Color Management

1. Color Management By D. B. Stovall 1 May 2014

2. Precision versus accuracy

3. Why color management? • Color management used to be closed loop • Print, evaluate, repeat until madness ensues • With modern technology, each step in the process can be controlled so the desired image can be reproduced with little trouble • IOW what appears on the monitor is close to what is in the print • Current technology is precise (repeatable), color management helps tie that to relative accuracy

4. Color • What is it? • Property of objects • Property of light • Occurs in the observer • This is the light-object-observer model • The reality • Happens in all 3 as an event • Sensation in the observer of the light from the light source as modified by the object

5. Light • Behaves as both a particle (photon) and as an electromagnetic wave • Wave behavior has frequency property • Sometimes 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 spectrum • High freqs (short wavelengths) are blue end of spectrum • About 700 nm for red, 400 nm for blue (nm = 10-9 m) • Spectrum order for low high wavelength is ROYGBV • IR is below red, UV is above violet

6. Color temperature • Uses theoretical blackbody radiator heated to various temperatures • If heated to certain temperatures will emit light with spectrum dependent on temperature alone • Thermal energy is measured • Uses degrees Kelvin • K = °C + 273.15

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 types • Tungsten ~3000K • Daylight (sunlight as modified by atmosphere) ~5000K • Fluorescent • When excitation of a gas occurs, electrons changing energy state downwards emit a photon at a particular frequency • Usually a line or discontinuous spectrum • LEDs are part of this family – beware!

8. Object behavior • Absorbs or reflects at certain frequencies • Modifies the light source like a filter • Transmissive or reflective • Certain types of material fluoresce • In effect changes frequencies of the photons • E.g. brighteners in papers changing UV to blue

9. Observer • Color perception starts in the eye • Cones responsible for color • 3 types of cones, respond to long, medium, and short wavelengths • Trichromancy • Trichromatic retinal structure makes possible the 3 additive primaries

10. Opponency • Retina color components do not work independently but in opponent pairs • Light-dark • Red-green • Yellow-blue • Zone theory of color • 1st layer of retina has cones • 2nd layer translates these into the 3 opponent signals • Models incorporate both opponency and trichromancy Short Med Long B-Y L-D R-G

11. Additive primary colors • Red, green, blue from long short wavelengths • Black = no wavelengths • White = all wavelengths • All 3 added • Can get any other color with some combination of these 3 R BG

12. Subtractive primary colors • Cyan, magenta, yellow • No good freq correlation since magenta is not part of color spectrum • Subtracts wavelengths from otherwise white source • Black = all wavelengths • White = no wavelengths • Can also get any color from these 3 R MY BG C

13. Metamerism • 2 different color samples producing the same stimuli in an observer • Also the same color sample producing different stimuli in an observer • Dependent on illumination and/or observer • Color matching depends on the phenomena • E.g. a chrome on a viewer versus an image on a monitor • We can match under certain illumination conditions • But under other conditions a mismatch will be apparent • E.g. tungsten versus daylight • Metamerism is what enables 4 color inks to represent the full spectrum • Limited by gamut • Can also occur between different types of observers • E.g. scanners, cameras, and people

14. Colorimetry • Applying a numeric model to color and color perception • Current system created by CIE • System components • Illuminants like D50 or D65 • Standard Observer like 2° color observer of 1931 • Tristimulus response of human observer • XYZ primary system • Derived from Standard Observer • Imaginary primaries, Y as luminance • Distances are distorted • xyY primary system • Transform of XYZ • Shows additive relationships • Distances also distorted • Uniform color spaces • L*a*b* • L* is lightness, a* is red/green opponency, b* is blue/yellow opponency • Perceptually uniform • L*U*V • Not widely used today • Usually CIEXYZ or CIELAB used in color spaces • Difference 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 image • Color constancy • Perception of an object having a constant color even if the illuminant changes • Devices do not have color constancy • Color management can preserve the relationships between colors in an image • Perceptual versus colorimetric renderings

