3 Why color management? Color management used to be closed loop Print, evaluate, repeat until madness ensuesWith modern technology, each step in the process can be controlled so the desired image can be reproduced with little troubleIOW what appears on the monitor is close to what is in the printCurrent technology is precise (repeatable), color management helps tie that to relative accuracy
4 Color What is it? The reality Property of objects Property of light Occurs in the observerThis is the light-object-observer modelThe realityHappens in all 3 as an eventSensation in the observer of the light from the light source as modified by the object
5 LightBehaves as both a particle (photon) and as an electromagnetic waveWave behavior has frequency propertySometimes described in terms of wavelength (c/f) since frequency unit is unwieldy here (e.g. 750 THz)For visible light:Low freqs (long wavelengths) are red end of spectrumHigh freqs (short wavelengths) are blue end of spectrumAbout 700 nm for red, 400 nm for blue (nm = 10-9 m)Spectrum order for low high wavelength is ROYGBVIR is below red, UV is above violet
6 Color temperatureUses theoretical blackbody radiator heated to various temperaturesIf heated to certain temperatures will emit light with spectrum dependent on temperature aloneThermal energy is measuredUses degrees KelvinK = °C
7 White light Pure white light is equal amounts of photons at all freqs White light as we can obtain it is not pure but of several typesTungsten ~3000KDaylight (sunlight as modified by atmosphere) ~5000KFluorescentWhen excitation of a gas occurs, electrons changing energy state downwards emit a photon at a particular frequencyUsually a line or discontinuous spectrumLEDs are part of this family – beware!
8 Object behavior Absorbs or reflects at certain frequencies Modifies the light source like a filterTransmissive or reflectiveCertain types of material fluoresceIn effect changes frequencies of the photonsE.g. brighteners in papers changing UV to blue
9 Observer Color perception starts in the eye Cones responsible for color3 types of cones, respond to long, medium, and short wavelengthsTrichromancyTrichromatic retinal structure makes possible the 3 additive primaries
10 OpponencyRetina color components do not work independently but in opponent pairsLight-darkRed-greenYellow-blueZone theory of color1st layer of retina has cones2nd layer translates these into the 3 opponent signalsModels incorporate both opponency and trichromancyB-YR-GL-DLongMedShort
11 Additive primary colors Red, green, blue from long short wavelengthsBlack = no wavelengthsWhite = all wavelengthsAll 3 addedCan get any other color with some combination of these 3RB G
12 Subtractive primary colors Cyan, magenta, yellowNo good freq correlation since magenta is not part of color spectrumSubtracts wavelengths from otherwise white sourceBlack = all wavelengthsWhite = no wavelengthsCan also get any color from these 3RM YB GC
13 Metamerism2 different color samples producing the same stimuli in an observerAlso the same color sample producing different stimuli in an observerDependent on illumination and/or observerColor matching depends on the phenomenaE.g. a chrome on a viewer versus an image on a monitorWe can match under certain illumination conditionsBut under other conditions a mismatch will be apparentE.g. tungsten versus daylightMetamerism is what enables 4 color inks to represent the full spectrumLimited by gamutCan also occur between different types of observersE.g. scanners, cameras, and people
14 Colorimetry Applying a numeric model to color and color perception Current system created by CIESystem componentsIlluminants like D50 or D65Standard Observer like 2° color observer of 1931Tristimulus response of human observerXYZ primary systemDerived from Standard ObserverImaginary primaries, Y as luminanceDistances are distortedxyY primary systemTransform of XYZShows additive relationshipsDistances also distortedUniform color spacesL*a*b*L* is lightness, a* is red/green opponency, b* is blue/yellow opponencyPerceptually uniformL*U*VNot widely used todayUsually CIEXYZ or CIELAB used in color spacesDifference calculations usually represented by ΔE
15 Model failures and color management Sometimes using colorimetry can get a color match in one color at the expense of other colors in the imageColor constancyPerception of an object having a constant color even if the illuminant changesDevices do not have color constancyColor management can preserve the relationships between colors in an imagePerceptual versus colorimetric renderings
16 Numerical color representation Either RGB or CMYKNumbers represent amount of colorant, not colorColorant is what is used to make a colorPigment, dye, light from a monitor phosphor, etc.Scanners (RGB) and cameras (RGB) for inputPrinters (CMYK) for outputMonitors (RGB) for both input and outputNo 2 scanners or monitors will produce color in exactly the same way
17 Digitally encoding the RGB or CMYK Usually by even byte boundary (byte = 8 bits) structure1 byte gives 256 levels (28)RGB (3 channels) gives 2563 theoretical colors (1.6 x 106)More bits increases fidelity and adds editing headroomE.g. 2 bytes (16 bits) gives 216 levels per channelAdding bits does not increase available dynamic range or produce more colors…These are controlled by the device itself…but decreasing the number of bits can reduce them!
