Minimizing Profiling Error

1. Minimizing Profiling Error ICC Color Symposium 13/11/2013 · Hong Kong John Seymour Applied Math., QuadTech Ragy Isaac Quality Director, Goss Co-organizers Implementing Agent Organizer

2. Agenda • Image-setter linearization. • Press calibration and TVI determination. • Image-setter / Press topics: • Goal • Test, measure, and analyze • System check • Choosing system components • Summary

3. Main Theme Same input Produces Same result Within a variability

4. Blocks Form TVI Form Blocks/TVI Test Forms Paper Motion Direction

5. Magnified Cyan TVI Form

6. Data scatter - Variability • The same 40% CCD measurements will not be exactly the same. • Data scatter about the average 40% dot Average

7. Image-Setter System: Goal • Computer file dot = plate dot • Provide a methodology to allow: • Linearization: • Force image-setter to produce intended dot on plate • Understanding system’s natural variability • System Diagnosis if off target • Choice of a winning system

8. Error = File Dot – Plate Dot

9. TVI Form Image-Setter: Test & Measure • Set the image-setter to mfg specs. • Create a plate with the TVI test form. • With a CCD measuring device: • Measure and input in spreadsheet every plate spot.

10. Image-Setter: Analyze • Graph the data in Excel • Calculate average & variability (standard deviation) for every dot%. • Analyze data per color and per dot% • Combine colors & re-analyze per dot% • Similar average values in step 1 & 2 ?? • Pre-press treats CMYK the same. Cyan Dot = Magenta Dot = Yellow Dot = Black Dot

11. Image-Setter: RIP Input • If CMYK %dots are equal for same %dots • Combine all CMYK dots • Set the RIP linearization curve to the average dot values. • If CMYK %dots are NOT equal: • Create separate linearization curves for each color. • Determine variability for every dot %

12. Daily System Check • Daily check: should include 4 strips • Average all 4 strips for each dot % • If within linearized values ± variability • Do nothing. Do not re-adjust your RIP. • Adjusting the RIP is tampering • Tampering pushes the system off target

13. Daily System Check: Example Upper Control Limit = 40.7 40% Lower Control Limit = 39.3

14. Daily System Check: Off Range? • If system correctly linearized: • Do not re-linearize! • Identify root causes • New consumable batch, • Room temperature, • Maintenance, …etc • Rectify the issue.

15. How to Choose Best Components? • The best system should be: • Digital values = Average dots on plate • The lowest variability across the plate for every dot % • Applies to: • Image-setter • Aluminum plate • Chemistries

16. Prepress system Summary • We have a methodology to: • Linearize the system • Quantify & control performance • Choose the perfect system

17. Press System: Goal • Same image/key value = Same density for: • All presses, press keys, press speeds, solids & dot% • Understanding system’s natural variability • System Diagnosis if off target • Choice of a winning system

18. Press: Testing • Three press tests: • Zeroing test with no image and no water • Blocks Test: uniform solid (100%) image • TVI Test: uniform variable dot image light EVEN print

19. Press: Testing – All Tests • To reduce variables & speed-up testing: • Set all press ink keys the same • Use the same image across the entire press, • Set all press keys the same • Evaluate inking with & without dampening light EVEN print

20. Press: Zeroing Test • Test conditions: • Set press to mfg specs. • Blank plate, no image • No dampener, one color at a time • Full ink fountain • All press keys set to zero • Use any paper • Low press speed to conserve paper. • Needed result: very light EVEN print light EVEN print

21. Press: Block Test • Test conditions: • Uniform image plate • With calibrated dampener • Full ink fountain • All press keys set the same • Use any paper • Production press speed. • Needed result: EVEN density

22. Press: Measure Densities • Measure solid density inside each square • Record data in a spreadsheet

23. Press: Analyze Densities • Graph densities in Excel and view pattern • Density variation has no pattern?? • Yes: press is set-up correctly • No: systematic press issue • Correct to eliminate pattern • Calculate average & variability (standard deviation.)

24. Solid Density

25. How to Choose Best Components? • The best system should be: • Higher average solid densities for the same ink rheology • The lowest variability across the press • Applies to: • Ink, dampening solution, blankets, … etc

26. Press: TVI Test • Test conditions: • Uniform image plate • With calibrated dampener • Full ink fountain • Set Solids to density specs • Use frequently used paper • Production press speed • Needed result: required solid density

27. Density versus CIELAB • Density is a single number, so it is user-friendly • It is useful because • Density relates (loosely) to ink film thickness • Density relates (loosely) to the color that we see

28. Density does not predict color Pure yellow ink Yellow withcyan Yellow withmagenta These patches might have the same density

29. Density and CIELAB • ISO 12647 specifies L*a*b* of CMYK solids • ISO 12647 does not specify density

30. Quick review of CIELAB • Colors can be characterizedin terms of • Chroma • Hue

31. Quick review of CIELAB • Or in terms of • a* • b* b* a*

32. Quick review of CIELAB • All colors of aparticular hue canbe characterizedin terms of • Lightness • Chroma

33. Quick review of CIELAB • Color is threedimensional • Lightness • Hue • Chroma

34. Quick review of CIELAB • Color is threedimensional • Lightness • Hue • Chroma(L*a*b*)

35. Density and CIELAB • ISO 12647 specifies L*a*b* of CMYK solids • ISO 12647 does not specify density • How do you find the correct density? • We need to find the relationship between the two

36. Cyan color as density changes What density should I run to? 0.73D 1.00D 1.26D Target a*b* 1.48D

37. Different view of the same data What density should I run to?

38. ISO 12647 control Keeping in this range What density should I run to?

39. ISO 12647 control Keeping in this range will keep DE < 4 What density should I run to?

40. Ink trajectory The green line is the estimated ink trajectory. This was computed from the spectra at 1.09D, using Beer’s law.

41. Ink trajectory Beer’s law can be used to estimate the “perifarbe” density from a single measurement.

42. Press: Measure Densities • Measure solid & dotdensities • Record data in a spreadsheet

43. Press: Analyze Densities • Graph densities & view pattern perdot% • Calculate average & variability • Exclude columns if solid density is off-spec • Calculate TVI for on-spec columns • Input TVI values in prepress • FORCE TVI to match ISO Specs • Reprint with new TVI settings • Revaluate TVI

45. How to Choose Best Components? • The best system should be: • Provides lower average TVI • Lower TVI variability for each dot% • Higher average solid densities for the same ink rheology • Applies to: • Ink, dampening solution, blankets, … etc

46. Press system Summary • We have a methodology to: • Use simple and effective testforms • Calibrate a press with and without water • Evaluate TVI & match ISO spec • Choose the perfect system

47. Comparison of halftone dots Stochastic Conventional, ideal hard dots Conventional, real dots

48. Comparison of halftone dots Stochastic,gravure, and ink jet True continuoustone Conventional, ideal hard dots Conventional, real dots Increasing TVI

49. Cyan ink – comparison of different screening Stochastic Conventional 50% 50%

50. Cyan ink – comparison of different screening Stochastic has much higher TVI (tone value increase) Stochastic is greener; conventional is bluer