Color Basics Part 7 – What is Color Management?
Managing color in today’s digital workflow is a concern because of the differences in color rendition from one device to another. Color management is a process that controls the exchange and reproduction of images across a wide range of devices.
The difference in color gamuts was discussed earlier in this series. Let’s look again at some examples. Although computer monitors have a relatively large color gamut, they are often poor at displaying certain colors, such as warm yellows. Most proofing devices have a larger gamut, or somewhat different gamut, of colors from a printing press. Presses using standard process inks cannot easily print deep blues and deep reds because these colors are outside the color gamut of the inks.
When using a computer as a design tool, it is important for the colors on your monitor to look the same day-to-day and week-to-week. This is known as “Calibration.” It is also necessary to understand that some of the colors in your design, such as bright and saturated colors, may not be printable on a press. So, to manage color better, your monitor should be able to simulate the printed results as closely as possible. In color management terms, this is known as “characterizing.”
During production, it is common to send a digital file of a color image to someone else (down the hall, across the street, or across the country) who needs to view it on a computer monitor. Ideally, the other person’s monitor has also been calibrated and characterized properly. Calibration and Characterization of color devices help ensure that during the production process an image is reproduced as accurately as possible. This is just one example of color management minimizing color differences.
Every electronic prepress device has its own unique color gamut and tone reproduction characteristic. Tone is the lightness/darkness value of a color. As an image moves from a scan to a proof to final print, each device along the workflow can introduce its own color and tone variations. Also, each device likely comes from a different manufacturer, which means that no single piece of equipment is completely compatible with the device that preceded it in the workflow.
The International Color Consortium (ICC) was formed in 1993 to address this dilemma. The goal of the ICC is to create, promote, and encourage the evolution and standardization of open-vendor platform, color management system architecture and components known as color management systems. A color management system, or CMS, allows any color device (monitor, scanner, camera, printer) to be defined in terms of a standard model of color space through a device “profile.” ICC Profiles provide color management systems with the information necessary to transform the native color data of a device to device-independent color space. Guidelines set by the ICC define profile structures and other rules of today’s color management systems. System-level color management modules (CMMs) provide utilities that any software can use for any color transformation task.
Characterization or Profiling
Characterization is the process of identifying the relationship between a device-dependent color gamut and device-independent color. After a device has been calibrated, characterizing is the next process (sometimes referred to a profiling). Any production device that scans, displays or prints a standard target comprised of many different solids and tints can be characterized. (Normally an IT8 target or some other variation of this standard calibration target is used.) Once scanned, displayed or imaged, this target is measured. Measurements are converted using software into a standard model of device-independent color space, commonly called CIELAB.
Monitor (Display) Profiles
To profile a monitor, a range of RGB patches are displayed and measured with a colorimeter or spectrophotometer. (Display profiles are bi-directional because a monitor can be both a source and a destination device.)
Scanner (Input) Profiles
Scanner profiles translate raw, digital, RGB values into a standard color space like CIELAB. Profiling software combines the digital scanner values with data measured from the target to form a “source” profile, which translates RGB values into CIELAB color space values.
Printer (Output) Profiles
Presses, desktop printers, wide-format printers and film recorders are profiled by printing a color target and measuring the printed patches with a spectrophotometer. Profiling software combines the measured CIELAB data with the printed CMYK percentages to form a “destination,” or “output” profile. (Printer profiles are bi-directional because a printer can be both a source and a destination device)
Gamut Compression and Transformation
Gamut compression handles out-of-gamut colors. When certain colors of an original image are outside the gamut of an output device, this process attempts to attain the best overall simulation of the original. Gamut compression is built into the output device profile, but can be customized to suit a particular image in some ICC-based, color management, image-editing packages. Good gamut compression usually results in the most pleasing reproduction of an original, but it may not give a truly accurate match of any specific color. Without gamut compression, in-gamut colors are matched more accurately, but out of gamut colors may be lost completely.
Transformation means changing colors from one color space to another, for instance from RGB to CMYK, using an independent color space like CIELAB in the conversion. A system-wide color management module, or CMM (ie. Colorsync), builds a link that directly converts RGB to CMYK. To create the link, the CMM looks up each RGB value in the scanner profile, then looks up the resulting CIELAB value in the press profile to get equivalent CMYK values.
Many of these transformation formulas are built-in to software like Photoshop and Illustrator, making this process very simple and easy.