How Inkjet Peinters Work
Home How Inkjet Printers Work
Origins
Although inkjet printers were first mass-produced in the 1980s, it was only in the 1990s that prices dropped low enough for that technology to be brought into the mass consumer market. Canon claims to have invented what it calls 'bubble jet' 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. And so began the development of a new printing method.

Inkjet printers have made rapid technological advances in recent years. First, the three-color printer succeeded in making color inkjet printing an affordable option; but as the superior four-color models became cheaper to produce and sell, it wound up being the standard and users' choice.

Inkjet printing has two chief benefits over laser printers: lower printer cost and color-printing capabilities. But while inkjet printers are priced much less than laser printers, they are actually more expensive to use and maintain. Cartridges need to be changed more frequently and the special coated paper required to produce high-quality output is very expensive. At a cost per page level, inkjet printing costs about 10 times more than laser printing.
Operation
Inkjet printing, like laser printing, is a non-impact process. Ink is emitted from nozzles while they pass over media. The operation of an inkjet printer is easy to visualize: liquid ink in various colors being squirted onto paper and other media, like plastic film and canvas, to build 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 up 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 8 1/2"-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 color laser printers (3 to 4 pages per minute in color, 12 to 14ppm in monochrome). 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, 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 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. Canon and Hewlett-Packard favor this method.

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). These deliver drop volumes of around 8 to 10 picolitres (a picolitre is a million millionth of a liter), 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-color (cyan, magenta and yellow) print head. Several small color 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 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. This is usually around 12MHz and half an inch respectively, giving print speeds of between 4 to 8 ppm for monochrome text and 2 to 4 ppm for color text and graphics.
Thermal Technology Thermal Technology

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 a cooling, which adds a to the overall length of printing time.

Piezo-Electric Technology
Epson's 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. So whenever a dot is required, a current is applied to the Piezo element which 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. This enables greater freedom in developing new types of inks.

Epson's newest mainstream inkjets have black print heads with 128 nozzles and color print heads with 192 nozzles (64 for each color), addressing a native resolution of 720 by 720dpi. Because 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.
Piezo-Electric Technology Piezo-Electric Technology

Color Perception
Visible light falls between 380 Nm(violet) and 780 Nm (red) on the electromagnetic spectrum. 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 color. Leaves, for example, are usually seen as green because chlorophyll absorbs light at the blue and red ends of the spectrum and reflects back the green part in the middle.

The temperature of a light source, measured in Kelvin (K), affects an object's perceived color. White light, as emitted by the fluorescent lamps in a viewing box or by a photographer's flash, doesn't distort colors and has an even distribution of wavelengths corresponding to a temperature of around 6,000 K. Standard light bulbs, on the other hand, emit less light from the blue end of the spectrum, corresponding to a temperature of around 3,000K, which causes objects to appear more yellow.

Humans perceive color via a layer of light-sensitive cells at the back of the eye called the retina. The key retinal cells in the eye are the cones, which 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 perceived color image.
Creating Color
Like computer monitors or television screens, printers produce colors by tightly positioning the key primary colors in a process that is called dithering.

Monitors and printers, however, differ in how this is accomplished because monitors are light sources, whereas printing output is light reflecting. Monitors mix the light from phosphors through the primary additive colors: red, green and blue (RGB), while printers use inks from the primary subtractive colors: cyan, magenta and yellow (CYM). In both cases the basic primary colors are dithered to form the entire spectrum. Dithering breaks a color pixel into an array of dots so that each dot is either made up of one of the basic colors or intentionally left blank.

Creating Color
Reproducing color viewed on a monitor to exactly match printer output is known as color matching. Colors vary from monitor to monitor and the colors on the printed page don't always correspond exactly with what is displayed on screen. The color generated on the printed page is dependent on the color system used and the particular printer model; not by the colors shown on the monitor.

Modern inkjets are able to print in color and black, but the way they switch between the two modes varies according to their capabilities. Printers enabled for four-color printing - cyan, yellow, magenta, and black (CMYK) - can switch between black and color images all on the same page with no problem. Printers equipped with only three colors cannot.

