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The appearance of an object is theresult of a complex interaction of thelight incident on the object, the optical

characteristics of the object, andhuman perception. Given that manu-factured products are meant to fulfillan intended purpose, their appearance

is one of their most important commer-cial attributes. Appearance often deter-mines the acceptability of a product toits seller, and ultimately to the con-sumer or end-user. The quality of aproducts appearance is psychologically related to its expected performanceand useful life. It therefore determinesits reception (or rejection) by potentialpurchasers.

All manufacturing industries areconcerned with the appearance of theirproducts. Appearance involves all visual

phenomena such as color, gloss, shape,texture, shininess, haze, and translu-cency that characterize objects. Allother things being equal, when con-sumers have a choice, they buy whatlooks best. Appearance is the foremostand most impressive product message.

Appearance is the foremost and most impressive product message.

Buyers also expect uniformity of appearance in any group of the sameproduct. When consumers see a differ-ence among the same products on dis-play, that difference is associated withpoor quality or out-of-date packaging. Visual appeal and uniformity of appear-

ance have such importance that quanti-tative identifications of appearance aredemanded by every marketplace.

Durability and resistance to fading

or degradation are also important fac-tors when considering the expected lifeof a product. Subjective commentsabout light fastness and durability areoften looked upon with great skepti-cism, but standardized testing proce-dures and quantitative measurementsbefore and after exposure testing canserve as a basis for comparison andhelp educated consumers make moreinformed purchase decisions.

The behavior of light interacting with products such as inks, paints, coat-

ings, papers, textiles, plastics, metals,ceramics, pharmaceuticals, cosmetics,and food varies depending on many physical characteristics. Using the rightinstrumentation and problem-solvingtechniques, it is possible to measurethe distinctive appearance attributesof a wide variety of products.

Interaction of Objects andMaterials

While light sources are visible by

their own emitted light, objects andmaterials appear to the eye accordingto how they affect the light that falls onthem (incident light). The objects ormaterials may be a printed surface, asheet of paper, an apple, or any of a great variety of different things.

Light (Figure 1) is defined as visu-ally evaluated radiant energy of wave-lengths from about 380 to 770 nm.

An Introduction to Appearance Analysis

P rinters and graphic arts ser-vice providers who deal dailywith such practices as colorcorrecting images, matchingproofs and press sheets, control-ling register, or using densitome-ters, spectrophotometers, orprofiles and color management arealready familiar with the concept of appearance analysis. They are alsofamiliar with that very subjectiveaspect of appearance analysis thatcomes from a customer who says,Well, it just doesnt look rightto me.

While the judgment of a prod-ucts appearance inevitablyincludes subjective opinion andcontextual issues, an element of science, technology, and numbersand measurements also applies,especially when it comes to colorand the factors that affect it. Andthat is what author Richard Haroldclarifies in this article. Harold, whohas been involved with appearanceanalysis for over 30 years and withGATF for two years, first describesthe interaction of light with objects,

which results in the human percep-tion of appearance. He then goeson to connect this perception tothe instruments and measure-ments that have been developedto analyze appearance attributes.

by Richard W. Harold

SS Number 84 A reprint from GATFWorld, the magazine of the Graphic Arts Technical Foundation

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Different wavelengths have differentcolors, and some wavelengths are visi-bly more intense than others. The eyes varying response to the same amountof energy at different wavelengths canbe represented by the luminosity curveof the human eye (Figure 2). Thesegraphical representations, known asspectral curves, can describe theamount of light or radiation at each wavelength, or our response to it, asin the luminosity curve.

Light can be produced by heatingobjects (e.g., the filament in a lightbulb) to incandescence or by theexcitation of atoms and molecules(e.g., when the heating coil in a elec-tric stove begins to glow red). Fluores-cence, where light is converted fromone spectral region to another, is aspecial case.

A completely radiating source,called a blackbody radiator, can beused as a reference

standard for identi-fying the color of incandescent lightsources. The corre-lated color temper-ature (CCT) of alight source is thetemperature of ablackbody radiator visually closest tothe appearance of the light source.For example, a typ-

ical incandescent(tungsten filament)lamp would havea CCT of about2856 K.

