A Guide to Understanding Color Communication Table of Communicating Color

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A Guide to Understanding Color Communication Table of Communicating Color ...... 2

Contents Ways to Measure Color ...... 3

Integrated Color – Throughout the Supply Chain ...... 4-5

Applications ...... 6

Attributes of Color Hue ...... 7 Chroma ...... 7 Lightness...... 8

Scales for Measuring Color The Munsell Scale ...... 9 CIE Color Systems...... 9-10 Chromaticity Values...... 11

Expressing Colors Numerically CIELAB (L*a*b*) ...... 12 CIELCH (L*C*h°) ...... 12-13

Color Differences, Notation and Tolerancing Delta CIELAB and CIELCH...... 14 CIE Color Space Notation...... 15 Visual Color and Tolerancing ...... 15 CIELAB Tolerancing ...... 15 CIELCH Tolerancing ...... 16 CMC Tolerancing ...... 16-17 CIE94 Tolerancing ...... 18 Visual Assessment vs. Instrumental ...... 18 Choosing the Right Tolerance ...... 18

Other Color Expressions White and Yellow Indices...... 19

Glossary ...... 20-24

1 Communicating How would you describe the color But even without such physical of this rose? Would you say it’s considerations, each observer Color yellow, sort of lemon yellow or interprets color based on personal maybe a bright canary yellow? references. Each person also verbally defines an object’s color Your perception and interpretation differently. of color are highly subjective. Eye fatigue, age and other physiolog- As a result, objectively communi- ical factors can influence your cating a particular color to color perception. someone without some type of standard is difficult. There also must be a way to compare one color to the next with accuracy.

The solution is a measuring instrument that explicitly identifies a color. That is, an instrument that differentiates a color from all others and assigns it a numeric value.

2 Ways to Today, the most commonly used not reflect back to the eye. A 0/45 instruments for measuring color instrument, more effectively than Measure Color are spectrophotometers. any other, will remove gloss from the measurement and measure the Spectro technology measures appearance of the sample exactly reflected or transmitted light at as the human eye would see it. many points on the visual spectrum, which results in a curve. Since the curve of each color is as Multi-Angle unique as a signature or fingerprint, the curve is an excellent tool In the past 10 or so years, car for identifying, specifying and makers have experimented with matching color. special effect colors. They use special additives such as mica, Sample The following information can help pearlescent materials, ground up Viewing Specular you to understand which type of seashells, microscopically coated Port Port instrument is the best choice for colored pigments and interference 8˚ 8˚ S specific applications. p pigments to produce different Reference h e Beam r colors at different angles of view. e Port Spherical Large and expensive goniometers were traditionally used to measure Spherically based instruments these colors until X-Rite introduced have played a major roll in formula- a battery-powered, hand-held, tion systems for nearly 50 years. multi-angle instrument. X-Rite Sample Being Measured Most are capable of including the portable multi-angle instruments Spherical “specular component” (gloss) while are used by most auto makers and measuring. By opening a small their colorant supply chain, world- trap door in the sphere, the “spec- wide. ular component” is excluded from Colorimeter Light Source the measurement. In most cases, databases for color formulation are Colorimeters are not spectropho- r more accurate when this compo- e R iv e tometers. Colorimeters are tristim- e c nent is a part of the measurement. c e e iv ulus (three-filtered) devices that R e Spheres are also the instrument of r choice when the sample is make use of red, green, and blue textured, rough, or irregular or filters that emulate the response of approaches the brilliance of a first- the human eye to light and color. In surface mirror. Textile manufac- some quality control applications, these tools represent the lowest Sample Being Measured turers, makers of roofing tiles or acoustic ceiling materials would all cost answer. Colorimeters cannot 0/45 likely select spheres as the right compensate for metamerism (a tool for the job. shift in the appearance of a sample due to the light used to illu- minate the surface). As colorime- 0/45 (or 45/0) ters use a single type of light (such 45˚ Light 25˚ Source as incandescent or pulsed xenon) 15˚ 75˚ No instrument “sees” color more and because they do not record Specular like the human eye than the 0/45. the spectral reflectance of the This simply is because a viewer media, they cannot predict this 110˚ does everything in his or her power shift. Spectrophotometers can to exclude the “specular compo- compensate for this shift, making 45˚ 45˚ nent” (gloss) when judging color. spectrophotometers a superior When we look at pictures in a Sample Being Measured choice for accurate, repeatable glossy magazine, we arrange color measurement. Multi-angle ourselves so that the gloss does

