Communication, Measurement, and Control for Digital Imaging - PDF document

Communication, Measurement, and Control for Digital Imaging COLORGUIDE and Glossary Communication, Measurement, and Control for Digital Imaging and Graphic Arts www.x-rite.com© X-Rite,Incorporated 1998ALL RIGHTS RESERVED X-Rite¨ ,X-RiteColor¨,the X-RiteColor logo,Digital Swatchbook¨,X-Scan¨,andQuickInk¨ are registered trademarks ofX-Rite,Incorporated.Adobe,Adobe PageMaker,and PostScript are registered trademarks and Adobe Photoshopand Adobe Illustrator are trademarks ofAdobe Systems Incorporated.EncapsulatedPostScript (EPS) is a trademark ofAltsys Corporation.FreeHand is a trademark ofMacromedia,Inc.QuarkXPress is a registered trademark ofQuark,Inc.All other brandor product names are trademarks or registered trademarks oftheir respective holders.Mention ofthird-party products is for informational purposes only and constitutes neitheran endorsement nor a recommendation.X-Rite,Incorporated assumes no responsibilityfor the performance or use ofthese third-party products.Apple and Macintosh are trademarks ofApple Computer,Inc.,registered in the UnitedStates and other countries.Mac and ColorSync are trademarks ofApple Computer,Inc. TABLE OF CONTENTS Understanding Color4The CIEColor Systems14Spectral Data vs. Tristimulus Data19 Color Measurement and Control Instrumentation22Measurement in the Graphic Arts Workflow26Color Specification27Color Management28Color Formulation35Color Control35Color Verification37 3Glossary 1 Color Communication Color communicates.Color sells.Color is the sizzle that drives the sale ofvirtuallyevery consumer product in the world.It evokes a wide range ofemotions that drawthe buyer to the product.As design,graphics,and imaging professionals,we knowthat color is a crucial part ofthe selling process because it is such an important partofthe buying decision .Ifwe use color effectively in the manufacturing and market-ing ofan item,potential buyers will perceive added value in that product. These GATFtest images demonstrate colors that must be repro-grass green,or food items are ÒoffÓby even a small margin, the To use color effectively,it must be kept under tight control .The color workflow beginswith the designerÕs ideas and the customerÕs specifications.From there,these colorsmust be communicated among several different individuals who will render andreproduce the colors on many different devices.At each stage ofproduction, output from the previous step becomes the for the next process.Every exchange bringsthe color into a new color spaceÑfrom photographic film to monitor RGBto CMYKprocess proofing and printing on a variety ofsystems.And every evaluation is madeby a different viewer under new viewing conditions.So,how do we ensure that our original ideas and specifications will remain intactthroughout this complicated process?This book is designed to answer that veryquestion.In short,the answer is color measurement Ñifyou can measure color,you control it.The remainder ofthis booklet explains the fundamentals ofcolorcommunication,measurement,and control. The Color Guide and GlossaryThe Challenge:Color Communication Consider the many different individuals who Òpass the batonÓofresponsibility forkeeping the customerÕs color specifications intact: ¥ Content Specifier/Client Defines message;determines imageconcept;provides general or specific color and paper specifications. ¥ Graphic Designer/Photographer Provides image,art,and page files;and printed or digital color specifications. ¥ Pre-Press Service Provider Provides final color-separated films;color break information;printed or digital color specifications. ¥ Printing Ink Supplier Provides inks that meet colorspecifications;considers paper specification. ¥ Printing Company Provides final printed piece;meets color specifications.Each step in the color reproduction process adds value and content to the message.Good color specification ensures that each process provides accurate color contentbased on the input received.As we strive to create dazzling,high-quality color documents and designs,we struggleto control color at each production phase.Each viewing situation presents its owninterpretation ofthe same color.For example: ¥ Our original scene contains a wide range ofnatural,vivid colors. ¥ A photograph ofthe scene captures much ofthe sceneÕs color;however,some ofthe dazzling tones are lost when the image is scanned into RGBdata.Still more colors are lost or changed when the scan is displayed on a monitorÑand the scene appears slightly different on different monitors. ¥ As we move our artwork between imaging,illustration,and layout programs,the colors are specified in different ways.For example,specifying 87% magenta / Color Communication 91% yellow produces a slightly different color in Photoshopª,FreeHandª,and QuarkXPress¨. ¥ When we print our artwork,the colors get color-separated from RGB data into CMYKdata.The colors are interpreted a bit differently on different devicesÑon our laser copier,our trade shopÕs proofing system,and on press. ¥ When we check our output,we view the colors under different lighting conditions that affect color appearance in different ways.Also,different individuals perceive colors based on their own vision skills and memory.The common question throughout this process is:which device is telling the truth Unfortunately,no individual viewers,programs,or devices can reveal the true identityofa color.They simply perceive the colorÕs appearance,which can be affected bylighting and other factors. The Solution:Color Measurement and Control Measurement is the key to total production control.Consider this:we measure size ininches or millimeters;weight in pounds and grams;and so on.These scales allow usto establish precise measurement standards that can be repeated in the productionprocess.This ensures that all manufactured items are identical and within our qualitytolerances.Using measured color data,we can do the same for colorÑwe can monitorcolor at each stage ofproduction and check the ÒclosenessÓofcolor matches usingrepeatable,standardized numerical data.So,what properties ofcolors allow them tobe discretely identified and measured?LetÕs find out by examining these propertiesÑhow color happens in nature andin our minds;how it is reproduced on screen and on paper;and how color can be as reflectance values ( spectral data )and as three-dimensional values tristimulus data 3 The Color Guide and GlossaryUNDERSTANDING COLOR To help you clearly understand how color is measured,we should first study thefundamentals ofcolorÕs physical and physiological properties.Color results from an interaction between light,object ,and the viewer .It is light has been modified by an object in such a manner that the viewer Ñsuch as the humanvisual systemÑperceives the modified light as a distinct color.All three elements mustbe present for color as we know it to exist.LetÕs examine colorÕs origins in more detailby first studying light . LightÑWavelengths and the Visible Spectrum Light is the visible part ofthe lectromagnetic spectrum Light is oftendescribed as consisting ofwaves.Each wave is described by itswavelength - the length from wave crest to adjacent wave crest.Wavelengths are measured in nanometers (nm).A nanometer isone-millionth ofa millimeter.The region ofthe electromagnetic spectrum visible to the human eye ranges fromabout 400 to 700 nanometers.This amounts to a mere slice ofthe massive electro-magnetic spectrum.Although we canÕt see them,we use many ofthe invisible wavesbeyond the visible spectrum in other waysÑfrom short-wavelength x-rays to thebroad wavelengths that are picked up by our radios and televisions. Our eyes have light sensors that are sensitive to the visible spectrumÕs wavelengths.When light waves strike these sensors,the sensors,the senors send signals to thebrain.Then,these signals are perceived by the brain as a particular color.Exactlywhich color is perceived depends on the composition ofwavelengths in the lightwaves.For example,ifthe sensors detect all visible wavelengths at once,the brainperceives white light.Ifno wavelengths are detected,there is no light present and thebrain perceives . Visible Spectrum 5Color Communication Now we know how our eyes and brain respond to the presence of visiblewavelengths or wavelengths.Next,letÕs examine how our vision system respondsto each individual wavelength.Passing a beam ofwhite light through a prism disperses the light so that we can seehow our eyes respond to each individual wavelength.This experiment demonstratesthat different wavelengths cause us to see different colors.We can recognize the visiblespectrumÕs dominant regions ofred,orange,yellow,green,blue,indigo,and violet;andthe ÒrainbowÓofother colors blending seamlessly in between.When our visual system detects a wavelength around 700nm,we see Òred;Ówhen awavelength around 450-500nm is detected,we see Òblues;Óa 400nm wavelength givesus Òviolet;Óand so on.These responses are the basis for the billions ofdifferent colorsthat our vision system detects every day.However,we rarely see wavelengths at once (pure white light),or just wavelength at once.Our world ofcolor is more complex than that.You see,color is part oflightÑit light.When we see color,we are seeing light that has into a new composition ofmany wavelengths.For example,when we seea red object,we are detecting light that contains mostly ÒredÓwavelengths.This is howall objects get their colorÑby modifying light.We see a world full ofcolorful objectsbecause each object sends to our eyes a unique composition ofwavelengths.Next,letÕsexamine how objects affect light. The Color Guide and GlossaryObjectsÑManipulating Wavelengths When light waves strike an object,the objectÕs surface absorbs some ofthe spectrumÕs energy,while other parts ofthe spectrum are reflected back from the object.Themodified light that is reflected from the object has an entirely new composition ofwavelengths.Different surfaces containing various pigments,dyes,and inks generatedifferent,unique wavelength compositions.Light can be modified by striking a reflective object such as paper;or by passingthrough a transmissive object such as film or a transparency.The light sourcesthemselves - emissive objects such as artificial lighting or a computer monitor -also have their own unique wavelength composition.Reflected,transmitted,or emitted light is,in the purest ofterms,Òthe color oftheobject.