16. Numerical color representation • Either RGB or CMYK • Numbers represent amount of colorant, not color • Colorant is what is used to make a color • Pigment, dye, light from a monitor phosphor, etc. • Scanners (RGB) and cameras (RGB) for input • Printers (CMYK) for output • Monitors (RGB) for both input and output • No 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) structure • 1 byte gives 256 levels (28) • RGB (3 channels) gives 2563 theoretical colors (1.6 x 106) • More bits increases fidelity and adds editing headroom • E.g. 2 bytes (16 bits) gives 216 levels per channel • Adding 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 brightness • Monitor phosphors or printer inks • Total range is color gamut • White point color • Black point density • Tone curve • Gamma curve in scanners, cameras, and monitors • Dot gain curve in printers • Sometimes a lookup table (LUT) used in place of a curve

19. Color models highlights • RGB and CMYK are device specific models • A given color triplet (x, y, z) will represent differently on different devices • CIE models like CIELAB are device independent • Represent perceived color • All devices are limited by gamut and dynamic range • Mismatches between devices require manipulation of some kind to match target device • E.g. from digital camera to printer

20. Transfer functions • Operate on input data to produce output data • Change the data in a consistent, time-invariant way • Color management is based on the concept of a transfer function f(x) g(x) h(x)

21. Color management systems • Determine perceived color from RGB or CMYK inputs • Attempt to keep colors consistent from device to device • PCS = profile connection space Outputs Printer Monitor Whatever Inputs Camera Scanner Whatever PCS

22. Color management components • PCS • CIELAB or CIEXYZ are mandated by ICC (International Color Consortium) but PCS are not limited to these • Profiles • Can be for a device, class of devices, or abstract color space • Basically a lookup table or mathematical transform • Describes behavior but does not alter the device • CMM (color management module) • Software engine • Converts from RGB or CMYK to PCS using data in the profile • Several different ones in use • ICC compliant ones are interchangeable but can differ subtly

23. Profile flow Data CMM Adjusted data Profile Rendering intent

24. Rendering intents • Handles out of gamut situations • E.g. camera to printer • Perceptual preserves color relationships, alters all • Saturation keeps colors saturated and vivid • Good for graphics • Relative colorimetric maps white of sources to destination and clips out of gamut colors • Preserves more of the original colors than perceptual • Absolute colorimetric same as relative but does not map white point • Mainly for proofing

25. Using profiles • If the image has no profile • Assigning a profile is for use within that application • Embedding a profile attaches to the file so the profile is available for use within different applications • Assigning or embedding does not change colorant values, just how they are interpreted • If an image already has an embedded profile • Converting a profile for an image does change the colorant values • Need to specify a target profile

26. Profile types • Input • Device space to PCS • Backward transform • Scanners and digital cameras • Display • Device space to PCS and back • Forward and backward transform • Monitors • Output • 2 way transform like display • Printers and presswork

27. Profile internals • Either 3x3 matrix or LUT • Matrix • Uses CIEXYL • For input or display • LUT • Also for input or display • Profile size much larger • Required for output profile • Adds 4th channel • Usually at least 6 tables • Perceptual, relative colorimetric, saturation • 1 for each direction

28. Building a profile • Sending known color values to a device and see what is actually measured • Monitors are generally profiled using a colorimeter • Printers profiled with either a colorimeter of spectrophotometer • Use known targets such as IT8 • Profiles are only as accurate as measurements and only describe a gamut, not enlarge it

29. Families of profiles • Device specific • Parameters are locally measured • Generic • Constructed from average device behavior or average media characteristics • Not as good as specific but may be adequate • Generic monitor profiles the least useful due to inherent unstable behavior • Color space profiles • E.g. CIELAB or CIEXYZ • Device independent profiles are similar, useful for editing • Adobe 98, EktaSpace, ProPhoto • Typically wider gamut except for sRGB

30. Profiling versus calibration • Profiling • Characterizing a device or media • Describes the device • Calibration • Sets the device to target characteristics • Controls the device • As 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 HW • Calibration adjusts 4 things • White luminance • White color • Black luminance • Not all calibration systems adjust this • Response curve • CRT monitors once in wide use, easier to calibrate due to control of electron guns • Now CRTs are gone and LCD/LEDs predominate • LCDs adjust with both monitor control and video LUT adjust • In video control SW

