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Glossary of Selected Light and Color Terms


"a" = Redness-greenness coordinate in certain transformed color spaces (Hunter L,a,b or CIELAB), generally used as the difference in "a" between a specimen and a standard reference color. If "a"  is positive, there is more redness than greenness; if "a" is negative,there is more greenness than redness. It is normally used with b as part of the chromaticity or chromaticity color difference.

Absorption = decrease in directional transmittance of incident radiation (such as light), resulting in a modification or conversion of the absorbed energy, into heat, for example. Light incident on a specimen may be partially reflected, partially transmitted, or partially absorbed.

Accuracy = the closeness of agreement between a test result and an accepted reference value (often used as a color instrument specification).

Achromatic = perceived as having no hue, that is, as white, gray, or black.

Additive Color Mixture = superposition or other nondestructive combination of lights of different perceived colors.

Angle Of Illumination =, angle between the specimen and the illuminator axis.

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, which are claimed to reproduce the color and spectral distribution of daylight. A more specific definition of the light source is to be preferred.

"b" =  yellowness-blueness coordinate in certain color spaces (Hunter L,a,b & CIELAB), generally used as the difference in "b" between a specimen and a standard reference color, normally used with "a" or "a" as part of the chromaticity difference. Generally, if "b" is positive, there is more yellowness than blueness; if "b" is negative, there is more blueness than yellowness.

Basic Color Terms = a group of 11 color names found in anthropological surveys to be in wide use in fully developed languages: white, black, red, green, yellow, blue, brown, gray, orange, purple, pink.

Beer’s Law = the absorbence of a homogeneous sample containing an absorbing substance is directly proportional to the concentration of the absorbing substance, often used in mixture prediction of transparent materials.

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

Brightness = (1) aspect of visual perception whereby an area appears to emit more or less light;  (2) of an object color, combination of lightness and saturation; (paper, reflectance of an infinitely thick specimen (reflectivity) measured for blue light with a centroid wavelength of 457 nm under specified spectral and geometric conditions of measurement).

C or Delta c  = abbreviations for chromaticity or chromaticity difference, respectively.

Calibrate, = to find and eliminate systematic errors of an instrument scale or method of instrument by use of material standards and techniques traceable to an authorized national or international measurement system.

Chroma =  (1) attribute of color used to indicate the degree of departure of the color from a gray of the same lightness. (2) C*, (in the CIE 1976 L*, a*, b* or L*, u*, v* system) the quantity C*ab = (a*2 + b*2)1/2 or C*uv = (u*2 + v*2)1/2.  (3) attribute of a visual perception, produced by an object color that permits a judgment to be made of the amount of pure chromatic color present, irrespective of the amount of achromatic color.

Chromatic = perceived as having a hue; not white, gray, or black. (opposite of achromatic)

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, 380-770 nm). It has many useful properties for comparing colors of both luminous (light emitting) and nonluminous (reflective) materials.

CIE = the abbreviation for the French title of the International Commission on Illumination, Commission Internationale de l’Eclairage.

CIE 1931 Standard Observer   = ideal colorimetric observer with color matching functions x-(y), y-(y), z-(y) corresponding to a field of view subtending a 2º angle on the retina; commonly called the "2º standard observer."

CIE 1964 Supplementary Standard Observer  = ideal colorimetric observer with color matching functions x-10(y), y-10(y), z-10(y) corresponding to a field of view subtending a 10º angle on the retina; commonly called the "10º standard observer."

CIELAB Color Difference = color difference calculated by using the CIE 1976 L* a* b* opponent-color scales (also referred to as CIELAB), based on applying a cube-root transformation to CIE 1931 tristimulus values X, Y, Z or CIE 1964 tristimulus values X10, Y10, Z10.

Clarity  = the characteristic of a transparent body whereby distinct high-contrast images or high-contrast objects (separated by some distance from the body) are observable through the body.