ÓThere are as many different colors as there are different object surfacesÑeach object affects light in its own unique way.The pattern ofwavelengths that leavesan object is the objectÕs spectral data ,which is often called the colorÕs Òfingerprint.ÓSpectral data results from a close examinationÑor measurement Ñofeach wave-length.This examination determines the percentage ofthe wavelength that is reflect-ed back to the viewerÑits reflectance intensity You can visually examine a colorÕs spectral properties by plotting its measurement spectral curve .This type ofdata can be gathered only by using a spectropho-tometer such as X-RiteÕs Digital Swatchbook,model 938 Spectrophotometer,Colortron,DTP41,or Auto-Tracking Spectrophotometer (ATS) system. 7Color CommunicationSpectral Data Spectral data can be plotted as a spectral curve,providing a visual representationofa colorÕs fingerprint.LightÕs wavelengths and reflectance intensity provide twoabsolute points ofreference for plotting a curve:the 300 nanometers ofdifferentwavelengths comprise the horizontal axis,and the level ofreflectance intensitycomprises the vertical axis.Using ColorShopÕs Spectral Compare tool,you can compare a colorÕs curve shapeÑwhere it rises and dipsÑto the points along the wavelength axis. To compute spectral data,spectrophotometers examine a number ofpoints along thewavelength axis (Digital Swatchbook,for example,reports 31 points spaced 10nmapart),then determine the amount ofreflectance intensity at each wavelength. the most complete and infallible description ofa color you can achieve .Later,weÕlldemonstrate spectral dataÕs power and precision further as we compare it with othercolor models and specification methods.So far,weÕve studied light ; objects ;how objects affect light to generate different colors;and how a spectrophotometer can be used to directly measure how different objectsaffect light.To completely define color as we know it ,we must conclude by studying viewer Ñthe human eye and other devices that sense or render color. The Color Guide and GlossaryViewerÑSensing Wavelengths as ÒColorÓ For our visual palette ofcolors to exist,all three elements ofcolorÑ light , object ,and viewer Ñmust be present.Without light there would be no wavelengths;without objects there would be only white,unmodified light;and without the viewer therewould be no sensory response that would recognize or register the wavelengths as aunique Òcolor.ÓThere is a well-known riddle that asks:ÒIfa tree falls in the woods and no one isthere to hear it,does it make a sound?ÓActually,a similar question can be asked inregards to color:ÒIfa red rose is not seen,does it have color?ÓThe answerÑwhichmay surprise youÑis .Technically,color there in the form ofwavelengths (thespectral data).However,the color we know as ÒredÓonly happens in our minds,afterour visual sensory system has responded to those wavelengths. The basis for human vision is the network oflight sensors in our eyes.These sensorsrespond to different wavelengths by sending unique patterns ofelectrical signals tothe brain.In the brain,these signals are processed into the sensation of sight light and ofcolor.As our memory system recognizes distinct colors,we thenassociate a name with the color.So,do our brains also examine discrete wavelength information and plot curves forevery color we see?Not exactly.The human visual system must work far too quicklyto do all that,given the deluge ofnew wavelength information that it receives everysecond.Instead,this systemÕs miraculous design uses a more efficient method forÒmass-processingÓwavelengths.It breaks the visible spectrum down into its mostdominant regions of red , green ,and ,then concentrates on these colors tocalculate color information. RGBÑColorÕs Additive Primaries By mixing these dominant colorsÑcalled the additive primaries Ñin different combi-nations at varying levels ofintensity,the full range ofcolors in nature can be veryclosely simulated.Ifthe reflected light contains a mix ofpure red,green,and bluelight,the eye perceives white;ifno light is present,black is perceived.Combining twopure additive primaries produces a subtractive primary.The subtractive primaries cyan,magenta,and yellow are the opposing colors to red,green,and blue. The human eyeÕs three-value color system has been imitatedand exploitedÑby inventors ofcolor scanners,monitors,andprinters.The color rendering methods used by these devicesare based directly on our response to stimuli ofred,green,and blue light. The Color Guide and Glossary Like the human eye,these devices must also process a large amount ofcolor informa-tion at onceÑon screen or on paper.In logical fashion,these devices imitate the eyeÕsresponse to the additive primaries to create a colorful illusion:For example,a monitorblends varying intensities ofred,green,and blue light at each ofits tiny pixels.Thesepixels are so small and tightly packed that the eyeÕs RGB response is ÒfooledÓinto theperception ofmany different colors when really there are only three. CMYand CMYKÑThe Subtractive Primaries Monitors and scanners can employ the additive color system becausethey are emissive devicesÑthey can directly red,green,and bluelight to darkness. Printers ,on the other hand,must render colors onpaper and other substrates,so they must work with reflected light.To do this,printers employ the opposing subtractive primaries ofcyan,magenta,and yellow.In the visible spectrum, is directly opposed to red; magenta opposite ofgreen;and yellow is the opposite ofblue.When cyan,magenta,and yellow pigments are deposited on a white,reflective sub-strate,each completely absorbsÑor subtracts Ñits opposing counterpartfrom the oncoming white light.For this reason,the printing processuses cyan,magenta,and yellow inks to control the amount ofred,green,and blue light that is reflected from white paper. primary is produced.*Theoretically, when all threemuddy gray. For this reason, Color Communication These colors are printed on paper as separate layers ofhalftone dot patterns.Theillusion ofdifferent colors and tones is created by varying the size,balance,and angleofthe dots.The effects ofvarying dot sizes is similar to the varying intensities ofamonitorÕs red,green,and blue phosphors.This diagram demonstrates how the subtractive primaries remove their additivecounterpart from light to produce the appearance ofa color: HSLÑThe Three Dimensions of Color So far,weÕve learned that color consists ofcomplex wavelength information,andthat the human eye,monitors,and printers,convert this complex information intothree-value systems ofprimary colors in order to simplify processing and renderingofthat information.Another way to simplify color description is to describe its threeattributes or Òdimensions:Ó ¥ Hue Ñits basic color,such as red,pink,blue,or orange. ¥ Saturation Ñits vividness or dullness. ¥ Lightness Ñits brightness or darkness. Light waves also have three attributes that directly affect the attributes ofhue,saturation,and lightness.Ofcourse,wave length determines the colorÕs ;wave purity determines saturation ;and wave amplitude (height) determines lightness . The Color Guide and Glossary Spectral curves demonstrate the relationship between wave attributes and the waywe perceive these attributes.Vibrant,colorful objects reflect a distinct part ofthe spectrum at high intensity;objects that are near-white or light gray reflect most ofthe spectrum uniformly andat high intensity;dark gray,dark brown,and black objects absorb most ofthe spec-trumÕs energy;and so on. Color SpaceÑMapping ColorÕs Dimensions Hue,saturation,and lightness demonstrate that visible color is three-dimensional.These attributes provide three coordinatesthat can be used to ÒmapÓvisible color in color space .The early-20th Century artist A.H.MunsellÑcreator ofthe MunsellColor Charts Ñis credited as a pioneer ofintuitive three-dimensional color spacedescriptions.There are many different types ofcolor spaces that are based on orresemble MunsellÕs designs.Basically,a color space based on hue,saturation (or chroma ),and lightness (or value uses cylindrical coordinates.Lightness is the center vertical axis and saturation is thehorizontal axis that extends from the lightness axis.Hue is the angle at which the sat-uration axis extends from the lightness axis.We can apply the relationship between wave attributes and color attributes to a Color Communication three-dimensional color space.Wave determines a colorÕs position on thelightness axis;wave purity determines its location on the saturation axis;and wave-length determines hue angle.Around the ÒequatorÓlie vibrant,pure hues.As the huesblend together toward the center,they become less pure and lose saturation.On thevertical axis,colors ofdifferent hue and chroma become lighter or darker.The light-ness extremes ofwhite and black lie at the Òpoles.