Colour management
Content
• Light and colour
• Colour management
• Role of paper in colour management
• Standardised printing
Light and colour
World of Colour - psychological
• Human visual system can discriminate about
• 200 hues
• 500 levels of brightness
• 20 levels of saturation
• � Altogether, we can distinguish 2 million different colours
• However, we only have about 7500 words for different colours
• It is not sure that people see colours in the same way– they may just have learned to call certain coloursby certain names
World of colour - technical
• Light is electromagnetic radiation
400 nm 700 nm
pm nm um cm m
Illumination and reflection
Illumination affects colours
Under daylight illumination Under incandescent illumination
Creating colours
380
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700
720 nm
Green
380
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700
720 nm
RGB
Cyan
Yellow
CMY
Colour spaces
• RGB
• used for digital images or for colour displays
• e.g. scanners and digital cameras operate in RGB space
• CMYK
• used for four-colour prints
Colour spaces
• CIE L*a*b* colour space is the most commonly used device independent (scientific) colour space in the graphic arts industry
• Colours are defined by the perception of human visual system
• In CIE L*a*b* colour space colours can be defined with chroma,
lightness and hue
Three dimensions of colour
• Hue
• Chroma or saturation
• Lightness
How to define colour difference
• The most common way to measure color difference is ∆E
• It is distance between two colours in CIE Lab – colour space
• We can distinguish colourdifference of ~1-2 ∆E
• ∆E is not consistent againsthuman perception across the whole gamut
• Newer formulas are suggested like∆E2000
• ∆E2000 will in the future ISO standards
X
X∆E
∆E – an example
∆E = 1
∆E = 2
∆E = 3
∆E = 4
∆E = 5
However - don’t trust your senses
However - don’t trust your senses
Colour management
The importance of colour in printed
products
• Colour is used to impressand strengthen the brand of advertised products or the magazine itself
• Colour accuracy in of utmost importance in advertisements and catalogues of colour criticalproducts such as cosmetics, clothes and furniture
What is colour management
• Standardising how colour is reproduced in differentstages of the workflow from camera to proofing and printing
• At the same time colour management ensures thatfull potential of the materials and processes in question is utilised
The benefits of good colour management
• Quality of printed products increases
• Quality consistency increases
• Time/money savings in prepress through decreasedretouching of images and remaking of proofs
• Time/money savings in printing through faster colourok in makeready
From colour space to colour gamut
• Colour space is a mathematical model to describethe way colour is represented
• RGB, CMYK
• L*a*b*
• Colour gamut is the volume in chosen colour spacethat a particular device can reproduce
• Colour gamut is usually presented in Lab colour space
Colour gamuts
-150
-100
-50
0
50
100
150
-140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120
b*
a*
Adobe RGB
Coated Offset
Uncoated Offset
ICC-profiles
• ICC-profiles can
• Describe how a particular device produces colour or
• It can be a abstract colour space (like working RGB colour
space)
• There are different kinds of profiles
• input profiles (e.g. for scanners and cameras)
• display profiles (e.g. for monitors)
• output profiles (e.g. for proofers and presses)
• device link profiles (mainly for repurposing cmyk)
ICC output profiles (press profiles)
• ICC output profiles have two functions
1. To separate images for
printing
2. To simulate printing in
proofing
UCR, GCR, TAC
• UCR (under color removal) and GCR (grey component replacement) are methods to determine, how black is reproduced RGB to CMYK conversion
• UCR,GCR, TAC are defined in ICC profile
• UCR � black replaces CMY in neutral (achromatic) tones
• GCR � black replaces CMY also in chromatic tones
• TAC (total area coverage) �defines the maximum inkcoverage
Black separation
C 7%
M 36%
Y 45%
K 1%
C 0%
M 33%
Y 42%
K 10%
Isocoated v2 300 Maximum GCR
-100
-50
0
50
100
-100 -50 0 50 100
a*
b*
Gamut mapping with ICC-profiles
• Different rendering intents
• Perceptual
• Relative colorimetric
• Absolute colorimetric
• Saturation
• PerceptuaI
• “Moves" also the colours inside colour gamut in order to maintain the image as natural as possible
• Relative
• Does not move colours inside the gamut
original's gamut
printer's gamut
Gamut mapping – an example
Perceptual
Relative
RGB
CMYK
History of proofing
Flat-bed proofing
Analog proofers
1st generation of digital
proofers
Modern digital proofers
Proofing
• Inkjet proofers use more than 4 colour to producecolours
• CMYK + light cyan + light magenta + different blacks
• Proofing uses absolute colorimetric � white point of the ICC profile is taken into account
Proofing
• How to match paper shade and light tones in proofing
• Use same paper in proofing as in printing
• Use proofing paper with similar shade and optical brightener
amounts
• Simulate paper shade in proofing with ink
• Most common way in modern inkjet proofing
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-5
-3
-1
1
3
5
7
-7 -5 -3 -1 1 3 5 7
a*
b*
Printing paper shade
Proofing paper shade
Simplified workflow
Role of paper in colourmanagement
Significance of paper in colour management
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-50
0
50
100
-100 -50 0 50 100
a*
b*
Colour gamut / paper shade Dot gain
Influenced mainly by paper!
