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.