Tài liệu A Guide to Understanding Color Communication pdf

8 Scales for Measuring Color The Munsell Scale.. Since the curve of each color is asunique as a signature or finger-print, the curve is an excellent toolfor identifying, specifying andma

A Guide to Understanding

Color Communication

Communicating Color 2

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

Table of

Contents

© X-Rite, Incorporated 2002

How would you describe the color

of this rose? Would you say it’syellow, sort of lemon yellow ormaybe a bright canary yellow?

Your perception and interpretation

of color are highly subjective Eyefatigue, age and other physiolog-ical factors can influence yourcolor perception

But even without such physicalconsiderations, each observerinterprets color based on personalreferences Each person alsoverbally defines an object’s colordifferently

As a result, objectively cating a particular color tosomeone without some type ofstandard is difficult There alsomust be a way to compare onecolor to the next with accuracy

communi-The solution is a measuring ment that explicitly identifies acolor That is, an instrument thatdifferentiates a color from allothers and assigns it a numericvalue

instru-Communicating

Color

Ways to Measure Color

Today, the most commonly usedinstruments for measuring colorare spectrophotometers

Spectro technology measuresreflected or transmitted light atmany points on the visual spec-trum, which results in a curve

Since the curve of each color is asunique as a signature or finger-print, the curve is an excellent toolfor identifying, specifying andmatching color

The following information can helpyou to understand which type ofinstrument is the best choice forspecific applications

Spherical

Spherically based instrumentshave played a major roll in formula-tion systems for nearly 50 years

Most are capable of including the

“specular component” (gloss) whilemeasuring By opening a smalltrap door in the sphere, the “spec-ular component” is excluded fromthe measurement In most cases,databases for color formulation aremore accurate when this compo-nent is a part of the measurement

Spheres are also the instrument ofchoice when the sample is

textured, rough, or irregular orapproaches the brilliance of a first-surface mirror Textile manufac-turers, makers of roofing tiles oracoustic ceiling materials would alllikely select spheres as the righttool for the job

not reflect back to the eye A 0/45instrument, more effectively thanany other, will remove gloss fromthe measurement and measure theappearance of the sample exactly

as the human eye would see it

Multi-Angle

In the past 10 or so years, carmakers have experimented withspecial effect colors They usespecial additives such as mica,pearlescent materials, ground upseashells, microscopically coatedcolored pigments and interferencepigments to produce differentcolors at different angles of view.Large and expensive goniometerswere traditionally used to measurethese colors until X-Rite introduced

a battery-powered, hand-held,multi-angle instrument X-Riteportable multi-angle instrumentsare used by most auto makers andtheir colorant supply chain, world-wide

Colorimeter

Colorimeters are not tometers Colorimeters are tristim-ulus (three-filtered) devices thatmake use of red, green, and bluefilters that emulate the response ofthe human eye to light and color Insome quality control applications,these tools represent the lowestcost answer Colorimeters cannotcompensate for metamerism (ashift in the appearance of asample due to the light used to illu-minate the surface) As colorime-ters use a single type of light (such

spectropho-as incandescent or pulsed xenon)and because they do not recordthe spectral reflectance of themedia, they cannot predict thisshift Spectrophotometers cancompensate for this shift, makingspectrophotometers a superiorchoice for accurate, repeatablecolor measurement

Sample Being Measured

Beam

Port

Sph

Color – Throughout the

X-Rite products are designed forintegration and compatibilitythroughout the supply chain Forexample a large installation may useintegrated, networked color formula-tion and quality assurance software,such as X-RiteColor®Master, andseveral X-Rite sphere instrumentsthroughout the shop A smallsupplier with X-Rite QA-Master Iinstalled on a single computer andone SP62 spectrophotometer will

be compatible with the largerinstallation

Color control is required in a widevariety of applications, in variedscopes This is why X-Rite offersthe following process solutions:

Color Formulation and Quality Assurance

From basic quality assurancefunctions to the most sophisti-cated color formulation needs, X-RiteColor Master software,combined with X-Rite instruments,provides the ultimate flexibility toscale software packages to uniqueneeds now and over time Multiplemath engines can easily and accu-rately formulate opaque, translu-cent and transparent colors atfixed loads or with minimizedpigment usage With all databasesoperating from the same structure

in a network installation, managingcolor standards and measure-ments makes X-RiteColor Masterthe most efficient software forenterprise and supply chainprocesses

Special Effect and Pearlescent Paint

The X-Rite MA68II tometer offers a full range ofangular viewing (15˚ to 110˚) foraccurate evaluation of the changesexhibited in metallic, pearlescentand special effect paint finishes.The unique dynamic rotationalsampling (DRS) technology utilizes

spectropho-a simple, robust opticspectropho-al systemwhich provides simultaneousmeasurement of all angles TheMA68II interfaces with X-RiteColorMaster software for complete colorquality control applications

Sphere and 0/45 Instruments

X-Rite offers a wide range ofsphere and 0/45 spectrophotome-ters in portable and countertopmodels that offer superb inter-instrument agreement andrepeatability These instrumentsare easy to use and can be setupfor streamlined, automated capture

of color data

Non-Contact Color

Measurement

The X-Rite TeleFlash system provides

online color measurement and

evalua-tion of color deviaevalua-tion to the running

production line TeleFlash can

accu-rately measure the color of products

that are textured, finely patterned or glossy, such as extruded vinyl, bulk

goods, coil coatings, synthetic films, paints (wet and dry), textiles,

carpeting, granules, food pigments, paper, powders, glass, ceramics,

metal, minerals and plaster

TeleFlash offers a measuring distance of up to five feet, tolerating small

variations in the measuring distance from system to sample The system’s

thermochromism compensation allows for color measurement without the

time usually required for cooling and stabilizing

Multi-User, Network Installations and Portable Data

The networkability of X-Rite software makes it easy to communicate data

and share standards across an enterprise This ease translates into

effi-ciency which has a direct effect on profitability For applications without

networked computers, X-Rite Color-Mail can be used for fast, easy

communication of color data via standard e-mail ColorMail can be a

seamless part of X-RiteColor Master software

Calibrated, On-Screen Color

X-Rite offers the only color formulation and quality assurance software to

use the International Color Consortium’s (ICC) standard device profiles for

on-screen color This means that colors will be consistently displayed on

different computers, so long as ICC profiles are used Use X-Rite monitor

optimizers and auto-scan densitometers for complete color calibration and

control on computers, printers and scanners

Retail Color Matching Systems

MatchRite color matching systems are used worldwide in retail paint sales

and home decor services With networkable installation, portable

measure-ment instrumeasure-ments and hundreds of available paint databases (plus the

ability to create new databases), MatchRite is the most widely installed

color matching system

Spectrophotometry’s applicationsare seemingly boundless Color-matching measurements are madeevery day by those comparing areproduced object to a referencepoint Spectrophotometry-assistedcolor measurement can be useful

in areas such as:

• Corporate logo standardization

• Color testing of inks

• Color control of paints

• Control of printed colors onpackaging material and labels

• Color control of plastics andtextiles throughout thedevelopment and manufacturingprocess

• Finished products like printedcans, clothing, shoes,automobile components, plasticcomponents of all types

Applications

Each color has its own distinctappearance, based on threeelements: hue, chroma and value(lightness) By describing a colorusing these three attributes, youcan accurately identify a particularcolor and distinguish it from anyother.

