Printing Conformance Assessment – Harmonizing Grey Reproduction & Tolerances

Bob Chung, RIT March 28, 2011 ISO/TC 130/WG13, Berlin, Germany

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WG13 N 0009

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Motivation

•  Harmonizing conformance assessment activities is a major task for TC130/WG13.

•  The starting point for executing this task is interpreting the ‘shalls’ contained in the TC130’s standards.

–  Most aims are clearly defined. Grey Reproduction aims are the exception to this rule.

–  Tolerances are more challenging. The tolerances contained in TC130 standards share a number of limitations:

•  Within the body of TC130 standards, the metrics (ΔEab, ∆E00, ΔH, ∆Ch, etc.) used to set tolerances for similar aims lack consistency.

•  One tolerance is assumed to work in all applications, in spite of the fact some applications are substantially more color critical than others.

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Background & Purpose

•  TC130/WG3 has overall responsibility for setting aims and tolerances in Graphic Arts standards.

–  New insights to overcoming the limitations of current aims and tolerances are evolving.

•  The purpose of this presentation is to: –  Present current work in the areas of Grey Reproduction and Tolerance

Harmonization.

–  Determine if WG13 feels this work could contribute to its mission of harmonizing assessment procedures.

–  Determine if WG13 believes we should seek the leadership of WG3 to incorporate these concepts into its portfolio of standards.

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Harmonizing Grey Reproduction

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Grey Reproduction •  Solid tone and TVI control the behavior of individual

printing units.

•  Grey reproduction extends Solids & TVI control in several important ways:

–  It captures the impact of overprinting effects, i.e., ink transparency & trapping.

–  It includes the impact of paper color on printed neutrals.

•  For these reasons, grey reproduction is becoming an important process control parameter.

–  As the need to assess grey reproduction conformance increases, the limitations of current assessment methods are increasingly problematic.

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Current Assessment Methods •  There are different approaches in specifying grey

reproduction conformity.

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Limitations of Current Methods

•  Current  methods  compare  printed  neutrals  using  predefined  triplets  to  theore5cal  grey  ramps.  

•  This  method  introduces  several  limita5ons:  1.  The  predefined  triplets  are  not  necessarily  neutral.  

2.  Conformance  to  the  theore5cal  grey  ramps  is  judged  qualita5vely  -­‐-­‐  there  are  no  numeric  tolerances.  

3.  The  assessment  of  neutral  appearance  is  limited  to  the  chroma5c  dimension  (a*,  b*),  not  L*  dimension.  

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Current Method – Version 1

•  Use  pre-­‐defined  triplet  device  values.  •  Measure  printed  patches  colorimetrically.  •  Plot  grey  reproduc5on  (a*/b*  vs  %Dot)  &  compare  to  

the  substrate-­‐based  grey  ramp  (SBGR)  as  done  by  IDEAlliance.  

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Current Method – Version 2

•  Grey  reproduc5on  may  be  ploVed  using  a*/b*  vs.  L*  as  proposed  in  ISO/WD  12647-­‐2  (2010).    •  Varia5on  of  prin5ng  are  expressed  as  non-­‐overlapping  data  points.  •  No  pass/fail  decision  because  there  are  no  colorimetric  aims.  

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Overcoming Limitations Using a Dataset

1.  Star5ng  from  pre-­‐defined  CMY  triplets.  

•  e.g.,  Media  Wedge,  P2P  

2.  Finding  their  colorimetric  aims  via  the  ICC  profile  of  the  target  dataset.  

•  e.g.,  ISO  Coated  v2,  GRACoL  (2006)  

•  Absolute  Colorimetric  Rendering  intent  

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Substrate-corrected Colorimetric Aims (SCCA)

3.  Use  tris5mulus  linear  equa5ons  in  ISO  13655  (Annex  A)  to  adjust  the  target  dataset  for  produc5on  stock.  –  This  method  works  for  any  color,  including  grey.  

Data  Set  (Paper_2)    

XYZ_2  –  to  –  LAB_2  •                                    •  •                                    •  •                                    •  

   

Data  Set  (Paper_1)    

LAB_1  –  to  –  XYZ_1  •                                    •  •                                    •  •                                    •  

 

Tris;mulus  Linear  Correc;on      

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Conformance Assessment with Dataset  4.  This  chart  shows  the  SCCA  in  terms  of  substrate  

corrected  grey  reproduc5on  aims.    –  Grey  reproduc5on  conformance  can  now  be  assessed  versus  the  

colorimetric  values  of  the  triplets,  not  just  based  on  the  paper  color.  

