Out of Gamut: Color-Correct Vocabulary

Out of Gamut: Color-Correct Vocabulary Color guru Bruce Fraser unrolls the creativepro.com glossary of color terminology. (Creativepro)By Bruce Fraser, creativepro,contributing editor

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.com copyright, of course), but don't fear: We'll be adding this glossary as a permanent feature of creativepro.com, and linking it to from future Out of Gamut columns. We'll also be adding to the glossary now and then as needed.

Enjoy!

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:



red+green=yellow

red+blue=magenta

green+blue=cyan

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.

Device-dependent: Describes a color space defined in terms of physical colorants, such as a monitor's RGB or a printing press' CMYK. So called because the actual color produced from a set of device-dependent values depends on the colorants and physical properties of the device in question.

Device-independent: Describes a color space defined using synthetic primaries based on human perception, independent of the properties of any physical device. Device-independent color models provide an unambiguous description of perceived color, unlike device-dependent color models.

Dye: A soluble colorant (as opposed to pigments, which are insoluble). Dyes are capable of producing brighter colors than pigments, but are less stable and less resistant to fading over time.

Dynamic Range: The range of density that a film stock, digital camera, scanner, or measuring instrument can detect, from the lowest to the highest, usually expressed in O.D. (Optical Density) units. The lowest density is termed dMin, the highest density is termed dMax.

Gamut: The range of color a device can produce, or the range of color a color model can represent.

Gamut Compression: The process where a large color gamut (for example, that of transparency film) is reduced to fit the smaller gamut of a print or display process (for example, color printing).

HSB Color Model: A color model that describes color in terms of hue, saturation, and brightness.

Hue: The property of a color that is identified by a color name, such as "red," "green," or "blue." Used as a primary in the HSB (Hue, Saturation, Brightness) color model.

ICC (International Color Consortium): A group of hardware and software vendors dedicated to developing cross-platform standards for color communication and consistency.

ICC Profile: A standard format developed by the ICC for a data file that describes the color behavior of an input, display, or output device, or a color model, by referencing it to a device-independent color model such as CIE XYZ or CIELAB. Used in almost all current color-management systems.

Illuminant: A light source with known spectral power distribution.

Kelvin (K): Unit of measurement of color temperature. The Kelvin scale starts at absolute zero (-273° Celsius).

Intent: See Rendering Intent.

LCH Color Model: A derivative of CIELAB that uses cylindrical coordinates of lightness, chroma, and hue instead of the rectangular coordinate system of Lab.

Light: That small part of the electromagnetic spectrum whose wavelengths lie in the range of 380 to 720 nanometers, and hence are detectable by the human eye.

Lightness: The degree to which a color sample appears to reflect light. This attribute of color is used in the LCH (Lightness, Chroma, Hue) color model.

Magenta: One of the subtractive primary colors. Magenta absorbs all green light, reflecting red and blue.

Metamerism: The phenomenon where two color samples appear to match under one light source, and differ under another. Two such samples are called a metameric pair.

Nanometer: A unit of length equal to one-millionth of a millimeter. Visible light wavelengths are measured in nanometers.

Optical Density: The ability of a material to absorb light. The darker the material, the higher the density. Density is usually expressed on a logarithmic scale of Optical Density (O.D.) units.

Perceptual Rendering: One of the four ICC-specified rendering intents used for handling out-of-gamut colors in color matching. Perceptual rendering attempts to compress the gamut of the source space into the gamut of the destination space in such a way that the overall relationships between the colors -- and hence the overall image appearance -- is preserved, even though all the colors may change in the process.

Phosphors: Chemical compounds that emit light when struck by a beam of electrons. The amount of light emitted is proportional to the intensity of the electron beam. RGB monitors use three different phosphors to produce red, green, and blue light.

Photoreceptor: A mechanism that emits an electrical or chemical signal that varies in proportion to the amount of light that strikes it. CCD (charge-coupled device) sensors in desktop scanners and digital cameras, PMT (photomultiplier tubes) in drum scanners, and the rods and cones in the human retina are all photoreceptors.

