Glossary

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a* Red - green coordinate in CIE L*a*b* color space. A positive a* value indicates redness and a negative a* value indicates greenness.

absolute white In theory, a material that perfectly reflects all light energy at every visible wavelength. In practice, a solid white with known spectral reflectance data that is used as the reference white for all measurements of absolute reflectance. When calibrating a spectrophotometer, often a white ceramic plaque is measured and used as the absolute white reference.

absorb/absorption Dissipation of the energy of electromagnetic waves into other forms (e.g., heat) as a result of its interaction with matter; a decrease in directional transmittance of incident radiation, resulting in a modification or conversion of the absorbed energy.

achromatic color A neutral color - typically white, gray, or black, that has no chroma and no hue.

additive primaries Red, green and blue light. When all three additive primaries are combined at 100% intensity, white light is produced. When these three are combined at varying intensities, a gamut of different colors is produced. Combining two primaries at 100% produces a subtractive primary, either cyan, magenta or yellow:
100% red + 100% green = yellow
100% red + 100% blue = magenta
100% green + 100% blue = cyan
See subtractive primaries

appearance Manifestation of the nature of objects and materials through visual attributes such as size, shape, color, texture, glossiness, transparency, opacity, etc.

artificial daylight Term loosely applied to light sources, frequently equipped with filters, that try to reproduce the color and spectral distribution of daylight. A more specific definition of the light source is preferred.

attribute Distinguishing characteristic of a sensation, perception or mode of appearance. Colors are often described by their attributes of hue, chroma (or saturation) and lightness.

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b* Yellow - blue coordinate in CIE L*a*b* color space. A positive b* value indicates yellowness and a negative b* value indicates blueness.

black In theory, the complete absorption of incident light; the absence of any reflection. In practice, any color that is close to this ideal in a relative viewing situation i.e., a color of very low saturation and very low luminance.

brightness The dimension of color that refers to an achromatic scale, ranging from black to white. Also called lightness, luminous reflectance or transmittance (q.v.). Because of confusion with saturation, the use of this term should be discouraged.

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C* Chromaticity coordinate in CIE L*C*h color space. A chroma of 0 (zero) indicates a perfectly neutral color. A larger C* value indicates a more chromatic or saturated color

chroma The intensity or saturation level of a particular hue, defined as the distance of departure of a chromatic color from the neutral (gray) color with the same value. In an additive color-mixing environment, imagine mixing a neutral gray and a vivid red with the same value. Starting with the neutral gray, add small amounts of red until the vivid red color is achieved. The resulting scale obtained would represent increasing chroma. The scale begins at zero for neutral colors, but has no arbitrary end. Munsell originally established 10 as the highest chroma for a vermilion pigment and related other pigments to it. Other pigments with higher chroma were noted, but the original scale remained. The chroma scale for normal reflecting materials may extend as high as 20, and for fluorescent materials it may be as high as 30.

chromatic Perceived as having a hue not white, gray or black.

chromaticity That part of color specification which does not involve luminance. Chromaticity is two-dimensional and is specified by pairs of numbers such as dominant wavelength and purity.

chromaticity coordinates (CIE) The ratios of each of the three tristimulus values X, Y and Z in relation to the sum of the three designated as x, y and z respectively. They are sometimes referred to as the trichromatic coefficients. When written without subscripts, they are assumed to have been calculated for illuminant C and the 2 (1931) standard observer unless specified otherwise. If they have been obtained for other illuminants or observers, a subscript describing the observer or illuminant should be used. For example, x10 and y10 are chromaticity coordinates for the 10 observer and illuminant C.

chromaticity diagram (CIE) A two-dimensional graph of the chromaticity coordinates (x as the abscissa and y as the ordinate), which shows the spectrum locus (chromaticity coordinates of monochromatic light, 380-770nm). It has many useful properties for comparing colors of both luminous and non-luminous materials.

