Before actually using color for digitally printed applications, it is
necessary to understand some of the factors that influence the
appearance of color on a printed document. Described below are some of
the important points to consider.
|
 Spot
color is usually considered to be any color used on a document that is
not a process color. It can be used simply as a secondary color or as a
color to provide emphasis to a specific area of a printed piece. Spot
colors are often printed in conjunction with four-color printing because
many digital presses and direct imaging presses have the capability of
printing five or more colors at one time.Specialty digital inks can also be used as a spot color to provide even greater emphasis to an area. Some examples of specialty inks are: fluorescent varieties, fade resistant inks, opaque inks, and metallic inks. Many of the specialty inks demand unique considerations for their proper use at the press, especially when they are used on direct imaging presses. Metallic inks may require that the press run at a slower rate and the preparation and clean up time may be longer. Fluorescent inks are very transparent and may require that the printed document be run a second time through the direct imaging press to achieve the desired coating of ink. |
Subtractive Color Theory
Red, Green and Blue ( RGB), are the primary colors of white light and are called the Additive Primary Colors. (*For complete information on Additive Color, see "Color Models" in the article, Color on Digitally Printed Applications.) We are able to see color because different objects reflect and absorb, or subtract, the primary colors of light differently. For example, we see an object as yellow because it absorbs or subtracts
blue light from white light. Since the blue light has been absorbed,
the red and green light is leftover and is reflected back to our eyes.
The red and green light combine to make yellow and so we see the color
of the object as yellow. This is known as the Subtractive Color Process
because portions of the visible light spectrum are subtracted from
white light to reveal color. If 100% of the red light is subtracted from
white light the resulting color is cyan. (The green light and blue
light are the remaining primary light colors after the red is removed
and the green and blue combine to form cyan). If 100% of the green light
is subtracted from white light the result is magenta (red and blue
light form magenta) and when 100% of the blue light is subtracted from
white light the result is yellow (red and green light form yellow). Cyan, Magenta, and Yellow ( CMY) are the Subtractive Primary Colors, and are used as the primary colors for creating pigments for color reproduction in process color printing. |
| Ink Color |
Light Color(s) Absorbed |
Light Color(s) Reflected |
Color the Human Eye Sees |
| C | Red Light | Green and Blue Light |
Cyan |
| M | Green Light | Red and Blue Light |
Magenta |
| Y | Blue Light | Red and Green Light |
Yellow |
| M + Y | Green and Blue Light |
Red Light | Red |
| C + Y | Red and Blue Light |
Green Light | Green |
| C + M | Red and Green Light |
Blue Light | Blue |
| Four-Color Printing Pigments Since light cannot be used to reproduce color on the printed page, colored pigments must be used. The combination of the subtractive primary color pigments (cyan, magenta, and yellow) in varying degrees is what creates the illusion of a full-color printed image. In the lightest and whitest areas of the printed image, very little of the primary colors are present so these areas appear very light. In the darkest areas of the printed image, much more of each color is present, making the areas appear darker. The combination of the three subtractive primaries, each at 100% strength, should create black, but since cyan, magenta, and yellow color pigments are not perfect, there is no way of creating a true black color when they are combined. If full strength cyan, magenta, and yellow are combined, the resulting color is actually a dark, muddy brown. Even if it were possible to combine all three primaries at full strength to create black, printing 100% of any or all of the subtractive primaries can cause such an extreme saturation of color on the printed surface that it can result in very poor print quality. Because of this problem, the addition of black is necessary to assist the cyan, magenta, and yellow in creating the darkest hues, deepening shadows, and producing a true black text. Cyan, Magenta, Yellow, and Black, (CMYK), form the basis of the four-color printing process. The "K" is used to signify black to avoid confusion with blue. |
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Because the mixture of the primary ink colors do not make a true black, it is necessary to add black ink to achieve deeper shadows and true black text. |
| The mixture of 100% of any two primary colors will result in a secondary color. 100% yellow and cyan will result in green; 100% cyan and magenta result in blue; and 100% magenta and yellow result in red. The secondary colors are also the same as the primary light colors (RGB) represented as pigments. |
| Separating the Individual Color Components In order to reproduce a full-color image on the printed page using the four process colors, the image is first divided into the individual subtractive primary color components. Traditionally, a complicated procedure of producing individual photographic separations was necessary in order to reproduce a full-color image using conventional printing processes, such as offset. Today, digital technology has resulted in the elimination of this cumbersome process for digital printing; however, photographic and electronic color separations are still required for the enormous quantity of print applications produced with conventional printing processes. Described below are the three primary methods used in the printing industry for separating color components. 1. Photographic Separations Using a large process camera, a full color image is converted into halftone negatives, which contain a series of dots of various sizes that represent shades of gray. The dots are achieved with the use of special screens placed over the negative material during exposure. When printed, the smaller dots create the lighter areas of the image, with the lightest appearing white, and the larger dots make up the darker areas of the image, with the darkest appearing black. This system of using the halftone screens directly with the film is known as the Direct Screen Photographic Color Separation method. A halftone negative is made for each of the separate subtractive color components of the image (cyan, magenta, and yellow) and black. Different filters are used that allows the camera to record onto the film only the components of one of the primary colors at a time. The four resulting halftone negatives are records, in varying shades of gray dots, for each of the separate color components and are known as Color Separations. Another photographic technique known as the Indirect Photographic Color Separation method is also used. The original image is separated into the individual primary color components and continuous tone films are made for each of the separations. From the separated films, the halftone negatives or positives are created. The advantage in first creating continuous tone separations before creating the halftones is that the continuous tone separations can be reduced or enlarged and corrected before the actual halftone films are produced. The color separations films created with photographic methods are used to prepare analog plates for traditional printing processes. Each plate corresponds to one of the four colors, cyan, magenta, yellow, or black. The plates are coated with the corresponding ink colors during the printing process and the inked image from the plates is transferred to the substrate. The varying sizes of the halftone dots on the plates, determines the quantity of ink that each component of the image on the plate will receive. The printed dots from each of the plates overlap on the substrate to produce the full-color image. |
