| Color Management Systems |
| What is a Color Management System? |
The challenge of color publishing is to reproduce colors the eye sees on a series of devices that have progressively diminishing color capabilities. Even the best photographic film can capture only a small portion of the colors discernible to the human eye. A computer monitor can display only a small fraction of those colors, and a printing press can reproduce fewer colors still.
A color management system (CMS) is a collection of software tools designed to reconcile the different color capabilities of scanners, monitors, printers, imagesetters, and printing presses to ensure consistent color throughout the print production process. Ideally, this means that the colors displayed on your monitor accurately represent the colors of the final output. It also means that different applications, monitors, and operating systems will display colors consistently.
A CMS is most beneficial when designing publications for output devices with small color gamuts, such as printing presses, proofers, and desktop printers. A CMS maps colors from a device with a large color gamut, such as a monitor, to a device with a smaller color gamut, such as a proofer or printing press; consequently, all colors on the monitor represent colors that the output device can reproduce.
This discussion is divided into the following sections:
The Need for Open Color Management
Before desktop publishing, high-end prepress operators used proprietary, or closed, systems, where all devices were integrated and calibrated to known values in order to work together. Color specialists were highly trained professionals who could work these systems to make a wide variety of adjustments to the color in a scanned image and predict, with reasonable accuracy, what the final printed piece would look like based on their manipulations.
Certain factors in the prepress, printing, film, and video industries have made these high-end proprietary solutions less viable. Desktop publishing has brought about the increase of open production systems. The design and production workflow is no longer confined to a closed system, but may be distributed across many different systems made up of devices from different vendors.
Because each device reproduces color differently, the color you see at one stage of design and production rarely matches what you see at another. In other words, color is device-dependent—the color you see depends on the device producing it. A scanner interprets an image as certain RGB values according to its particular specifications; a particular monitor displays RGB colors according to the specifications of its phosphors; a color desktop printer outputs in RGB or CMYK according to its own specifications. And, each press produces printed output according to the specifications followed (e.g., SWOP, TOYO, DIC) and the type of inks used.
Thus the need for an open color management system to communicate color reliably between different devices and operating systems. Open color management lets you compensate for the differences in these devices and communicate color in a device-independent manner.
Why WYS Isn't WYG
Perhaps the most frustrating aspect of working with digital files for color output is that WYSIWYG (what you see is what you get) doesn't always apply. The color you worked so hard to get "just right" on your monitor doesn't look correct when you print it. The reason is simple.
By their very natures, monitors and printing presses reproduce color in completely different ways. A monitor uses the RGB color model. This is an additive color model where red, green, and blue light is combined to create colors, and combining full intensities of all three make white:
A printing press, by contrast, uses the CMYK color model, in which three colors of transparent ink (cyan, magenta, and yellow) are combined—along with black (noted as K)—in varying amounts to create colors. CMYK is a subtractive color model where the inks filter the white light that reflects back from the paper and subtract some of the red, green, and blue light from the spectrum. The color we see is what is left in the spectrum. Subtracting all colors by combining the CMY inks at full saturation should, in theory, render black:
However, impurities in the existing CMY inks make full and equal saturation impossible, and some RGB light does filter through, rendering a muddy brown color. Hence, the addition of black ink to CMY.
Moreover, RGB and CMYK have different color gamuts, or ranges of reproducible colors, as this illustration shows:
RGB monitors can display more colors than can be matched in print. Conversely, some CMYK colors cannot be matched on-screen. Moreover, RGB gamuts vary widely between devices with some gamuts being considerably wider than others. While this may seem beneficial, wider RGB gamuts can be problematic when outputting to a press. The colors in the RGB gamut that are outside the CMYK gamut must be compressed (i.e., mapped to a space within the CMYK gamut). This always entails a loss to the quality of the original design and underscores the feeling that what you see is not what you get.
Device-Independent Color
As the previously explained, color varies depending on the device that produces it. In a sense, each device speaks its own color language, which it can't communicate well to another device. What is needed is an interpreter.
To illustrate this, imagine four people in a room. Each person is assigned a task that requires agreement among them all. One speaks Swahili, one speaks French, one speaks Mandarin, and one uses sign language. For the group to communicate, they need an interpreter who knows all four languages, as well as an agreed-upon neutral language. All discussion must first go through the interpreter who then translates it to the neutral language that all can understand. Each will continue to use his or her own native language, but will communicate with the others by using the neutral language.
A color management system works in much the same way, using a device-independent color model as the neutral color language by which all color information is referenced. The particular color model used is CIELAB, developed in 1976 by the Commission Internationale de l'Eclairage (International Committee on Illumination, or CIE). CIE's standard for measuring color is based on how the human eye perceives it.
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