3 Integrated Since color is a key element of a consumer's buying decisions, how Color – do you handle color consistency and quality in a global environment? Throughout the How do you keep in step with Supply Chain consumer color preferences while ensuring color options are available and easy to reproduce? The solution: X-Rite color measurement technology.

Accurate Color. On Time. Every Time.

Whether you're working with When you streamline your work- coatings, plastics or textiles, flow system, it can greatly reduce X-Rite understands the challenges the complexity of your supply unique to your business. Because chain. With a digital supply chain customer needs—not off-the-shelf solution for color control, you can products—drive the solutions we shift color management responsi- bring, our industry experts take the bilities to your organization. This time to understand your business. means you control color data work- The unique combination of X-Rite's flow. The result: data flows faster, technology, our vertically integrated which provides the information manufacturing capabilities and the needed to optimize choice of industry's largest global presence suppliers, and respond quickly to allows us to invent and adapt specific market pressures. solutions for you, wherever you are.

4 Controlling Color throughout the Process

X-Rite's product portfolio offers solutions which connect color accurately throughout the entire process. Our solutions save you money by reducing scrap, production downtime, off-color product shipments and rework. We simplify X-Rite is a global leader in color the process of managing color measurement, management and through-out your global supply communication solutions, and can chain, whether with multiple help you get the right color, every locations or at a single facility, time, from the earliest stages of helping you protect the quality production to final product shipment. of corporate brands.

Offering a broad range of benchtop, Calibrated, portable or non-contact systems, On-Screen Color X-Rite instruments can be found on the desktop, in retail settings, in X-Rite offers the only color for- laboratories or on production lines, mulation and quality assurance all managing color reliably and software to use the International accurately. Your total solution Color Consortium’s (ICC) standard includes software that enhances device profiles for on-screen color. instrument functionality by adding This means that colors will be quality control, profiling functions, consistently displayed on different color matching, or color manage- computers, so long as ICC profiles ment packages. Complimented are used. Use X-Rite monitor with our Macbeth Lighting offering optimizers and auto-scan densito- and visual evaluation tools from meters for complete color calibration Munsell Color, X-Rite offers the and control on computers, printers complete portfolio for all of your and scanners. color needs.

5 Applications Spectrophotometry’s applications • Corporate logo standardization are seemingly boundless. Color- matching measurements are made • Color testing of inks every day by those comparing a • Color control of paints reproduced object to a reference point. Spectrophotometry-assisted • Control of printed colors on color measurement can be useful packaging material and labels in areas such as: • Color control of plastics and textiles throughout the development and manufacturing process • Finished products like printed cans, clothing, shoes, automobile components, plastic components of all types

Attributes Each color has its own distinct Hue appearance, based on three of Color elements: hue, chroma and value When asked to identify the color of (lightness). By describing a color an object, you’ll most likely speak using these three attributes, you first of its hue. Quite simply, hue is can accurately identify a particular how we perceive an object’s color color and distinguish it from any — red, orange, green, blue, etc. other. The color wheel in Figure 1 shows the continuum of color from one hue to the next. As the wheel illus- trates, if you were to mix blue and green paints, you would get blue- green. Add yellow to green for Yellow yellow-green, and so on.

Chroma

Chroma describes the vividness or Green Red dullness of a color — in other words, how close the color is to either gray or the pure hue. For example, think of the appearance of a tomato and a radish. The red of Blue the tomato is vivid, while the radish appears duller.