Discussion Based on the lightness, hue, chroma version of CIELAB, it incorporates chroma and hue-angle correction terms for improved visual spacing and variable weighting factors for lightness (l) and chroma (c) relative to hue for improved correlation depending on type of judgment (acceptability, perceptibility) and application (textiles, others). CMC reports the equivalent of D E as a weighted function of the ratio of L:Ch as Performance Factor=PF.

Color = (1) of an object, aspect of object appearance distinct from form, shape, size, position, or gloss that depends upon the spectral composition of the incident light, the spectral reflectance of transmittance of the object, and the spectral response of the observer, as well as the illuminating and viewing geometry.  (2) perceived, attribute of visual perception that can be described by color names such as white, gray, black, yellow, brown, vivid red, deep reddish purple, or by combinations of such names.  (3) psychophysical, characteristics of a color stimulus (that is, light producing a sensation of color) denoted by a colorimetric specification with three values, such as tristimulus values.

Color Constancy =  the general tendency of the colors of an object to remain constant when the color of the illumination is changed.

Color Difference = 1) perceived, the magnitude and character of the difference between two colors described by such terms as redder, bluer, lighter, darker, grayer, or cleaner.  (2) computed, the magnitude and direction of the difference between two psychophysical color stimuli and their components computed from tristimulus values, or chromaticity coordinates and luminance factor, by means of a specified set of color-difference equations.

Colorimeter Tristimulus = instrument that measures psychophysical color, in terms of tristimulus values, by the use of filters to convert the relative spectral power distribution of the illuminator to that of a standard illuminant, and to convert the relative spectral responsively of the receiver to the responsivities prescribed for a standard observer. See spectrocolorimeter.

Colorimetry = the science of color measurement.

Color Match = (1) condition existing when colors match within a specified or agreed tolerance. Sometimes called commercial color match. (2) condition existing when colors are indistinguishable; a normal observer is usually implied. Sometimes called an exact color match.

Color Matching = procedure for providing, by selection, formulation, adjustment, or other means, a trial color that is indistinguishable from, or within specified tolerances of, a specified standard color under specified conditions.

Color Measurement = physical measurement of light radiated, transmitted, or reflected by a specimen under specified conditions and mathematically transformed into standardized colorimetric terms that can be correlated with visual evaluations of color relative to one another. Although the term "color measurement" is normally used, color itself cannot be measured.

Color Perception = subjective impression of color, as modified by the conditions of observation and by mental interpretation of the stimulus object.

Color Space = a geometric space, usually of three dimensions, in which colors are arranged systematically.

Color Specification = notation or set of three color-scale values used to designate a color in a specified color system. Practical color specifications may include color tolerances as well as target color designation, measuring instrument, instrument settings, measurement procedures and sample preparation procedures.

Color Stimulus = a radiant flux capable of producing a color perception.

Color Temperature = of a light source, the temperature, usually expressed in kelvins, of a full radiator (perfect, theoretical black substance that when heated would not affect the "color" of the light emitted) which emits light of the same chromaticity as the source. Average daylight color temperature is often expressed as D65, for 6,500 degrees Kevin.

Color Tolerance = the permissible color difference between sample and specified color (Standard).

Contrast, = objective, the degree of dissimilarity of a measured quantity such as luminance of two areas, expressed as a number computed by a specified formula.

Contrast Ratio, =   In color measurement, a sample is measured over a white background, then over a black background and the ratio expressed either as  a perctentage (where 100% = complete opacity) or as a ratio to 1. 

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 830 nm. They are described in terms of the correlated color temperature. The most important is D6500 (often referred to as average daylight) because of the closeness of its correlated color temperature to that of Illuminant C = 6774 K. D7500, bluer than D6500 and D5000, yellower than D6500, are also used.

Densitometer = instrument designed for measuring optical density of a photographic negative or positive or a printed image; not suitable for colorimetry.

