Transcript Color Management - Photo Gathering

By D. B. Stovall
1 May 2014
Precision versus accuracy
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
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
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
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
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!
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
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
Opponency
 Retina color components do not work independently
but in opponent pairs
Short
Med
Long
 Light-dark
 Red-green
 Yellow-blue
B-Y
L-D
 Zone theory of color
R-G
 1st layer of retina has cones
 2nd layer translates these into the 3 opponent signals
 Models incorporate both opponency and
trichromancy
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
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
B
Y
G
C
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
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
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
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
Digitally encoding the RGB or
CMYK
 Usually by even byte boundary (byte = 8 bits) structure
 1 byte gives 256 levels (28)
3
 RGB (3 channels) gives 256 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!
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
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
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
Color management systems
 Determine perceived color from RGB or CMYK inputs
 Attempt to keep colors consistent from device to
device
Inputs
Outputs
Camera
Scanner
Whatever
Printer
Monitor
Whatever
PCS
 PCS = profile connection space
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
Profile flow
Data
CMM
Profile
Rendering intent
Adjusted
data
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
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
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
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
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
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
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
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
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
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
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.
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!
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
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
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
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
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
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
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/