A Guide to Understanding Communication Part 3 © X-Rite, Incorporated 2015

Delta CIELAB and CIELCH The expressions for these color Color differences are ∆L* ∆a* ∆b* or DL* Differences, Assessment of color is more than a Da* Db*, and ∆L* ∆C* ∆H* or DL* numeric expression. Usually it’s an DC* DH* (∆ or D symbolizes Notation and assessment of the color difference “delta,” which indicates difference). (delta) from a known standard. ∆ ∆ ∆ Tolerancing CIELAB and CIELCH are used to Given L* a* b*, the total differ- compare the of two objects. ence or distance on the CIELAB diagram can be stated as a single value, known as ∆E*.

2 2 2 1/2 ∆E*ab = [(∆L ) + (∆a ) + (∆b )]

Let’s compare the color of Flower A to Flower C, pictured below. Separately, each would be classi- fied as a rose. But what is their relationship when set side by side? How do the colors differ?

Using the equation for ∆L* ∆a* ∆b*, the color difference between Flower A and Flower C can be expressed as:

∆L* = +11.10 ∆a* = –6.10 ∆b* = –5.25

Flower A: L* = 52.99 a* = 8.82 b* = 54.53 The total color difference can be expressed as ∆E*=13.71

The values for Flowers A and C are shown at the bottom of this page. On the a* axis, a reading of

–6.10 indicates greener or less . On the b* axis, a reading of –5.25 indicates bluer or less yellow. On the L* plane, the measurement differ- ence of +11.10 shows that Flower C is lighter than Flower A.

If the same two flowers were compared using CIELCH, the color differences would be expressed as:

∆L* = +11.10 ∆C* = –5.88

∆H* = 5.49

Referring again to the flowers Flower C: L*=64.09 a*=2.72 b*=49.28 ∆ shown below, the C* value of –5.88 indicates that Flower C is less Color difference of Flower C to A chromatic, or less saturated. The ∆H* value of 5.49 indicates that ∆L* = +11.10, ∆a* = –6.10, ∆b* = –5.25 Flower C is greener in than 2 2 2 1/2 Flower A. The L* and ∆L* values are ∆E* = [(+ 11.1) + (–6.1) + (–5.25) ] ab identical for CIELCH and CIELAB. ∆E*ab = 13.71

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CIE Notation

∆L* = difference in /darkness value + = lighter – = darker

∆a* = difference on red/ axis Lightness + = redder – = greener Chroma ∆b* = difference on yellow/ axis

+ = yellower – = bluer

∆C* = difference in chroma + = brighter – = duller

∆H* = difference in hue Figure 13: Tolerance ellipsoid

∆E* = total color difference value

Refer to Figure 11 on page 10.

a* Visual Color and Tolerancing b*

Poor color memory, eye fatigue, and viewing Lightness Standard conditions can all affect the human eye’s ability to distinguish color differences. In addition to those limitations, the eye does not detect differences in hue (red, yellow, green, blue, etc.), (L*) chroma (saturation) or lightness equally. In fact, the average observer will see hue differences first, chroma differences second and lightness differences last. Visual acceptability is best represented by an ellipsoid (Figure 13).

As a result, our tolerance for an acceptable color match Figure 14: CIELAB tolerance box consists of a three-dimensional boundary with varying limits for lightness, hue and chroma, and must agree with visual assessment. CIELAB and CIELCH can be used to create those boundaries. Additional tolerancing formulas, known as CMC and CIE94, produce ellipsoidal tolerances.

Samples within the box b* and not in the ellipsoid are CIELAB Tolerancing numerically correct but visually unacceptable When tolerancing with CIELAB, you must choose a difference limit for ∆L* (lightness), ∆a* (red/green), and ∆b* (yellow/blue). These limits create a rectangular tolerance box around the ∆b* standard (Figure 14).