Dominant Wavelength = the wavelength of a spectrally pure light that, when added to a reference achromatic (white) light, will produce a combination that matches the color of a specimen light.

Delta D  = indicates deviation or difference.

Delta E, Delta e, D E, D e  =  the total color difference computed with a color difference equation. It is generally calculated as the square root of the sum of the squares of the chromaticity difference, D a + D b, and the lightness difference, D L; in CMC identified as the "commercial factor" (CF).

Flop = a difference in color and appearance of a material viewed over two widely different aspecular angles.

Fluorescence = a process by which radiant flux of certain wavelengths is absorbed and reradiated nonthermally at other, usually longer, wavelengths. (this phenomenon creates colors that show an abnormal color response and are used for packaging and other dramatic effects).

Foot Candle = unit of illuminance equal to one lumen per square foot.

Gardner Color Scale = a color scale for clear, light-yellow fluids, defined by the chromaticities of glass standards numbered from 1 for the lightest to 18 for the darkest.

Gray Scale = 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 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.

Hue = the attribute of color by means of which a color is perceived to be red, yellow, green, blue, purple, etc. Pure white, black, and grays possess no hue.

Hunter Color Difference = color difference calculated by the use of the Hunter equations, based on the opponent-color coordinates, L, a, b, applied to CIE 1931 tristimulus values for CIE standard illuminant C, and by extension to the CIE 1964 standard observer and other CIE standard illuminants.

Illuminant = mathematical description of the relative spectral power distribution of a real or imaginary light source, that is, the relative energy emitted by a source at each wavelength in its emission spectrum (the data entered into a color computer, used to predict the effect of different light sources on the perceived color).

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

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

Index of Refraction  = the numerical expression of the ratio of the velocity of light in a vacuum to the velocity of light in a substance (gas, liquid, solid), at a specified wavelength.

Interference Filter = filter constructed of extremely thin alternate layers of high and low refractive-index material and capable of transmitting narrow spectral bands formed by constructive interference within the desired waveband and destructive interference at other wavelengths (used in filter colorimeters and some abridged spectrophotometers).

Just-Perceptible Difference = color difference that is just large enough to be perceived by an observer in almost every trial.

Light =  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 nm to 780 nm. Thus, it is incorrect to speak of ultraviolet or infrared "light" because the human observer cannot see radiant energy in the ultraviolet and infrared regions.

Lightness = (1) the attribute of color perception by which a non-self-luminous body is judged to reflect more or less light.  (2) the attribute by which a perceived color is judged to be equivalent to one of a series of grays ranging from black to white.

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, or the emission may be described in terms of its correlated color temperature.

Lovibond Tintometer = instrument for evaluating the colors of materials by visual comparison with the colors of glasses of the Lovibond color system.

Luminescence = emission of light ascribable to nonthermal excitation.

Match = to provide, by selection, formulation, adjustment, or other means, a trial color that is indistinguishable from, or within specified tolerances of, a specified standard color under specified conditions.

Metameric = (1) pertaining to spectrally different objects or color stimuli that have the same tristimulus values. (2) pertaining to objects, having different spectrophotometric curves that match when illuminated by at least one specific  illuminant (viewing condition) and observed by a specific observer.

Metamerism = property of two specimens that match under a specified illuminator (illuminant) and to a specified observer and whose spectral reflectances or transmittances differ in the visible wavelengths and may appear to be a miss match under a second specified illuminant to the same specified observer.

Munsell Color System = a system of specifying colors of surfaces illuminated by daylight and viewed by an observer adapted to daylight, in terms of three attributes: hue, value, and chroma, using scales that are perceptually approximately uniform.

Munsell Notation = (1) the Munsell hue, value, and chroma assigned to the color of a specimen by visually comparing the specimen to the chips in the Munsell Book of Color.  (2) a notation in the Munsell color system, derived from luminous reflectance Y and chromaticity coordinates x and y in the 1931 CIE system for standard illuminant C, by the use of scales defined by the Optical Society of America Subcommittee on the Spacing of the Munsell Colors.

