Epson inkjet cartridge Color 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 color. Leaves, for example, are their familiar color because chlorophyll absorbs light at the blue and red ends of the spectrum and reflects the green part in the middle.

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The 'temperature' of the 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 flashlight, has an even distribution of wavelengths, corresponding to a temperature of around 6,000K, and doesn't distort colors. 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 color 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 color image.

epson Creating color


Creating color accurately on paper has been one of the major areas of research in color printing. Like monitors, printers closely position different amounts of key primary colors which, from a distance, merge to form any color; 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 colors: red, green and blue (RGB), while printers use inks made of the primary subtractive colors: cyan, magenta and yellow (CMY). White light is absorbed by the colored inks, reflecting the desired color. In each case, 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 made up of one of the basic colors or left blank.

The reproduction of color from the monitor to the printer output is also a major area of research known as color-matching. Colors vary from monitor to monitor and the colors on the printed page do not always match up 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. Printer manufacturers have put lots of money into the research of accurate monitor/printer color-matching.

Modern inkjets are able to print in color 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 colors - cyan, yellow, magenta, and black (CMYK) - can switch between black and white text and color images all on the same page with no problem. Printers equipped with only three colors, canít.

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 you canít 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 color, you must physically swap over 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. 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.

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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 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 maximizing 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 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 (or toner) dots can be mixed together to make intermediate colors, then a binary CMYK printer can only print eight 'solid' colors (cyan, magenta, yellow, red, green and blue, plus black and white). Clearly this isn't a big enough palette to deliver good color 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 colors rather than just a few.

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.

Recently, 'six-color' 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-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 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 colors.

Long-time market leader Hewlett-Packard has consistently espoused 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 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 colors (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 color 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 colors per dot.

epson Color management


The human eye can distinguish around a million colors, the precise number depending on the individual observer and viewing conditions. Color devices create colors in different ways, resulting in different color gamuts.

Color can be described conceptually by a three-dimensional HSB model:

bulletHue (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 in degrees, between 0 to 360.
bulletSaturation (S), also referred to as chroma, refers to the intensity of the dominant colors; it is measured as a percentage from 0 to 100 percent - at 0% the color would contain no hue, and would be gray, at 100%, the color is fully saturated.
bulletBrightness (B) refers to the color'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 color models. CRT monitors use the former, creating color by causing red, green, and blue phosphors to glow; this system is called additive color. Mixing different amounts of each of the red, green or blue, creates different colors, and each can be measured from 0 to 255. If all red, green and blue are set to 0, the color is black, is all are set to 255, the color 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 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%, 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 color space model. This defined color as a combination of three axes: x, y, and z, with, in broad terms, x representing 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's the basis for 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*v models released in 1978, map the distances between color co-ordinates more accurately to the human color perception system.

For color is to be an effective tool, it must be possible to create and enforce consistent, predictable color 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 colors. It is in the field of color management that all of this color modeling effort comes into its own. This uses the device-independent CIE color space to mediate between the color gamuts of the various different devices. Color management systems are based on generic profiles of different color 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, 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 they are computationally intensive and mutually incompatible. Recognizing the problems of cross-platform color, the ICC (International Color 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 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; the contains information about the profile itself, such as what device created the profile and when and the second is colourmetric 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.

epson 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 colors 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 color. With color printing, because different inks are mixed, 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 theyíve 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 color 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 colors 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. 

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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 color. Leaves, for example, are their familiar color 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 color. 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 colors. 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 color 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 color image.

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