UnderstandingColor Management

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CO LO R M A N AG E M E N T

Why do we need colormanagement? Why dowe have a problem withmatching and control-

ling colors in digital imaging? Why can’twe just scan a picture, look at it on thescreen, and print it out and have thecolor match throughout? The answer isthat every imaging device has its ownparticular characteristics. Color imagingwill only produce accurate results if wetake into account the personal charac-teristics of each device.

Each scanner or digital camera cap-tures the same scene slightly differently.Each printer prints the same digital filein a different way. We need to quantifyand compensate for device variability.Included in this variability is the issueof color gamut. No device can repro-duce all colors.

Device CharacteristicsScanner Variability: An image from ascanner will generally be an RGB imagein which each pixel in the image isspecified by three numbers corre-sponding to red, green, and blue. If youuse different scanners to scan the samesample, you will get different results.

Printer Variability: Device variabilityalso occurs when we print an image. Ifyou sent the CMYK image to threeprinters, each device would receivethese CMYK instructions, which tell theprinter to drop varying amounts ofcyan, magenta, yellow, and black inkon the paper. Because of the differ-ences in the printing systems printedresults can be dramatically different.

There is also variation among moni-tors. Thus, we see that there is vari-ability in every part of the imagingchain. To get accurate color, we need toknow about the device being used toscan, print or display an image. Color iscontrolled by knowing about thebehavior/characteristics of a device.

Two Methods of Color ControlTwo ways to make allowances fordevice characteristics are the old way,which is closed-loop color, and the newway, which is known as open-loopcolor—color management.

Closed-Loop Color: Digital color oncewas the preserve of high-end systems.The same manufacturer would sell asystem that included the monitor, soft-ware, scanner, output, and so on. The

Every imaging device has its own “personality,”the color imaging will only produce accurateresults if we take into account the personalcharacteristics of each device.BY ABHAY SHARMA

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image was acquired from one scanner, it was dis-played on the same monitor, and images were des-tined for one type of print process. In this controlledsituation, it was relatively easy to obtain the color wewanted. However, two important conditions had tobe met: skilled personnel and a fixed workflow. Asthe conditions for a closed-loop system disintegrated,something had to be done to get consistent, accu-rate color. The answer is an open-loop environment.

Open-Loop Color Management: Modern-day colormanagement uses a central hub system to commu-nicate color between devices, as shown in the aboveillustration. In today’s workflows, images come froma number of places, are viewed on different displays,and are printed on different printer technologies.Instead of connecting every device to every otherdevice, a color management system connects alldevices into and out of a central connection space—the profile connection space (PCS). This illustrationis the single most important diagram to understandin color management. It provides the conceptualbasis for all color management.

In color management, every device must have aprofile. A golden rule of color management is image+ profile. In a typical workflow, images from a scannermay be “brought into” the profile connection spaceusing the scanner profile. To print the image it wouldbe “sent out” from the connection space to a printerusing a printer profile. Thus, color management oper-

ations require a source and a destination profile—weneed to know where the image is coming from andwhere it is going.

In color management it is easy to add a new deviceto the workflow. All that is needed for a new deviceis a single profile that connects the device (and itsimages) to the central space. A profile describes thecharacteristics of a device, thus as soon as a deviceand its profile are available to the network, the char-acteristics of the device become known to all otherdevices in the system.

Color SpacesColor specification falls into two main categories—device-dependent color and device-independent color.

Device-Dependent Color: RGB values from a scannerare not a universal truth but are dependent onwhich scanner was used to scan the original. Suchvalues (including CMYK values for printers) areknown as device-dependent color values becausethe colors you obtain are dependent on the devicebeing used. There is nothing wrong with specifyinga pixel value in terms of RGB or CMYK. As scannersuse red, green and blue filters, they truly generateRGB pixel values when they scan an image. Simi-larly CMYK instructions are appropriate for mostprinters. RGB and CMYK are instructions for adevice and are necessarily in units that the devicecan understand and use.

Profileconnection

space

A color management systemconnects all devices into andout of a central connectionspace—the profile connectionspace—to communicate colorbetween devices.