CIE (Commission Internationale de lEclairage) The International Commission on Illumination, the primary international organization concerned with color and color measurement.

CIE 1976 L*a*b* color space A uniform color space utilizing an Adams-Nickerson cube root formula, adopted by the CIE in 1976 for use in the measurement of small color differences.

CIE 1976 L*u*v* color space A uniform color space adopted in 1976. Appropriate for use in additive mixing of light (e.g., color TV) and when an associated chromaticity is required.

CIE chromaticity coordinates The ratios of each of the tristimulus values of a color to the sum of the tristimulus values. In the CIE systems they are designated by x, y, and z.

CIE L*a*b* (or CIELAB) Color space in which values L*, a*, and b* are plotted at right angles to one another to form a three-dimensional coordinate system. Equal distances in the space approximately represent equal color differences. Value L* represents Lightness; value a* represents the Red/Green axis; and value b* represents the Yellow/Blue axis. CIE L*a*b* is a popular color space for use in measuring reflective and transmissive objects.

CIE L*u*v* (or CIELUV) Color space in which values L*, a*, and b* are plotted at right angles to one another to form a three-dimensional coordinate system. Equal distances in the space approximately represent equal color differences. Value L* represents Lightness; value a* represents the Red/Green axis; and value b* represents the Yellow/Blue axis. CIE L*a*b* is a popular color space for use in measuring reflective and emissive objects.

CIE Luminosity Function (Y) A plot of the relative magnitude of the visual response as a function of wavelength from about 380 to 780 nm, adopted by CIE in 1924.

CIE Standard Illuminants Known spectral data established by the CIE for four different types of light sources. When using tristimulus data to describe a color, the illuminant and observer must also be defined. These standard illuminants are used in place of actual measurements of the light source.

CIE Standard Observer A hypothetical observer having the tristimulus color-mixture data recommended in 1931 by the CIE for a 2 visual field. A supplementary observer for a larger visual field of 10 was adopted in 1964.

CIE Tristimulus Values Amounts of the three components necessary in a three-color additive mixture required for matching a color: in the CIE System, they are designated as X, Y, and Z. The illuminant and standard observer color matching functions used must be designated; if they are not, the assumption is made that the values are for the 1931 CIE 2 Standard Observer and Illuminant C.

CIE xy Chromaticity Diagram A two-dimensional graph of the chromaticity coordinates, x as the abscissa and y as the ordinate, which shows the spectrum locus (chromaticity coordinates of mono-chromatic light, 380-770nm). It has many useful properties for comparing colors of both luminous and non-luminous materials.

CIELAB (or CIE L*a*b*, CIE Lab) Color space in which values L*, a* and b* are plotted using Cartesian coordinate system. Equal distances in the space approximately represent equal color differences. Value L* represents lightness; value a* represents the red/green axis; and value b* represents the yellow/blue axis. CIELAB is a popular color space for use in measuring reflective and transmissive objects.

CMC (Colour Measurement Committee of the Society of Dyes and Colourists of Great Britain) Organization that developed and published in 1988 a more logical, ellipse-based equation based on L*C*h color space for computing DE (see delta E*) values as an alternative to the rectangular coordinates of the CIELAB color space.

color One aspect of appearance; a stimulus based on visual response to light, consisting of the three dimensions of hue, saturation and lightness.

color attribute A three-dimensional characteristic of the appearance of an object. One dimension usually defines the lightness, the other two together define the chromaticity.

color correction A photographic or electronic process that is used to compensate for the unwanted absorptions of process inks and also the deficiencies of the color separation process. A color correction can also refer to any color change requested by the client.

color difference The magnitude and character of the difference between two colors under specified conditions.

color-matching functions Relative amounts of three additive primaries required to match each wavelength of light. The term is generally used to refer to the CIE standard observer color-matching functions.

color measurement Physical measurement of light radiated, transmitted or reflected by a specimen under specified condition and mathematically transformed into standardized colorimetric terms. These terms can be correlated with visual evaluations of colors relative to one another.