Pigment: An insoluble colorant (as opposed to dyes, which are soluble). Pigments generally have better fade-resistance and permanence than dyes.

Primaries: The components of a color in a color model. They may be actual primary colors perceivable by humans, as in RGB or CMYK, or they may be imaginary mathematical constructs, as with CIE XYZ (1931) or CIELAB.

Primary Colors: The colors from which all other colors can be made. The primary colors of light are red, green, and blue. These are the additive primaries, used for transmissive or emissive color. The primary colors of pigments are cyan, magenta, and yellow, used for reflective color. It's possible to create all colors from primary colors because the human eye contains three different types of color-sensitive photoreceptors, which are sensitive to the individual primary colors.

Profile: A data file that describes the color behavior of a physical device (such as a scanner, monitor, or printer) or that defines the color of an abstract color space (such as Adobe RGB 1998 or ColorMatch RGB) in terms of a device-independent color model (such as CIELAB or CIE XYZ). Used by color-management systems to define and match color.

Relative Colorimetric Rendering: One of the four ICC-specified rendering intents used for handling out-of-gamut colors in color matching. Relative Colorimetric rendering first scales the white of the source space to the white of the target space, adjusting all other colors relative to that white. Then it matches the adjusted 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.

Rendering Intent: A method of handling out-of-gamut colors when matching one color space to another. The ICC profile specification specifies four rendering intents: Perceptual, Absolute Colorimetric, Relative Colorimetric, and Saturation.

Saturation: The property of a color that makes it appear strongly colored. Black, white, and gray have no saturation. A red tomato has high saturation. Pastel colors have low saturation. Also known as Chroma. (This attribute of color is used in the HLS (Hue, Lightness, Saturation) and HSB (Hue, Saturation, Brightness) color models.

Saturation Rendering: One of the four ICC-specified rendering intents used for handling out-of-gamut colors in color matching. Saturation rendering maps fully-saturated colors in the source space to fully saturated colors in the target space, sacrificing hue and lightness.

Spectral Data: The most complete and precise means of describing a color, by specifying the amount of each wavelength that the sample reflects. Typically, spectral data records the amount of reflected light in 10-nanometer or 20-nanometer bands.

Spectral Power Distribution: The amount of light a light source produces at each wavelength.

Spectrophotometer: An instrument that measures the amount of light a color sample reflects or transmits at each wavelength, producing spectral data.

Standard Illuminant: See CIE Standard Illuminants.

Standard Observer: See CIE Standard Observer.

Subtractive Primaries: Cyan, magenta, and yellow. Used to create reflective color. Cyan absorbs (subtracts) all red light, reflecting blue and green. Magenta absorbs all green light, reflecting blue and red. Yellow absorbs all blue light, reflecting red and green.

Tristimulus: The practice of specifying or creating colors using three stimuli. These may be additive (RGB) or subtractive (CMY) primary colorants; three attributes such as Lightness, Chroma, and Hue; or three purely synthetic mathematical constructs, as with CIE XYZ (1931) or CIELAB.

Tristimulus Data: The three values used to define or create a color, such as Red 255, Green 0, Blue 0. Tristimulus values alone do not define a color unambiguously: The illuminant (light source) must also be defined. In the case of device-dependent tristimulus values such as monitor RGB, the primaries must also be defined in a device-independent system such as CIE XYZ or CIELAB. Tristimulus values can always be computed from spectral data, but spectral data cannot be inferred from tristumulus values.

Visible Spectrum: The portion of the electromagnetic spectrum with wavelengths between 380 and 720 nanometers. Wavelengths in this range provoke the sense of color when they impinge on the photoreceptors in the human retina. The shorter wavelengths within this range produce blue and violet sensations; the longer wavelengths produce orange and red sensations.

Yellow: One of the subtractive primary colors. Yellow absorbs all blue light, reflecting red and green.