18 Main variables of a color system Colorant color and brightnessMonitor phosphors or printer inksTotal range is color gamutWhite point colorBlack point densityTone curveGamma curve in scanners, cameras, and monitorsDot gain curve in printersSometimes a lookup table (LUT) used in place of a curve
19 Color models highlights RGB and CMYK are device specific modelsA given color triplet (x, y, z) will represent differently on different devicesCIE models like CIELAB are device independentRepresent perceived colorAll devices are limited by gamut and dynamic rangeMismatches between devices require manipulation of some kind to match target deviceE.g. from digital camera to printer
20 Transfer functions f(x) g(x) h(x) Operate on input data to produce output dataChange the data in a consistent, time-invariant wayColor management is based on the concept of a transfer function
21 Color management systems Determine perceived color from RGB or CMYK inputsAttempt to keep colors consistent from device to devicePCS = profile connection spaceInputsCameraScannerWhateverOutputsPrinterMonitorWhateverPCS
22 Color management components PCSCIELAB or CIEXYZ are mandated by ICC (International Color Consortium) but PCS are not limited to theseProfilesCan be for a device, class of devices, or abstract color spaceBasically a lookup table or mathematical transformDescribes behavior but does not alter the deviceCMM (color management module)Software engineConverts from RGB or CMYK to PCS using data in the profileSeveral different ones in useICC compliant ones are interchangeable but can differ subtly
24 Rendering intents Handles out of gamut situations E.g. camera to printerPerceptual preserves color relationships, alters allSaturation keeps colors saturated and vividGood for graphicsRelative colorimetric maps white of sources to destination and clips out of gamut colorsPreserves more of the original colors than perceptualAbsolute colorimetric same as relative but does not map white pointMainly for proofing
25 Using profiles If the image has no profile Assigning a profile is for use within that applicationEmbedding a profile attaches to the file so the profile is available for use within different applicationsAssigning or embedding does not change colorant values, just how they are interpretedIf an image already has an embedded profileConverting a profile for an image does change the colorant valuesNeed to specify a target profile
26 Profile types Input Display Output Device space to PCS Backward transformScanners and digital camerasDisplayDevice space to PCS and backForward and backward transformMonitorsOutput2 way transform like displayPrinters and presswork
27 Profile internals Either 3x3 matrix or LUT Matrix LUT Uses CIEXYL For input or displayLUTAlso for input or displayProfile size much largerRequired for output profileAdds 4th channelUsually at least 6 tablesPerceptual, relative colorimetric, saturation1 for each direction
28 Building a profileSending known color values to a device and see what is actually measuredMonitors are generally profiled using a colorimeterPrinters profiled with either a colorimeter of spectrophotometerUse known targets such as IT8Profiles are only as accurate as measurements and only describe a gamut, not enlarge it
29 Families of profiles Device specific Generic Color space profiles Parameters are locally measuredGenericConstructed from average device behavior or average media characteristicsNot as good as specific but may be adequateGeneric monitor profiles the least useful due to inherent unstable behaviorColor space profilesE.g. CIELAB or CIEXYZDevice independent profiles are similar, useful for editingAdobe 98, EktaSpace, ProPhotoTypically wider gamut except for sRGB
30 Profiling versus calibration Characterizing a device or mediaDescribes the deviceCalibrationSets the device to target characteristicsControls the deviceAs devices change over time, must recalibrate and/or re-profile to make sure response will be as expected
31 Display calibration“Display” consists of monitor, video drivers, video card or HWCalibration adjusts 4 thingsWhite luminanceWhite colorBlack luminanceNot all calibration systems adjust thisResponse curveCRT monitors once in wide use, easier to calibrate due to control of electron gunsNow CRTs are gone and LCD/LEDs predominateLCDs adjust with both monitor control and video LUT adjustIn video control SW
32 Display calibration methods VisualE.g. Adobe Gamma applicationPretty much useless but beats nothingBundled monitor and calibratorE.g. LaCie Blue-EyeOne button calibrationMay or may not include colorimeter (“puck”)Standalone calibration packagesUseful on any monitor but rely on manual control of monitor and video SWUsually includes puck
33 Viewing environment Good idea to use a monitor hood Ambient light affects light level so use a hood and a low light level in the roomSome users paint walls gray and do other things but is more important to have a consistent environment
34 Calibrating a monitor - 1 User inputsWhite point, e.g. 5000K or 6500K, or maybe a direct K inputMy own viewing hood was measured at 5300K so I use thatSome say always use 6500K but YMMVGammaEither 2.2 or 1.8Most use 2.2 today1.8 was originally used by Mac to match to LaserWriterBlack point if availableI use 0.2 Cd/m2Make sure the monitor is set the way you want before you startResolution, refresh rate, etc.