ÓAnd ofcourse,at the center ofitall lies neutral grayÑwhere white,black,and all hues meet and blend together. Tristimulus Data A color space can be used to describe the range ofvisible or reproducible colorsÑ gamut Ñofa viewer or device.This three-dimensional format is also a veryconvenient way to compare the relationship between two or more colors.Later,weÕll see how we can determine the perceptual ÒclosenessÓoftwo colors by the distance between them in a color space.Three-dimensional color models and three-valued systems such as RGB,CMY,and HSLare known as tristimulus data Locating a specific color in a tristimulus color space such as RGBor HSL is similarto ÒnavigatingÓaround a city using a map.For example,on the HSLcolor spaceÒmap,Óyou first locate the intersection where the Hue angle meets the Saturation .Then,the Lightness value tells you what ÒfloorÓthe color is located on:from deep below ground (black) to street level (neutral) to a high-rise suite (white).In many applications,the intuitiveness oftristimulus color descriptions makes thema convenient measurement alternative to complex (yet more complete and precise)spectral data.For example,instruments called colorimeters measure color by imitat-ing the eye to calculate amounts ofred,green,and blue light.These RGBvalues areconverted into a more intuitive three-dimensional system where relationshipsbetween several color measurements can be easily compared.However,any system ofmeasurement requires a repeatable set ofstandard scales.For colorimetric measurement,the RGBcolor model cannot be used as a standard The Color Guide and Glossary because it is not repeatableÑthere are as many different RGBcolor spaces as thereare human viewers,monitors,scanners,and so on (it is,as weÕll discuss later, device-dependent ).For a set ofstandard colorimetric measurement scales,we turn to therenowned work ofthe Commission Internationale dÕEclairage Having explored the measurable properties and attributes ofcolor,letÕs now studythe established CIEstandards upon which most industrial color communication andmeasurement is based. THE CIE COLOR SYSTEMS In 1931,the established standards for a series ofcolor spaces that represent thevisible spectrum.Using these systems,we can compare the varying color spaces ofdifferent viewers and devices against repeatable standards The CIE color systems are similar to the other three-value models weÕve discussed inthat they utilize three coordinates to locate a color in a color space.However,the CIEspacesÑwhich include CIE XYZ,CIE L*a*b*,and CIE L*u*v*Ñare device-indepen-dent ,meaning the range ofcolors that can be found in these color spaces is notlimited to the rendering capabilities ofa particular device,or the visual skills ofaspecific observer. CIEXYZand The Standard Observer The basic CIE color space is CIE XYZ .It is based on the visual capabilities ofa Standard Observer ,a hypothetical viewer derived from the CIEÕs extensive researchofhuman vision.The CIE conducted color-matching experiments on a number ofsubjects,then used the collective results to create Òcolor-matching functionsÓand aÒuniversal color spaceÓthat represents the average humanÕs range ofvisible colors.The color matching functions are the values ofeach light primaryÑred,green,andblueÑthat must be present in order for the average human visual system to perceiveall the colors ofthe visible spectrum.The coordinates X , Y ,and Z were assigned tothe three primaries. Color Communication From these XYZ values,the CIE constructed the xyY Chromaticity Diagram define the visible spectrum as a three-dimensional color space.The axes ofthiscolor space are similar to the HSLcolor space;however,the xyY space could not bedescribed as cylindrical or spherical.The CIE found that we do not see all colorsuniformly,and therefore the color space they developed to ÒmapÓour range ofvision is a bit skewed.On our rendering ofthe xydiagram,we have demonstrated the limitations ofcolorspaces developed using coordinates ofmonitor RGBand printer CMYK.To lead us The xy chromaticitydiagram has a ÒnaturalÓ shape because weYou can see how the upper left of the diagram Òstretches outÓin the greens and yellows, white,reds, and purples are packed tightly together. The Color Guide and Glossary into our next discussion,we must also point out that the RGBand CMYKgamutsshown here are not standard gamuts.These descriptions would change for every indi-vidual device.However,the XYZgamut is a device-independent, repeatable standard. CIEL*a*b* The ultimate goal ofthe CIEwas to develop a repeatable system ofcolor communi-cation standards for manufacturers ofpaints,inks,dyes,and other colorants.ThesestandardsÕmost important function was to provide a universal framework for colormatching.The Standard Observer and XYZcolor space were the foundations ofthisframework;however,the unbalanced nature ofthe XYZ spaceÑas demonstrated bythe xyY chromaticity diagramÑmade these standards difficult to clearly address. As a result,the CIEdeveloped more uniform color scales called CIEL*a*b* L*u*v* .Ofthese two models,CIEL*a*b*is the most widely used.The well-balancedstructure ofthe L*a*b* color space is based on the theory that a color cannot beboth green and red at the same time,nor blue and yellow at the same time.As aresult,single values can be used to describe the red/green and the yellow/blue attrib-utes.When a color is expressed in CIE L*a*b*,L* defines lightness;a* denotes thered/green value;and b* the yellow/blue value.In many ways,this color spaceresembles three-dimensional color spaces like HSL. Color Communication CIEL*C*H¡ The L*a*b*color model uses rectangular coordinates based on the perpendicularyellow-blue and green-red axes.The CIEL*C*H¡ color model uses the same XYZ-derived color space as L*a*b*,but instead uses cylindrical coordinates of Lightness , Chroma ,and Hue angle.These dimensions are similar to the Hue,Saturation(Chroma),and Lightness.Both L*a*b* and L*C*H¡attributes can be derived from ameasured colorÕs spectral data via direct conversion from XYZ values,or directlyfrom colorimetric XYZvalues.When a set ofnumerical values are applied to eachdimension,we can pinpoint the colorÕs specific location in the L*a*b*color space.The diagram below shows the L*a*b*and L*C*H¡coordinates graphed atop theL*a*b*color space.WeÕll revisit these color spaces later when we examine colortolerancing and verification.These three-dimensional spaces provide a logical framework within which therelationship between two or more colors can be calculated.The ÒdistanceÓbetweentwo colors in these spaces identifies their ÒclosenessÓto each other. The Color Guide and Glossary As you will recall,the viewerÕs gamut is not the only element ofcolor that changeswith every different viewing situation. Lighting conditions also influence the appear-ance ofcolors.When describing a color using tristimulus data,we must also describethe reflectance data ofthe light source.But which light source do we use?Again,theCIEhas stepped in to establish standard illuminants ,as well. CIEStandard Illuminants Defining the properties ofthe illuminant is an important part ofdescribing color inmany applications.The CIEÕs standards provide a universal system ofpre-definedspectral data for several commonly-used types.The CIE standard illuminants were first established in 1931 as a set ofthree,identified as A,B,and C: ¥ Illuminant A represents incandescent lighting conditions with acolor temperature ofabout 2856¡K; ¥ Illuminant B represents direct sunlight at about 4874¡K;and ¥ Illuminant C represents indirect sunlight at about 6774¡K.Later,the CIE added a series ofD illuminants,a hypothetical E illuminant,and aseries ofF illuminants.The D illuminants represent different daylight conditions,asmeasured by color temperature.Two such illuminantsÑ D D Ñare com-monly used as the standard illuminants for graphic arts viewing booths (Ò50ÓandÒ65Órefer to color temperatures 5000¡K and 6500¡K,respectively). These illuminants are represented in color calculations as spectral data.The spectralreflectance power oflight sourcesÑwhich are emissive objectsÑis really no differentthan the spectral data ofa reflective colored object.You can recognize the influenceofcertain colors in different types oflight sources by examining their relative powerdistribution as spectral curves. Color Communication Tristimulus color descriptions rely heavily on CIEstandard color systems and illumi-nants.Spectral color descriptions,on the other hand,do not directly require this addi-tional information.However,CIEstandards do play an important role in the conver-sion ofcolor information from tristimulus to spectral data.Next,letÕs explore furtherby examining the relationship between spectral data and tristimulus data. SPECTRAL DATA VS. TRISTIMULUS DATA We have examined the principle methods for describing color.These methods canbe separated into two distinct categories: ¥ There is spectral data ,which actually describes the surface properties ofthecolored object by demonstrating how the surface affects (reflects,transmits,or emits) light.Conditions such as lighting changes,the uniqueness ofeach humanviewer,and different rendering methods have no effect on these surface properties. ¥ And,there is tristimulus data ,which simply describes how the color ofthe objectappears to a viewer or sensor,or how the color would be reproduced on a device such as a monitor or printer,in terms ofthree coordinates or values.CIEsystems such as XYZand L*a*b*locate a color in a color space using three-dimensional coordinates;while color reproduction systems such as RGBand CMY(+K) describea color in terms ofthree values that can are mixed to generate the color.As a color specification and communication format,spectral data has some distinctadvantages over conventional tristimulus formats such as RGBand CMY(+K) values.Most importantly,spectral data is the only true description ofthe actual colored object.RGBand CMYK color descriptions,on the other hand,are dependent upon viewingconditionsÑon the type ofdevice that is rendering the color;and on the type oflighting under which the color is viewed. The Color Guide and Glossary As we discovered in our color space comparison,every color monitor has its ownrange (or gamut) ofcolors that it can generate using its RGB phosphorsÑevenmonitors made in the same year by the same manufacturer.The same goes for print-ers and their CMYK colorants,which in general have a more limited gamut than To precisely specify a color using RGB or CMYK values,you must also define thecharacteristics ofthe specific device where you intend the color to appear. As we discussed earlier,different illuminants such as incandescent light and daylighthave their own spectral characteristics.A colorÕs appearance is greatly affected bythese characteristicsÑthe same object will often appear differently under differenttypes oflighting. To precisely specify a color using tristimulus values,you must also define thecharacteristics ofthe illuminant under which you intend the color to appear. Device- and Illuminant- I Measured spectral data,on the other hand,is both device- and -independent : ¥ Spectral data measures the composition oflight reflected from an before it is interpreted by a viewer or device. Color Communication ¥ Different light sources appear differently when they are reflected froman object because they contain different amounts ofthe spectrum ateach wavelength.However,the object always absorbs and reflects the percentage ofeach wavelength,regardless ofamount.Spectral data is a measurement ofthis percentage So,the two components ofcolor that change with every viewing conditionÑthelight source and the viewer or deviceÑare Òbypassed,Óand the ever-stable propertiesofthe object surface are measured,instead.To accurately specify color,spectral datais all we needÑit is,in short,Òthe real thing.