32. Display calibration methods • Visual • E.g. Adobe Gamma application • Pretty much useless but beats nothing • Bundled monitor and calibrator • E.g. LaCie Blue-Eye • One button calibration • May or may not include colorimeter (“puck”) • Standalone calibration packages • Useful on any monitor but rely on manual control of monitor and video SW • Usually 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 room • Some users paint walls gray and do other things but is more important to have a consistent environment

34. Calibrating a monitor - 1 • User inputs • White point, e.g. 5000K or 6500K, or maybe a direct K input • My own viewing hood was measured at 5300K so I use that • Some say always use 6500K but YMMV • Gamma • Either 2.2 or 1.8 • Most use 2.2 today • 1.8 was originally used by Mac to match to LaserWriter • Black point if available • I use 0.2 Cd/m2 • Make sure the monitor is set the way you want before you start • Resolution, refresh rate, etc.

35. Calibrating a monitor - 2 • SW will set white luminance first • Some apps do this automatically, some use a user desired set point • I use 120 Cd/m2 • Then black luminance • Can be iterative process if controlling the monitor/SW is being done manually • But SW should walk you through it • Then color temperature • Again either set by user or automatically to target point by SW • Lastly the SW displays color patches and the puck reads and feeds back measurements so profile can be built • Make sure the profile is saved in a place the OS can find it • Windows is /System32/Spool/Drivers/Color • Mac depends on OS • Once 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 kind • Reflective spectrophotometer is the best • Best to use a device the profiling SW can talk to • Try to get 4 mm to 8 mm measurement aperture • Handheld ones are cheaper but measuring swatches can be tedious • XY plotter types do this automatically but are expensive • Do not bother with printer profilers that involve using your scanner

37. Output (printer) profiles - 2 • General flow • Read master target • Print target from file • Read printed target • SW builds profile • Verify by printing a target using new profile and reading • Will probably be at least 300 swatch reads and may have to do several so SW can average • Usually SW package provides target and target file • If using SW that does not support measuring instrument, have to read into text file or spreadsheet and import into SW • Good luck with that! • Targets are IT8.7/3 or proprietary • Make 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 use • In Photoshop, select Photoshop manages colors • In printer driver, select no color management • A profile will be for 1 printer/paper combination • Changing anything requires a new profile • Be aware of things like drying time • Save the profile as stated above • Can use canned ones from paper manufacturer as well • May have to tweak your prints occasionally but will be close • I use them with good results

39. Input (scanner) profiles • Camera profiling difficult unless using controlled lighting in studio • Also very brand specific • Transparency and reflective only • No color negative targets available • Useless since orange mask varies with exposure • Need physical target and target description file (TDF) • Individual TDF from specific target is best but most expensive • More often use TDF from target “run” • Common transmissive is IT8.7/1 • Usually 1 target suffices for all films except Kodachrome since dye structures are similar • Common reflective is IT8.7/2 • Make sure to use consistent settings and all equipment is warmed up • Turn off all adjustments • ICE and GEM do not generally interfere so can leave on if desired • Scan the target and let the SW build the profile • Save to the proper place as above • I have had good luck with the canned profiles from Epson but YMMV

40. Evaluating what you have done • Depends on viewing conditions • 5000K viewing hood with adjustable intensity is ideal • ICC profiles are based on D50 illuminant • Some profiling applications also measure viewing light and factor that into the profile • Before utilizing print to monitor matching • Match brightness, not color temperature • Do not put monitor and viewing hood in same field of view • I violate this with good results since I set monitor white point to color temp of viewer but YMMV • Various methods of validating the profiles and calibration too complex to go into in this presentation • Excellently summarized in Chapter 9 of Real World Color Management by Fraser, Murphy, Bunting • Book is a bit dated now but still an excellent primer on the subject • Beyond 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 process • Can present as RGB but errors in profiling may put on a slight color cast • Since I do not have to deal with this I need to do more research in this area

42. More information • www.cambridgeincolour.com/tutorials/color-management1.htm • www.xrite.com/documents/literature/en/L11-176_Guide_to_CM_en.pdf • www.lacie.com/us/technologies/technology.htm?id=10029 • spyder.datacolor.com/