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Device-Independent Color: If RGB and CMYK are notreliable descriptors of color, what is? CIE (Interna-tional Commission on Illumination) systems are reli-able measurement systems. The CIE organization hasover the years specified a number of color measure-ment systems. CIE systems use a measuring instru-ment to sample a color and produce a numeric result.The measuring instrument, for example, does notneed to know about the printer that produced thesample, it just measures a color patch. Thus CIE mea-surements are independent of any particular printer,scanner, or monitor, and we can say they are device-independent measurements. CIE systems are stan-dardized and dependable systems, so when a coloris specified by one of these systems it means the samething to any user anywhere.

CIE systems form the basis for color management.Two CIE systems popular in color management areshown above. The LAB system is used in profile gen-eration and is also used in Photoshop. The Yxysystem is often used to analyze and compare thecolor gamut of different devices. The CIE LAB(L*a*b*) system specifies a color by its position in a3D color space. The coordinate L* stands for light-ness, a* represents the position of the color on a red-green axis, and b* represents the position of thecolor on a yellow-blue axis. In the Yxy system, a coloris specified by its x and y coordinates on a graphcalled the chromaticity diagram.

LAB and Yxy are device-independent color metricsthat are not related to any specific device. A CIE valuecan more accurately be thought of as a description or

specification of a color as viewed by a human observer.CIE systems (especially LAB) are important ways tomeasure and specify color and are used implicitly andexplicitly in many color management operations.

Internal Color ScaleBecause scanners produce different RGB values, eachscanner has its own internal color scale. Thus, thepixel values are specified according to a particularscanner. We cannot simply hand these numbers onto a printer without providing some informationabout the scale on which they are based. If we simplypass on a set of RGB values, there is no guarantee theprinter will correctly interpret them.

We would have the same problem with printers—each printer would have its own CMYK color space.Working with each device’s individual color scale isdifficult and largely impractical.

Device ProfilesInstead of trying to convert data from every deviceto every other device, we relate each device to a cen-tral scale. If we are presented with an RGB pixelvalue, we want to know what color it really refers to.We use a profile to interpret the color representedby RGB pixel values. A profile provides data thatallows us to evaluate what color the pixel values rep-resent. A profile relates device dependent RGBvalues to device-independent, unambiguous LABvalues. Thus, a profile contains data to convertbetween the RGB value each scanner produces andthe LAB number for that color. Each scanner will

CO LO R M A N AG E M E N T

Color management uses well-defined CIE color systems LAB(left) and Yxy (right).

+b

-a +a

-b

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need to have a different profile and the profile mustaccompany the image from that scanner to allow itsdevice-dependent RGB values to be correctly inter-preted. The correct interpretation of the RGB scanvalues greatly improve an image, giving it much bettercontrast, enhanced tone reproduction, vivid saturatedcolors, and generally better overall color reproduction,as shown in the images to the right.

When you print an image, the process is reversed.You specify an LAB color and the profile establishesthe necessary CMYK instructions specific to thatprinter to produce that color.

International Color Consortium (ICC)The framework for the profile connection space andthe format of profiles is specified by the ICC, a regu-latory body that supervises color management pro-tocols between software vendors, equipmentmanufacturers and users. Today’s color managementis basically “ICC-color management.”

Inside a ProfileProfiles are an essential part of any color manage-ment system. A profile can be embedded in animage or be used as standalone files. Profiles allowthe correct interpretation of image data (pixelvalues). Because the ICC specifies a standard profileformat, profiles are neither vendor specific nor plat-form specific. The same profile can be accepted byvarious software programs and operating systems.Software such as Photoshop, QuarkXPress, andInDesign can use images with device profiles fromany manufacturer and for any device. The same pro-file can be used on Mac and Windows. Because pro-files have a standard format, there are a number of“profile inspector” utilities that can be used to viewthe content of a profile.

Profile-Making SoftwareA profile is needed to link an image to the centralconnection space, thus to implement color manage-ment it is necessary to have an ICC profile for eachdevice in your imaging chain. There are three mainways to obtain a profile for your device: Custom,Generic, and Process.

Custom Profiles: Color management is mostly donewith custom profiles. A custom profile refers to a pro-file that is made specifically for your device and thestate it is in. To make a custom profile, you need atest chart, a measuring instrument, and profile-

making software. Profiling a device involves creatinga map between device-dependent RGB/CMYK anddevice-independent LAB. Thus, all profiles—monitor,scanner, or printer—require that these two sets ofdata be available.

Custom-made profiles are the most accurate typeof profile because they describe the characteristicsof your device and the state it is in. Custom pro-filing is one of the most important operations incolor management.