Although theappearance of materials, includingprinted ones,results from very complex factors,the problem can besimplified for analysis by separatingchromatic (color)

attributes from geometric (gloss, haze,texture, etc.) attributes, and by separat-ing diffuse from specular light distribu-tions. With the help of this breakdown,it is possible to identify nearly any attribute and prescribe the measuringinstrument and techniques needed toanalyze it.

The light striking an object will beaffected by its interaction with theobject in several ways. The light distri-butions (types of reflection or transmis-sion) that result after light strikes anobject give us our impressionsof what the object looks like. Specularreflection, for example, makes anobject look glossy or shiny. Metals areusually distinguished by stronger spec-ular reflection than that from othermaterials, and smooth surfaces arealways shinier than rough ones.

Geometric Attributes of Appearance

Unlike color, the geometricattributes of appearance, often associ-ated with surface properties, cannot becompletely defined in any simple coor-dinate arrangement. Fortunately, if only relatively flat, uniform, surfaceareas and over-simplified specular anddiffuse distributions of light are consid-ered, some meaningful simplificationof geometric attributes is possible.

First, light may be characterized asreflected or transmitted by an object.Reflected light is light that reboundsfrom an illuminated surface. Transmit-ted light is light that passes through theobject and is viewed from the exit side.Transmitted and reflected light may each be further divided into diffusedand undiffused light, giving four mainkinds of light distribution from objects:diffuse reflection, specular reflection,diffuse transmission, and regular trans-mission (see Figure 3). This separationinto diffuse and specular componentsprovides a good approximation (ade-quate for many analyses) of the geo-metric distribution of reflected ortransmitted light.

Selective absorption of certain wavelengths is what results in our per-ception of color. When absorption isthe dominant process, the resulting col-ors are not intense. If all wavelengthsare absorbed, black results. And if all wavelengths are reflected, whiteresults.

All the following processes operateon the majority of objects:s Specular (shiny) reflections Diffuse reflections Regular transmissions Diffuse transmission by scattering,and absorption.

Physical analyses of the combinedresults of these processes are madeusing measurements from spectro-photometers and goniophotometers(see Figure 4).

Spectrophotometric curves are ameasure of the reflection or transmis-sion of light, wavelength by wave-length, over the visible spectrum.

A n I n t r o d u c t i o n t o A p p e a r a n c e A n a l y s i s

2 No. 84 2001 Graphic Arts Technical Foundation

Figure 2. Relative spectral sensitivity of human vision. We see light in the yellow-green portion of the spectrum around 550 nm much more easily than elsewhere. The 1924 Photopic Vision curve represents the eyes sensitivity under daylight viewing conditions. The 1951 Scotopic Vision curve showsnighttime vision sensitivity with a shift toward the blue end of the spectrum.

Figure 1. The electromagnetic spectrum. Our eyes are sensitive to a limited range of the electromagnetic spectrum, the wavelengths from about 380 to 780 nm, which contain all the colorswe know.

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Spectral curves thus relate to color andcan be used to help identify the com-ponent dyes or pigments used to pro-duce the color.

Goniophotometric curves describe

how light is reflected from or transmit-ted through objects as a function of varying angles, and they relate to geo-metric attributes such as gloss andhaze. Although spectrophotometricand goniophotometric measurementsdo not provide conclusive values

describing appearance, they quantify the light-object interaction part of theobserving situation.

Chromatic Attributesof Appearance

The Physics of Color Color is associated with light waves,

specifically, their wavelength distribu-tions. These distributions are most

often referred to as the spectrophoto-metric characteristics. Visible wave-lengths are those between the violetand red ends of the spectrum, near400 and 700 nm, respectively (see Fig-

ure 1). The selective absorption of dif-ferent amounts of the wavelengths within these limits ordinarily deter-mines the colors of objects. Wave-lengths not absorbed are reflected ortransmitted (scattered) by objects andthus visible to observers. In other

2001 Graphic Arts Technical Foundation

Figure 3. Types of light reflection and transmission.

Diffuse reflection is character- istic of light that is redirected (scattered) over a range of angles from a surface onwhich it is incident. Diffuse reflection accounts for more of the color than any other type of distribution because most objects are opaque and reflect light diffusely.

Specular reflection is reflectionas from a mirror. It is highly directional instead of diffuse,and the angle of reflection is the same as the angle of the inci-

dent light striking the object.Specular reflection is what givesobjects a glossy or mirrorlike appearance. There are a variety of ways to assess or see thisglossy appearance.