When comparing this tolerance box with the visually accepted Samples within ellipsoid, some problems emerge. A box-shaped tolerance the ellipsoid around the ellipsoid can give good numbers for unacceptable are visually acceptable color. If the tolerance box is made small enough to fit within a* the ellipsoid, it is possible to get bad numbers for visually ∆a* acceptable color (Figure 15). Figure 15: Numerically correct vs. visually acceptable

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Color Differences, Notation and Tolerancing continued

CIELCH Tolerancing

CIELCH users must choose a difference limit for ∆L* (lightness), ∆C*

(chroma) and ∆H* (hue). This creates a wedge-shaped box around the Standard standard. Since CIELCH is a polar-coordinate system, the tolerance box can be rotated in orientation to the hue angle (Figure 16).

∆H* When this tolerance is compared with the ellipsoid, we can see that it more closely matches human perception. This reduces the amount of ∆L*

Lightness disagreement between the observer and the instrumental values (Figure 17).

CMC Tolerancing

CMC is not a color space but rather a tolerancing system. CMC toler- ancing is based on CIELCH and provides better agreement between visual assessment and measured color difference. CMC tolerancing was developed by the Colour Measurement Committee of the Society of Dyers Figure 16: CIELCH tolerance and Colourists in Great Britain and became public domain in 1988. wedge

The CMC calculation mathematically defines an ellipsoid around the stan- dard color with semi-axis corresponding to hue, chroma and lightness. The ellipsoid represents the volume of acceptable color and automatically varies in size and shape depending on the position of the color in color space.

Figure 18 (on page 17) shows the variation of the ellipsoids throughout b* color space. The ellipsoids in the area of color space are longer ∆H* and narrower than the broader and rounder ones in the green area. The size and shape of the ellipsoids also change as the color varies in chroma ∆C* and/or lightness. ∆H*

The CMC equation allows you to vary the overall size of the ellipsoid to better match what is visually acceptable. By varying the commercial factor ∆C*

(cf), the ellipsoid can be made as large or small as necessary to match visual assessment. The cf value is the tolerance, which means that if ∆ cf=1.0, then E CMC less than 1.0 would pass, but more than 1.0 would ∆H* fail (see Figure 19 on page 17). ∆C*

Since the eye will generally accept larger differences in lightness (l) than in a* chroma (c), a default ratio for (l:c) is 2:1. A 2:1 ratio will allow twice as much difference in lightness as in chroma. The CMC equation allows this Figure 17: CIELCH tolerance ratio to be adjusted to achieve better agreement with visual assessment ellipsoids (see Figure 20 on page 18).

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Yellow

Tolerance ellipsoids are tightly packed in the orange region.

Green Red

Tolerance ellipsoids are larger in the green region.

Blue Figure 18: Tolerance ellipsoids in color space

Cross sections Hue and tolerances of the ellipsoid become smaller as lightness increases or decreases

Chroma

Hue

Chroma

Hue Standard

cf = 0.5 cf = 1

Figure 19: Commercial factor (cf) of tolerances

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Color Differences, Notation and Tolerancing continued

CIE94 Tolerancing

In 1994 the CIE released a new tolerance method called CIE94. Like (1.4:1) CMC, the CIE94 tolerancing method also produces an ellipsoid. The user has control of the lightness (kL) to chroma (Kc) ratio, as well as the (2:1) commercial factor (cf). These settings affect the size and shape of the ellipsoid in a manner similar to how the l:c and cf settings affect CMC.

Lightness However, while CMC is targeted for use in the textile industry, CIE94 is targeted for use in the paint and coatings industry. You should consider the Chroma type of surface being measured when choosing between these two toler- ances. If the surface is textured or irregular, CMC may be the best fit. If the surface is smooth and regular, CIE94 may be the best choice.

Visual Assessment vs. Instrumental

Though no color tolerancing system is perfect, the CMC and CIE94 equa- tions best represent color differences as our eyes see them. Figure 20: CMC tolerance ellipsoids % Agreement Tolerance Method with Visual

CIELAB 75% CIELCH 85% CMC or CIE94 95%

Choosing the Right Tolerance

When deciding which color difference calculation to use, consider the following five rules (Billmeyer 1970 and 1979):

1. Select a single method of calculation and use it consistently.

2. Always specify exactly how the calculations are made.

3. Never attempt to convert between color differences calculated by different equations through the use of average factors.

4. Use calculated color differences only as a first approximation in setting tolerances, until they can be confirmed by visual judgments.