Generic Profiles: Manufacturers usually supply ageneric profile for a device, but the profile may notaccurately represent the device you have in front ofyou. A custom profile generated for your device islikely to give you better results. There is one good usefor a generic profile, however, color managementoperations cannot be initiated without a profile: anda generic profile can be used as a starting pointbecause the profile will be of the correct type anddescription for the device.

sRGB, SWOP, and Other Process Profiles: Anotheroption is to use a process profile. One popular way ofworking is to say your device is representative of atypical process, so you can justify using a processprofile. A number of process profiles are availablesuch as sRGB for monitors and SWOP for printers. Ifyou believe your monitor is average, you can use ansRGB profile as your monitor profile.

Process profiles are very common in commercialprinting. A printing press can be run so it producesdensity values that are in accordance with acceptedstandards for printing, known as “reference printing

An image withoutand an image with ascanner profile. Noticehow a profile improvesthe appearance of theimage. The image wasscanned on a FujiQuattro scanner with aprofile made in FujiColourKit profilingsoftware.

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conditions.” Some of the common settings includeSNAP, GRACol, and SWOP. If your print process oper-ates according to SWOP specifications, in a colormanaged workflow, you can use a SWOP profile asyour printer profile. As long as a process profile is agood representation of your device, process profilesprovide flexibility and convenience to end-users.

Device GamutsA color gamut is defined as the range of colors adevice can produce. The gamut of a device is part ofthe device’s characteristics and is derived during theprofiling process. Thus, profiles contain informationabout a device including its gamut.

Some devices create colors using RGB—the pri-mary (or additive) color set. The technologies that useRGB are computer monitors, flatbed scanners, slidescanners, slide writers, and large-screen video pro-jectors. CMY (commonly known as CMYK becauseprinter technologies also use black ink) is called thesecondary (or subtractive) color set. The CMYK set isused by nearly all output devices. An accepted gen-eralization is that RGB systems have a larger colorgamut and can cope with more colors than CMYKsystems. As images are passed into and out of thecentral connection space, they may be sent to adevice with a small color gamut. Thus we have to dealwith colors that are out-of-gamut of the destinationprocess. Color management provides a number ofways to deal with out-of-gamut colors.

Rendering IntentsGamut issues affect many color management opera-tions. Images from a scanner or digital camera willhave colors a printer cannot reproduce. If a colorcannot be printed, we would like a color managementsystem to find a replacement. The ICC has specifiedfour color-replacement schemes, called renderingintents: perceptual, relative, absolute, and saturation.

Perceptual Rendering Intent: Perceptual rendering isused to process photographic-type images. This intentprocesses colors in an image so that the printedreproduction is pleasing. Perceptual rendering is likelyto change the color of an image from original toreproduction, but the relationship between colors isretained and this creates the best-looking images.

Relative and Absolute Colorimetric Intent: The colori-metric intents are used in instances where we desirethe highest accuracy in color reproduction, forexample in logos where color is critical.

Saturation Intent: The saturation intent makes theimage more colorful by utilizing the full gamut of thedestination device. This intent has total disregard forany genuine representation of color. This intent is usedfor business graphics such as graphs and pie charts,for which it is important to have bright, vivid colors.

Version 4 ProfilesThe rendering intents just described have until nowbeen primarily associated with printer profiles. Fol-lowing the Version 4 revision of the ICC specification,it is now possible to create rendering intents for alltypes of profiles (scanner, monitor, and printer). Manyvendors are now starting to incorporate different ren-dering intents in their monitor and scanner profiles.

Three Cs of Color ManagementThe whole process of color management can be neatlydefined in terms of three Cs—Calibration, Characteri-zation, and Conversion. Calibration involves estab-lishing a fixed, repeatable condition for a device.Anything that alters the color of the image must beidentified and “locked-down.” Calibration involvesestablishing some known starting condition and somemeans of returning the device to that state. After adevice has been calibrated, its characteristic responseis studied in a process known as characterization. Incolor management, characterization refers to theprocess of making a profile. During the profile gener-ation process, the behavior of the device is studied bysending a reasonable sampling of color patches (a testchart) to the device and recording the device’sresponse. Thus, the typical behavior, gamut and char-acteristics of the device are ascertained, and thisinformation is stored in the device profile. Creating aprofile is the characterization part of the process.

