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 colour
A neutral colour that has no hue (white, grey or black)
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 colours 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
A visual perception through which an object is seen to have attributes such as size, shape, colour, texture, glossiness, transparency, opacity, etc.
Artificial daylight
Term loosely applied to light sources, frequently equipped with filters, that try to reproduce the colour 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. Colours are often described by their attributes of hue, chroma (or saturation) and lightness.
Black
In theory, the complete absorption of incident light; the absence of any reflection. In practice, any colour that is close to this ideal in a relative viewing situation — i.e., a colour of very low saturation and very low luminance.
Brightness
The dimension of colour 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.
C*
Abbreviation for chromaticity.
Chroma/Chromaticity
The intensity or saturation level of a particular hue, defined as the distance of departure of a chromatic colour from the neutral (grey) colour with the same lightness value. In an additive colour-mixing environment, imagine mixing a neutral grey and a vivid red with the same value. Starting with the neutral grey, add small amounts of red until the vivid red colour is achieved. The resulting scale obtained would represent increasing chroma. The scale begins at zero for neutral colours, 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, grey or black.
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 aresometimes 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 nm -770 nm). It has many useful properties for comparing colours of both luminous and non-luminous materials.
CIE (Commission Internationale de l’Eclairage)
Die Internationale Beleuchtungskommission, die erste internationale Organisation für Farbe und Farbmessung.
CIE 1976 L*a*b* colour space
A uniform colour space utilising an Adams-Nickerson cube root formula, adopted by the CIE in 1976 for use in the measurement of small colour differences.
CIE 1976 L*u*v* colour space
A uniform colour space adopted in 1976. Appropriate for use in additive mixing of light (e.g., colour TV).
CIE chromaticity coordinates
See chromaticity coordinates (CIE).
CIE chromaticity diagram
See chromaticity diagram (CIE).
CIE daylight illuminants
See daylight illuminants (CIE).
CIE luminosity function (y)
See luminosity function (CIE).
CIE standard illuminants
See standard illuminants (CIE).
CIE standard observer
See standard observer (CIE).
CIELAB (or CIE L*a*b*, CIE Lab)
Colour space in which values L*, a* and b* are plotted using Cartesian coordinate system. Equal distances in the space approximately represent equal colour differences. Value L* represents lightness; value a* represents the red/green axis; and value b* represents the yellow/blue axis. CIELAB is a popular colour space for use in measuring reflective and transmissive objects.
CMC (Colour Measurement Committee of the Society of Dyes and Colourists of Great Britain)
Organisation that developed and published in 1988 a more logical, ellipse-based equation based on L*C*h colour space for computing rE (see delta E*) values as an alternative to the rectangular coordinates of the CIELAB colour space.
Colour
One aspect of appearance; a stimulus based on visual response to light, consisting of the three dimensions of hue, saturation and lightness.
Colour attribute
A three- dimensional characteristic of the appearance of an object. One dimension usually defines the lightness, the other two together define the chromaticity.
Colour difference
The magnitude and character of the difference between two colours under specified conditions.
Colour-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 colour-matching functions.
Colour measurement
Physical measurement by visual or instrumental means of light radiated, transmitted or reflected by a specimen under specified condition and transformed into numeric colorimetric terms describing its numeric colour attributes.
Colour model
A colour-measurement scale or system that numerically specifies the perceived attributes of colour. Used in computer graphics applications and by colour measurement instruments.
Colour order systems
Systems used to describe an orderly three- dimensional arrangement of colours. Three bases can be used for ordering colours: 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 colour mixture basis (i.e., a psychophysical basis); examples are the CIE System and the Ostwald System; 3) an orderly subtractive colour mixture basis; an example is the Plochere Colour System based on an orderly mixture of inks.
Colour space
Three-dimensional solid enclosing all possible colours. The dimensions may be described in various geometries, giving rise to various spacings within the solid.