ÓOn the other hand,RGBand CMYKdescriptions are subject to ÒinterpretationÓby different viewers and devices. Detecting Metamerism Another advantage ofspectral data is its ability to predict the effects ofdifferent lightsources on an objectÕs appearance.As we discussed earlier,different light sources have their own compositions ofwavelengths,which in turn are affected by theobject in different ways.For example,have you ever matched a pair ofsocks andpants under fluorescent department store lighting,then later discover that they donot match as well under your homeÕs incandescent lighting? This phenomenon is metamerism The following example compares two shades ofgray that are a metameric match.Under daylight conditions,these grays appear to match quite well.However,underincandescent lighting,the first gray sample takes on a reddish cast.These changescan be demonstrated by plotting the spectral curves for the different grays and the The Color Guide and Glossary different illuminants,then comparing where their strongest reflectance power occursin relation to each other and to the visible spectrum wavelengths:When our samples are illuminated by daylight,the relationship between these twocolors is enhanced in the blue region (the highlighted area),where the curves areclose together.Incandescent light,on the other hand,distributes more reflectancepower in the red region,where the two sample curves happen to separate sharply.So,under cooler lighting the differences between the two samples are not so appar-ent;but the differences are quite apparent when viewed in warmer lighting.Our eye-sight can be fooled by these changes in lighting conditions.Because tristimulus datais illuminant-dependent,these formats cannot demonstrate the effects ofthesechanges.Only spectral data can clearly detect these characteristics. Color Measurement and Control Now that weÕve learned the fundamentals ofcolor and the different ways we cancommunicate color data,letÕs look at the ways we can this data.WeÕve alreadytouched on two instruments that measure colorÑ spectrophotometers colorime-ters .First,weÕll take a more detailed look at these instruments,along with a thirdcommonly used graphic arts instrument,the densitometer .Then,weÕll take a look atdifferent types ofcolor measurements and how they are used during specific phasesofthe digital imaging and graphic arts production workflow. INSTRUMENTATION We have discussed many scales for communicating and describing colorÑeitherby its primary color attributes,its perceptual attributes,or its actual spectral data.These models provide us with units ofmeasurement similar to ÒinchesÓandÒounces.ÓAll we need is a set ofÒrulersÓthat can measure a color in terms ofnumeric expressions such as CIE L*a*b*.Today,the most commonly used instru-ments for measuring color are densitometers , colorimeters ,and spectrophotometers . Gathering Color Measurements Color measurement instruments ÒreceiveÓcolor the same way our eyes receive color:by gathering and filtering the manipulated wavelengths oflight that are reflectedfrom an object.Earlier,we demonstrated how this combination oflight,object (inour case,a rose),and viewer caused us to perceive a ÒredÓrose.When an instrumentis the viewer,it ÒperceivesÓthe reflected wavelengths as a numeric value.The scopeand accuracy ofthese values depend on the measuring instrumentÑthey can beinterpreted as a simple density value by a densitometer;a tristimulus value by acolorimeter;or as spectral data by a spectrophotometer. Assigning Numeric Values to Colors Each type ofcolor measurement instrument does something that our eye cannotdo:assign a specific value to the color that can be consistently analyzed in termsofnumeric tolerances and control limits.Each instrument makes this conversiondifferently. ¥ Ofthese instruments,a densitometer is the most commonly used.A densitometer is a photo-electric device that simply measures andcomputes how much ofa known amount oflight is reflected fromÑortransmitted throughÑan object.It is a simple instrument used primari-ly in printing,pre-press,and photographic applications to determinethe strength ofa measured color. Color Measurement and Control Densitometers such as X-RiteÕsATD (above) and 361TR (left) simplyfrom or transmitted through the object D1.17 the press operator make necessary Auto TrackingDensitometer (ATD)361T Transmission The Color Guide and Glossary ¥ colorimeter also measures light,but it instead breaks the light downinto its RGB components (in a manner similar to that ofthe human eye,a monitor,or a scanner).A colorÕs numeric value is then determinedusing the CIE XYZ color space or one ofits derivatives,such as CIEL*a*b* or CIE L*u*v*.These measurements are visually interpreted ina color space graph. X-RiteÕs model 528 object. Using CIEXYZasCIEL*a*b* value isÒpinpointedÓas: L* 51.13 (2¡Standard Observer Color Measurement and Control ¥ spectrophotometer measures spectral dataÑthe amount oflightenergy reflected from an object at several intervals along the visiblespectrum.These measurements result in a complex data set ofreflectance values which are visually interpreted in the form ofaspectral curve.Because a spectrophotometer gathers such complete color information,this information can be translated into colorimetric or densitometricdata with just a few calculations.In short,a spectrophotometer is themost accurate,useful,and flexible instrument available. such as X-RiteÕs Digital The Color Guide and GlossaryMEASUREMENT APPLICATIONS IN THE GRAPHIC ARTS WORKFLOW Different types ofcolor measurement instruments are used in various stages ofthegraphic arts production workflow.A precise measurement program can ensureconsistent color results from initial ideas to the final printed pieceÑand all theexchanges from device-to-device in between.Different types ofmeasurement areappropriate for specific production stages.For example,spectral data is the bestmeasurement format for pinpoint color specifications;while simple density measure-ments are more appropriate for monitoring press sheet color bars over the course ofafour-color process press run.First,we should re-emphasize this important point:the typical RGBcolor space ismuch smaller than the range ofcolors that is visible to the human eye;and the CMYKprinting process can achieve an even smaller gamut.Also,lighting conditions and mate-rials such as colorants and substrates place additional limits on the gamut ofrepro-ducible color.Scanning and display technology continues to improve color bit depth andpush the capabilities ofRGB outward;and new printing technologies such as HiFi colorhave widened the process printing gamut.However,variations will always exist betweenoriginal natural colors,their reproduction via scanners and monitor display,and theirreproduction via different printing processes. Color measurement allows us to achieve the color production results: ¥ Minimal color variation between devices and production stages; ¥ these variations are predictable,and overall outputs are consistent;and ¥ any problematic color variations are quickly identified and corrected with little waste oftime or materials.Next,weÕll discuss how specific types ofcolor measurements can be applied to optimizecolor consistency and quality in some key stages ofthe production workflow: ¥ Specification (by client and content creator) ¥ Color Management (by content creator and service provider) ¥ Formulation (by ink supplier and printer) ¥ Control (by printer) ¥ Verification (by printer,client,and content creator)Note that this workflow is a full circleÑthe key is to present a finished product thatmatches the clientÕs original color specifications as closely as possible. Color Measurement and ControlCOLOR SPECIFICATION The most complete way to define a color is with spectral data.Now that technologicaladvances have made spectrophotometers widely available,spectral data is the logicalbest solution for describing,specifying,or identifying colors.Spectral measurementsare especially crucial for colors outside the traditional CMYK color descriptionÑsuchas out-of-gamut spot colors and HiFi process colors.Spectral descriptions remain thesame at any workflow stage because they are device-independent.In addition,RGB,CMYK,and custom ink formulations can be accurately derived from spectral data.X-RiteÕs DIGITALWATCHBOOKsystem allows you to Òpoint and clickÓits hand-heldspectrophotometer on a color sample,then instantly view the color on your computermonitor.The measured colorÕs spectral data is stored as a digital color.A collectionofmeasured colors can be saved in a Òpalette,Ówhich can then be imported intoother graphics programs such as Adobe Illustratorª.These palettes are also accessiblefrom Photoshopª via the Apple Color Picker.Beginning the color production work-flow with spectral descriptions means this precise,device-independent data can beutilized at other phases in the processÑat your service provider,by your client,andby your printer. part of ColorShopÕs digital Òpalette.