- December 01, 2002, Sheila Nysko, Agfa Corp.
Just as perception of color varies from one individual to the next, each device in the imaging workflow—input, display, output—relies on a different method to process colors.
The technology used by each device in the imaging workflow limits the range of colors that particular machine can display. This range of colors is known as its color gamut. Regardless of the device, there will be certain colors outside of its color gamut. For instance, presses using CMYK inks (the cyan, magenta, yellow, and black inks used in four-color printing) cannot print deep blues and deep reds easily, as these are outside of the presses' color space. Monitors often are poor at displaying certain colors, such as warm yellows, accurately.
Differences in color space can play havoc with an image as it moves along the workflow. A specific blue might be inside the color space of the designer's monitor but outside that of the standard four-color press. On the monitor, this blue will appear quite saturated. Yet once it is printed, the blue will be too desaturated.
Even different four-color printers relying on the same technology may have different color spaces. A specific color may print well on one type of printer but not on the other. The colors of commercial-grade printing inks tend to be relatively impure. Other types of printers, particularly those used for shorter runs, such as ink jets or digital presses, often use inks or toners boasting much purer colors than those of press inks. Since pure colors yield a wider color space, these printers will boast broader color spaces than ordinary ink presses and can print colors that an ink press cannot. This can cause difficulties for a designer who might be using an ink-jet printer at the studio. It can be a shock when we print a job and discover the colors of an image have changed dramatically somewhere between the scan and the printed piece.
In an ideal scenario, we would have perfect color consistency and predictability. We could scan a photo, display it on a monitor, and find no color difference between the original and what is displayed. Also, were we to print that image, there would be no differences between the printed result and what was displayed on the monitor. Unfortunately, this type of color consistency and predictability does not occur naturally; we must work to achieve it.
There are two hazards for good color predictability: (1) differences in color gamuts among the different devices in the workflow, and (2) deviations from the standard performance of any device in the workflow.
Fortunately, a color management system (CMS) can remedy these problems and provide the color consistency and predictability we seek.
In general, the term CMS usually is reserved for those systems that use the internationally accepted CIE system of color measurement as a reference. The CMS manages the difference in the color spaces of the workflow devices. The CMS transforms the scanned RGB (red, green, blue) data of the colors to the CIE color space and converts them into CMYK separations. This transformation relies on professional conversion algorithms. The CMS also can convert CMYK image data into other CMYK data for a specific output device or for reproduction. The CMS can be “taught” the characteristics of the particular device you are using. This is called characterization. The characteristics of each device in your workflow will be stored by the CMS as a profile. Once your CMS establishes the profiles for the devices, it corrects the color by factoring in these characteristics automatically.
Most color management systems allow you to map your colors using different methods depending on the kind of images you are using, such as continuous tone, spot color, or artificially created images.
More information on this topic is contained in Agfa's booklet, The Secrets of Color Management, available at agfabooks.com.
Sheila Nysko is packaging manager of Agfa Corp. Ridgefield Pk. NJ; 201/440-2500; firstname.lastname@example.org