Colour specification
Tristimulus values, chromaticity coordinates and luminance value, or other colour-scale values, used to designate a colour numerically in a specified colour system.
Colour temperature
A measurement of the colour 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.
Colour wheel
The visible spectrum’s continuum of colours arranged in a circle, where complementary colours such as red and green are located directly across from each other.
Colourants
Materials used to create colours — dyes, pigments, toners, waxes, phosphors.
Colorimeter
An optical measurement instrument that responds to colour 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 coloured lights or receptors — red, green and blue.
Colourist
A person skilled in the art of colour matching (colourant formulation) and knowledgeable concerning the behavior of colourants in a particular material; a tinter (q.v.) (in the American usage) or a shader. The word “colourist†is of European origin.
Complements
Two colours that create neutral grey when combined. On a colour wheel, complements are directly opposite from each other: blue/yellow, red/green and so on.
Contrast
The level of variation of a measured quantity such as lightness between two areas in an image. This is expressed as a number computed by a specified formula.
D65
The CIE Standard Illuminant that represents a colour temperature of 6504K. This is the colour 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 830 nm. They are described in terms of the correlated colour temperature. The most important is D65 because of the closeness of its correlated colour temperature to that of illuminant C, 6774K. D75 bluer than D65 and D55 yellower than D65 are also used.
Delta (D or r)
A symbol used to indicate deviation or difference.
Delta E*, rE*
The total colour difference computed with a colour difference equation (rE*ab or rEcmc).
Dye
A soluble colourant — as opposed to pigment, which is insoluble.
Dynamic range
An instrument’s range of measurable values, from the lowest amount it can detect to the highest amount it can handle.
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 780 nm 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.
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, energises the phosphors, causing them to glow.
Gloss
attribute due to surface reflected light, at specular angle to illumination, responsible for thre degree to which reflected highlights or images of objects may be seen as superimposed on a surface. An additional parameter to consider when determining a colour 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 Colour Tolerance Set. The general rule for evaluating the gloss of a colour sample is the higher the gloss unit, the darker the colour sample will appear. Conversely, the lower the gloss unit, the lighter a sample will appear. Gloss can be measured in gloss units, which are expressed using the angle of measurement and the gloss value (e.g. 60º gloss = 29.8). A 60º geometry is recommended by the American Society for Testing and Materials (ASTM) D523 standard for the general evaluation of gloss
Greyscale
An achromatic scale ranging from black through a series of successively lighter greys 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 colours.
Hue
Attribute of a visual sensation according to which an area appears to be similar to one, or to proportions of two of the perceived colours, red, yellow, green and blue. 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 colours in this circle may be mixed to obtain continuous variation from one hue to another. Colours defined around the hue circle are known as chromatic colours. 2) The attribute of colour by means of which a colour is perceived to be red, yellow, green, blue, purple, etc. White, black and grey possess no hue.
Illuminant
Mathematical description of the relative spectral power distribution of a real or imaginary source — i.e., the relative energy emitted by a source at each wavelength in its emission spectrum that can in illuminating objects affect their perceived colours. Often used synonymously with “light source†or “lamp,†though such usage is not recommended.
Illuminant A (CIE)
Incandescent illumination, yellow-orange in colour, with a correlated colour temperature of 2856K. It is defined in the wavelength range of 380 nm to 830 nm.
Illuminant C (CIE)
Tungsten illumination that simulates average daylight, bluish in colour, with a correlated colour temperature of 6774K.
Illuminants D (CIE)
Daylight lluminants, defined from 300 nm to 830 nm (the UV portion 300 nm to 380 nm being necessary to correctly describe colours that contain fluorescent dyes or pigments). They are designated as D, with a subscript to describe the correlated colour temperature; D65 is the most commonly used, having a correlated colour 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.
Kelvin (K)
Unit of measurement for colour temperature. The Kelvin scale starts from absolute zero, which is -273° Celsius.
