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Colour management
- 2. Content • Light and colour • Colour management • Role of paper in colour management • Standardised printing
- 4. 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
- 5. World of colour - technical • Light is electromagnetic radiation 400 nm 700 nm pm nm um cm m
- 7. Illumination affects colours Under daylight illumination Under incandescent illumination
- 9. 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
- 10. 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
- 11. Three dimensions of colour • Hue • Chroma or saturation • Lightness
- 12. 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 colour difference of ~1-2 ∆E • ∆E is not consistent against human perception across the whole gamut • Newer formulas are suggested like ∆E2000 • ∆E2000 will in the future ISO standards X X ∆E
- 13. ∆E – an example ∆E = 1 ∆E = 2 ∆E = 3 ∆E = 4 ∆E = 5
- 14. However - don’t trust your senses
- 15. However - don’t trust your senses
- 17. 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
- 18. 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
- 19. 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
- 20. 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
- 21. 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
- 22. 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)
- 23. ICC output profiles (press profiles) • ICC output profiles have two functions 1. To separate images for printing 2. To simulate printing in proofing
- 24. 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
- 25. Black separation C 7% M 36% Y 45% K 1% C 0% M 33% Y 42% K 10% Isocoated v2 300 Maximum GCR
- 26. -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
- 27. Gamut mapping – an example Perceptual Relative RGB CMYK
- 28. History of proofing Flat-bed proofing Analog proofers 1st generation of digital proofers Modern digital proofers
- 29. 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
- 30. 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 -7 -5 -3 -1 1 3 5 7 -7 -5 -3 -1 1 3 5 7 a* b* Printing paper shade Proofing paper shade
- 32. Role of paper in colour management
- 33. Significance of paper in colour management -100 -50 0 50 100 -100 -50 0 50 100 a* b* Colour gamut / paper shade Dot gain Influenced mainly by paper!
- 34. 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
- 35. 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
- 36. Paper – the 5th colour
- 37. 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
- 38. 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
- 39. • 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, % Roughness-PPS 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
- 40. Paper classification – shade vs. dot gain properties Paper shade and brightness Inkdemand(dotgain-TVI)increases WFC MWC Std LWC MFC UWF SC Hi-brite LWC
- 41. 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
- 44. Sappi prepress datasheets • Recommendations for each grade for ICC-profiles as well as additional information on optical propesties
- 46. 1. Paper groups 2. Lab targets for CMYK, RGB 3. Dot gain targets 4. Tolerances for Lab, dot gain ISO 12647-2:2004
- 47. 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
- 48. 1. Paper groups 2. Lab targets for CMYK, RGB 3. Dot gain targets 4. Tolerances for Lab, dot gain ISO 12647-2:2004
- 49. -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
- 50. 1. Paper groups 2. Lab targets for CMYK, RGB 3. Dot gain targets 4. Tolerances for Lab, dot gain ISO 12647-2:2004
- 51. 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
- 52. 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 dotgain 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
- 53. 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
- 54. 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 dotgain 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
- 55. How to control dot gain in CM workflow dot gain is influenced=
- 56. 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
- 58. Normal densityDensity too low Density too high
- 59. Normal dot gainDot gain too low Dot gain too high
- 60. Magenta dot gain too highTarget print conditions Magenta print density decreased
- 62. 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
- 63. 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!
- 64. 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)