Ó The Color Guide and Glossary Earlier we noted that there are as many RGBcolor spaces as there are monitors,andas many CMYKcolor spaces as there are printers.This situation creates a great dealofambiguity and guesswork for designers who create and proofcolors on theirdesktop devices.Scanned colors donÕt look the same when they are displayed on amonitor;on-screen colors do not match the printed proof;and the colors in imagefiles display and output differently at each production site(design studio,service bureau,printer). Color managementsystems (CMS) help solve these problems at the desktop level,and in turn provide solutions Òdownstream,Óas well.A color management system identifies the RGBand CMYKcolor spaces that are crucial to your workÑthose belongingto your scanner,monitor,and printer.Descriptions ofthesedevices are appropriately named profiles ,or also referred to as characterizations .Macintosh and Mac OS- compatible comput-ers provide a built-in frameworkÑcalled AppleColorSync for implementing and handling these device profiles.Colormeasurement instruments are used in conjunction with theCMS and CMS-supported software to gather the importantperformance data that comprises the device profiles,and toperiodically monitor and adjust the performance ofthe devices.Utilizing your CMS,CMS-compatible software utilities andPlug-Ins,and color measurement instrumentation,you canachieve desktop color consistency in two major stepsÑdevice calibration and device characterization . Device Calibration Device calibration is the first step in the desktop color management process.Yourmonitor and output device performance capabilities can change over timeÑphos-phor instability is a principle cause ofmonitor drift;and changes in colorants androom humidity can throw printer performance offcourse.Monitor and printercalibration procedures utilize different types ofdevices. Color Measurement and Control analyzed to determine where any performance drift has occurred.Your monitorÕsgamma,white and black point,and color balance are adjusted and correctedaccordingly.Finally,the software saves a monitor profile in the ColorSync Profilesfolder in your system folder.In addition to calibration,you can do some other things to ensure reliable monitorviewing:choose a neutral gray pattern for your on-screen Òdesktop;Óavoid locatingbrightly-colored artwork adjacent to your monitor;avoid locating your workstationnear windows or room lighting that is glaring or that changes frequently;and evenshield your monitor on top and at the sides with a cardboard ÒawningÓ.You can setyour brightness and contrast knobs at the desired levels before calibration. Monitor Optimizer Monitor calibration is most accurately achieved using a colorimeterÑsuch asX-RiteÕs Monitor Optimizer or model DTP92Ñand compatible calibration software.For example,the Monitor Optimizer sensor attaches directly to your monitor,positioned over a color target displayed on screen by the ColorShop MonitorCalibrator control panel.The target area flashes a series ofcolors,the instrumentmeasures each patch;then the software collects the measurement data.This data is The Color Guide and Glossary Output device calibration is typically achieved using a densitometer (or,increasingly using a colorimeter or spectrophotometer) and accompanying software.Calibration adjusts a deviceÕs output to correlate with the values requested by thesoftware.In the case ofa color printer,calibration ensures that the correct levels ofcyan,magenta,yellow,and black colorants are printed.A typical test image featuresrows ofpatchesÑone row for each color the device can print.Each rowÕs patchesrepresent different percentages,usually arranged in 5% or 10%increments fromsolid to zero coverage.In the case offilm imagesetters,on the other hand,outputvalues are verified for a single separation filmÕs tone values.These patches are measured to calculate the deviceÕs linearity Ñits ability to properlyimage the percentages assigned from the calibration software.An auto-scanningdensitometer such as X-RiteÕs model DTP32 makes these measurements fast andeasy by automatically scanning an entire row with one pass through the reading slot.The resulting measurements are communicated back to the software,where internaladjustments are made to the PostScript commands that control the color values sentto the output device. Device Characterization / Profiling Device characterization is the second step in the color management process,follow-ing device calibration.Characterization is the process ofactually creating deviceprofiles for your scanner,monitor,and printer.While many device manufacturers X-Rite DTP32 densitometer Color Measurement and Control IT8 Target for ship factory-generated,generic profiles on disk with their products, profilesthat you create for your specific devices are more accurate and reliable,and thereforewill yield better color results. Scanner characterization involves using a scanned test print or transparencysuch as an IT8 Target,and then running a scanner characterization utility program.The IT8 test pattern consists ofdozens ofdifferent color patches that represent auniform sampling ofthe CIE XYZor L*a*b*color space.The target comes with adata file containing the XYZvalues for each patch.The utility compares theseknown values to the scannerÕs device-dependent RGBrepresentation ofeach color.Any differences between the two values are calculated.From this data,the scannerÕscolor space can be determined.This unique color space information is saved as partofyour scannerÕs custom profile. Monitor characterization is accomplished using the same instrumentation (suchas Monitor Optimizer)and on-screen target sequence that is used for calibration.For characterization,the colorimetric data from the device is compared to themonitorÕs ability to render these colors,so the software can calculate how themonitorÕs color space relates to the XYZcolor space.This unique information isthe central component ofthe monitorÕs custom profile. The Color Guide and Glossary Printer characterization is similar to scanner characterization in that it measuresa test pattern to determine the deviceÕs range ofachievable colors.For printers,thetest pattern is a uniform sampling ofoverprinted CMYKtints that are imagedusing the output device.Software for printer characterization uses a test image with 500 or more coloredpatches.This image is output to the printer.The patches are then measured,and theresulting colorimetric data is calculated into color space information for that specificprinter,as it relates to the CIEXYZ or CIELABcolor space.This informationbecomes the central component in the printerÕs custom profile.Because characterization is concerned with the printerÕs ability to render a rangeofdifferent process colorsÑnot specific colorant densitiesÑa colorimeter orspectrophotometer must be used to gather the measurements (X-RiteÕs DigitalSwatchbook spectrophotometer,or DTP41 Auto Scan Spectrophotometer,for example). Proofing system and press characterizations designers accurately predict the way colors will reproduce at later stages ofthe production process.Service bureaus and printers who utilize color measure-ment and management systems can consider supplying clients with custom pro-files oftheir output devices.Knowing the capabilities ofall the output devices in theworkflow can further enhance your ability to make important color control decisionsduring the desktop design stage ofproduction.Achieving color control early in theprocess can save review cycle time and wasted materials downstream. X-Rite DTP41Auto ScanSpectrophotometer ColorMeasurement and Control A device color space is ÒconstructedÓbased on its ability to scan,display,or renderdifferent points in the CIExy chromaticity chart.Most target patches represent various at maximum saturation Ñthe first two color space dimensions (recall ourdiscussion ofhue,saturation,and lightness in the previous chapter).Various tintsofblack and the primary colors are also included to determine the deviceÕs capabilitiesfor rendering different levels of lightness ,as well.The characterization software ÒknowsÓthe device-independent values ofthe targetÕspatches,which represent a device gamut.These known color values are compared tothe deviceÕs actual,measured performance.The amount ofdifference at each point isdetermined,and the measured points are ÒmappedÓin relation to the known points.The resulting information provides the characterization software with a detaileddescription ofthe deviceÕs unique capabilities. Profile-generating systems store device profiles in a specific location in your operatingsystem software.Programs that utilize device profilesÑsuch as the ColorShop software,Adobe¨ Illustratorª,Adobe¨ PageMakerª,Adobe¨ FreeHandª,Adobe¨ PhotoShopª,QuarkXPress¨ Ñ allow you to activate the desired device profiles from the storagelocation via menus within the programÕs operating environment. The Color Guide and GlossaryHow Color Management Systems Works The diagram on the previous page showing smaller RGBand CMYKcolor spaces ÒmappedÓinside the xygamut demonstrates the process of gamut compression .This process happens frequently when we move colorsthrough the production process:our original scene contains colors thatare not captured on photographic film;some colors in the photograph arenot within the scannerÕs color space,or gamut ;and still more colors are lost orreplaced when the scan is displayed in a monitorÕs gamut.By the time our image isprinted on proofing devices and on press,its original gamut has beencompressed considerably.At each stage,out-of-gamut colors are replaced withthe nearest approximate achievable colors.For example,Apple ColorSync helps you keep gamut compression predictableand under control.It utilizes your peripheralsÕprofile information to calculate aÒcommon groundÓcolor space within the framework ofCIEXYZ.When you useyour profiled peripherals in conjunction with ColorSync,you work only with colorsthat are in the device color space areas that Òoverlap.