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-color cartridge (CMY) for color printing, but they can't do 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 color, you must physically swap the cartridges. When you use black on a color page, it will be made up from the three colors, which tends to result in an unsatisfactory dark green or gray color usually referred to as composite black.
Print Quality
The two main determinants of color 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.

In practice, most printers make a trade-off between opting for 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, while general business users will require reasonably high resolution so as to achieve good text quality and reasonable image quality.

The simplest type of color 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 dots can be mixed together to make intermediate colors, then a binary CMYK printer can print only eight 'solid' colors (cyan, magenta, yellow, red, green and blue, plus black and white). Clearly this isn't a sufficient palette to deliver quality color printing, which is why there are half tones.

Half toning 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 half toning printer can 'fool' the human eye into seeing a palette of millions of colors rather than just a few colors.

In continuous tone printing, there's an unlimited palette of solid colors. In practice, 'unlimited' means 16.7 million colors, 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 color, 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 colors and smoother halftones. Such devices are referred to as "contone" printers.

Six-color inkjet printers have now appeared on the market, specifically geared for 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 tiny (and therefore light) dots. Six-color inkjets produce more subtle flesh tones and finer color 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 picoletres. Smaller 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 colors.

Market-leader Hewlett Packard has consistently advocated the advantages of improving color print quality by increasing the number of colors that can be printed on an individual dot rather than simply increasing dpi, arguing that the latter approach sacrifices both speed and causes problems arising from excess ink - especially on plain paper. In 1996 HP manufactured the first inkjet printer to print more than eight colors (or two drops of ink) on a dot. Its DeskJet 850C being capable of printing up to four drops of ink on a dot. Over the years it has progressively refined its PhotoREt color layering technology to the point where, by late 1999, it was capable of producing an extremely small 5 pl drop size and up to 29 ink drops per dot representing over 3,500 printable colors per dot.
Color Management
The human eye can distinguish around a million colors; the precise number depending on the individual observer and viewing conditions. Color can be described conceptually by the following three-dimensional HSB model: Color Management

  • Hue (H) refers to the basic color in terms of one or two dominant primary colors (red, or blue-green, for example). It is measured as a position on the standard color wheel, and is described as an angle between 0 to 360 degrees.
  • Saturation (S), also called Chroma, refers to the intensity of the dominant colors. It is measured from 0 to 100 percent: at 0% the color would contain no hue and would be gray, at 100%, the color is fully saturated.
  • Brightness (B) refers to the colors 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 color models. CRT monitors use the former, in a system of additive color, creating color by causing red, green, and blue phosphors to glow. By mixing varying amounts of each of the red, green or blue, different colors 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 color is black; if they are set to 255, the color 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 color. The basic printing ink colors 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" colors a large number of different colors can be produced. Here the level of ink is measured from 0% to 100%; orange, for example being represented by 0% cyan, 50% magenta, 100% yellow and 0% black.

The CIE (Commission Internationale de l'Eclairage), which was formed early in the last century to develop standards for the specifications of light and illumination, defined the first color space model. This model describes color as a combination of three axes: x, y, and z. In broad terms, x represents the amount of redness in a color, 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 is the foundation most other color-space models. The most familiar refinement is the Yxy model; in which the near triangular xy planes represent colors with the same lightness, with lightness varying along the Y-axis. Subsequent developments, such as the L*a*b and L*u*vmodels released in 1978, map the distances between color coordinates more accurately to the human color perception system.

In printing, for color is to be an effective tool, one must be able to create and enforce consistent, predictable color throughout the production chain: scanners, software, monitors, desktop printers, external PostScript output devices, pre-press service bureaus, and printing presses. The dilemma is that different devices are simply not capable of creating the identical range of colors. That's where color management comes in, whereby the device-independent CIE color space to mediate between the color gamut of the various devices. Color management systems are based on generic profiles of different color devices, which describe imaging technology, gamut and operational methods. These profiles are then fine-tuned by calibrating actual devices to measure and correct any deviations from ideal performance. Finally, colors are translated from one device to another, with mapping algorithms choosing the optimal replacements for out-of-gamut colors that can't be handled.