5. Always remember that nobody accepts or rejects color because of numbers — it is the way it looks that counts.

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Other and Yellow Indices white a material should appear, be it photographic and printing paper Color Certain industries, such as paint, or plastics. textiles and paper manufacturing, Expressions evaluate their materials and prod- In some instances, a manufacturer ucts based on standards of white- may want to judge the yellowness ness. Typically, this whiteness or tint of a material. This is done to index is a preference rating for how determine how much that object’s color departs from a preferred white toward a bluish tint.

The effect of whiteness or yellow- ness can be significant, for example, when printing inks or dyes on paper. A blue ink printed on a highly-rated white stock will look different than the same ink printed on newsprint or another low-rated stock.

The American Standards Test Methods (ASTM) has defined whiteness and yellowness indices. The E313 whiteness index is used for measuring near-white, opaque materials such as paper, paint and plastic. In fact, this index can be used for any material whose color appears white.

The ASTM’s E313 yellowness index is used to determine the degree to which a sample’s color shifts away from an ideal white. The D1925 yellowness index is used for measuring plastics.

The same blue ink looks like a different color when printed on paper of various whiteness

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absolute white – In theory, a mate- – In theory, the complete Glossary rial that perfectly reflects all absorption of incident light; the energy at every visible wavelength. absence of any reflection. In prac- In practice, a solid white with known tice, any color that is close to this spectral reflectance data that is used ideal in a relative viewing situation — as the “reference white” for all meas- i.e., a color of very low saturation urements of absolute reflectance. and very low luminance. When calibrating a spectropho- tometer, often a white ceramic brightness – The dimension of color plaque is measured and used as the that refers to an achromatic scale, absolute white reference. ranging from black to white. Also called lightness, luminous absorb/absorption – Dissipation of reflectance or transmittance (q.v.). the energy of electromagnetic waves Because of confusion with satura- into other forms (e.g., heat) as a tion, the use of this term should be result of its interaction with matter; a discouraged. decrease in directional transmittance of incident radiation, resulting in a c* – Abbreviation for chromaticity.

modification or conversion of the chroma/chromaticity – The inten- absorbed energy. sity or saturation level of a particular

achromatic color – A neutral color hue, defined as the distance of that has no hue (white, gray or black). departure of a chromatic color from the neutral (gray) color with the additive primaries – Red, green and same value. In an - blue light. When all three addi- tive mixing environment, imagine mixing primaries are combined at 100% a neutral gray and a vivid red with intensity, white light is produced. the same value. Starting with the When these three are combined at neutral gray, add small amounts of varying intensities, a of red until the vivid red color is different colors is produced. achieved. The resulting scale Combining two primaries at 100% obtained would represent increasing produces a subtractive primary, chroma. The scale begins at zero for either cyan, magenta or yellow: neutral colors, but has no arbitrary 100% red + 100% green = yellow end. Munsell originally established 100% red + 100% blue = magenta 10 as the highest chroma for a 100% green + 100% blue = cyan vermilion and related other to it. Other pigments with See subtractive primaries higher chroma were noted, but the

original scale remained. The chroma appearance – An object’s or mate- rial’s manifestation through visual scale for normal reflecting materials attributes such as size, shape, color, may extend as high as 20, and for texture, glossiness, transparency, fluorescent materials it may be as opacity, etc. high as 30.

artificial daylight – Term loosely chromatic – Perceived as having a applied to light sources, frequently hue — not white, gray or black. equipped with filters, that try to reproduce the color and spectral chromaticity coordinates (CIE) – distribution of daylight. A more The ratios of each of the three tris- specific definition of the light source timulus values X, Y and Z in relation is preferred. to the sum of the three — desig- nated as x, y and z respectively. attribute – Distinguishing character- They are sometimes referred to as istic of a sensation, perception or the trichromatic coefficients. When mode of appearance. Colors are written without subscripts, they are often described by their attributes of assumed to have been calculated for hue, chroma (or saturation) and illuminant C and the 2° (1931) stan- lightness. dard observer unless specified otherwise. If they have been