color model A color-measurement scale or system that numerically specifies the perceived attributes of color. Used in computer graphics applications and by color measurement instruments.

color order systems Systems used to describe an orderly three-dimensional arrangement of colors. Three bases can be used for ordering colors: (1) an appearance basis (i.e., a psychological basis) in terms of hue, saturation and lightness; an example is the Munsell System; (2) an orderly additive color mixture basis (i.e., a psychophysical basis); examples are the CIE System and the Ostwald System; and (3) an orderly subtractive color mixture basis; an example is the Plochere Color System based on an orderly mixture of inks.

color separation T he conversion of the red, green, and blue color information used in a computer into cyan, magenta, yellow, and black channels that are used to make printing plates.

color space Three-dimensional solid enclosing all possible colors. The dimensions may be described in various geometries, giving rise to various spacings within the solid.

color specification Tristimulus values, chromaticity coordinates and luminance value, or other color-scale values, used to designate a color numerically in a specified color system.

color temperature A measurement of the color of light radiated by a black body while it is being heated. This measurement is expressed in terms of absolute scale, or degrees Kelvin. Lower Kelvin temperatures such as 2400K are red; higher temperatures such as 9300K are blue. Neutral temperature is white, at 6504K.

color wheel The visible spectrums continuum of colors arranged in a circle, where complementary colors such as red and green are located directly across from each other.

colorants Materials used to create colors dyes, pigments, toners, waxes, phosphors.

colorimeter An optical measurement instrument that responds to color in a manner similar to the human eye by filtering reflected light into its dominant regions of red, green and blue.

colorimetric Of, or relating to, values giving the amounts of three colored lights or receptors red, green and blue.

colorist A person skilled in the art of color matching (colorant formulation) and knowledgeable concerning the behavior of colorants in a particular material; a tinter (q.v.) (in the American usage) or a shader. The word colorist is of European origin.

complements Two colors that create neutral gray when combined. On a color wheel, complements are directly opposite from each other: blue/yellow, red/green and so on.

contrast The level of variation between light and dark areas in an image.

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D65 The CIE standard illuminant that represents a color temperature of 6504K. This is the color temperature most widely used in graphic arts industry viewing booths. See Kelvin (K).

daylight illuminants (CIE) Series of illuminant spectral power distribution curves based on measurements of natural daylight and recommended by the CIE in 1965. Values are defined for the wavelength region 300 to 830nm. They are described in terms of the correlated color temperature. The most important is D65 because of the closeness of its correlated color temperature to that of illuminant C, 6774K. D75 bluer than D65 and D55 yellower than D65 are also used.

delta (D) A symbol used to indicate deviation or difference.

delta E*, delta e* The total color difference computed with a color difference equation (delta Eab or delta Ecmc). In color tolerancing, the symbol DE is often used to express Delta Error.

delta Ecmc Developed by the Color Measurement Committee of the Society of Dyes and Colourists in Great Britain. DEcmc is an ellipse-based equation for computing DE values as an alternative to the rectangular coordinates of the CIE L*a*b* color space. DEcmc more closely matches human vision when judging the acceptability of color difference.

dye A soluble colorant as opposed to pigment, which is insoluble.

dynamic range An instruments range of measurable values, from the lowest amount it can detect to the highest amount it can handle.

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electromagnetic spectrum The massive band of electromagnetic waves that pass through the air in different sizes, as measured by wavelength. Different wavelengths have different properties, but most are invisible and some completely undetectable to human beings. Only wavelengths that are between 380 and 720 nanometers are visible, producing light. Waves outside the visible spectrum include gamma rays, x-rays, microwaves and radio waves.

emissive object An object that emits light. Emission is usually caused by a chemical reaction, such as the burning gasses of the sun or the heated filament of a light bulb.

