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The following information has been accumulated by us over a lengthy period from our own knowledge or long-lost sources and much, much more is available on hundreds of sites on the Internet. For your convenience, we have paraphrased, assembled and collated some of the more interesting and relevant aspects here. ....As it was in the beginningInkjet printers were first mass-produced in the 1980s and by the 1990s prices dropped low enough for that concept to be brought into the consumer market. Apparently the idea of a jet printer has its basis in the last century it seems, when two Englishmen, Kelvin and Raleigh, formulated mathematical equations describing the conditions under which a stream of liquid can break into droplets. Canon® apparently "invented" its 'bubble jet'TM technology in 1977, when a researcher accidentally touched an ink-filled syringe with a hot soldering iron and the heat forced a drop of ink out of the needle. This began the development of a new printing method.Inkjet printers have made rapid technological advances in recent years. First, the three-colour printer succeeded in making colour inkjet printing an affordable option; but as the superior four-colour models became cheaper to produce and sell*, it wound up being the standard and users' choice. *These days we believe that the printer manufacturers are almost giving their printers away at cost or less because of the enormous profits they make from their "genuine" replacement cartridges. Inkjet printing has two chief benefits over laser printers: lower printer purchase cost plus colour-printing capabilities. While inkjet printers are priced much less than laser printers, they are actually more expensive to use and maintain (when using the "genuine" cartridges from the printer manufacturer). back to top OperationInkjet printing means ink is emitted from nozzles while they pass over the paper. The operation of an inkjet printer is easy to visualise: liquid ink in various colours being squirted onto paper (and other media, like plastic film and canvas), to create an image. A print head scans the page in horizontal strips, using the printer's motor assembly to move it from left to right and back again, while the paper is rolled past in vertical steps, again by the printer. A strip (or row) 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.For most inkjet printers, the print head takes about half a second to print the strip across a page. On a typical A4 (210mm / 8¼" wide) page, the print head operating at 300 dpi deposits at least 2,475 dots across the page. This translates into an average response time of about 1 / 5000th of a second - quite a technological feat! In the future, however, advances will allow for larger 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 4 pages per minute in colour, 12 to 14ppm in B&W). In other words, declining costs for improving technology. There are several types of inkjet printing. The most common is "drop on demand" (DOD), which means squirting small droplets of ink onto paper through tiny nozzles; like turning a water hose on and off 5,000 times a second. The amount of ink propelled onto the page is determined by the print driver software that dictates which nozzles shoot droplets, and when. 
The nozzles used in inkjet printers are hairbreadth-fine and on early models they became easily clogged. On modern inkjet printers this is rarely a problem -providing you follow the "rules" and don't let everything dry out. 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.
back to top Thermal TechnologyCANON and HEWLETT-PACKARD favour this method. Most inkjets use thermal technology, whereby heat is used to fire ink onto the paper. There are three main stages in this process. 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.