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obtained for other illuminants or in the space approximately represent applications and by color measure- observers, a subscript describing the equal color differences. Value L* ment instruments. observer or illuminant should be represents lightness; value a* repre- used. For example, x10 and y10 are sents the red/green axis; and value color order systems – Systems chromaticity coordinates for the 10° b* represents the yellow/blue axis. used to describe an orderly three- observer and illuminant C. CIELAB is a popular color space for dimensional arrangement of colors. use in measuring reflective and Three bases can be used for chromaticity diagram (CIE) – A transmissive objects. ordering colors: 1) an appearance two-dimensional graph of the chro- basis (i.e., a psychological basis) in maticity coordinates (x as the CMC (Colour Measurement terms of hue, saturation and light- abscissa and y as the ordinate), Committee of the Society of Dyes ness; an example is the Munsell which shows the spectrum locus and Colourists of Great Britain) – System; 2) an orderly additive color (chromaticity coordinates of mono- Organization that developed and mixture basis (i.e., a psychophysical chromatic light, 380-770nm). It has published in 1988 a more logical, basis); examples are the CIE System many useful properties for ellipse-based equation based on and the Ostwald System; and 3) an comparing colors of both luminous L*C*h˚ color space for computing DE orderly mixture and non-luminous materials. (see delta E*) values as an alterna- basis; an example is the Plochere tive to the rectangular coordinates of Color System based on an orderly CIE (Commission Internationale de the CIELAB color space. mixture of inks. l’Eclairage) – The International – One aspect of appearance; a Commission on Illumination, the color color space – Three-dimensional primary international organization stimulus based on visual response to solid enclosing all possible colors. concerned with color and color light, consisting of the three dimen- The dimensions may be described in measurement. sions of hue, saturation and light- various geometries, giving rise to ness. various spacings within the solid. CIE 1976 L*a*b* color space – A – A three-dimen- uniform color space utilizing an color attribute color specification – Tristimulus Adams-Nickerson cube root formula, sional characteristic of the appear- values, chromaticity coordinates and adopted by the CIE in 1976 for use ance of an object. One dimension luminance value, or other color-scale in the measurement of small color usually defines the lightness, the values, used to designate a color differences. other two together define the chro- numerically in a specified color maticity. system. CIE 1976 L*u*v* color space – A – The magnitude uniform color space adopted in 1976. color difference – A measure- Appropriate for use in additive mixing and character of the difference ment of the color of light radiated by of light (e.g., color TV). between two colors under specified a while it is being heated. conditions. This measurement is expressed in CIE chromaticity coordinates – color-matching functions – terms of absolute scale, or degrees See chromaticity coordinates (CIE). Kelvin. Lower Kelvin temperatures Relative amounts of three additive such as 2400K are red; higher CIE chromaticity diagram – See primaries required to match each chromaticity diagram (CIE). wavelength of light. The term is temperatures such as 9300K are generally used to refer to the CIE blue. Neutral temperature is white, at CIE daylight illuminants – See standard observer color-matching 6504K. daylight illuminants (CIE). functions. – The ’s continuum of colors arranged in a CIE luminosity function (y) – See color measurement – Physical circle, where luminosity function (CIE). measurement of light radiated, trans- such as red and green are located CIE standard illuminants – See mitted or reflected by a specimen directly across from each other. standard illuminants (CIE). under specified condition and mathe- matically transformed into standard- colorants – Materials used to create CIE standard observer – See stan- ized colorimetric terms. These terms colors — dyes, pigments, toners, dard observer (CIE). can be correlated with visual evalua- waxes, . tions of colors relative to one CIE tristimulus values – See tris- colorimeter – An optical measure- another. timulus values (CIE). ment instrument that responds to

color in a manner similar to the CIELAB (or CIE L*a*b*, CIE Lab) – – A color-measurement human eye — by filtering reflected Color space in which values L*, a* scale or system that numerically light into its dominant regions of red, and b* are plotted using Cartesian specifies the perceived attributes of green and blue. coordinate system. Equal distances color. Used in computer graphics