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fluorescent lamp A glass tube filled with mercury gas and coated on its inner surface with phosphors. When the gas is charged with an electrical current, radiation is produced. This, in turn, energizes the phosphors, causing them to glow.

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gloss An additional parameter to consider when determining a color standard, along with hue, value, chroma, the texture of a material and whether the material has metallic or pearlescent qualities. Gloss is an additional tolerance that may be specified in the Munsell Color Tolerance Set. The general rule for evaluating the gloss of a color sample is the higher the gloss unit, the darker the color sample will appear. Conversely, the lower the gloss unit, the lighter a sample will appear.

Gloss is measured in gloss units, which use the angle of measurement and the gloss value (e.g. 60 gloss = 29.8). A 60 geometry is recommended by the American Society for Testing and Materials (ASTM) D523 standard for the general evaluation of gloss.

grayscale An achromatic scale ranging from black through a series of successively lighter grays to white. Such a series may be made up of steps that appear to be equally distant from one another (such as the Munsell Value Scale), or it may be arranged according to some other criteria such as a geometric progression based on lightness. Such scales may be used to describe the relative amount of difference between two similar colors.

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h Hue angle coordinate in CIE L*C*h color space. The angle refers to the hue's location in the CIE L*C*h color wheel, where red is 0, yellow is 90, green is 180, and blue is 270.

hue (1) The first element in the color-order system, defined as the attribute by which we distinguish red from green, blue from yellow, etc. Munsell defined five principal hues (red, yellow, green, blue and purple) and five intermediate hues (yellow-red, green-yellow, blue-green, purple-blue and red-purple. These 10 hues (represented by their corresponding initials R, YR, Y, GY, G, BG, B, PB, P and RP) are equally spaced around a circle divided into 100 equal visual steps, with the zero point located at the beginning of the red sector. Adjacent colors in this circle may be mixed to obtain continuous variation from one hue to another. Colors defined around the hue circle are known as chromatic colors. (2) The attribute of color by means of which a color is perceived to be red, yellow, green, blue, purple, etc. White, black and gray possess no hue.

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illuminant Mathematical description of the relative spectral power distribution of a real or imaginary light source i.e., the relative energy emitted by a source at each wavelength in its emission spectrum. Often used synonymously with light source or lamp, though such usage is not recommended.

illuminant A (CIE) Incandescent illumination, yellow-orange in color, with a correlated color temperature of 2856K. It is defined in the wavelength range of 380 to 770nm.

illuminant C (CIE) Tungsten illumination that simulates average daylight, bluish in color, with a correlated color temperature of 6774K.

illuminants D (CIE) Daylight illuminants, defined from 300 to 830nm (the UV portion 300 to 380nm being necessary to correctly describe colors that contain fluorescent dyes or pigments). They are designated as D, with a subscript to describe the correlated color temperature; D65 is the most commonly used, having a correlated color temperature of 6504K, close to that of illuminant C. They are based on actual measurements of the spectral distribution of daylight.

integrating sphere A sphere manufactured or coated with a highly reflective material that diffuses light within it.

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Kelvin (K) Unit of measurement for color temperature. The Kelvin scale starts from absolute zero, which is -273 Celsius.

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light (1) Electromagnetic radiation of which a human observer is aware through the visual sensations that arise from the stimulation of the retina of the eye. This portion of the spectrum includes wavelengths from about 380 to 770nm. Thus, to speak of ultraviolet light is incorrect because the human observer cannot see radiant energy in the ultraviolet region. (2) Adjective meaning high reflectance, transmittance or level of illumination as contrasted to dark, or low level of intensity.

light source An object that emits light or radiant energy to which the human eye is sensitive. The emission of a light source can be described by the relative amount of energy emitted at each wavelength in the visible spectrum, thus defining the source as an illuminant. The emission also may be described in terms of its correlated color temperature.

lightness Perception by which white objects are distinguished from gray, and light-colored objects from dark-colored.

luminosity function (y) (CIE) A plot of the relative magnitude of the visual response as a function of wavelength from about 380 to 780nm, adopted by CIE in 1924.

