Out of Gamut: Color-Correct Vocabulary
For some time now I’ve been explicating the finer points of color management in this column, and while I’ve strived to define terminology when needed, the conviction that a permanent, at-your-ready glossary would help tremendously has grown each and every month. In this column, we’re introducing just that — a glossary of key terms that every student (and master) of color-management should understand.
Feel free to read, bookmark, save, print (use the print-friendly format), and otherwise consume the glossary now to your heart’s content (without violating the creativepro copyright, of course).
Absolute Colorimetric Rendering: One of the four ICC-specified rendering intents used for handling out-of-gamut colors in color matching. Absolute Colorimetric rendering matches those colors in the source space that are inside the gamut of the target space exactly, and clips out-of-gamut colors to the nearest reproducible hue, sacrificing lightness and saturation.
Additive Primary Colors: The primary colors of light, from which all other colors can be made — red, green, and blue. Adding 100 percent of all three produces white light, while adding lesser intensities produces a gamut of different colors. Combining 100 percent of two additive primaries produces a subtractive primary:
See also Primary Colors, Subtractive Primaries.
Black: The absence of light. The color that is produced when an object absorbs all light.
When the maximum intensity of the subtractive primaries — cyan, magenta, and yellow — are combined, the resulting color should, in theory, be black. Color film, for example, produces black using only cyan, magenta, and yellow dyes. Printing inks, however, are less colorimetrically pure than film dyes, and combining 100 percent cyan, magenta, yellow inks yields a muddy brown; hence, black ink is added as a fourth color ink. Black is abbreviated as “K” in CMYK to avoid confusion with “B” for blue.
Blue: One of the three additive primary colors, centered around a wavelength of approximately 436 nanometers.
Brightness: The degree to which a color sample appears to reflect light. This attribute of color is used in the HSB (Hue, Saturation, Brightness) color model.
Calibration: The act of adjusting a device to bring its behavior into accordance with a known specification. For example, monitors are calibrated to a specific color temperature, gamma, and black and white luminance. Imagesetters and platesetters are calibrated to make sure that they deliver the requested dot percentage accurately. Calibration is typically accomplished by measuring the behavior of a device with an instrument such as a colorimeter or densitometer, comparing the measured behavior with the standard to which the device is being calibrated, then adjusting the device so that it behaves in accordance with that standard.
Characterization: The act of describing a device’s behavior through software. In color management, this typically means creating an ICC profile.
Chroma: The property of a color that makes it appear saturated, or strongly colored. Black, white, and gray have no chroma. A red tomato is high in chroma. Pastel colors are low in chroma. This attribute of color is used in the LCH (Lightness, Chroma, Hue) color model.
Chromaticity Coordinates: Coordinates that describe the hue and saturation, or red-greenness and yellow-blueness, of a color, excluding its lightness. Usually plotted on a two-dimensional plane of constant lightness. See CIE xy Chromaticity Diagram.
CIE (Commission Internationale de l’Eclairage): The international standards organization responsible for setting standards for color and color measurement. (The French name translates to “International Commission on Illumination.”)
CIE XYZ (1931): The first of a series of mathematical models produced by the CIE that describe color in terms of synthetic primaries based on human perception. The primaries are imaginary mathematical constructs that model our eyes’ response to different wavelengths of light. Such models allow us to specify perceived color unambiguously, unlike models such as RGB and CMYK, which define amounts of colorants rather than actual colors.
CIELAB (CIE L* a* b*, CIE Lab): A mathematical derivative of CIE XYZ (1931) that describes colors using three synthetic primaries: L* (which indicates Lightness), a* (which indicates red-greenness), and b* (which indicates yellow-blueness).
CIE Standard Illuminants: A series of spectral data sets that describe the spectral components of different types of light sources. Used in conjunction with tristimulus values such as XYZ or Lab to define a color.
CIE Standard Observer: A hypothetical observer that represents “normal” human color vision, defined in terms of the eye’s color-matching functions. The CIE defines two such standard observers — the 2-degree observer and the 10-degree observer — because color vision is most acute in the center of the visual field.
CIE Tristimulus Values: Amounts of the three primaries required to match a color sample. When specifying tristimulus values, the standard observer and standard illuminant must also be specified.
CIE xy Chromaticity Diagram: A two-dimensional graph of chromaticity coordinates that shows the location of a color on a plane of constant lightness.
CMM (Color Matching Method): A software component that adjusts the numerical values that get sent to, or received from, different devices so that the perceived color they produce remains consistent. The “engine” in color management systems.
CMY: Cyan, magenta, and yellow — the subtractive primary colors — or a color space that describes colors in terms of their cyan, magenta, and yellow components.
Color: The human perceptual response to different wavelengths of light impinging on the photoreceptors in the retina.
Color Management: A set of software technologies that seeks to match color across input, display, and output devices by referencing their color behavior to a known standard by means of device profiles. The signals each device receives are adjusted in such a way that the perceived color remains consistent.
Color Matching Functions: The relative amounts of three additive primaries needed to match each wavelength of light, usually based on the CIE Standard Observer. The human eye, digital cameras, and scanners all have color matching functions.
Color Model: A means of specifying color numerically, usually in terms of varying amounts of primary colors. Examples include RGB, CMYK, and CIELAB.
Color Space: A three-dimensional representation of the colors that can be produced by a color model. The universe of colors a color model can produce.
Color Temperature: A measurement of the color of white light, expressed in Kelvins. (The Kelvin scale is a measure of temperature, starting from absolute zero.) The color temperature is the color of light a perfect black-body radiator emits when heated to that temperature. Computer monitors typically have a color temperature of 5000-9300 Kelvins: 5000 Kelvins is a yellowish-white, 9300 Kelvins is a blue white.
Colorants: Materials used to produce color, such as dyes, inks, pigments, toners, or phosphors.
Colorimeter: An optical instrument that measures the relative intensities of red, green, and blue light reflected or emitted from (or transmitted through) a color sample. Typically used to measure color from computer monitors.
ColorSync: The color management system built into Apple’s Macintosh operating system.
Cones: The specialized photoreceptors in the human eye that allow us to discriminate between different wavelengths of light. Our eyes contain three distinct types of cones, designated the L, M, and S cones because they are primarily sensitive to long, medium, and short wavelengths of light. (The other type of photoreceptor in the eye are known as rods. They are primarily used in low-light and peripheral vision and do not contribute to color vision.)
Cyan: One of the subtractive primary colors. Cyan colorants absorb all red light, reflecting green and blue.
D50: The CIE Standard Illuminant that represents a daylight-correlated color temperature of 5000 Kelvins. Widely used as a standard for viewing booths in the printing industry.
D65: The CIE Standard Illuminant that represents a daylight-correlated color temperature of 6500 Kelvins. Widely used as a standard color temperature for calibrated monitors.
Delta Error (delta-E): A measurement of color difference. In theory, delta-E is the smallest color change someone with normal color vision can detect.
Densitometer: An instrument that measures optical density.
Density: See optical density.