Diffuse transmission occurswhen light penetrates anobject, scatters, and emergesdiffusely on the other side. Aswith diffusely reflected light,diffusely transmitted light leaves the object surface in all directions. Diffuse transmission is seen visually as cloudi- ness, haze or translucency,each of which is of interest inappearance measurement.

Regular transmission refers to light passing through an object without diffusion. Regular transmission measurementsare widely used in chemical

analysis and color measurement of liquids. Potential appearance attributes important for regular transmissionshould be roughly analogouswith gloss attributes associated with specular reflection.

Diffuse reflection

Figure 4. Instruments used for physical analysis of light reflection and transmission. The goniophotometer (left) measures light scatter as a function of variable angles of illumination or observation. This instrument is useful for studies of gloss, luster, surface smoothness (or rough- ness), haze, and distinctness of image. A spectrophotometer measures and analyzes the reflected light from a surface wavelength by wavelength as a means of determining the color of that surface. The unit shown here (right) is an automated scanning spectrophotometer that measures both reflected and transmitted light. Handheld spectrophotometers about the size of densitometers are also available.

Specular reflection Regular transmission

Diffuse transmission

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words, yellow objects characteristically absorb blue light; red objects absorbgreen light, and so on.

Physically, the color of an objectis measured and represented by spec-trophotometric curves, which are plotsof fractions of incident light (that isreflected or transmitted) as a functionof wavelength throughout the visiblespectrum relative to a reference. Thetypical reference is a white standard

that has been calibrated relative to theperfect white reflecting diffuser (100%reflectance at all wavelengths). Figure5 shows spectrophotometric curves forsome typical colored surfaces.

The white, gray, and black curvesare nearly straight, horizontal lines atthe top, middle, and bottom of thegraph, respectively, while each of the

curves representingchromatic colors ishighest in that part of the spectrum associ-ated with that color

(i.e., light that is scat-tered) and drops tolower values at other wavelengths (i.e., lightthat is absorbed).

Physiology of Color Psychologically and

physiologically, coloris a perception in thebrain, resulting fromsignals brought to it by light receptors in the

eyes. The color of any material results fromthe effect that the pigments, dyes, orother absorbing materials in the per-ceived object have on light. The eyedoes not see wavelength analyses, suchas the curves in Figure 5; rather, it syn-thesizes the responses of three colorreceptors (for red, green, and bluelight) in the eye. A skilled colorimetristcan estimate from curves, such as thosein Figure 5, what a specimens color will look like, but an unskilled personcannot.Attributes of Color As Seen by an Observer

What an artist sees when examininga color is neither its spectrophotomet-ric curve nor the separate responsesof the eyes red, green, and blue lightreceptors. If asked to identify the colorof an object, the artist will speak first of its hue. Hue is the attribute that corre-sponds to whether the object is red,orange, yellow, green, blue, or violet.This attribute is often related to thehue circle, which has been recognizedby artists, color technologists, and dec-orators for years.

A second attribute of color, and areadily appreciated one, is saturation.Saturation is determined by how farfrom the gray (lightness) axis towardthe pure hue at the outer edge that acolor is perceived to be. A pastel tint,for example, is said to have a low satu-

ration while a pure color is said to havehigh saturation.

A third attribute or dimensionof color is associated with an objectsluminous intensity (usually light-

reflecting or transmitting capacity).This attribute is variously called light-ness, value, and sometimes, althoughincorrectly, brightness.

The three attributes of an objectscolor, then, are hue, saturation, andlightness. They are related to eachother as shown in Figure 6. One of thebest-known surface-color systems isthe Munsell System of Color Notationillustrated in Figure 7. In this system,the three visually perceived dimensionsof color appearance are designated as

hue, value (lightness), and chroma(saturation). In color communication,particularly when discussing color dif-ferences, lightness, chroma and hue(LCH) are the most frequently usedterms.

Illuminants The light source (the illuminant)

will affect the perception of color.Both natural daylight and artificial sim-ulated daylight are commonly used for visually examining the color differencebetween materials. A window facingnorth (to be free of direct sunshine)is the natural illuminant normally employed. Artists are known for theirpreference for studios with northlight. However, natural daylight variesgreatly in spectral quality with time of day, weather conditions, direction of view, time of year, and geographicallocation. Because of this variability, thetrend in industrial testing has beentoward using a simulated daylightsource that can be standardized andremain relatively stable in spectralquality.