 Tiny heating elements are used to eject ink droplets from the print head's nozzles. Most thermal inkjets have print heads containing a total of between 300 and 600 nozzles, each about the diameter of a human hair (approx. 70 microns). The photo at right shows the 3-colour print-head (the small square in the centre) of a Hewlett Packard colour cartridge. These deliver drop volumes of around 8 to 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 three-colour (cyan, magenta and yellow) print head. Several small colour ink drops - typically between four and eight - are typically combined to deliver a variable dot size. 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 600 dpi, while enhanced resolutions of 1200 dpi are increasingly becoming available. Print speed (ppm, or oages per minute) is chiefly a function of the frequency with which the nozzles can be made to fire ink drops and the width of the swath / travel printed by the print head. This is usually around 12MHz and 12mm respectively, giving print speeds of between 4 to 8 ppm for B&W text and 2 to 4 ppm for colour text and graphics. Thermal technology, meanwhile, imposes the limitation that whatever type of ink is used; it must be heat-resistant because the firing process is heat-based. Using heat in thermal printers conversely also creates a need for cooling, which adds to the overall length of printing time. back to top Piezo-Electric TechnologyEPSON favours this method. Epson'sTM proprietary inkjet technology uses a Piezo crystal at the rear of the ink reservoir. This is rather like a loudspeaker cone that flexes when an electric current flows through it. Whenever a dot is required, a current is applied to the Piezo element. This element then flexes and in so doing, forces a drop of ink out of the nozzle. There are several advantages to the Piezo method. The process permits greater control over the shape and size of the released ink droplet. The minuscule fluctuations in the crystal allow for smaller droplet sizes and hence higher nozzle density. Unlike thermal technology, the ink does not have to be heated and cooled between each cycle. This saves time, and the ink itself is geared more for its absorption properties than its ability to withstand high temperatures. Epson's newer consumer inkjets have black print heads with 128 nozzles and colour print heads with 192 nozzles (64 for each colour), a native resolution of 720 by 720dpi. As the Piezo process can deliver small and perfectly formed dots with extreme 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 inks that 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 fine print quality, especially on coated or glossy paper. Yes, Virginia, Epson do make good printers. back to top Colour PerceptionIf you can remember back to your science or physics classes at school, white light comprises roughly equal proportions of all the visible wavelengths and when this light is shined on or through an object, some wavelengths are absorbed while others are reflected or transmitted. It's the reflected or transmitted light that gives the object its perceived colour.Plant leaves are seen as green because the chlorophyll absorbs light at the blue and red ends of the spectrum and reflects back the green part in the middle.   
Again, if you cast your mind back to biology classes at school, we humans perceive colour via a layer of light-sensitive cells at the back of the eye (the retina). The key retinal cells in the eye are the "cones" which contain photo pigments that make them sensitive to red, green or blue (RGB) 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 perceived colour image. back to top ColourMost of the colour descriptions ( RGB, CMYK, HSB, gamma, dithering etc ) referred to here are found in virtually all graphics-related software programs.  
Our eyes can distinguish around one million colours, depending on the person and viewing conditions. The term "Colour" can be described by the following "HSB" model:
As we said earlier, television / PC monitors use the former (RGB) in a system of additive colour, creating colour by causing red, green, and blue phosphors to glow. By mixing varying amounts of each of the red, green or blue, different colours are created, and each can be measured in a scale from 0 to 255. If all red, green and blue are set to 0, the colour is black; if they are set to 255, the colour is white.     
In printing, 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. This is known usually as "4-colour process", as used in commercial brochure and magazine printing. By using varying amounts of these "process" colours a large number of different colours can be produced. The amount of ink is measured from 0% to 100% and ORANGE, for example being represented by 0% Cyan, 50% Magenta, 100% Yellow and 0% Black.    
back to top Creating ColourLike computer monitors or television screens, printers produce colours by tightly positioning the key primary colours in a process that is called dithering.Computer monitors (and TV sets) and printers, however, differ in how this is accomplished because monitors are light sources, whereas printing output is light reflecting.  As we said TV sets and computer monitors mix the light from phosphors through the primary additive colours, Red, Green and Blue (RGB), while printers use inks from the primary subtractive colours, Cyan, Magenta and Yellow (CYM). In both cases 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 either made up of one of the basic colours or intentionally left blank.
 Reproducing colour viewed on a monitor to exactly match printer output is known as colour matching. Colours vary from monitor to monitor and the colours on the printed page don't always correspond exactly 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. We have probably all experienced this and complain it's not "WYSIWYG" (What you see is what you get!).