Figure 2 shows how chroma changes as we move from center to Figure 1: Hue the perimeter. Colors in the center are gray (dull) and become more saturated (vivid) as they move Less Chroma More toward the perimeter. Chroma also is known as saturation.

oma Chr (Saturation)

Figure 2: Chromaticity

Attributes of Color continued

Lightness

The luminous intensity of a color — i.e., its degree of lightness — is called its value. Colors can be classified as light or dark when comparing their value.

For example, when a tomato and a radish are placed side by side, the red of the tomato appears to be much lighter. In contrast, the radish has a darker red value. In Figure 3, the value, or lightness, characteristic is represented on the vertical axis.

White White Lightness

Black Black

Figure 3: Three-dimensional color system depicting lightness

8 Scales for The Munsell Scale color differences that our eyes detect? Measuring In 1905, artist Albert H. Munsell originated a color ordering system CIE Color Systems Color — or color scale — which is still used today. The Munsell System of The CIE, or Commission Color Notation is significant from a Internationale de l’Eclairage historical perspective because it’s (translated as the International based on human perception. Commission on Illumination), is the Moreover, it was devised before body responsible for international instrumentation was available for recommendations for photometry measuring and specifying color. and colorimetry. In 1931 the CIE The Munsell System assigns standardized color order systems numerical values to the three prop- by specifying the light source (or erties of color: hue, value and illuminants), the observer and the chroma. Adjacent color samples methodology used to derive values represent equal intervals of visual for describing color. perception. The CIE Color Systems utilize The model in Figure 4 depicts the three coordinates to locate a color Munsell Color Tree, which provides in a color space. These color physical samples for judging visual spaces include: color. Today’s color systems rely on instruments that utilize mathematics • CIE XYZ to help us judge color. • CIE L*a*b* Figure 4: Munsell Color Tree • CIE L*C*h° Three things are necessary to see color: To obtain these values, we must • A light source (illuminant) understand how they are calculated. • An object (sample) As stated earlier, our eyes need • An observer/processor three things to see color: a light source, an object and an We as humans see color because observer/processor. The same our eyes process the interaction of must be true for instruments to see light hitting an object. What if we color. Color measurement instru- replace our eyes with an instrument ments receive color the same way —can it see and record the same our eyes do — by gathering and 120

100 120 er

w 100

80 o

60 al P 80

40 60 ercent Reflectance e Spectr P 20 40

Relativ 20

400 500 600 700 400 500 600 700 Wavelength (nm) Wavelength (nm) Figure 5: Spectral curve from a measured sample Figure 6: Daylight (Standard Illuminant D65/10˚)

9 Scales for Measuring Color continued

filtering the wavelengths of light reflected from an object. The instrument perceives the reflected light wavelengths as numeric values. These values are recorded as points across the visible spectrum and are called spectral data. Spectral data is represented as a spectral curve. This curve is the color’s fingerprint (Figure 5).

Once we obtain a color’s reflectance curve, we can apply mathematics to map the color onto a color space.

To do this, we take the reflectance curve and multiply the data by a CIE standard illuminant. The illuminant is a graphical representation of the light source under which the samples are viewed. Each light source has a power distribution that affects how we see color. Examples of different illuminants are A — incandescent, D65 — daylight (Figure 6) and F2 — fluorescent.

We multiply the result of this calculation by the CIE standard observer. The CIE commissioned work in 1931 and 1964 to derive the concept of a standard observer, which is based on the average human response to wavelengths of light (Figure 7).

In short, the standard observer represents how an average person sees color across the visible spectrum. Once these values are calculated, we convert the data into the tristimulus values of XYZ (Figure 8). These values can now identify a color numerically.

2° Observer (CIE 1931) 2.0 10° Observer (CIE 1964) alues

V z(λ) A spectrophotometer measures 1.5 spectral data – the amount of

ulus light energy reflected from an y(λ) 1.0 object at several intervals along istim

r the visible spectrum. The

T x(λ) spectral data is shown as 0.5 a spectral curve.