In order to define the artificial lightsources used in appearance evaluation,the Commission Internationale delEclairage (CIE) established standardilluminants, which have spectral char-acteristics similar to natural lightsources and are reproducible in thelaboratory (CIE, 1931): Illuminant Adefines light typical of that from an



Figure 6. How hue, saturation, and lightness are related to each other in a three- dimensional color system.

WhiteYellow

Red

Gray

Blue Green

Black

% R

e l a t

i v e

R e f l e c t a n c e

100

0400 700Wavelength, nanometers (nm)

Figure 5. Spectrophotometric curves for some typical colored surfaces. The white, gray, and black curves are nearly horizontal lines, while each of the curves representing a chromatic color is

highest in the part of the spectrum associated with that color.

L i g h t n e s s

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Opponent-Colors (L, a, b-type)

Color Scales Because the CIE scales do not pro-

vide even reasonably uniform estimatesof perceived color differences or colorand relationships, scientists have devel-oped so-called uniform color scales.Most, although not all, are opponent-colors (L,a,b-type) scales.

The opponent-colors theory of color vision had its beginnings withThomas Young in 1807, Hermann VonHelmholtz in 1857, and Ewald Heringin 1878. It was refined in 1930 by G.E.Meller, and since then, those whohave applied Mellers principleshave created several highly usefultechniques.

The opponent-colors theory pre-sumes that, in the human eye, there isan intermediate signal-switching stagebetween when the light receptors in theretina receive color signals and whenthe optic nerve takes those color signalsto the brain (see Figure 10). During thisswitching stage, it is presumed that redresponses are compared with green togenerate a red-to-green color dimen-sion. The green response (or red andgreen together, depending on the the-ory) is compared in a similar manner with the blue to generate a yellow-to-blue dimension. These two dimensionsare widely, although not always, associ-ated with the symbols a and b,respectively. The necessary thirddimension, L for lightness, is a non-

linear functionsuch as thesquare or cuberoot of the CIEY value, which

is percentreflectance (ortransmission).

The scien-tific validity of the opponent-colors system isstrongly sup-ported by experimentalevidence. Forexample, in

1966 Russell L. De Valois of the Pri-

mate Vision Laboratory at the Univer-sity of California (Berkeley) attachedelectrodes to individual optic nervefibers of monkeys and identified L, a,and b correlating signals, rather than X,Y, and Z correlating signals. The wideacceptance and use of the opponent-colors system by practicing color tech-nologists also supports its validity.

Figure 11 shows the dimensions of the L, a, b opponent-colors coordinatesystem. The earliest of these L, a, b-type scales was the original HunterL, a, b scale developed and refined by

Richard S. Hunter between 1942 and1958. Many other opponent-colors-typescales were developed between 1958and the early 1970s, but they are rarelyused today.

In 1976, the CIE adopted anotherL, a, b-type scale identified as theCIE 1976 L*a*b* scale. Often abbre- viated as the CIELAB scale, it is thecurrent recommended color scale inalmost all domestic and internationalcolor measurement test methods andspecifications.

Communicating Color byNumbers: Color and ColorDifference Scales

The industrial use of color measurement, formulation, and specification habecome a common practice to ensuremore consistent production without visible variation. Customers have cometo expect to see the same color every time they purchase the same product, whether it is days or months betweenpurchases. To achieve this degree of color control, numerical tolerances aredeveloped to ensure that if productionfalls within the specified tolerance,

there will be minimal chance of



Figure 10. The opponent-colors theory presumes, that, in the human eye, there is an intermediate signal-switching stage between when the light receptors in the retina receive color signals and when the optic nerve takes those color signals to the brain. During this switching stage, it is presumed that red responses are compared with green to generate a red-to- green color dimension. The green response (or red and green together, depending on the theory) is compared in a similar manner with the blue to generate a blue-to-yellow signal.

Figure 9. Comparison of CIE 2 and 10 Standard Observer.

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customer complaints about colormatching.