The third C of color management is conversion, aprocess in which images are converted from one colorspace to another. The conversion process relies onapplication software (e.g., Photoshop), system-levelsoftware (e.g., ColorSync), and a CMM (e.g., AdobeCMM). The three Cs are hierarchical—each process isdependent on the preceding step. Thus characteriza-tion is only valid for a given calibration condition.

Color Management WorkflowsWorkflows typically involve source and destinationdevices and color management operations such asimage conversions or processing. A color manage-ment system can consist of input, output, and dis-

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play devices connected to a central connection spacevia profiles. There are a number of ways of processingimages through such a system.

The real power of color management is evident inprinter-based workflows. This is where the imagefrom a digital camera or scanner is brought into thecentral space and then sent to the printer. In thiscase, the software applies the input profile, and thesystem uses an output profile. Typically, this opera-tion converts the image from RGB to LAB and thenfrom LAB to CMYK. The printed result is thus sim-ilar to the original image.

Soft proofing involves previewing the printedimage on a monitor to get a color-accurate previewof what the printed result will look like. In this sce-nario, the image is brought into the central connec-tion space using the input profile. The image isprocessed through the output profile, which imbuesthe image with the look and feel of the printer. Fromthe printer space, the image is brought back into thecentral space and sent to the monitor. Simply bychoosing a printer profile, you can immediately seeon the screen what your print will look like.

The press proof process simulates a printing pressresult on a local desktop printer. The image isbrought into the central space via the input profile.The image is processed to the press profile, broughtback in from the press, and finally sent out to a localinkjet printer. The inkjet device provides a renditionof the printed product, providing a preview of theprinted result that can be used to detect any prob-lems with the print job.

RGB Versus CMYK WorkflowIn modern color management, it is preferable toretain the image in its original color space, usuallyRGB. The advantages of working in RGB are:kRGB images are three-channel images and have asmaller file size compared to four-channel CMYKcounterpart.kThere are more Photoshop filters in RGB mode thanin CMYK mode.kRGB images are not “committed” to a print processand are therefore not gamut compressed.kWorking in RGB reduces the number of color con-versions to which an image may be subjected.

The gamut of RGB scanners is relatively large. Ifthe image is converted to a printer CMYK that has asmaller gamut, any colors in the RGB set but not inthe CMYK set must be thrown away. This process is

irreversible. Suppose a client wants an Internet ver-sion of a printed catalog. It is possible to take theprocessed CMYK images back to the connectionspace and then to web RGB, however, you don’t haveas many colors and are not fully exploiting the capa-bilities of the display medium. It would be better totake the input RGB data to web RGB.

In color management you do not have to committo a particular print process to see what the printedimage will look like. It is not necessary to convert theimage from RGB to CMYK. It is possible to simulatethis process. Software allows you to preview and evenedit the image as if it were in the destination space.

Why Do We Need Color Management?Color management gets us very close, very quickly.It removes the need for endless iterations. There areof course many instances where a particular imagewill need further editing and “tweaking.” Color man-agement provides a way to get to a good baselinefrom which to make further, aesthetic changes. Atthe end of the day color management saves time andmoney—the bottom line with which few can argue.Color management provides flexibility, the ability toproof at remote locations, and the ability to use inkjetprinters instead of high-end proofers. In the finalanalysis it facilitates a quicker, less expensive work-flow, which is what counts.

Understanding Color Management

by Abhay sharma

This article is excerpted from chapter 1 of Abhay Sharma’s popular UnderstandingColor Management. The book provides an in-depth look at CIE color systems;quantifying color difference; measuring instruments; and scanner, camera,monitor, press and printer profiles, as well as Apple utilities and colormanagement in Photoshop. Additional key features:

•Show how to make and use profiles for monitors, scanners, and printers.

•Describe how to set up and use color management in Photoshop.

•Include updates on Version 4.0 ICC profiles.

•Examine color management in Mac OS X.

The book is available for purchase through IPA. Please email donna@ipa.org forfurther information. Sharma, in conjunction with IPA, has conducted a series ofcolor management webinars and ICC Profiling webinars that have been archivedand are available on-demand or on CD. Go to www.ipa.org/webinars for moreinformation or to order the webinar series.

Copy for this article is from Understanding Color Management 1st edition bySHARMA. ©2004. Reprinted with permission of Delmar Learning, a division ofThomson Learning: www.thomsonrights.com. Fax 800-730-2215.

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