or water-fast enough. Pigments, based on bigger (50 to 100nm) molecules, are more waterproof and fade-resistant, but can't yet deliver the range of colours that dyes do and aren't transparent. This means that pigments are currently only used for the black ink. Future developments will concentrate on creating water-fast and light-fast CMY inks based on smaller pigment-type molecules.
Paper
Most of the current generation of inkjet printers require high-quality coated or glossy paper for the production of photo-realistic output, and this can be very expensive. One of the ultimate aims of inkjet printer manufacturers is to make colour printing media-independent, and the attainment of this goal is generally measured by the output quality achieved on plain copier paper. This has vastly improved over the past few years, but coated or glossy paper is still needed to achieve full-colour photographic quality. Some printer manufacturers, like Epson, even have its own proprietary paper, which is optimised for use with its piezo-electric technology.
Inkjet printers can become expensive when printer manufacturers tie you to their proprietary consumables. Paper produced by independent companies is much cheaper than that supplied directly by printer manufacturers, but it tends to rely on its universal properties and rarely takes advantage of the idiosyncratic features of particular printer models.
A great deal of research has gone into the production of universal paper types, which are optimised specifically for colour inkjet printers. PLUS Colour Jet paper, produced by Wiggins Teape, is a coated paper produced specifically for colour inkjet technology, and Conqueror CX22 is designed for black ink and spot-colour business documents and is optimised both for inkjet and laser printers.
Paper pre-conditioning seeks to improve inkjet quality on plain paper by priming the media to receive ink with an agent that binds pigment to the paper, reducing dot gain and smearing. A great deal of effort is going in to trying to achieve this without incurring a dramatic performance hit - if this yields results, one of the major barriers to widespread use of inkjet technology will have been removed.
Manageability and Cost
There's no doubt that the inkjet printer has been one of desktop computing's success stories of the late 1990s. It's first phase of development was the monochrome inkjet of the late 1980s - a low-cost alternative to the laser printer. The second spanned the arrival of colour and its development to the point of effective photographic quality - giving the inkjet an all-round capability unmatched by any other printer technology. However, when it comes to manageability and running costs, the inkjet trails its rival laser technology by some distance and it is on improving these aspects of the technology that the inkjet's third phase of development will focus.
Hewlett-Packard's HP2000C inkjet, launched in late 1998, signalled encouraging progress in this direction. Most inkjet printers combine the ink reservoir and the print head in one unit. When the ink runs out its necessary to replace both - even though print heads can have a lifetime many times that of ink reservoirs. The HP2000C differs radically from traditional designs, using a modular system in which the ink cartridges and print heads are kept as separate units. The printer uses four pressurised cartridges, which hold 8cm3 of ink each and remain static underneath a hinged cover at the front of the printer. These are connected by tubes which are integrated with the standard ribbon-style cable that runs to the print head carriage. Internal smart chips monitor the supply, activating a plunger on the relevant cartridge when it requires a refill. Each ink cartridge can keep track of how much ink it has used and how much remains, even if it is moved between printers. The print heads are also self-monitoring - triggering an alert when they need to be replaced. The whole system can look at the requirements for a particular print job and only start if it determines there is sufficient ink to complete it.
Wasted ink is also a problem, which adversely affects running costs. With printers which combine the cyan, yellow and magenta inks from a single tri-colour cartridge, the emptying of one reservoir requires the replacement of the whole cartridge, regardless of how much ink is left in the other two reservoirs. The solution to this problem, deployed by a number of printers already, is to have a separate, independently-replaceable, ink cartridge for each colour. The downside is increased maintenance effort - an inkjet printer that uses four cartridges typically requiring twice the attention of one where the three colours are combined.
In terms of manageability, the HP2000C includes another innovative feature. The incorporation of a second paper tray means that two paper types can be kept in the printer at once to minimise user attention. This is essential in a networked environment - as is the ability to warn of impending ink depletion.
Print capacities also have to improve. At the end of 1998 the standard for personal laser printers was around 3,000 pages from a toner/drum cartridge. Typically the best an inkjet could manage was around 500 to 900 pages from a single black ink cartridge. Colour ink use fared even worse - supporting a capacity of between 200 and 500 pages only. Print speeds are expected to reach 10ppm by the year 2000, and with these increased print speeds will come increased cartridge capacities. Inkjet manufacturers are expected to introduce workgroup colour printers with much larger secondary ink containers linked to small primary ink reservoirs close to or in the print head. These printers will automatically replenish the small primary reservoir from the secondary as needed.
Another area in which reductions in running costs can be made is paper. The expectation is that the recent preoccupation with outright photographic quality on high-cost glossy paper will diminish as inkjet technologists start to focus on getting better results from plain paper for the next generation of inkjet printers.
LASER Printer
LASER Printer
In the 1980s, dot matrix and laser printers were pre-dominant, with inkjet technology not emerging in any significant way until the 1990s. Hewlett-Packard introduced the laser printer 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 competitive inkjet technology. They produce much better quality black text documents than inkjets, and they tend to be designed more for the long load - 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.
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 synchronized 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 stick 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 pads which cleans the drum and the corona wire need to be changed regularly.
LED Printers
LED (light-emitting diode) page printing - invented by Casio, championed by Oki and also used by Lexmark - was touted as the next big thing in laser printing in the mid-1990s. However, five years on - notwithstanding its environmental friendliness - the technology had yet to make a significant impact in the market.
The technology produces the same results as conventional laser printing and uses the same fundamental method of applying toner to the paper. A static charge is applied to a photo-receptive drum and, when the light from the LED hits it, the charge is reversed, creating a pattern of dots that corresponds to the image that will eventually appear on the page. After this, electrically charged dry toner is applied, which sticks to the areas of the drum that have had their charge reversed, and then applied to the paper as it passes past the drum on its way to the output tray. The difference between the two technologies lies in the method of light distribution.
LED printers function by means of an array of LEDs built into the cover of the printer - usually more than 2,500 covering the entire width of the drum - which create an image when shining down at 90 degrees. A 600-dpi LED printer will have 600 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 and, consequently, the technology presents a cheaper alternative to conventional laser printers. The LED system also has the benefit of being compact in relation to conventional lasers. Colour devices have four rows of LEDs - one each for cyan, magenta, yellow and black toners - allowing colour prints speeds the same as those for monochrome units.
The principal disadvantage of LED technology is that the horizontal resolution is absolutely fixed, and while some resolution enhancements can be applied, none of them will true lasers offer as good as the possible resolution upgrades. Moreover, an LED printer's drum performs at its best in terms of efficiency and speed when continuous, high-volume printing is called for. In much the same was, as a light bulb will last less long the more it is switched on and off, so an LED printer's drum lifetime is shortened when used often for small print runs.
LCD printers work on a similar principle, using a liquid crystal panel as a light source in place of a matrix of LEDs.
Colour Laser
Laser printers are usually monochrome devices, but as with most mono technologies, laser printing can be adapted to colour. It does this by using cyan, magenta and yellow in combination to produce the different printable colours. Four passes through the Electro-photographic process are performed, 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 print heads 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 be set 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 photo-conductive (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 authorized 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 the user can alter this period of time 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, recognizing 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.
Post Script
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 that 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 are 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 vectors descriptions giving WYSIWYG on the desktop. PCL 5 also utilized 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
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