Modern inkjets are able to print in colour and black but the way they switch between the two modes varies according to their capabilities. Printers with four-colour printing - cyan, yellow, magenta, and black (called CMYK ) - can switch between black and colour images all on the same page with no problem. Older printers equipped with only three colours cannot. back to top Print QualityThe two main governing factors 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 the higher the resolution and the more levels per dot, the better the overall print quality.Most printers compromise between higher resolution and providing more levels per dot. This is often determined by the printer's intended use. Graphic arts professionals, for example, are interested in maximizing the number of levels per dot to deliver higher 'photographic' image quality. Business and home PC users will require reasonably high resolution so as to achieve good text quality and reasonable image quality. The simplest type of colour printer is a "binary" device (0,1 or ON-OFF) in which the cyan, magenta, yellow and black dots are either "on" (printed) or "off" (not printed), with no intermediate levels possible. If ink dots can be mixed together to make intermediate colours, then a binary CMYK printer can print only eight 'solid' colours (cyan, magenta, yellow, red, green and blue, plus black and white). This won't give quality colour printing, which is why there are half tones.  Half toning divides a printer's dot resolution into a grid of halftone cells and then turns 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 half toning printer can 'fool' our eye into seeing millions of colours rather than just a few. In continuous tone printing, there is effectively unlimited colours - which really means 16.7 million colours, - about 17 times more than our eyes can distinguish.
To achieve this, the printer must create and overlay 256 shades per dot per colour, which obviously requires precise control over dot creation and placement. Continuous tone printing is largely by dye sublimation printers, but all the mainstream printing technologies can produce multiple shades (usually between 4 and 16) per dot, allowing them to deliver a huge range of solid colours and smoother halftones. Six-colour inkjet printers are specifically geared for delivering photographic-quality printing. These printers use two extra inks - light cyan and light magenta - to compensate for current inkjet technology's inability to create very tiny dots. Six-colour inkjets produce more subtle flesh tones and finer colour graduations than standard printers but may become obsolete when ink drop volumes shrink even further. Smaller ink-drop sizes will also reduce the amount of half toning required, as a wider range of tiny drops can be combined to create a broader palette of solid colours. back to top Operational considerations … and CostsAs we know, inkjet operating and running costs are high and way behind the laser printer.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. Some printers (like EPSON® ) use a system in which the ink cartridges and print heads are kept as separate units. Inkjet manufacturers are already starting to introduce printers with much larger secondary ink tanks linked to small primary ink reservoirs close to or in the print head. These printers will automatically replenish the primary reservoir as needed. Internal "smart chips" monitor the supply, activating a plunger on the relevant cartridge when it requires a refill and each ink cartridge can keep track of how much ink it has expended, even if it is transferred to another printer. The print heads are also self-monitoring - triggering an alert when they need to be replaced. The whole system can also survey the requirements of a particular print job and only begin the process if it determines there is sufficient ink to complete it. There should be more of it - if the costs are reasonable, of course. Again, as we well and truly know, wasted ink is also a problem. Where (most) printers combine cyan, yellow and magenta inks in a single tri-colour (3-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, already used by some models is to have a separate, independently replaceable ink cartridge for each colour. Inkjet performance or capacity also needs to improve. Not long ago the standard output for a standard laser printers was around 3,000+ pages from a toner / drum cartridge (some are now up to 6,000 and even 15,000 pages). Typically the best an inkjet could manage was around 500 to 900 pages from a single black ink cartridge and colour-ink use is even worse - typically only between 200 and 500 pages (we wish ours was that good). Like print speed (ppm) this depends on how you measure it, too. These figures are usually based on perhaps 15% coverage of the page, NOT a full A4 photograph. Print speeds are getting up to around 10+ ppm now but some of the manufacturers' currently-quoted figures seem impossible to achieve … and with those higher speeds we also need better cartridge capacities. back to top Any Trademarks or Registered Trademarks referred to remain the property of their respective owners and are used solely for product identification. The information contained herein has been accumulated by Ink Sales Plus over a lengthy period from our own knowledge or long-lost sources and much, much more is available on hundreds of sites on the internet. For your convenience, we have paraphrased, assembled and collated some of the more interesting and relevant aspects here. It is given as a matter of general interest and Ink Sales Plus makes no representation as to its accuracy. Some sections have been reproduced in part or in whole with permission from InkTec & Image Specialists Inc. |
How inkjet printers work 
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Copyright © 2002 Ink Sales Plus ABN 32 971 644 638. Any brands or registered trademarks referred to here remain the property of their respective owners and are used soley for product identification |
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