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Glossary continued

colorimetric – Of, or relating to, – The be arranged according to some other values giving the amounts of three massive band of electromagnetic criteria such as a geometric progres- colored or receptors — red, waves that pass through the air in sion based on lightness. Such scales green and blue. different sizes, as measured by may be used to describe the relative wavelength. Different wavelengths amount of difference between two colorist – A person skilled in the art have different properties, but most similar colors. of color matching (colorant formula- are invisible — and some completely hue – 1) The first element in the tion) and knowledgeable concerning undetectable — to human beings. color-order system, defined as the the behavior of colorants in a partic- Only wavelengths that are between attribute by which we distinguish red ular material; a tinter (q.v.) (in the 380 and 720 nanometers are visible, American usage) or a shader. The from green, blue from yellow, etc. producing light. Waves outside the Munsell defined five principal word “colorist” is of European origin. visible spectrum include gamma (red, yellow, green, blue and ) complements – Two colors that rays, x-rays, microwaves and radio and five intermediate hues (yellow- create neutral gray when combined. waves. red, green-yellow, blue-green, On a color wheel, complements are emissive object – An object that purple-blue and red-purple. These 10 directly opposite from each other: emits light. Emission is usually hues (represented by their corre- blue/yellow, red/green and so on. caused by a chemical reaction, such sponding initials R, YR, Y, GY, G, contrast – The level of variation as the burning gasses of the sun or BG, B, PB, P and RP) are equally between light and dark areas in an the heated filament of a light bulb. spaced around a circle divided into 100 equal visual steps, with the zero image. – A glass tube filled with mercury gas and coated point located at the beginning of the – The CIE standard on its inner surface with phosphors. red sector. Adjacent colors in this that represents a color temperature When the gas is charged with an circle may be mixed to obtain contin- of 6504K. This is the color tempera- electrical current, radiation is uous variation from one hue to ture most widely used in graphic produced. This, in turn, energizes the another. Colors defined around the arts industry viewing booths. See phosphors, causing them to glow. hue circle are known as chromatic Kelvin (K). colors. 2) The attribute of color by gloss – An additional parameter to means of which a color is perceived daylight illuminants (CIE) – Series consider when determining a color to be red, yellow, green, blue, purple, of illuminant spectral power distribu- standard, along with hue, value, etc. White, black and gray possess tion curves based on measurements chroma, the texture of a material and no hue. of natural daylight and recommended whether the material has metallic or by the CIE in 1965. Values are pearlescent qualities. Gloss is an illuminant – Mathematical descrip- defined for the wavelength region additional tolerance that may be tion of the relative spectral power 300 to 830nm. They are described in specified in the Munsell Color distribution of a real or imaginary terms of the correlated color temper- Tolerance Set. The general rule for light source — i.e., the relative ature. The most important is D65 evaluating the gloss of a color energy emitted by a source at each because of the closeness of its sample is the higher the gloss unit, wavelength in its emission spectrum. correlated color temperature to that the darker the color sample will Often used synonymously with “light of illuminant C, 6774K. D75 bluer appear. Conversely, the lower the source” or “lamp,” though such usage than D65 and D55 yellower than D65 gloss unit, the lighter a sample will is not recommended. are also used. appear. illuminant A (CIE) – Incandescent illumination, yellow-orange in color, ∆ – A symbol used to Gloss is measured in gloss units, delta (D or ) with a correlated color temperature indicate deviation or difference. which use the angle of measurement of 2856K. It is defined in the wave- and the gloss value (e.g. 60˚ gloss = length range of 380 to 770nm. delta E*, delta e* – The total color 29.8). A 60˚ geometry is recom- difference computed with a color mended by the American Society for illuminant C (CIE) – Tungsten illumi- difference equation (∆Eab or ∆Ecmc). Testing and Materials (ASTM) D523 nation that simulates average In color tolerancing, the symbol DE standard for the general evaluation daylight, bluish in color, with a corre- is often used to express Delta Error. of gloss. lated color temperature of 6774K. dye – A soluble colorant — as – An achromatic scale illuminants D (CIE) – Daylight illu- opposed to pigment, which is insol- ranging from black through a series minants, defined from 300 to 830nm uble. of successively lighter grays to white. (the UV portion 300 to 380nm being dynamic range – An instrument’s Such a series may be made up of necessary to correctly describe range of measurable values, from steps that appear to be equally colors that contain fluorescent dyes the lowest amount it can detect to distant from one another (such as or pigments). They are designated as the highest amount it can handle. the Munsell Value Scale), or it may D, with a subscript to describe the

