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? The reality 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

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
B-Y
R-G
L-D
Long
Med
Short

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
B G

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
M Y
B G 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 f(x) g(x) h(x)
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

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
Inputs
Camera
Scanner
Whatever
Outputs
Printer
Monitor
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 Display Output 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 LUT 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 Generic Color space profiles
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
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
spyder.datacolor.com/