0.0 380 430 480 530 580 630 680 730 780 Wavelength (nm) Figure 7: CIE 2° and 10° Standard Observers

120 300 120 120 300 300 120 100 120 120 250 100 100 250 250 100 2° Observer (CIE 1931)2° Observer (CIE 1931) 80 100 100 2° Observer (CIE 1931) 80 80 200 10° Observer (CIE 1964) 200 200 2.0 10° Observer10° Observer(CIE 1964) (CIE 1964) X = 62.04 80 60 80 80 z(λ) z(λ) z(λ) 60 60 1501.5 150 150 40 60 60 40 40 60 y(λ) Y = 69.72 1001.0y(λ) y(λ) = 100 100

Reflectance Intensity 40

Reflectance Intensity X Percent Reflectance Reflectance Intensity X 40 20 40 20 20 Tristimulus Values x(λ) x(λ) x(λ) Reflectance Intensity Reflectance Intensity Reflectance Intensity Reflectance Intensity 0.550 Reflectance Intensity Reflectance Intensity 20 50 20 20 50 Z = 7.34 Relative Spectral Power 0.00 0 0 400 500 400 600 500 700 600 700400 500 400 600 500 700 600 700 380 430 480 530 580 630 680 730 780 400 500 600 700 400 500 600 700 380 430380 480 430 530 480 580 530 630 580 680 630 730 680 780 730 780 Wavelength (nm)Wavelength (nm) Wavelength (nm)Wavelength (nm) Wavelength (nm) Wavelength (nm) WavelengthWavelength (nm)Wavelength (nm) (nm)

Spectral Curve D65 Illuminant Standard Observer Tristimulus Values

Figure 8: Tristimulus values

10 Chromaticity Values

Tristimulus values, unfortunately, have limited use as color specifications because they correlate poorly with visual attributes. While Y relates to value (lightness), X and Z do not correlate to hue and chroma.

As a result, when the 1931 CIE standard observer was established, the commission recommended using the chromaticity coordinates xyz. These coordinates are used to form the chromaticity diagram in Figure 9. The notation Yxy specifies colors by identifying value (Y) and the color as viewed in the chromaticity diagram (x,y).

As Figure 10 shows, hue is represented at all points around the perimeter of the chromaticity diagram. Chroma, or saturation, is represented by a movement from the central white (neutral) area out toward the diagram’s perimeter, where 100% saturation equals pure hue.

H u e y Figure 9: CIE 1931 (x, y) Saturation chromaticity diagram

Figure 10: Chromaticity diagram x

11 Expressing To overcome the limitations of at the same time. As a result, chromaticity diagrams like Yxy, the single values can be used to Colors CIE recommended two alternate, describe the red/green and the uniform color scales: CIE 1976 yellow/blue attributes. Numerically (L*a*b*) or CIELAB, and CIELCH (L*C*h°). CIELAB (L*a*b*) These color scales are based on When a color is expressed in the opponent-colors theory of color CIELAB, L* defines lightness, a* vision, which says that two colors denotes the red/green value and cannot be both green and red at b* the yellow/blue value. the same time, nor blue and yellow Figures 11 and 12 (on page 13) show the color-plotting diagrams for L*a*b*. The a* axis runs from left to right. A color measurement movement in the +a direction depicts a shift toward red. Along the b* axis, +b movement represents a shift toward yellow. The center L* axis shows L = 0 (black or total absorption) at the bottom. At the center of this plane is neutral or gray.

To demonstrate how the L*a*b* values represent the specific colors of Flowers A and B, we’ve plotted their values on the CIELAB Color Chart in Figure 11.

The a* and b* values for Flowers Flower A: L* = 52.99 a* = 8.82 b* = 54.53 A and B intersect at color spaces identified respectively as points A and B (see Figure 11). These points specify each flower’s hue (color) and chroma (vividness/dullness). When their L* values (degree of lightness) are added in Figure 12, the final color of each flower is obtained.