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0 10 20 30 40 50 60 70 80 90 100

a* b*

L*

a* b* Grey_a* Grey_b*

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Conformance Assessment with Dataset 5.  Compare  measured  grey  reproduc5on  to  the  substrate-­‐

corrected  grey  reproduc5on  aims.  •  By  adop5ng  this  method,  we  can  introduce  quan5ta5ve  tolerances  for  

chroma5c  and  L*  errors.  •  Two  case  studies  demonstrate  how  the  use  of  SCCAs  overcomes  the  

limita5ons  of  current  conformance  assessment  methods.  

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Case Study I – Grey Reproduction

•  IDEAlliance  database  analysis  (Chung  &  Wang,  2011)  –  Filtered;  n  =  85;  23  triplets  

0 10 20 30 40 50 60 70 80 90 100−15

−10

−5

0

5

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Dot area (%)

Mea

sure

d a* / b

*

Measured a*

Measured b*

Student Version of MATLAB

Measurement vs. %Dot

0 10 20 30 40 50 60 70 80 90 100−15

−10

−5

0

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Dot area (%)

Mea

sure

d a* / b

* − A

im a

* / b*

Measured a* − Aim a*

Measured b* − Aim b*

Student Version of MATLAB

(Measurement – SCCA) vs. %Dot

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Case Study I – Grey Reproduction •  Paper color

–  Bluish paper (top row); Yellowish paper (bottom row)

•  Grey reproduction convergence –  Good convergence (left); Partial convergence (middle)

–  Poor convergence (right) is a function of ink set.

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•  ∆L* and ∆Ch are parameters describing the extent of the neutrality between substrate-corrected colorimetric aims (SCCA) and measurement of the triplets.

∆L* = L1* - L2*

L1, a1, and b1 are substrate-corrected colorimetric aims (SCCA). L2, a2, and b2 are measurements of the CMY triplets.

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Case Study I – Grey Reproduction

!Ch = a1 *!a2 *( )2 + b1 *!b2 *( )2

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Grey Reproduction Conformance

•  Option 1 –  Compute weighted ∆Ch for all 23 triplets and determine pass/fail

based on the average not to exceed 1.5 ∆Ch and max not to exceed 3.0 ∆Ch.

•  Option 2 –  Grey reproduction conformance, based on 3 triplets, has 82%

agreement with Option 1 when the following tolerances are applied.

•  (25C 19M 19Y) at 2 ∆Ch

•  (50C 40M 40Y) at 3 ∆Ch

•  (75C 66M 66Y) at 4 ∆Ch

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Case Study II – Grey Reproduction

•  RIT PSA Survey database (Chung & Chen, 2011) –  Unfiltered; n = 35; 3 triplets

–  CRF of ∆L* & ∆Ch (quarter-tone, midtone & three quarter-tone)

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!!

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Case Study I & II – Grey Reproduction

•  Summary –  Having predefined triplets, available from standard color control

targets, simplifies grey reproduction conformance assessment.

–  Three triplets, at quarter-tone, midtone and three quarter-tone, e.g., as specified in ISO/WD 12647-2 (2010), are sufficient for grey reproduction conformance assessment.

–  Standard-writing bodies can specify target colorimetric aims for these triplets from the target dataset.

–  End users can convert target colorimetric aims into Substrate-Corrected Colorimetric Aims for conformance assessment.

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Harmonizing Tolerances

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Conceptual Framework

•  One of limitations in the tolerances contained in TC130 standards is that

–  One tolerance is assumed to work in all applications, in spite of the fact some applications are substantially more color critical and other applications are less.

•  Work conducted over the past three months has opened a path for overcoming these limitations.

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Universal Tolerance Schema

* This schema assumes the use of substrate corrected colorimetric aims (SCCA) to overcome the limitations of target aims tied to standard paper types.

•  The consistency of tolerance metrics could be improved by adopting a universal tolerance schema.

•  To demonstrated the concept, we created this chart:

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Conceptual Framework

•  Building on such a schema, it is possible to create a family of tolerances for each relevant aim.

–  Tolerances would range from relaxed to normal to stringent, designed to balance application needs and process capability.

•  Tolerances for grey reproduction (∆Ch, ∆L*) are good candidates for creating such a family. The table below illustrates the concept.

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Conceptual Framework

•  Finally, the tolerance schema could be published as a separate standard.

•  Such a tolerance standard would allow other Graphic Arts Standards to focus on aims.

•  Standard specific aims could then be combined with relevant tolerances from the tolerance standard to provide the basis for conformance assessment.

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Discussion

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•  Does WG13:

–  Agree that clarifying Grey Reproduction and Harmonizing Tolerances could contribute to its mission of harmonizing assessment procedures?

–  Believe that it should seek the leadership of WG3 to incorporate these concepts into TC130’s portfolio of standards?

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Robert Chung

Professor School of Print Media

RIT

Rochester, NY, USA

Phone: 585-475-2722

email: rycppr@rit.edu

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Thanks for Listening