ÓWithin this area,color spaceinformation can be easily translated from one device color space to the next.Forexample,you can more accurately predict your output colors based on what you seeon your monitor. RGBCMYK converts scanner,monitor, and printertion into CIEXYZUsing CIEXYZas a RGBvalues that more your printerÕs specific Color Measurement and ControlCOLOR FORMULATION Custom formulation for special spot colors is based on spectrophotometricmeasurements ofvarious ink and paper combinations.This is typically doneby the ink manufacturer.Now,technological advances in measurement instrumentation and software have brought ink formulation to theprinting site,where the actual production paper can be calculatedinto a custom ink formula that will match the customerÕs specifications.These affordable solutions,such as X-RiteÕs QuickInk system,utilize suppliedspectral data,specification from existing color guides,or measurement ofthe actual sample or swatch. COLOR CONTROL Color controlÑor process control Ñis critical to achieving consistent,quality colorthroughout an entire print job,across different shifts,between printing pressoperators,or between batches ofmaterials.In any printing or imaging application,color can vary on a single printed page,and from one page to the next.Measurement information can be used to control these color variations.For example,densitometers are used to read color bars ,which are basically smallversions oftest forms that are printed in unused areas ofthe printed page.Generally,color bars provide sample patches (ofsolid inks,tints,overprints,and specialpatterns) to test critical print characteristics.Calculations such as density,dot area,dot gain,print contrast,and apparent trap allow press operators to troubleshoot on-press color problems.Comparing color bar measurements between printed sheetsclearly identifies any changes in printing characteristics. softwareformulatescustom inksto match measuredcolor data The Color Guide and Glossary These densitometric measurements indicate how the press is performing at thattime.By comparing measurements ofseveral press sheet color bars at variousintervals during the press run,the press operator can: ¥ monitor overall press performance over time; ¥ monitor the performance ofthe individual ink keys over time;and ¥ document print quality for clients.Measurements are analyzed in relation to control limits that have been establishedfor the press.Any measurement data that is not within acceptable range ofthe con-trol limits indicates a possible problem with the process or equipment.Having thisinformation close at hand allows operators to quickly pinpoint problem areas andmake fast,seamless adjustments to press settings with minimal waste ofmaterials. X-RiteÕs Auto TrackingSpectrophotometer (ATS)intervals throughout thepanying ATSsoftware TodayÕs newest printing technologies such as HiFi color can often be monitored andcontrolled more effectively with colorimetric or spectral measurement.HiFi printingapplications that use CMYK+RGB,or custom touch-plate or bump colors are espe-cially well-suited to process control using these tools,such as X-RiteÕs model 938hand-held spectrophotometer,or the ATSSystem.As the achievable gamut ofHiFicolor printing expands,spectral data will play an increasing role in controlling HiFiÕsexpanded palette ofachievable process colors. Color Measurement and ControlControl Limits As we mentioned earlier,any press run will vary in its color output from sheet-to-sheet,from start to finish.Some variation is normal and acceptable.Control limitsare established to ensure that the press runÕs variation remains normal and accept-able.They are similar to the lines on either side ofa street laneÑsome variationwithin the lines is acceptable,as drivers typically make subtle steering adjustments.Problems can occur,however,ifthe vehicleÑor the pressÕperformanceÑsuddenlyveers beyond the lines.Control limits are most commonly monitored using frequent densitometricmeasurements taken from press sheet color bars.For example,the Auto TrackingSpectrophotometer system features an accompanying software package that displaysthe measurement data in graphical formats showing press performance trends overtime.These linear graphs quickly identify any ink density measurements that aremuch stronger or weaker than acceptable. the ATSsoftware COLOR VERIFICATION Another key benefit ofcolor measurement is the ability to monitor color accuracyat each step ofthe reproduction workflow,and ultimately verify that customerspecifications have been achieved as closely as possible.Verifying that the actual ink colors are correctÑespecially non-process ink colorsÑrequires the capabilities ofa colorimeter or spectrophotometer (a densitometer canalso be used on these special colors,but typically only to measure strength).Becausespectrophotometers can function as densitometers and colorimeters,they are themost logical and versatile method for controlling and verifying the quality ofcolor reproduction. The Color Guide and Glossary Color Tolerances Verification between color specifications and actual color results is achieved by usingtolerances that are based on numeric color measurement data.Color tolerancinginvolves comparing the measurements ofseveral color samples (the color output)tothe data ofa known color standard (the specification or input).Then,the ÒclosenessÓofthe samples to the standard is determined.Ifa sampleÕs measured data is not closeenough to the desired standard values,it is unacceptable and adjustments to theprocess or equipment may be required.(While control limits and color tolerances are separate considerations,the product-tion workflow and print job should be set up with both parameters in mind.Ingeneral,a project should never have customer specifications that cannot be achievedwithin the printerÕs control limits.)The amount ofÒclosenessÓbetween two colors can be calculated using a variety ofcolor tolerancing methods.These methods calculate the ÒdistanceÓbetween two setsofmeasurement coordinates within a three-dimensional color space such as The most common methods are CIELABand CMC. CIELAB Tolerancing Method CIELABcalculations are based on the L*a*b*color space we examined earlier.UsingCIELAB,the standard colorÑor original specificationÑis pinpointed by its measure-ment data in the L*a*b*color space.Then,a theoretical Òtolerance sphereÓis plottedaround the color.This sphere represents the acceptable amount ofdifference betweenthe standard and other measured samples (the color output).Data that falls within thetolerance sphere represents an acceptable color.Measurements that fall outside thetolerance sphere are unacceptable. Color Measurement and Control The size ofthe tolerance sphere is determined by customerÕs specifications for acceptablecolor difference,which is expressed in delta () units such as E (delta error).A typicalcustomer tolerance in the graphic arts industry usually lies between 2 and 6 E.Thismeans,for example,that samples outside the tolerance box lie more than 6 units awayfrom the standard.Tolerances ofless than 2 units are typically unachievable given nor-mal process variation,while a high tolerance could result in visible mismatches betweenspecifications and results (highly dependent on the image).Differences between colors inan image that are within 4 units ofeach other often are not visible to most viewers. Elliptical Tolerancing Methods Our eyes accept color matches inside elliptical regions,as opposed to the ÒsphericalÓregions used in the CIELAB tolerancing method.For this reason,the CIELAB method canoften provide misleading results.For example,an ÒacceptableÓcolor that falls within aCIELAB tolerance might actually lie outside the elliptical region ofacceptability. The CMC and CIE94 tolerancing methods directly addresses our ÒellipticalÓperceptionofcolor difference,and therefore is regarded in many industries as a more logical andaccurate tolerancing system than CIELAB.A similar color difference calculation calledCIE94 is growing in popularity and also uses ellipsoids.CMC and CIE94 are not new color spacesÑthey are simply tolerancing systems that arebased on the L*a*b* color space.The calculations mathematically define an ellipsoidaround a standard color in the color space.This ellipsoid consists ofa semi-axis that cor-responds to the attributes ofhue,chroma,and lightness.It represents the area ofaccep-tance in relation to the standard,the same way the CIELAB ÒsphereÓdefines acceptabledifference limits.In CMC and CIE94,the size ofthe ellipsoid varies depending on itsposition in the color spaceÑfor example,in the orange region,ellipsoids are narrower,while in the green region,ellipsoids are wider.Also,ellipsoids in high-chroma regions are The Color Guide and Glossary larger than those in low-chroma regions. SUMMARY This ColorGuide has introduced you to the subjects ofcolor communication,measure-ment,and control in a format that we hope has been clear and interesting.Behind eachconcept and process we briefly covered in this book,there is much additional informa-tion and technical data that can be added to your knowledge ofcolor production.However,the information you have learned in this booklet will help you get started inthe world ofcolor measurement and control,by providing a basic explanation ofcolorscience and theory,the different tools used to measure color,and the different stages ofthe production process where color measurement is important.With this knowledge inhand,we recommend that you continue your studies by reading the excellent literaturelisted in our bibliography inside the back cover.The key point we want you to remember is this:Ifyou can measure color,you can control color.Without measurement,describing and verifying color can be ambiguous and unre-liable.With numerical measurement data,however,colors can be described and verifiedwith precision and confidence. Glossary Glossary Dissipation ofthe energy ofelectromagnetic waves into otherforms as a result ofits interaction with matter;a decrease in directional transmittance ofincident radiation,resulting in a modification or conversion ofthe absorbed energy. Additive Primaries: Red,green,and blue light.When all three additive primaries are combined at 100% intensity,white light is produced.When these three are combined at varying intensities,a gamut ofdifferent colors is pro-duced.Combining two primaries at 100% produces a subtractive primary,either cyan,magenta,or yellow:100% red + 100% green = yellow;100% red + 100% blue = magenta;100% green + 100% blue = cyan Subtractive Primaries. Manifestation ofthe nature ofobjects and materials through visualattributes such as size,shape,color,texture,glossiness,transparency,opacity,etc. Distinguishing characteristic ofa sensation,perception or mode ofappearance.Colors are often described by their attributes ofhue,saturation or chroma,and lightness. B The absence ofall reflected light;the color that is produced when an objectabsorbs all wavelengths from the light source.When 100% cyan,magenta,and yellow colorants are combined,the resulting color-theoretically-is