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metamerism A phenomenon exhibited by a pair of colors that match under one or more sets of illuminants (be they real or calculated), but not under all illuminants.

Munsell Color System The color identification of a specimen by its Munsell hue, value and chroma as visually estimated by comparison with the Munsell Book of Color.

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nanometer (nm) Unit of length equal to 10^-9 meter (a.k.a. one billionth of a meter, or a milli-micron).

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observer The human viewer who receives a stimulus and experiences a sensation from it. In vision, the stimulus is a visual one and the sensation is an appearance.

observer, standard The spectral response characteristics of the average observer defined by the CIE. Two such sets of data are defined, the 1931 data for the 2 visual field (distance viewing) and the 1964 data for the annular 10 visual field (approximately arm's length viewing).

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radiant energy A form of energy consisting of the electromagnetic spectrum, which travels at 299,792 kilometers/second (186,206 miles/second) through a vacuum, and more slowly in denser media (air, water, glass, etc.). The nature of radiant energy is described by its wavelength or frequency, although it also behaves as distinct quanta (corpuscular theory). The various types of energy may be transformed into other forms of energy (electrical, chemical, mechanical, atomic, thermal, radiant), but the energy itself cannot be destroyed.

reflectance The ratio of the intensity of reflected radiant flux to that of incident flux. In popular usage, it is considered the ratio of the intensity of reflected radiant energy to that reflected from a defined reference standard.

reflectance, specular Reflectance of a beam of radiant energy at an angle equal but opposite to the incident angle; the mirror-like reflectance. The magnitude of the specular reflectance on glossy materials depends on the angle and on the difference in refractive indices between two media at a surface and may be calculated from the Fresnel Law.

reflectance, total Reflectance of radiant flux reflected at all angles from the surface, thus including both diffuse and specular reflectances.

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saturation The attribute of color perception that expresses the amount of departure from a gray of the same lightness. All grays have zero saturation (ASTM). See chroma/chromaticity.

scattering Diffusion or redirection of radiant energy encountering particles of different refractive index. Scattering occurs at any such interface, at the surface, or inside a medium containing particles.

spectral power distribution curve Intensity of radiant energy as a function of wavelength, generally given in relative power terms.

spectrophotometer Photometric device that measures spectral transmittance, spectral reflectance or relative spectral emittance.

spectrophotometric curve A curve measured on a spectrophotometer; a graph with relative reflectance or transmittance (or absorption) as the ordinate, plotted with wavelength or frequency as the abscissa.

spectrum Spatial arrangement of components of radiant energy in order of their wavelengths, wave number or frequency.

specular gloss Relative luminous fractional reflectance from a surface in the mirror or specular direction. It is sometimes measured at 60 relative to a perfect mirror.

specular reflectance Reflectance of a beam of radiant energy at an angle equal but opposite to the incident angle; the mirror-like reflectance. The magnitude of the specular reflectance on glossy materials depends on the angle and the difference in refractive indices between two media at a surface. The magnitude may be calculated from Fresnels Law.

specular reflectance excluded (SCE) Measurement of reflectance made in such a way that the specular reflectance is excluded from the measurement; diffuse reflectance. The exclusion may be accomplished by using 0 (perpendicular) incidence on the samples. This then reflects the specular component of the reflectance back into the instrument by use of black absorbers or light traps at the specular angle when the incident angle is not perpendicular, or in directional measurements by measuring at an angle different from the specular angle.

specular reflectance included (SCI) Measurement of the total reflectance from a surface, including the diffuse and specular reflectances.

standard A reference against which instrumental measurements are made.

standard illuminants (CIE) Known spectral data established by the CIE for four different types of light sources. When using tristimulus data to describe a color, the illuminant must also be defined. These standard illuminants are used in place of actual measurements of the light source.

