Bài giảng colour management kho tài liệu bách khoa

Colour managementContent • Light and colour • Colour management • Role of paper in colour management • Standardised printing Color Management Presented by Nguyen Long Giang 2016 Light an

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Colour management

Content

•   Light and colour

•   Colour management

•   Role of paper in colour management

•   Standardised printing

Color Management Presented by Nguyen Long Giang 2016

Light and colour

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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 colours

by certain names

World of colour - technical

•   Light is electromagnetic radiation

Illumination and reflection

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Illumination affects colours

Creating colours

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 n

40

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72 Green 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 nm 40

42

48

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Cyan

Yellow

C M Y

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

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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

• Chroma or saturation

• Lightness

How to define colour difference

• It is distance between two colours in CIE

Lab – colour space

human perception across the

• Newer formulas are suggested like

∆E2000

X

X ∆E

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However - don’t trust your senses

Colour management

The importance of colour in printed

products

•   Colour is used to impress

and strengthen the brand of

advertised products or the

magazine itself

•   Colour accuracy in of

utmost importance in

advertisements and

catalogues of colour critical

products such as cosmetics,

clothes and furniture

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What is colour management

•   Standardising how colour is reproduced in different stages of the workflow from camera to proofing and printing

•   At the same time colour management ensures that full 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 decreased retouching of images and remaking of proofs

•   Time/money savings in printing through faster colour

ok in makeready

From colour space to colour gamut

•   Colour space is a mathematical model to describe the way colour is represented

•   RGB, CMYK

•   L*a*b*

•   Colour gamut is the volume in chosen colour space that a particular device can reproduce

•   Colour gamut is usually presented in Lab colour space

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Colour gamuts

150b*

100

50 a*

0 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 -50

-100

Uncoated Offset Adobe RGB

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

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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 ink

coverage

Black separation

Isocoated v2 300

C 7%

M 36%

Y 45%

K 1%

Maximum GCR

C 0%

M 33%

Y 42%

-100

-50

0

50

100

a*

Gamut mapping with ICC-profiles

•   Perceptual

•   Relative colorimetric

•   Absolute colorimetric

•   Saturation

•   “Moves" also the colours inside colour gamut

in order to maintain the image as natural as possible

•   Does not move colours inside the gamut

original's gamut •

printer's gamut

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Gamut mapping – an example

Relative

RGB

Perceptual

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 produce colours

•   CMYK + light cyan + light magenta + different blacks

•   Proofing uses absolute colorimetric € white point of the ICC profile is taken into account

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•   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

-7

Pr o

7 b*

5

3

1

a*

-5 -3 -1 1 3 5 7

-1

fing paper shade

-3

-5 Printing paper shade

Simplified workflow

Role of paper in colour management

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Significance of paper in colour management

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-100

a*

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

89

90

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96

*

Paper shade and luminance

SC

Hi-brite LWC MWC

WFC

-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

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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 gain properties of the paper

Double coated fine paper High brightness LWC Film coated LWC

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Matte-coated in the standard actually refers to silk grades (semi- matt)

grades is much smaller than with

"true" matt grades

Matt

Silk

Glossy

4,5

4

3,5

3

2,5

2

1,5

1

0,5

0

Roughness and dot gain

•

Paper classification – shade vs dot gain properties

MWC WFC

Paper shade and brightness

Std

LWC

UWF

SC

MFC

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

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ECI Standard ICC-profiles

Classification examples

Standardised

printing

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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

ISO 12647-2:2004

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0 10

0 8 6 4 2

0 -120 -100 -80 -60 -2

0

0 4 80 100

120 -20 -40 -60 -80 -100 -120

a

Printing to Lab-targets

•   Using Lab-targets to calculate correct print density

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

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Dot gain vs screen ruling

increasing screen

ruling

5 %

0 %

10 %

15 %

20 %

25 %

30 %

dot percentage

52 l/cm 60 l/cm 70 l/cm 80 l/cm 90 l/cm

•   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

20 %

15 %

10 %

5 %

0 %

25 %

30 %

dot percentage

52 l/cm 60 l/cm 70 l/cm 80 l/cm 90 l/cm

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How to control dot gain in CM workflow

dot gain is

influenced

=

Standardised printing

1 Find target Lab-values (ISO

12647-2) and corresponding print

density levels

2 Compensate dot gain (TVI)

according to ISO 12647-2

3 Print to pre-defined print densities

and fine-tune print result by using

gray balance patches

K70 CMY70 K50 CMY50 K30 CMY30

Some examples

on ink adjustments

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Normal density

Normal dot gain

Magenta dot gain too high

Target print conditions Magenta print density decreased

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