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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-6
-4
-2
0
2
4
6
8
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
10
15
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
5
10
15
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: [email protected]
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Thanks for Listening