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

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400

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720 nm

Green

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660

680

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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

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0

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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

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-50

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50

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-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

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-7 -5 -3 -1 1 3 5 7

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b*

Printing paper shade

Proofing paper shade

Simplified workflow

Role of paper in colourmanagement

Significance of paper in colour management

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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

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L*

Paper shade and luminance

Coated fine and MWC's

SC

Hi-brite

LWC

MWC

WFC

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-4

-2

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-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

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0,5

1

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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

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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)