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correlated color temperature; D65 is identification of a specimen by its surface, or inside a medium containing the most commonly used, having a Munsell hue, value and chroma as particles. correlated color temperature of visually estimated by comparison 6504K, close to that of illuminant C. with the Munsell Book of Color. spectral power distribution curve They are based on actual measure- – Intensity of radiant energy as a ments of the spectral distribution of nanometer (nm) – Unit of length function of wavelength, generally daylight. equal to 10-9 meter (a.k.a. one given in relative power terms. billionth of a meter, or a milli-micron). integrating sphere – A sphere spectrophotometer – Photometric manufactured or coated with a highly observer – The human viewer who device that measures spectral trans- reflective material that diffuses light receives a stimulus and experiences mittance, spectral reflectance or rela- within it. a sensation from it. In vision, the tive spectral emittance. stimulus is a visual one and the Kelvin (K) – Unit of measurement sensation is an appearance. spectrophotometric curve – A for color temperature. The Kelvin curve measured on a spectropho- scale starts from absolute zero, observer, standard – See standard tometer; a graph with relative which is -273˚ Celsius. observer. reflectance or transmittance (or light – 1) Electromagnetic radiation absorption) as the ordinate, plotted – A form of energy of which a human observer is aware radiant energy with wavelength or frequency as the consisting of the electromagnetic through the visual sensations that abscissa. spectrum, which travels at 299,792 arise from the stimulation of the kilometers/second (186,206 – Spatial arrangement of retina of the eye. This portion of the spectrum miles/second) through a vacuum, components of radiant energy in spectrum includes wavelengths from and more slowly in denser media order of their wavelengths, wave about 380 to 770nm. Thus, to speak (air, water, glass, etc.). The nature of number or frequency. of ultraviolet light is incorrect radiant energy is described by its because the human observer cannot wavelength or frequency, although it specular gloss – Relative luminous see radiant energy in the ultraviolet also behaves as distinct quanta fractional reflectance from a surface region. 2) Adjective meaning high (“corpuscular theory”). The various in the mirror or specular direction. It reflectance, transmittance or level of types of energy may be transformed is sometimes measured at 60˚ rela- illumination as contrasted to dark, or into other forms of energy (electrical, tive to a perfect mirror. low level of intensity. chemical, mechanical, atomic, specular reflectance – Reflectance light source – An object that emits thermal, radiant), but the energy of a beam of radiant energy at an light or radiant energy to which the itself cannot be destroyed. angle equal but opposite to the inci- human eye is sensitive. The emission dent angle; the mirror-like reflectance. of a light source can be described by reflectance – The ratio of the inten- The magnitude of the specular the relative amount of energy sity of reflected radiant flux to that of reflectance on glossy materials emitted at each wavelength in the incident flux. In popular usage, it is depends on the angle and the differ- visible spectrum, thus defining the considered the ratio of the intensity ence in refractive indices between source as an illuminant. The emis- of reflected radiant energy to that two media at a surface. The magni- sion also may be described in terms reflected from a defined reference tude may be calculated from of its correlated color temperature. standard. Fresnel’s Law. lightness – Perception by which reflectance, specular – See spec- white objects are distinguished from ular reflectance. specular reflectance excluded gray, and light-colored objects from (SCE) – Measurement of reflectance dark-colored. reflectance, total – See total made in such a way that the spec- reflectance. ular reflectance is excluded from the luminosity function (y) (CIE) – A measurement; diffuse reflectance. plot of the relative magnitude of the saturation – The attribute of color The exclusion may be accomplished visual response as a function of perception that expresses the by using 0˚ (perpendicular) incidence wavelength from about 380 to amount of departure from a gray of on the samples. This then reflects 780nm, adopted by CIE in 1924. the same lightness. All grays have the specular component of the zero saturation (ASTM). See reflectance back into the instrument – A phenomenon chroma/chromaticity. by use of black absorbers or light exhibited by a pair of colors that traps at the specular angle when the match under one or more sets of illu- scattering – Diffusion or redirection of radiant energy encountering particles incident angle is not perpendicular, minants (be they real or calculated), or in directional measurements by but not under all illuminants. of different refractive index. Scattering occurs at any such interface, at the measuring at an angle different from – The color the specular angle.