35 Calibrating a monitor - 2 SW will set white luminance firstSome apps do this automatically, some use a user desired set pointI use 120 Cd/m2Then black luminanceCan be iterative process if controlling the monitor/SW is being done manuallyBut SW should walk you through itThen color temperatureAgain either set by user or automatically to target point by SWLastly the SW displays color patches and the puck reads and feeds back measurements so profile can be builtMake sure the profile is saved in a place the OS can find itWindows is /System32/Spool/Drivers/ColorMac depends on OSOnce you calibrate and profile a monitor, do not do any further adjustments to the monitor or video SW or you invalidate everything!
36 Output (printer) profiles - 1 Must have a measuring instrument of some kindReflective spectrophotometer is the bestBest to use a device the profiling SW can talk toTry to get 4 mm to 8 mm measurement apertureHandheld ones are cheaper but measuring swatches can be tediousXY plotter types do this automatically but are expensiveDo not bother with printer profilers that involve using your scanner
37 Output (printer) profiles - 2 General flowRead master targetPrint target from fileRead printed targetSW builds profileVerify by printing a target using new profile and readingWill probably be at least 300 swatch reads and may have to do several so SW can averageUsually SW package provides target and target fileIf using SW that does not support measuring instrument, have to read into text file or spreadsheet and import into SWGood luck with that!Targets are IT8.7/3 or proprietaryMake sure there are no profiles assigned or imbedded into target file
38 Output (printer) profiles - 3 When printing ensure no printer driver controls are in useIn Photoshop, select Photoshop manages colorsIn printer driver, select no color managementA profile will be for 1 printer/paper combinationChanging anything requires a new profileBe aware of things like drying timeSave the profile as stated aboveCan use canned ones from paper manufacturer as wellMay have to tweak your prints occasionally but will be closeI use them with good results
39 Input (scanner) profiles Camera profiling difficult unless using controlled lighting in studioAlso very brand specificTransparency and reflective onlyNo color negative targets availableUseless since orange mask varies with exposureNeed physical target and target description file (TDF)Individual TDF from specific target is best but most expensiveMore often use TDF from target “run”Common transmissive is IT8.7/1Usually 1 target suffices for all films except Kodachrome since dye structures are similarCommon reflective is IT8.7/2Make sure to use consistent settings and all equipment is warmed upTurn off all adjustmentsICE and GEM do not generally interfere so can leave on if desiredScan the target and let the SW build the profileSave to the proper place as aboveI have had good luck with the canned profiles from Epson but YMMV
40 Evaluating what you have done Depends on viewing conditions5000K viewing hood with adjustable intensity is idealICC profiles are based on D50 illuminantSome profiling applications also measure viewing light and factor that into the profileBefore utilizing print to monitor matchingMatch brightness, not color temperatureDo not put monitor and viewing hood in same field of viewI violate this with good results since I set monitor white point to color temp of viewer but YMMVVarious methods of validating the profiles and calibration too complex to go into in this presentationExcellently summarized in Chapter 9 of Real World Color Management by Fraser, Murphy, BuntingBook is a bit dated now but still an excellent primer on the subjectBeyond this, look at workflow and specific techniques, especially for press work
41 Profiling monochrome images At this point does not seem to be industry agreement on the processCan present as RGB but errors in profiling may put on a slight color castSince I do not have to deal with this I need to do more research in this area
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