Hue

When asked to identify the color of

an object, you’ll most likely speakfirst of its hue Quite simply, hue ishow we perceive an object’s color

— red, orange, green, blue, etc

The color wheel in Figure 1 showsthe continuum of color from onehue to the next As the wheel illus-trates, if you were to mix blue andgreen paints, you would get blue-green Add yellow to green foryellow-green, and so on

Chroma

Chroma describes the vividness ordullness of a color — in otherwords, how close the color is toeither gray or the pure hue Forexample, think of the appearance of

a tomato and a radish The red ofthe tomato is vivid, while the radishappears duller

Figure 2 shows how chromachanges as we move from center tothe perimeter Colors in the centerare gray (dull) and become moresaturated (vivid) as they movetoward the perimeter Chroma also

Figure 2: Chromaticity

Figure 3: Three-dimensional color system depicting 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

Lightness

Attributes

of Color

continued

The Munsell Scale

In 1905, artist Albert H Munselloriginated a color ordering system

— or color scale — which is stillused today The Munsell System ofColor Notation is significant from ahistorical perspective because it’sbased on human perception

Moreover, it was devised beforeinstrumentation was available formeasuring and specifying color

The Munsell System assignsnumerical values to the three prop-erties of color: hue, value andchroma Adjacent color samplesrepresent equal intervals of visualperception

The model in Figure 4 depicts theMunsell Color Tree, which providesphysical samples for judging visualcolor Today’s color systems rely oninstruments that utilize mathematics

to help us judge color

Three things are necessary to seecolor:

• A light source (illuminant)

• An object (sample)

• An observer/processor

We as humans see color becauseour eyes process the interaction oflight hitting an object What if wereplace our eyes with an instrument

—can it see and record the same

color differences that our eyesdetect?

CIE Color Systems

The CIE, or CommissionInternationale de l’Eclairage(translated as the InternationalCommission on Illumination), is thebody responsible for internationalrecommendations for photometryand colorimetry In 1931 the CIEstandardized color order systems

by specifying the light source (orilluminants), the observer and themethodology used to derive valuesfor describing color

The CIE Color Systems utilizethree coordinates to locate a color

in a color space These colorspaces include:

Figure 5: Spectral curve from a measured sample

Figure 4: Munsell Color Tree

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

A spectrophotometer measuresspectral data – the amount oflight energy reflected from anobject at several intervals alongthe visible spectrum Thespectral data is shown as

Figure 7: CIE 2° and 10° Standard Observers

300 250

200 150 100

50 0

380 430 480 530 580 630 680 730 780

z(λ) y(λ) x(λ)

Wavelength (nm)

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

400 500 600 700

Wavelength (nm)

120 100 80 60 40 20

Spectral Curve D65 Illuminant Standard Observer Tristimulus Values

Figure 8: Tristimulus values

2.0 1.5 1.0

0.5 0.0

x

Hue

SaturationFigure 9: CIE 1931 (x, y)

chromaticity diagram

Figure 10: Chromaticity diagram

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

To overcome the limitations ofchromaticity diagrams like Yxy, theCIE recommended two alternate,uniform color scales: CIE 1976(L*a*b*) or CIELAB, and CIELCH(L*C*h°).

These color scales are based onthe opponent-colors theory of colorvision, which says that two colorscannot be both green and red atthe same time, nor blue and yellow

at the same time As a result,single values can be used todescribe the red/green and theyellow/blue attributes

CIELAB (L*a*b*)

When a color is expressed inCIELAB, L* defines lightness, a*denotes the red/green value andb* the yellow/blue value

Figures 11 and 12 (on page 13)show the color-plotting diagramsfor L*a*b* The a* axis runs fromleft to right A color measurementmovement in the +a directiondepicts a shift toward red Alongthe b* axis, +b movement repre-sents a shift toward yellow Thecenter L* axis shows L = 0 (black

or total absorption) at the bottom

At the center of this plane isneutral or gray

To demonstrate how the L*a*b*values represent the specificcolors of Flowers A and B, we’veplotted their values on the CIELABColor Chart in Figure 11

The a* and b* values for Flowers

A and B intersect at color spacesidentified respectively as points

A and B (see Figure 11) Thesepoints specify each flower’s hue(color) and chroma (vividness/dull-ness) When their L* values(degree of lightness) are added inFigure 12, the final color of eachflower is obtained

an angular measurement

Expressing

Colors Numerically

Flower A:

L* = 52.99 a* = 8.82 b* = 54.53

Flower B:

L* = 29.00 a* = 52.48 b* = 22.23