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, to speak of ultraviolet light is incorrect because the human observer cannot see radiant energy in the ultraviolet region.
- 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 colour temperature.
Luminous efficiency (V(l)) function (CIE)
A plot of the relative magnitude of the visual response as a function of wavelength from about 380 nm to 780 nm, adopted by CIE in 1924.
Metamerism
A phenomenon exhibited by a pair of colours that match under one or more sets of illuminants (be they real or calculated), but not under all illuminants.
Munsell Colour System
The colour identification of a specimen by its Munsell hue, value and chroma as visually estimated by comparison with the Munsell Book of Colour.
Nanometer (nm)
Unit of length equal to 10-9 metre (a.k.a. one billionth of a metre, or a milli-micron).
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
See standard observer.
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 theoryâ€). 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
See specular reflectance.
Reflectance, total
See total reflectance.
Saturation
The attribute of colour perception that expresses the amount of departure from a grey of the same lightness. All greys 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
Ratio of reflectance from a surface in the mirror or specular direction to the incident flux. 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 Fresnel’s 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 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 different types of light sources. When using tristimulus data to describe a colour, the illuminant must also be defined. These standard illuminants are used in place of actual measurements of the light source.
Standard observer (CIE)
- A hypothetical ideal observer having either the tristimulus colour-mixture data recommended in 1931 by the CIE for a 2º viewing angle or for a larger angle of 10º as adopted in 1964.
- The spectral response characteristics of the average observer defined by the CIE. Two such sets of data are defined, the 1931 data for the 2º visual field and the 1964 data for the 10º visual field. By custom, the assumption is made that if the observer is not specified, the tristimulus data has been calculated for the 1931, or 2º field observer. The use of the 1964 data should be specified.
Subtractive primary colors
Cyan, magenta and yellow. Theoretically, all three subtractive primaries on white paper are 100% combined, black. When combined with different intensities, a scale is made up of different colors. When combining two primary colors at 100%, an additive primary color, red, green, or blue, is equivalent to:
- 100% cyan + 100% magenta = blue
- 100% cyan + 100% yellow = green
- 100% magenta + 100% yellow = red
Tint
- verb: To mix white pigment with absorbing (generally chromatic) colourants.
- noun: The colour produced by mixing white pigment with absorbing (generally chromatic) colourants. The resulting mixture is lighter and less saturated than the colour without the white added.
Total reflectance
Ratio to the incident flux of the 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 colour sensation.
Tristimulus colorimeter
An instrument that measures colour in terms of tristimulus values for a standard illuminant.
Tristimulus values (CIE)
Percentages of the components in a three-colour additive mixture necessary to match a colour; in the CIE system, they are designated as X, Y and Z. The illuminant and standard observer colour-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, 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 normalised to 100. These must then be normalised to equivalent CIE values. The filter measurements should be properly designated as R, G and B instead of X, Y and Z.
Value
Indicates the degree of lightness or darkness of a colour in relation to a neutral grey scale. The scale of value (or V, in the Munsell system of colour notation) ranges from 0 for pure black to 10 for pure white. The value scale is neutral or without hue.
X
- One of the three CIE tristimulus values; the red primary.
- Spectral colour-matching functions of the CIE standard observer used for calculating the X tristimulus value.
- One of the CIE chromaticity coordinates calculated as the fraction of the sum of the three tristimulus values attributable to the X value.
Y
- One of the three CIE tristimulus values, equal to the luminous reflectance or transmittance; the green primary.
- Spectral colour-matching function of the CIE standard observer used for calculating Y tristimulus value.
- One of the CIE chromaticity coordinates calculated as the fraction of the sum of the three tristimulus values, attributable to the Y value.
Z
- One of the three CIE tristimulus values; the blue primary.
- Spectral colour-matching function of the CIE standard observer used for calculating the Z tristimulus value.
- One of the CIE chromaticity coordinates calculated as the fraction of the sum of the three tristimulus values attributable to the Z primary.