standard observer (CIE) (1) A hypothetical observer having the tristimulus color-mixture data recommended in 1931 by the CIE for a 2 viewing angle. A supplementary observer for a larger angle of 10 was adopted in 1964. (2) The spectral response characteristics of the average observer defined by the CIE. Two such sets of data are defined, the 1931 data for the 2 visual field (distance viewing) and the 1964 data for the annular 10 visual field (approximately arms length viewing). By custom, the assumption is made that if the observer is not specified, the tristimulus data has been calculated for the 1931, or 2 field observer. The use of the 1964 data should be specified.

subtractive primaries Cyan, magenta and yellow. Theoretically, when all three subtractive primaries are combined at 100% on white paper, black is produced. When these are combined at varying intensities, a gamut of different colors is produced. Combining two primaries at 100% produces an additive primary, either red, green or blue:
100% cyan + 100% magenta = blue
100% cyan + 100% yellow = green
100% magenta + 100% yellow = red

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tint (1) verb: To mix white pigment with absorbing (generally chromatic) colorants. 2) noun: The color produced by mixing white pigment with absorbing (generally chromatic) colorants. The resulting mixture is lighter and less saturated than the color without the white added.

total reflectance Reflectance of radiant flux reflected at all angles from the surface, thus including both diffuse and specular reflectances.

transparent Describes a material that transmits light without diffusion or scattering.

tristimulus Of, or consisting of, three stimuli; generally used to describe components of additive mixture required to evoke a particular color sensation.

tristimulus colorimeter An instrument that measures tristimulus values and converts them to chromaticity components of color.

tristimulus values (CIE) Percentages of the components in a three-color additive mixture necessary to match a color; in the CIE system, they are designated as X, Y and Z. The illuminant and standard observer color-matching functions used must be designated; if they are not, the assumption is made that the values are for the 1931 observer (2 field) and illuminant C. The values obtained depend on the method of integration used, the relationship of the nature of the sample and the instrument design used to measure the reflectance or transmittance. Tristimulus values are not, therefore, absolute values characteristic of a sample, but relative values dependent on the method used to obtain them. Approximations of CIE tristimulus values may be obtained from measurements made on a tristimulus colorimeter that gives measurements generally normalized to 100. These must then be normalized to equivalent CIE values. The filter measurements should be properly designated as R, G and B instead of X, Y and Z.

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Uniform Chromaticity Scale Diagram Any one of a variety of transformations of the CIE chromaticity diagram on which all pairs of just noticeably different colors of equal luminance are represented by pairs of points separated by nearly equal distances.

Uniform Color Scale A scale wherein the units of color difference that are judged to be equally different, are separated by nearly equal distances.

Uniform Color Space Three-dimensional space wherein all pairs of colors judged to be equally different, are separated by nearly equal distances.

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value Indicates the degree of lightness or darkness of a color in relation to a neutral gray scale. The scale of value (or V, in the Munsell system of color notation) ranges from 0 for pure black to 10 for pure white. The value scale is neutral or without hue.

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X (1) One of the three CIE tristimulus values; the red primary. (2) Spectral color-matching functions of the CIE standard observer used for calculating the X tristimulus value. (3) One of the CIE chromaticity coordinates calculated as the fraction of the sum of the three tristimulus values attributable to the X value.

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Y (1) One of the three CIE tristimulus values, equal to the luminous reflectance or transmittance; the green primary. (2) Spectral color-matching function of the CIE standard observer used for calculating Y tristimulus value. (3) One of the CIE chromaticity coordinates calculated as the fraction of the sum of the three tristimulus values, attributable to the Y value.

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Z (1) One of the three CIE tristimulus values; the blue primary. (2) Spectral color-matching function of the CIE standard observer used for calculating the Z tristimulus value. (3) One of the CIE chromaticity coordinates calculated as the fraction of the sum of the three tristimulus values attributable to the Z primary.

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