Until Apple introduced ColorSync as a part of its System 7.x operating system in 1992, color management was left to specific applications. These high-end systems have produced impressive results, but the process is computationally intensive and the devices are mutually incompatible. Recognizing the problems of cross-platform color, the ICC (International Color Consortium, originally known as the ColorSync Profile Consortium) was formed in March 1994 to establish a common device profile formats. 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 color 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; information about the profile itself, such as what device created the profile and when, and a colorimetric device characterization, which explains how the device renders color. The following year Windows 95 became the first Microsoft operating environment to include color management and support for ICC-compliant profiles, via the ICM (Image Color Management) system.
Ink
Whatever technology is applied to printer hardware, the final product consists of ink on media, 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 colors and visible banding, to excellent, near-photographic quality. Ink

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 about 100 times faster. The former is generally better suited to straightforward monochrome printing, while the latter is typically used for color printing. Because different inks are mixed to create colors, they need to dry as quickly as possible to avoid blurring. If slow-drying ink is used for color printing, the colors tend to bleed into one another before they’ve dried.

The ink used in inkjet technology is water-based, and this caused the results from some of the earlier printer models to be prone to smudging and running. Oil-based ink is not really a solution for this 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 output from inkjet printers is still generally poorer than from laser printing.

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 light fastness and water fastness, and suitability for printing on a wide variety of media.

Today's inkjets use dyes, based on small molecules (<50 nm), for the cyan, magenta and yellow inks. These have high brilliance and wide color gamut, but are neither light- or water-fast enough. Pigments, based on larger (50 to 100 nm) molecules, are more waterproof and fade-resistant, but they aren't transparent and cannot yet deliver the range of colors available from dye-based inks. This means that pigments are currently only used for the black ink. Future developments will likely concentrate on creating water-fast and light-fast CMY inks based on smaller pigment-type molecules.
Paper
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 color-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-color photographic quality. Some printer manufacturers, like Epson, even offer proprietary paper, which is optimized for use with its Piezo-electric technology.

Inkjet printing can be costly 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 the specific manufacturer's printer models.

A great deal of research has gone into the production of universal paper types, which are optimized specifically for color inkjet printers. PLUS Color Jet paper, produced by Wiggins Teape, is a coated paper produced specifically for color inkjet technology, and Conqueror CX22 is designed for black ink and spot-color business documents and is optimized both for inkjet and laser printers.

Paper pre-conditioning seeks to improve inkjet-printing quality on plain paper by priming the media to receive ink with an agent that binds pigment to the paper, thereby reducing dot gain and smearing.
Manageability and Costs
There's no doubt that the inkjet printer has been one of desktop computing success stories of the late 1990s. Its first phase of development was the monochrome inkjet of the late 1980s - a low-cost alternative to the laser printer. The second resulted from the advent of color 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.

Manageability and Costs Hewlett-Packard's HP2000C inkjet model, launched in late 1998, signaled 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 pressurized cartridges, which each hold 8cm of ink and remain static underneath a hinged cover at the front of the printer. These are connected by tubes, integrated with the standard ribbon-style cable, which run 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 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.

Wasted ink is also a problem that adversely affects operating costs. With printers that combine the cyan, yellow and magenta inks in a single tri-color 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 deployed by a number of printers, is to have a separate, independently replaceable, ink cartridge for each color. The downside is increased maintenance effort - an inkjet printer that uses four cartridges typically requires twice the attention of one where the three colors are combined. In terms of manageability, the HP2000C includes another innovative feature. The incorporation of a second paper tray, which means that two paper types can be kept in the printer at once to minimize user attention.

Print capacities also have to improve. At the end of 1998 the standard output 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. Color-ink use fared even worse - supporting a capacity of only between 200 and 500 pages. Print speeds are expected to reach 10 ppm by the year 2001, and with these higher speeds there is the corresponding expectation of increased cartridge capacities. Inkjet manufacturers are already starting to introduce workgroup color 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 primary reservoir as needed.
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