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Glossary continued

specular reflectance included (SCI) lighter and less saturated than the X – 1) One of the three CIE tristim- – Measurement of the total color without the white added. ulus values; the red primary. 2) reflectance from a surface, including Spectral color-matching functions of total reflectance – Reflectance of the diffuse and specular reflectances. the CIE standard observer used for radiant flux reflected at all angles calculating the X tristimulus value. 3) from the surface, thus including both standard – A reference against One of the CIE chromaticity coordi- which instrumental measurements diffuse and specular reflectances. nates calculated as the fraction of are made. transparent – Describes a material the sum of the three tristimulus

that transmits light without diffusion values attributable to the X value. standard illuminants (CIE) – or scattering. Known spectral data established by Y – 1) One of the three CIE tristim- the CIE for four different types of tristimulus – Of, or consisting of, ulus values, equal to the luminous light sources. When using tristimulus three stimuli; generally used to reflectance or transmittance; the data to describe a color, the illumi- describe components of additive green primary. 2) Spectral color- nant must also be defined. These mixture required to evoke a partic- matching function of the CIE stan- standard illuminants are used in ular color sensation. dard observer used for calculating Y place of actual measurements of the tristimulus value. 3) One of the CIE light source. tristimulus colorimeter – An instru- ment that measures tristimulus chromaticity coordinates calculated as the fraction of the sum of the standard observer (CIE) – 1) A values and converts them to chro- three tristimulus values, attributable hypothetical observer having the tris- maticity components of color. to the Y value. timulus color-mixture data recom- tristimulus values (CIE) – mended in 1931 by the CIE for a 2˚ Percentages of the components in a Z – 1) One of the three CIE tristim- viewing angle. A supplementary three-color additive mixture necessary ulus values; the blue primary. 2) observer for a larger angle of 10˚ to match a color; in the CIE system, Spectral color-matching function of was adopted in 1964. 2) The spectral they are designated as X, Y and Z. the CIE standard observer used for response characteristics of the The illuminant and standard observer calculating the Z tristimulus value. 3) average observer defined by the color-matching functions used must One of the CIE chromaticity coordi- CIE. Two such sets of data are be designated; if they are not, the nates calculated as the fraction of defined, the 1931 data for the 2˚ assumption is made that the values the sum of the three tristimulus visual field (distance viewing) and are for the 1931 observer (2˚ field) values attributable to the Z primary. the 1964 data for the annular 10˚ and illuminant C. The values obtained visual field (approximately arm’s depend on the method of integration length viewing). By custom, the used, the relationship of the nature of assumption is made that if the the sample and the instrument design observer is not specified, the tristim- used to measure the reflectance or ulus data has been calculated for the transmittance. Tristimulus values are 1931, or 2˚ field observer. The use of not, therefore, absolute values char- the 1964 data should be specified. acteristic of a sample, but relative values dependent on the method subtractive primaries – Cyan, used to obtain them. Approximations magenta and yellow. Theoretically, when all three subtractive primaries of CIE tristimulus values may be are combined at 100% on white obtained from measurements made paper, black is produced. When on a tristimulus colorimeter that gives these are combined at varying inten- measurements generally normalized sities, a gamut of different colors is to 100. These must then be normal- produced. Combining two primaries ized to equivalent CIE values. The at 100% produces an additive filter measurements should be prop- primary, either red, green or blue: erly designated as R, G and B 100% cyan + 100% magenta = blue instead of X, Y and Z. 100% cyan + 100% yellow = green 100% magenta + 100% yellow = red value – Indicates the degree of light- ness or darkness of a color in rela- tint – 1) verb: To mix white pigment tion to a neutral gray scale. The with absorbing (generally chromatic) scale of value (or V, in the Munsell colorants. 2) noun: The color system of color notation) ranges produced by mixing white pigment from 0 for pure black to 10 for pure with absorbing (generally chromatic) white. The value scale is neutral or colorants. The resulting mixture is without hue.

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