CIELCH (L*C*h°)

While CIELAB uses Cartesian coordinates to calculate a color in a color space, CIELCH uses polar coordinates. This color expression can be derived from CIELAB. The L* defines lightness, C* specifies chroma and h° denotes hue angle, an angular measurement. Flower B: L* = 29.00 a* = 52.48 b* = 22.23

90˚ Yellow The L*C*h° expression offers an +b* advantage over CIELAB in that it’s very easy to relate to the earlier systems based on physical Hue samples, like the Munsell Color Scale.

1/3 L* = 116 (Y/Yn) – 16

1/3 1/3 a* = 500 [(X/Xn) – (Y/Yn) ]

1/3 1/3 b* = 200 [(Y/Yn) – (Z/Zn) ]

1/3 L* =116 (Y/Yn) – 16 C* = (a2 + b2)1/2 180˚ 0˚ Green Red h° = arctan (b*/a*) -a* +a*

Xn, Yn, Zn, are values for a reference white for the illumination/observer used.

Blue -b* 270˚ Figure 11: CIELAB color chart

Figure 12: The L* value is represented on the center axis. The a* and b* axes appear on the horizontal plane.

13 Color Delta CIELAB and CIELCH The expressions for these 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). Tolerancing (delta) from a known standard. CIELAB and CIELCH are used to Given ∆L* ∆a* ∆b*, the total differ- compare the colors 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 classified as a yellow 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 red. On the b* axis, a reading of –5.25 indicates bluer or less yellow. On the L* plane, the measurement difference 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 hue than Flower A. The L* and L* values are ∆E* = [(+ 11.1)2 + (–6.1)2 + (–5.25)2]1/2 ∆ ab identical for CIELCH and CIELAB. ∆E*ab = 13.71

CIE Color Space Notation

∆L* = difference in lightness/darkness value + – = lighter = darker s s e n t h g i

∆a* = difference on red/green axis L + = redder – = greener Hue Chroma ∆b* = difference on yellow/blue 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, color blindness and viewing L i Standard g conditions can all affect the human eye’s ability to distinguish h t n color differences. In addition to those limitations, the eye does e s not detect differences in hue (red, yellow, green, blue, etc.), s ( 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

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). s

s ∆H* e

When this tolerance is compared with the ellipsoid, we can see that it n t h

g ∆L* more closely matches human perception. This reduces the amount of i disagreement between the observer and the instrumental values L (Figure 17).

CMC Tolerancing

C hr om ∆C* CMC is not a color space but rather a tolerancing system. CMC toler- a 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 standard 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 orange 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).

Yellow

Tolerance ellipsoids are

Standard tightly packed in the orange region. s s ∆H* e n t h g ∆L* i L

C hr om ∆C* a Green Red

Tolerance ellipsoids are larger in the b* green region. ∆H*

∆C* ∆H* Blue Figure 18: Tolerance ellipsoids in color space ∆C*

∆H* Cross sections Hue and chromaticity tolerances of the ellipsoid ∆C* become smaller as lightness increases or decreases a*

e Chroma u H

Chroma

Standard e

cf = 0.5 cf = 1 u H Figure 19: Commercial factor (cf) of tolerances

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 s (2:1) s

commercial factor (cf). These settings affect the size and shape of the e n t ellipsoid in a manner similar to how the l:c and cf settings affect CMC. h g i L However, while CMC is targeted for use in the textile industry, CIE94 is Hue targeted for use in the paint and coatings industry.You should consider the Chroma type of surface being measured when choosing between these two tolerances. 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 equations 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.