| 2. Electronic Separations The electronic separation of an image into its individual color components has become the most popular and labor saving method of producing color separations. The separations can be created from an image produced with analog methods, such as photography, or from a digitally generated image. When creating separations from an image produced with analog methods, the image is first placed in a scanner, which converts the image into a digital record. The digital image can then be imported into an image editing software program where the color separations can be produced with the click of a mouse. The scanned image and/or the color separations can be quickly manipulated and edited in an infinite number of ways with the tools provided with the software. Color separations are also created from digitally produced images. A digital camera or images composed entirely in design or illustration software on a computer are used for this purpose. Digitally produced images eliminate the scanning step from the workflow. The color separations created electronically can be output to film to prepare analog plates for conventional printing processes or they can be used for Computer-to-Plate applications that skip the process of creating films. 3. Digitally Separated Color Components The technology of digital printing has removed the step of having to create color separations in any physical form whatsoever. There are no films or plates to hassle with. Consider a desktop printer and the process in which it creates full-color images. The user simply clicks a mouse on a computer to direct the printer to begin outputting a full-color document. The computer does the work of separating the color components so that the printing head of the desktop printer outputs the correct series of colored dots from the corresponding ink cartridges. The overlapping dots on the paper simulate the full-color image. A digital press operates in a similar fashion, only on a much larger scale. Rather than using ink cartridges and a single printing head, most digital presses use a separate rotary, reimageable printing unit (often referred to as a print engine) for applying each of the process colors to the substrate (as shown in the illustration below). The digital front end of the printing equipment processes the necessary data and directs each print engine to apply the required series of colored dots to the paper in order to reproduce the full-color image. |
| Before digital printing became commonplace, four-color printing was
rarely used for commodity items due to the expense of creating
conventional color separations and the time and skill required to print
the colors on a conventional press. Today, items as generic as an
account statement may incorporate four-color printing due to the ease in
which such items are printed using digital equipment. Since the public
has become so accustomed to seeing everyday items printed in color, it
has become difficult for printers to make a selling point of their
expertise in four-color printing, especially when inexpensive desktop
printers can achieve excellent results with static, low volume
applications. The use of high-fidelity color is a tool that printers can use to set themselves apart from the run-of-the-mill operation. There are numerous variations of hi-fi color systems designed for digital equipment as well as conventional printing presses. All provide an expanded color space, which produces more distinctive, vibrant colors. An example of a high-fidelity color system is the PANTONE Hexachrome® system which uses orange and green in addition to enhanced versions of cyan, magenta, yellow, and black. The use of the six high-fidelity colors increases the range of color by 20% and provides improvement in transitions between colors. The high-fidelity color system can reproduce more than 90% of the Pantone Matching System® colors. |
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| The image on the left was printed with standard four-color printing and the image on the right was printed with high-fidelity color. Note the vibrancy of the color and the range of hues and values of the hi-fi image that are not possible with standard four-color printing. |
| Of course, using additional colors means that the standard four-color
digital (or conventional press) must have additional print engines. A
number of digital press manufacturers offer models that employ hi-fi
color. The digital front ends included with these presses are able to
convert jobs to incorporate the increased color gamut. The cost of digital printing equipment that includes six or more print engines can be prohibitive so many printers are naturally reluctant to make such a large investment. For those printers that for the time being are basically stuck with the equipment they have, there are improved CMYK inks and toners available that provide an increased range of color over standard CMYK inks. If the quantity is small, printing a high-fidelity application digitally is usually less expensive than the same application printed with conventional equipment. There are fewer steps involved with the hi-fi digital printing process. When using digital printing, the price per piece is the same regardless of the quantity. When using conventional printing processes, the price per piece increases as the quantity becomes smaller. When hi-fi printing is used with conventional printing processes such as offset, the same prepress procedures that are used with four-color printing are followed except that additional color separations (photographic or electronic) must be made for each additional color used. This is another factor that makes hi-fi printing with non-digital processes much more expensive. Shown in the illustration below are the color separations (negatives), created photographically, that are necessary to produce the plate positives used when printing high-fidelity color with the conventional offset printing process. This separation process is achieved automatically by the digital front end of a digital press, which eliminates the necessity of creating physical color separations when using digital equipment for hi-fi printing. This is only one of the many advantages of using digital equipment for all types of color printing. |
- Pure violets, greens and oranges are very difficult to match using
four-color process printing. To achieve vibrant colors, a fifth spot
color or 6-color high-fidelity printing can be used.
- Fluorescent and metallic colors cannot be achieved by using
four-color process printing. A spot color is necessary for printing the
fluorescent and metallic colors.
- For a rich black use a mixture of 40%C, 30%M, 30%Y, & 100%K
instead of just 100% black. The combination of all of these colors in
the correct proportions will create a black with a darker appearance.
- Only 50% of the PANTONE colors can be closely simulated using
four-color process printing, but nearly 90% can be simulated with the
use of high-fidelity printing.
- Pantone colors in desktop software are not always the same. Always check the built-in percentages.
- Never trust the color on the monitor. It is always best to use the color values and percentages.
- Banding can often occur in gradient blends (vignettes). Banding is the visible lines between the color changes in the gradient. To help eliminate banding, limit the change in a color to no more than 75% from end to end. For example, instead of going from 0% cyan to 100% cyan, start at 10% and end at 80%. If you are using PhotoShop to create the gradient, try adding a pixel or two of noise from the noise filter.
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