Colour gamut
• Paper defines reachable print density level i.e. colour gamut
• smooth and dense paper can reach higher print densities than
rough and porous
Coated paper Uncoated paper
89
90
91
92
93
94
95
96
L*
Paper shade and luminance
Coated fine and MWC's
SC
Hi-brite
LWC
MWC
WFC
-6
-4
-2
0
2
4
6
-6 -4 -2 0 2 4 6
WFC
MWC
Hi-brite LWC
Std LWC
MFC
SC
MFC
Std
LWC
PT3
PT1
Spektrolino D50, 2°, white backing
Paper – the 5th colour
Dot gain (TVI, Tone Value Increase)
• Dot gain is an increase in the area of the halftone dot
40% dot on plate 60% dot on paper � 20% dot gain
Dot gain curves
Influence of paper on TVI
• Surface of the paper (mainly roughness) defines dot gainproperties of the paper
Double coated fine paper High brightness LWC Film coated LWC
• Matte-coated in the standard
actually refers to silk grades (semi-
matt)
• Difference between silk and glossy
grades is much smaller than with
"true" matt grades
Matt
Paper gloss, %
Roug
hness -
PP
S
Silk
Glossy
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
0 20 40 60 80
Roughness and dot gain
Paper classification – shade vs. dot gain properties
Paper shade and brightness
Ink d
em
and (
dot
gain
-T
VI)
incre
ases
WFC
MWC
Std
LWC
MFC
UWF
SC
Hi-brite
LWC
Paper classification in ISO 12647-2
• Classification in 2004 version is almost the same as in 1996 version
• New developments at ISO TC 130 coming up
ECI Standard ICC-profiles
Classification examples
Sappi prepress datasheets
• Recommendations for each grade for ICC-profiles as well as additional information on optical propesties
Standardisedprinting
1. Paper groups 2. Lab targets for CMYK, RGB
3. Dot gain targets
4. Tolerances for Lab, dot gain
ISO 12647-2:2004
Paper classification in ISO 12647-2
• Classification in 2004 version is almost the same as in 1996 version
• New developments at ISO TC 130 coming up
1. Paper groups 2. Lab targets for CMYK, RGB
3. Dot gain targets
4. Tolerances for Lab, dot gain
ISO 12647-2:2004
-120
-100
-80
-60
-40
-20
0
20
40
60
80
100
120
-120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120
a
b
Printing to Lab-targets
• Using Lab-targets to calculate correct print density
1. Paper groups 2. Lab targets for CMYK, RGB
3. Dot gain targets
4. Tolerances for Lab, dot gain
ISO 12647-2:2004
Dot gain
• Dot gain is influenced by
• Printing ink
• Ink concentration, tack, fount emulsion
• Paper
• Roughness, porosity � ink demand
• Press conditions
• Nip pressure, construction, etc..
• Print density level
• Printing plate � screen ruling / algorhitms
• Process disturbancies
• Doubling, slurring
Dot gain vs screen ruling
• Dot gain increases with
increasing screen
ruling
• Difference can be
compensated in CTP
0 %
5 %
10 %
15 %
20 %
25 %
30 %
0 % 20 % 40 % 60 % 80 % 100 %
dot percentage
do
t g
ain
52 l/cm 60 l/cm 70 l/cm 80 l/cm 90 l/cm
130 l/inch 150 l/inch 175 l/inch 200 l/inch 225 l/inch
CTP compensation
• Not only level but also shape of the dot gain curve is adjusted
• Usually iterative process to find average level of a certain press
• However paper and ink changes should only affect level
• should be quite easy to implementate
Earlier case - after compensation
• Not a perfect match
• Some fine tuning
needed
0 %
5 %
10 %
15 %
20 %
25 %
30 %
0 % 20 % 40 % 60 % 80 % 100 %
dot percentage
do
t g
ain
52 l/cm 60 l/cm 70 l/cm 80 l/cm 90 l/cm
130 l/inch 150 l/inch 175 l/inch 200 l/inch 225 l/inch
How to control dot gain in CM workflow
dot gain is
influenced=
Standardised printing
1. Find target Lab-values (ISO 12647-2) and corresponding printdensity levels
2. Compensate dot gain (TVI) according to ISO 12647-2
3. Print to pre-defined print densitiesand fine-tune print result by usinggray balance patches
K70 CMY70 K50 CMY50 K30 CMY30
Some exampleson ink
adjustments
Normal densityDensity too low Density too high
Normal dot gainDot gain too low Dot gain too high
Magenta dot gain too highTarget print conditionsMagenta print density
decreased
Sappi CM services
Sappi Offering in Colour Management
1. How to print on Sappi papers – Sappi Prepress
Recommendations
• Recommendations on correct printing conditions and use of correct ICC profiles with Sappi graphic papers
2. How to achieve high and consistent print quality – Colour
management consultation including e.g.
• Trainings
• Creating customer specific ICC profiles
• Printer auditing and consultation on standardised printing
• Evaluating the whole CM workflow
• Print quality evaluation of printed products (PQE – see next slide)
• Optimising proofing
Colour management services - PQE
• Colour quality check of printed product using
established Print Quality Evaluation (PQE)
method
• checking colour consistency and quality of chosen printed product
• recommendations based on results• print density levels
• dot gain compensation
• printability issues
• standardised PQE reports with several languages
• english, german, french, spanish, italian, polish, russian
• Can be used as throw-in to introduce customers to our offering!
CM offering ”pyramid”
Prepress datasheets, customer inquiries,
International co-operation (TC130, ECI,
Paperdam), own research
PQE measurement cases
(~10-20 / a)
CM
Projects
(1-2 / a)