black.In real-world applications,this combination produces a muddy grayor brown.In four-color process printing,black is one ofthe process inks.The letter ÒKÓis used to represent Black in the CMYK acronym to avoid confusion withBlueÕs ÒBÓin RGB. The attribute ofvisual perception in accordance with which an areaappears to emit or reflect more or less light (this attribute ofcolor is used in the colormodel HSBÑHue,Saturation,Brightness).See Lightness . C To check,adjust,or systematically standardize the graduations ofa device. Chroma: The attribute ofvisual perception in accordance with which an area appearssaturated with a particular color or hue-for example,a red apple is high in chroma;pastelcolors are low in chroma;black,white,and gray have no chroma (this attribute ofcolor isused in the color model L*C*HÑLightness,Chroma,Hue).Also referred to as Saturation . The Color Guide and GlossaryChromaticity, Chromaticity Coordinates: Dimensions ofa color stimulusexpressed in terms ofhue and saturation,or redness-greenness and yellowness-blueness,excluding the luminous intensity.Generally expressed as a point in a plane ofconstantluminance.See CIE xy Chromaticity Diagram . CIE (Commission Internationale de lÕEclairage): A French name thattranslates to International Commission on Illumination,the main international organizationconcerned with color and color measurement. The CIE94 tolerancing method utilizes three-dimensional ellipsoids as ÒcontainersÓfor color acceptance.CIE94 is conceptually similar to CMC2:1 but lacks some ofthe hueand lightness adjustments.It is expected that CIE94 will evolve over the next few years asadditional studies are performed. CIELAB (or CIE L*a*b*, CIE Lab): Color space in which values L*,a*,and b* areplotted at right angles to one another to form a three-dimensional coordinate system.Equaldistances in the space approximately represent equal color differences.Value L* representsLightness;value a* represents the Redness/Greenness axis;and value b* represents theyellowness/blueness axis.CIELAB is a popular color space for use in measuring reflectiveand transmissive objects. CIE Standard Illuminants: Known spectral data established by the CIE for fourdifferent types oflight sources.When using tristimulus data to describe a color,theilluminant must also be defined.These standard illuminants are used in place ofactual measurements ofthe light source. CIE Standard Observer: A hypothetical observer having the tristimulus color-mixturedata recommended in 1931 by the CIE for a 2¡viewing angle.A supplementary observer for a larg-er angle of10¡was adopted in 1964.Ifnot specified,the 2¡Standard Observer should be assumed.Ifthe field ofview is larger than 4¡,the 10¡Standard Observer should be used. CIE xy Chromaticity Diagram: A two-dimensional graph ofthe chromaticitycoordinates,x as the abscissa and y as the ordinate,which shows the spectrum locus(chromaticity coordinates ofmonochromatic light,380-770nm).It has many usefulproperties for comparing colors ofboth luminous and non-luminous materials. CIE Tristimulus Values: Amounts ofthe three components necessary in a three-coloradditive mixture required for matching a color:in the CIE System,they are designated as X,Y,and Z.The illuminant and standard observer color matching functions used must bedesignated;ifthey are not,the assumption is made that the values are for the 1931 CIE2¡Standard Observer and Illuminant C. CIE Chromaticity Coordinates: x and y values that specify the location ofa colorwithin the CIE chromaticity diagram. CMC (Color Measurement Committee): Ofthe Society ofDyes and Colouristsin Great Britain.Developed a more logical,ellipse-based equation for computing as an alternative to the spherical regions ofthe CIELAB color space. CMY: The subtractive primaries cyan,magenta,and yellow.See Subtractive Primaries . Color Management: Matching colors between an original image,scanner,monitor,color printer and final press sheet. Glossary Relative amounts ofthree additive primaries requiredto match each wavelength oflight.The term is generally used to refer to the CIE StandardObserver color matching functions designated.See CIE Standard Observer . Color Model: A color measurement scale or system that numerically specifies theperceived attributes ofcolor.Used in computer graphics applications and by colormeasurement instruments. Color Separation: The conversion ofthe red,green,and blue color informationused in a computer into cyan,magenta,yellow,and black channels that are used to makeprinting plates. Color Space: A three-dimensional geometric representation ofthe colors that can beseen and/or generated using a certain color model. Color Specification: Tristimulus values,chromaticity coordinates and luminancevalue,or other color-scale values,used to designate a color numerically in a specifiedcolor system. Color Temperature: A measurement ofthe color oflight radiated by an object whileit is being heated.This measurement is expressed in terms ofabsolute scale,or degreesKelvin.Lower Kelvin temperatures such as 2400¡K are red;higher temperatures such as9300¡K are blue.Neutral temperature is gray,at 6504¡K. Color Wheel: The visible spectrumÕs continuum ofcolors arranged into a circle,wherecomplementary colors such as red and green are located directly across from each other. Materials used to create colorsÐdyes,pigments,toners,phosphors. Built-in color management architecture for Apple Macintosh computers.Third-party vendors utilize the ColorSync framework to provide device calibration,devicecharacterization,and device profile-building methods. An optical measurement instrument that responds to color in a mannersimilar to the human eyeÐby filtering reflected light into its dominant regions ofred,green, Ofor relating to values giving the amounts ofthree colored lights orreceptorsÐred,green,and blue. The level ofvariation between light and dark areas in an image. Control Limits: The amount ofacceptable variation in press capabilities over thecourse ofa press run. One ofthe process ink colors for printing.Pure cyan is the ÒredlessÓcolor;itabsorbs all red wavelengths oflight and reflects all blue and green wavelengths. D D : The CIE Standard Illuminant that represents a color temperature of5000¡K.This is the colortemperature that is most widely used in graphic arts industry viewing booths.See Illuminants D . D : The CIE Standard Illuminant that represents a color temperature of6504¡K. The Color Guide and Glossary A symbol used to indicate deviation or difference. Delta Error ( In color tolerancing,the symbol E is used to express Delta Error,thetotal color difference computed using a color difference equation.The color difference is generally calculated as the square root ofthe combined squares ofthechromaticity differences,b*,and the Lightness difference,L.See A sensitive,photoelectric instrument that measures the density ofimages or colors. The ability ofa material to absorb lightÐthe darker it is,the higher the density. Describes a color space that can be defined only by using infor-mation on the color-rendering capabilities ofa specific device.For example,the RGB colorspace must be generated by a monitor,a device which has specific capabilities and limitationsfor achieving its gamut ofcolors.In addition,all monitors have different capabilities andlimitations,as do different scanners,printers,and printing presses. Describes a color space that can be defined using the full gamutofhuman vision,as defined by a standard observer,independent ofthe color-renderingcapabilities ofany specific device. Device Profile: Device-specific color information that is a characterization ofa deviceÕscolor rendering and reproduction capabilities.Monitor profiles,scanner profiles,and printerprofiles are utilized in a color management system such as Apple ColorSync to help thedevices communicate color information with each other.Profiles are created by calibrationand/or characterization method. A soluble colorant;as opposed to pigment,which is insoluble. Dynamic Range: An instruments range ofmeasurable values,form the lowest amountit can detect to the highest amount it can handle. E Electromagnetic Spectrum: The massive band ofelectromagnetic waves thatpass through the air in different sizes,as measured by wavelength.Different wavelengthshave different properties,but most are invisibleÐand some completely undetectableÐtohuman beings.Only wavelengths that are between 380 and 720 nanometers in size arevisible,producing light.Invisible waves outside the visible spectrum include gamma raysx-rays,microwaves and radio waves. Emissive Object: An object that emits light.Usually some sort ofchemical reaction,such as the burning gasses ofthe sun or the heated filament ofa light bulb. F Fluorescent Lamp: A glass tube filled with mercury gas and coated on its innersurface with phosphors.When the gas is charged with an electrical current,radiation isproduced which in turn energizes the phosphors,causing the phosphors to glow. Four-Color Process: Depositing combinations ofthe subtractive primaries cyan,magenta,yellow,and black on paper to achieve .These colorants are deposited as dots ofdifferent sizes,shapes,and angles to create the illusion ofdifferent colors.See CMY,Subtractive Primaries . Glossary G The range ofdifferent colors that can be interpreted by a color model orgenerated by a specific device. Gamut Compression: Or tonal range compression.The color space coordinates ofa color space with a larger gamut are reduced to accommodate the smaller gamut ofadestination color space.For example,the gamut ofphotographic film is compressed forrepresentation in the smaller CMYK gamut used for four-color process printing.See . Gamut Mapping: Converting the coordinates oftwo or more color spaces into acommon color space.Often results in tonal range compression.See Gamut Compression . H - I HiFi Printing: Process printing that expands the conventional four-color processgamut using additional,special ink colors. The basic color ofan object,such as Òred,ÓÒgreen,ÓÒpurple,Óetc.Defined by itsangular position in a cylindrical color space,or on a Color Wheel. ICC (International Color Consortium): A group ofhardware and softwarecompanies dedicated to the