Natural Color System = color order system based on resemblance’s of colors to up to four of six "elementary" colors red, yellow, green, blue, black, and white, in which the attributes of the colors are hue, chromaticness, and blackness.

Observer Metamerism = the property of specimens having different spectral characteristics and having the same color when viewed by one observer, but different colors when viewed by a different observer under the same conditions.

Opacity =  the ability of a specimen to prevent the transmission of light. 

Opaque =  transmitting no optical radiation, you can not see through the material.

Opponent-Color Scales = scales that denote one color by positive scale values, the neutral axis by zero value, and an approximately complementary color by negative scale values. Common examples include scales that are positive in the red direction and negative in the green direction (CIE a*, Hunter a) and scales that are positive in the yellow direction and negative in the blue direction (CIE b*, Hunter b).

Pearlescent  = a colorant exhibiting various colors depending on the angles of illumination and viewing, as observed in mother-of-pearl.

Photochromism = a reversible change in color of a specimen due to exposure to light.

Photometer = an instrument for measuring light.

Physical Standard = stable specimen having a value of a physical quantity assigned by accurate measurements under specified conditions, usually in a standards laboratory.

Precision = the closeness of agreement between test results obtained under prescribed conditions.

Primary Colorants = a small number (pallet) of colorants (dyes or pigments) that may be mixed to produce a large gamut of colors.

Radiant Energy = the form of energy consisting of the electromagnetic spectrum that travels at 115.890 kilometers/s (186.500 miles/s) through a vacuum, reducing this speed 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.

Reference Sstandard = a physical standard used to calibrate a group of laboratory standards.

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

Reflection = of radiant energy (in the case of color, light), the process by which radiant energy is returned from a material or object.

Refraction = change in the direction of light determined by change in the velocity of the light in passing from one medium to another.

Repeatability = (1)  the closeness of agreement between the results of successive measurements of the same test specimen, or of test specimens taken at random from a homogeneous supply, carried out on a single laboratory, by the same method of measurement, operator, and measuring instrument, with repetition over a specified period of time. This is the most important  aspect of sample presentation technique and   one of the most important specifications for a color instrument.

Reproducibility = the closeness of agreement between the results of successive measurements of the same test specimen, or of test specimens taken at random from a homogeneous supply, but changing conditions such as operator, measuring instrument, laboratory, or time. The changes in conditions must be specified.

Saturation  = the attribute of color perception that expresses the degree of departure from the gray of the same lightness. All grays have zero saturation. Commonly used as a synonym for chroma especially in graphic arts.

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.

Shade = a color slightly different from a reference color. Or ..

Shade = to adjust the color of a test specimen to be a closer color match to the standard.

Sheen = the specular gloss at a large angle of incidence for an otherwise matte specimen (used in textiles).

Spectral power Distribution Curve = intensity of radiant energy as a function of wavelength, generally gives in relative power terms.

Spectrocolorimeter = spectrophotometer, one component of which is a dispersive element (such as prism, grating, or interference filter or wedge) that is normally capable of producing as output only colorimetric data (such as tristimulus values and derived color coordinates) but not the underlying spectral data from which colorimetric data are derived.

Spectrophotometer = photometric device for the measurement of spectral transmittance, spectral reflectance, or relative spectral emittance.

Spectrophotometry  =quantitative measurement of reflection or transmission properties as a function of wavelength.

Spectrum  = spatial arrangement of components of radiant energy in order of their wavelengths, wave number or frequency.

Spectrophotometric Curve = a curve measured on a spectrophotometer: hence a graph of relative reflectance or transmittance (or absorption) as the ordinate, plotted versus wavelength or frequency as the abscissa. In color, usually covering the practical visual range from 400-700 nm.