Other White 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 yellowness 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

Glossary absolute white – In theory, a mate- black – In theory, the complete rial that perfectly reflects all light 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 spectrophotometer, 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 same value. In an additive color- and blue light. When all three addi- mixing environment, imagine mixing tive 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 gamut 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 pigment and related other pigments to it. Other pigments with See subtractive primaries higher chroma were noted, but the appearance – An object’s or mate- original scale remained. The chroma 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 — designated 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

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 subtractive color 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 color temperature – 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 black body while it is being heated. conditions. This measurement is expressed in CIE chromaticity coordinates – terms of absolute scale, or degrees See chromaticity coordinates (CIE). color-matching functions – Relative amounts of three additive Kelvin. Lower Kelvin temperatures CIE chromaticity diagram – See primaries required to match each such as 2400K are red; higher 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. color wheel – The visible spectrum’s continuum of colors arranged in a CIE luminosity function (y) – See color measurement – Physical circle, where complementary colors 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 mathematically 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, phosphors. tions of colors relative to one CIE tristimulus values – See tris- colorimeter – An optical measure- another. timulus values (CIE). ment instrument that responds to CIELAB (or CIE L*a*b*, CIE Lab) – color model – A color-measurement color in a manner similar to the Color space in which values L*, a* scale or system that numerically human eye — by filtering reflected and b* are plotted using Cartesian specifies the perceived attributes of light into its dominant regions of red, coordinate system. Equal distances color. Used in computer graphics green and blue.

Glossary continued

colorimetric – Of, or relating to, electromagnetic spectrum – The be arranged according to some other values giving the amounts of three massive band of electromagnetic criteria such as a geometric progres- colored lights 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, from green, blue from yellow, etc. American usage) or a shader. The producing light. Waves outside the Munsell defined five principal hues word “colorist” is of European origin. visible spectrum include gamma (red, yellow, green, blue and purple) rays, x-rays, microwaves and radio complements – Two colors that and five intermediate hues (yellow- waves. create neutral gray when combined. 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. fluorescent lamp – A glass tube point located at the beginning of the filled with mercury gas and coated D65 – The CIE standard illuminant red sector. Adjacent colors in this on its inner surface with phosphors. that represents a color temperature circle may be mixed to obtain contin- When the gas is charged with an of 6504K. This is the color tempera- uous variation from one hue to electrical current, radiation is ture most widely used in graphic another. Colors defined around the produced. This, in turn, energizes the arts industry viewing booths. See hue circle are known as chromatic phosphors, causing them to glow. 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 – Mathematical descrip- by the CIE in 1965. Values are pearlescent qualities. Gloss is an illuminant tion of the relative spectral power defined for the wavelength region additional tolerance that may be distribution of a real or imaginary 300 to 830nm. They are described in specified in the Munsell Color light source — i.e., the relative terms of the correlated color temper- Tolerance Set. The general rule for energy emitted by a source at each ature. The most important is D65 evaluating the gloss of a color wavelength in its emission spectrum. because of the closeness of its sample is the higher the gloss unit, Often used synonymously with “light correlated color temperature to that the darker the color sample will source” or “lamp,” though such usage of illuminant C, 6774K. D75 bluer appear. Conversely, the lower the is not recommended. than D65 and D55 yellower than D65 gloss unit, the lighter a sample will are also used. appear. illuminant A (CIE) – Incandescent illumination, yellow-orange in color, delta (D or ∆) – A symbol used to Gloss is measured in gloss units, 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 grayscale – 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

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 radiant energy – A form of energy of which a human observer is aware 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 spectrum – Spatial arrangement of retina of the eye. This portion of the 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 metamerism – 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 Munsell Color System – The color the specular angle.

Glossary continued

specular reflectance included lighter and less saturated than the X – 1) One of the three CIE tristim- (SCI) – 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- diffuse and specular reflectances. which instrumental measurements nates calculated as the fraction of are made. transparent – Describes a material the sum of the three tristimulus that transmits light without diffusion standard illuminants (CIE) – values attributable to the X value. 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 – An instru- tristimulus value. 3) One of the CIE light source. tristimulus colorimeter 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, magenta and yellow. Theoretically, used to obtain them. Approximations 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 lightness 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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