development ofa specification that is OS independent andprovides the digital imaging,printing and related industries with a data format fordefining the color and reproduction characteristics ofdevices and their related media. Incident luminous energy specified by its spectral distribution. Illuminant A (CIE): CIE Standard Illuminant for incandescent illumination,yellow-orange in color,with a correlated color temperature of2856¡K. Illuminant C (CIE): CIE Standard Illuminant for tungsten illumination that simulatesaverage daylight,bluish in color,with a correlated color temperature of6774¡K. Illuminants D (CIE): CIE Standard Illuminants for daylight,based on actual spec-tral measurements ofdaylight.D65 with a correlated color temperature of6504¡K ismost commonly used.Others include D50,D55,and D75. Illuminants F (CIE): CIE Standard Illuminant for fluorescent illumination.F2 repre-sents a cool white fluorescent lamp (4200 K),F7 represents a broad-band daylight fluorescentlamp (6500 K),and F11 represents a narrow-band white flourescent lamp (4000 K). Saturation or reflective energy as related to visible wavelengths oflight.Reflectance ofwavelengths at high intensity generates high saturation,or chroma. Series oftest targets and tools for color characterization established by ANSI(American National Standards Institute)Committee IT8 for Digital Data ExchangeStandards.Different IT8 targets are used to characterize different devices such asscanners and printers. K - L Kelvin (K): Unit ofmeasurement for color temperature.The Kelvin scale starts fromabsolute zero,which is -273¡Celsius. The Color Guide and Glossary A color space that is similar to CIELAB,except uses cylindrical coordinates oflightness,chroma,and hue angle instead ofrectangular coordinates. Electromagnetic radiation in the spectral range detectable by the human eye(approx.380 to 720nm). The attribute ofvisual perception in accordance with which an area appearsto emit or reflect more or less light.Also refers to the perception by which white objects aredistinguished from gray objects and light-from dark-colored objects. M One ofthe process ink colors for printing.Pure magenta is the ÒgreenlessÓcolor;it absorbs all wavelengths ofgreen from light and reflects all red and blue wavelengths. Metamerism, Metameric Pair: The phenomenon where two colors appear tomatch under on light source,yet do not match under a different light source.Two suchcolors are called a metameric pair. Monitor RGB: Same as RGB;monitor RGB simply refers specifically to the color space thatcan be achieved by a particular monitor using combinations ofred,green,and blue light. Munsell Color Charts: Athree-dimensional color system developed by AlbertMunsell that is based on the attributes Munsell Hue,Munsell Value,and Munsell Chroma. N - O - P Nanometer (nm): Unit oflength equal to 10meter,or one millionth ofa milli-meter.Wavelengths are measured in nanometers. On a press sheet color bar,overprints are color patches where two processinks have been printed,one atop the other.Checking the density ofthese patches allowspress operators to determine trap value.The term Overprint also applies to any objectprinted on top ofother colors. Materials that emit light when irradiated by cathode rays,or when placedin an electric field.The quantity ofvisible light is proportional to the amount ofexcitationenergy present. Pertaining to the electrical effects oflight or other radiationÐforexample,emission ofelectrons. Photoreceptor: The cone- and rod-shaped nuerons that cover the retina ofthe eye.Photoreceptors are excited by visible wavelengths,then send signals to the brain where thesensation ofcolor is perceived. An insoluble colorant;as opposed to a dye,which is soluble. A tiny picture element that contains red,green,and blue information for colorrendering on a monitor or a scanner.When generating colors,pixels are similar to dots ofink on paper.A monitor resolution description in terms ofpixels-per-inch (ppi) is similarto a printer resolution description in terms ofdots-per-inch (dpi). GlossaryPrimary Colors: The dominant regions ofthe visible spectrum:red,green,and blue;andtheir opposite colors cyan,magenta,and yellow. Additive Primaries,Subtractive Primaries . Triangular-shaped glass or other transparent material.When light is passedthrough a prism,its wavelengths refract into a rainbow ofcolors.This demonstrates thatlight is composed ofcolor,and indicates the arrangement ofcolors in the visible spectrum. Visible Spectrum . Process Control: Using densitometric and colorimetric measurement data frompress sheet color bars to monitor press performance throughout the press run.Data is ana-lyzed in relation to established control limits.See Control Limits R Reflective Object: A solid object that returns some or all ofthe wavelengths oflightthat strike its surface.A reflective object that returns 100% ofall light is called a perfectdiffuser-a perfectly white surface. The percentage oflight that is reflected from an object.Spectrophotometersmeasure an objectÕs reflectance at various intervals along the visible spectrum to determine theobject colorÕs spectral curve.See Spectral Curve , Spectral Data . The additive primaries red,green,and blue.See Additive Primaries . S The attribute ofcolor perception that expresses the amount ofdeparturefrom the neutral gray ofthe same lightness.Also referred to as chroma. The order in which inks are deposited on paper by a printing press. Spectral Curve: A colorÕs ÒfingerprintÓÑ a visual representation ofa colorÕs spectraldata.A spectral curve is plotted on a grid comprised ofa vertical axis-the level ofreflectanceintensity;and a horizontal axis-the visible spectrum ofwavelengths.The percentage ofreflectedlight is plotted at each interval,resulting in points that form a curve. Spectral Data: The most precise description ofthe color ofan object.An objectÕscolor appearance results from light being changed by an object and reflected to a viewer.Spectral data is a description ofhow the reflected light was changed.The percentage ofreflected light is measured at several intervals across its spectrum ofwavelengths.Thisinformation can be visually represented as a spectral curve. Spectrophotometer: An instrument that measures the characteristics oflightreflected from or transmitted through an object,which is interpreted as spectral data. Spatial arrangement ofelectromagnetic energy in order ofwavelength size. Electromagnetic Spectrum,Visible Spectrum Standard: An established,approved reference against which instrument measurementsofsamples are evaluated. Subtractive Primaries: Cyan,Magenta,and Yellow.Theoretically,when all threesubtractive primaries are combined at 100% on white paper,black is produced.When The Color Guide and Glossary three are combined at varying intensities,a gamut ofdifferent colors is produced.Combiningtwo primaries at 100% produces an additive primary,either red,green,or blue:100% cyan + 100% magenta = blue;100% cyan + 100% yellow = green;100% magenta + 100% yellow = red T Tolerance: The amount ofacceptable difference between a known correct standard(usually the customerÕs specifications) and a set ofmeasured samples.See Delta Error Transmissive Object: An object that allows light to pass through from one side tothe other.The color ofa transmissive object results from the manipulation ofwavelengthsoflight as they pass through. Tristimulus: A method for communicating or generating a color using three stimuliÐeither additive or subtractive colorants (such as RGB or CMY),or three attributes (such aslightness,chroma,and hue). Tristimulus Data: The three tristimulus values that combine to define or generate aspecific color,such as R 255/G 255/B 0.Tristimulus data does not completely describe acolorÐthe illuminant must also be defined.Also,in device-dependent color models such asRGB,the capabilities ofthe viewer or color-rendering device must also be defined.See Device-Dependent . V - W - X - Y Viewing Booth: A enclosed area with controlled lighting that is used in graphic artsstudios,service bureaus,and printing companies as a stable environment for evaluatingproofs and press sheets.Viewing booths are generally illuminated using graphic arts indus-try-standard D65 lighting,and are surfaced in neutral gray colors.See . Visible Spectrum: The region ofthe electromagnetic spectrum between 380 and720 nanometers.Wavelengths inside this span create the sensation ofcolor when they areviewed by the human eye.The shorter wavelengths create the sensation ofviolets,purples,and blues;the longer wavelengths create the sensation oforanges and reds. Wave: A physical activity that rises and then falls periodically as it travels through Wavelength: Light is made up ofelectromagnetic waves;wavelength is the crest(peak)-to-crest distance between two adjacent waves. White Light: Theoretically,light that emits all wavelengths ofthe visible spectrumat uniform intensity.In reality,most light sources cannot achieve such perfection. Yellow: One ofthe process ink colors for printing.Pure yellow is the ÒbluelessÓcolor;it absorbs all wavelengths ofblue from light and reflects all red and green wavelengths. Billmeyer,Fred W.Jr.,and Max Saltzman 1982. Principles ofColorTechnology .Second Edition.Chichester,England:John Wiley & Sons.Hunt,R.W.G.1991. Measuring Colour .Second Edition.Chichester,England:Ellis Horwood.Jackson,Richard,Ken Freeman,and Lindsay MacDonald 1994. ComputerGenerated Colour .First Edition.Chichester,England:John Wiley & Sons.Kieran,Michael 1994. Understanding Desktop Color .First Edition.Berkeley,California:Peachpit Press.Molla,R.K.1988. Electronic Color Separation .Montgomery,WV:R.K.Printing and Publishing.Southworth,Miles,Thad McIlroy,and Donna Southworth 1992. TheColor Resource Complete Color Glossary .Livonia,New York:The ColorResource. www.x-rite.com X-Rite, Incorporated Ð World Headquarters 3100 44th Street S.W. ¥ Grandville, Michigan 49418 USA¥ (616) 534-7663 ¥ (888) 826-3059 ¥ FAX (616) 534-8960 X-Rite Ltd. Lower Washford Mill ¥ Mill Street Buglawton ¥ Congleton Cheshire ¥ England CW12 2AD ¥ (44) 1260-279988 ¥ FAX (44) 1260-270696 X-Rite MŽditerranŽe X-Rite Asia Pacific Ltd. 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