Standardize = to adjust instrument output to correspond to a previously established calibration using one or more homogeneous specimens or reference materials. (calibrate, verify) This is the normal condition that is often referred to as "field calibration" of a color instrument.

Standard Observer = an ideal observer having visual response described by the CIE color-matching functions.

Subtractive Color Mixture = mixture of absorbing media or superposition of filters so that the spectral composition of light passing through the combination is determined by simultaneous or successive absorption.

Texture = the visible surface structure depending on the size and organization of small constituent parts of a material; typically, the surface structure of a woven fabric or surface finish of a painted part.

Thermochromism  = a change in color with temperature change. adj - thermochromatic

Tint = the color produced by the mixture of white pigment with absorbing (generally chromatic) colorants. The color of the resulting mixture is lighter and less saturated than the color without the addition of the white.

Tint = to adjust the color of a test specimen to be a closer color match to the standard.

Tolerance = the range of color difference that is acceptable to say that the color is a commercial match. Most usually an agreement  is made between buyer and seller concerning color acceptability.

Translucency = the property of a specimen by which it transmits light diffusely without permitting a clear view of objects beyond the specimen and not in contact with it.

Translucent  = transmitting light diffusely, but not permitting a clear view of objects beyond the specimen and not in contact with it.

Transmission = of radiant energy, the process whereby radiant energy passes through a material or object.

Transparency  = the degree of regular transmission, thus the property of a material by which objects may be seen clearly through a sheet of it.

Transparent = adjective to describe 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 Values – Match = the amounts of the three specified human response stimuli required to match a color.

Tristimulus Values, CIE, =  the amounts (in percent) 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 observer (2º field) and Illuminant C. The values obtained depend on the method of integration used and on the relationship of the nature of the sample and on 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, giving measurements generally normalized to 100, which 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.

Tungsten Light Source =  constant burning tungsten, traditional light source in color instruments.                   

Turbidity, n – reduction of transparency of a specimen due to the presence of particulate matter.

Ultraviolet = referring to radiant flux having wavelengths shorter than the visible wavelengths, about 10 nm to 380 nm.

Uniform Color Space = schematic arrangement of colors in space in which spatial intervals between points correspond to visual differences between colors represented by those points (goal of all color space/order systems that has yet to be achieved).

Viewing Conditions = the conditions under which a visual observation is made, including the angular subtense of the specimen at the eye, the geometric relationship of light source, specimen, and eye, the photometric and spectral character of the light source, the photometric and spectral character of the field of view surrounding the specimen, and the state of adaptation of the eye.

Visible = pertaining to that portion of the electromagnetic spectrum to which the eye is sensitive, approximately 390 to 710 nm in wavelength.

Visibility = the properties and behavior of light waves and objects interacting in the environment to produce light signals capable of evoking visual sensation.

Visual Colorimeter  = an instrument using the eye as detector that measures color stimuli produced by mixing one or more of at least three primary colors.

Visual Perception = the visual experience resulting from stimulation of the retina and the resulting activity of associated neural systems.

Wavelength =  (of an electromagnetic wave), the distance in the direction of propagation between nearest points at which the electric vector has the same phase; the distance as expressed in nanometers (nm, billionth of a meter) of the wavelength period or frequency (peak to peak or trough to trough) of the wavelength in question. The visible spectrum spans 400-700m.

Whiteness = attribute of color perception by which an object color is judged to approach the preferred white.

Working Standard  = an instrument standard or laboratory standard in routine use or a standard that is almost identical to the laboratory standard and the color difference is exaclty known from the  laboratory standard.

Xenon Light Source = high energy, pulsed light used in color instruments.

Yellowness = attribute of color perception by which an object color is judged to depart from colorless or a preferred white toward yellow.

X” = One of the three CIE tristimulus values; the red primary.

Y” = One of the three CIE tristimulus values; equal to the luminous reflectance or transmittance ; the green primary

Z” = One of the three CIE tristimulus values; the blue primary.