Although there are numerous waysto specify color tolerances, one increas-ingly popular method involves using atotal color difference formula based onthe CIELAB color scale and differ-ences in lightness, chroma, and huecalled E CMC (l:c). The CMC for-mula has appeared in several differentdomestic and ISO (International Stan-dards Organization) standards.

The CMC formula is being used inmany industries, including paints, plas-tics, graphic arts, paper, textiles, inks,and food products to name just a few.CMC values for an acceptable colormatch typically range from about 0.4(for some paints and textiles) to 2 to 4units (for some graphic arts appli-cations). The size depends, of course,on the nature of the product, the appli-cation, and customer requirements andexpectations.

A new total color difference for-mula, CIE E 2000, reportedly with

significant improvements over the pre- vious Eformulas ( E* CIELAB and ECMC), is being considered in CIECommittee for eventual public release.

ConclusionsThis overview is intended to clarify

the basic concepts of the science andtechnology of appearance measure-

ment. Proper applica-tion of these princi-ples to industrial andprinting situations canquantify appearance

and thus enable itsprecise control.Many color and

appearance instru-ment manufacturerscan provide assistancein applying this tech-nology for specificapplications. Severalleading universitiesand some instrumentmanufacturers offer

courses in appearance analysis andcolor measurement. See the referencesfor further information. s

References and Resources

1. R.S. Hunter, and R.W. Harold, The Measurement of Appearance, 2nd Edi-tion. Wiley-Interscience, New York,1987.

2. F.W. Billmeyer Jr. and M. Saltzman,Principles of Color Technology, 2nd Edi-tion. Wiley-Interscience, New York,1981.

3. Colorimetry, 2nd Edition., CIE Publica-tion No. 15.2, Bureau Central de la CIE,Vienna, Austria, 1986.

4. The Science and Technology of Appearance Measurement, Hunter Asso-ciates Laboratory, Inc., Marketing Publi-cation, Reston, Virginia, 1981.

5. R.L. De Valois, Physiological Basis of Color Vision, Color 69, Vol. 1, p. 29,Muster-Schmidt, Gttingen, 1970.

6. Colour Physics for Industry, 2nd Ed.,edited by R. McDonald, Society of Dyers

and Colourists, England, 1997.7. ASTM Standards on Color and Appear- ance Measurement, Sixth Edition, Ameri-can Society for Testing and Materials,Conshohocken, PA, 2000.

8. R.W. Harold and Gerald M. Kraai,Legal Protection for Color, GATFWorld,March/April 1999, p. 3136

9. Akihiro Kondo with R.W. Harold,Measuring the Effect of Substrate Prop-erties on Color Inkjet Images, GATF- World, Sep/Oct 1999, p. 3136.

10. R.W. Harold and D.Q. McDowell,

Standard Viewing Conditions for theGraphic Arts, GATF SecondSight reprint(Cat. No. SS72), 1999, 7 pages.

11. D.Q. McDowell and L. Warter,Viewing Conditions: Whats New?Should We Care? IPA Prepress Bulletin,Nov/Dec 1997, p. 17-20.

12. ISO3664:2000, Standard Viewing Conditions for Prints, is available fromNPES at www.npes.org/standards/ order.htm.

13. Gary G. Field, Color and its Repro-

duction, 2nd Edition, GATFPress, Pitts-burgh, 1999, chap. 26.

Richard W. Harold ([email protected]), director of color and appear-ance applications for BYK-Gardner USA,Columbia, MD, has worked extensivelywith pigments, dyes, inks, paints andcoatings, plastics and a host of otherindustries where color and appearanceare important. An organic chemistby training, he is convenor of ISO TC38/SC1/WG7 Colour Measurement and vice chairman of ASTM CommitteeE12 on Color and Appearance.

No. 84 7 2001 Graphic Arts Technical Foundation

Figure 11. Example of an opponent-colors L,a,b-type color system.

Graphic Arts Technical Foundation, 2001200 Deer Run RoadSewickley, PA 15143-2600Phone: 412-741-6860 Fax: 412-741-2311Printed in U.S.A. 434101 5-01 1.5 MInformation in this report may not be quotedor reproduced without prior written consent of GATF, except that up to 50 words may bequoted without advance permission providedthe quotation refers specifically to GATF asthe source and does not contain formulas orprocesses.

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