Color Theory

Color management is the process by which you try to match color conditions in a set of devices (e.g., scanner, monitor and printer). These devices produce color using different methods; therefore, they may display the same color value differently, and may not all display the same range of color values (see color gamut, below). In other words, color is device-dependent: the color that you see will change depending on the device that's producing it.

A color management system (CMS) is a collection of color management software tools used to try to make color device-independent. Ideally, the colors on your monitor should accurately represent the colors in a scanned image, and the colors you see on the final output. A CMS maps colors from the color gamut of one device into a device-independent color space, and then maps those colors to the color gamut of another device.

Calibration
The process of returning a device to known color conditions. Commonly done with devices that change color frequently, such as monitors (phosphors lose brightness over time) and printers (proofers and other digital printing devices can change output when colorant or paper stock is changed).

Characterization
The process of creating a profile that describes the unique color conditions found on a particular device. For monitors, this includes colorimetric descriptions of its phosphors and the color temperature of its white point. For printers, this includes descriptions of the inks, paper stock, and line ruling. In a CMS, you characterize the calibrated monitor by creating an ICC device profile.

CMM
Color Matching Method, also called a Color Engine. The specific software component in a CMS that does the color conversion calculations from one device's color space to another using the ICC device profiles (e.g., ColorSync or Kodak CMS).

Color Gamut
The total range of colors reproduced by a device. A color is said to be "out of gamut" when its position in one device's color space cannot be directly translated into another device's color space. For example, the total range of colors that can be reproduced with ink on coated paper is greater than that for uncoated newsprint, so the total gamut for uncoated newsprint is said to be smaller than the gamut for coated stock. The CMYK gamut is generally smaller than their RGB gamut.

Color Model
The dimensional coordinate system used to numerically describe colors. Some models include: Red, Green, Blue (RGB); Hue, Lightness, Saturation (HLS); Cyan, Magenta, Yellow, Black (CMYK); and Lightness, a, b (Lab).

Color Space
The specific range of colors in a particular model (e.g., RGB, Lab, CMYK). For example, if you have an RGB master image and convert it to CMYK using an ICC device profile for a CMYK composite printer, then, using the same master RGB document, convert the image to CMYK using a different ICC device profile, each resulting CMYK image will be in a different CMYK color space. Also called Gamut.

Colorimeter
A device that measures the luminosity of RGB light. When used with software, it can be used to create ICC device profiles for monitors. Many high-end monitors targeted at the pre-press professional contain such devices internally (e.g., Barco self-calibrating monitor) or have a colorimeter attached (e.g., Radius PressView).

ColorSync
A system-level CMS developed by Apple Computer, Inc. for Mac OS. Adobe PageMaker, Adobe Photoshop, and Adobe Illustrator currently support ColorSync 2.x and later.

Densitometer
A device that measures the density of exposed film (in transmissive mode) or printed inks (in reflective mode). They are used in the printing industry to measure the output accuracy of halftone screens as discreet dot percentages. For that reason, they are used as a calibration tool for imagesetters and printing presses.

ICC
The International Color Consortium established in 1993 by eight industry vendors (including Adobe Systems) to create, promote, and encourage the standardization and evolution of an open, vendor-neutral, cross-platform color management system architecture and components.

ICC Device Profile
A file that describes how a particular device (e.g., monitor, scanner, printer, or proofer) reproduces color (i.e., its specific color space). Profiles can be either generic or custom.

  • Generic device profiles are created by the device manufacturer, who examines the color characteristics of a group of the same devices under controlled conditions, using device profiling software and instruments.
  • Custom device profiles are created for a individual device, using a color measuring instrument (e.g., a spectrophotometer or colorimeter) and device profiling software. Custom device profiles are far more accurate than generic device profiles, but cost more to create.

Process Printing
Output based from a printing press that uses four colors (cyan, magenta, yellow, and black) to create the illusion of continuous tone images. For that reason, cyan, magenta, yellow, and black are also known as process colors (CMYK).

Proofer
A printing device a service bureau uses to create images as contracts (also called a contract proof) to match in the final output from the offset press. Traditional proofers create prints from negative separations; digital proofers create ink-jet (e.g., IRIS) or dye-sublimation (e.g., Kodak, 3M) composite prints.

Reference Color Space (RCS)
A reference color space is a device-independent theoretical color model used by a CMM (color engine) for translating colors from one device's gamut to another. The CIE Lab is an example of a Reference Color Space. This component is built into the CMM, it is neither alterable nor visible to users.

Render Intent
The method a CMM uses for converting (i.e., mapping) colors from one device's gamut to another. The three methods are called Perceptual, Saturation, and Relative Colorimetric. In PageMaker and Illustrator the names of the rendering intents are called Image, Graphics, and Colorimetric, but they do the same thing.

  • Perceptual/Image: Compresses the total gamut from one device's color space into the gamut of another device's color space when one or more colors in the original image is out of the gamut of the destination color space. This preserves the spectral relationship between colors and prevents "clipping," which is where distinctly different colors in one color space appear the same in the destination color space. The disadvantage of this render intent is that all of the original colors will change.
  • Saturation/Graphics: Reproduces the original image color saturation (vividness) when converting into the target device's color space. Primarily designed for business graphics, where exact relationships between colors (such as in a photographic image) are not as important as bright colors.
  • Relative Colorimetric/Colorimetric: When a color in the current color space is out of gamut in the target color space, it is mapped to the closest possible in gamut color in the target color space, while colors that are in gamut are not affected. This render intent can cause two colors which appear different in the current color space to be the same in the target color space. This is called "clipping." This is the method of color space translation built into Photoshop 4.0 and earlier.

Spectrophotometer
A device that measures either 16 or 32 discreet bandwidths of visible light. When used with software, it can be used to create ICC device profiles for monitors and output devices.



Educational Materials for Digital Printing