Measuring CMYK Halftones: The Case for Spot Color Tone Value … Presentation.pdf · 2020. 11. 21. · 70th Annual Technical Conference · Baltimore, MD · 2018 TVI varies widely

70th Annual Technical Conference · Baltimore, MD · 2018

Measuring CMYK Halftones: The Case for Spot Color Tone Value (SCTV) Formula

Mike StricklerMSP Graphic Services


§  Benefits:

§  Simplified calibration of all types of devices regardless of ink and screening type

§  Improved correspondence between measured values and visual impression

§  Consistent, unambiguous curves for all colors in process control software

PROPOSAL: Current density-based TVI should gradually be replaced with ISO 20654 (SCTV) colorimetric tone curves for CMYK printing.


Some background …


L* 80a* 20b* -5

Unlike CMYK: Spot color tints are usually undefined

50% Magenta (from profile)

L* ? a* ?b* ?

50% Pantone 239 C


Spot color gradients can be visually very nonlinear


TVI varies widely among spot colors

Figure 1 - TVImw curves of 15 randomly selected inks

The shapes of these curves in Figure 1 are all reasonable, but the magnitudes of some are ridiculous. One of the inks (not shown in Figure 1) had a 50% TVImw of 48.6%, in other words, according to this measure, the 50% patch looks like a 98.6%. It would be an understatement to call this plugging.

This one ink is not an anomaly. Figure 2 is a histogram showing 50% TVImw of all of the inks. There were 70 inks that had a 50% TVImw above 35%. I have not asked lately, but I am guessing that most prepress folks would balk if I requested plate curves that brought this down to 18%. Banding will be an issue.

Tone ramps of 15 spot inksscreened and printed identically

(Courtesy John Seymour)


Density measurement of spot tints can be misleading

Color: Reflex Blue

Tonal compression in the “cyan” range

Courtesy John Seymour


Or, 1-10-Dt

1-10-Ds

Density v Spot Color Tone (ISO 20654)

Where R is X, Y, or Z reflectance

X 100


Old Plot v New

TVI

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80 90 100

CyanMagentaYellowBlack


Old Plot v New

-6

-4

-2

0

2

0 10 20 30 40 50 60 70 80 90 100CyanMagentaYellowBlack

SCTV


Why SCTV for CMYK? Common visual reference for calibration of all devices and inks


Avoidance of ambiguity in “mixed” TVI displays

CMYK (TVI) + Spot Color (SCTV)

Spot Color (SCTV) with correction


Linear SCTV as a universal target?

SCTV = 0%


Comparison: coated substrate

GRACoL2013

ISO Curve A (from profile)

SCTV Linear


Fogra 52 (uncoated)

ISO Curve B (from profile)

SCTV Linear

Comparison: Uncoated substrate


Comparison: black

GRACoL 2013

ISO Curve B (from profile)

SCTV Linear

Fogra 52

ISO Curve C (from profile)

SCTV Linear


For backward compatibility, simplified correction curves

0

2

4

6

0 10 20 30 40 50 60 70 80 90 100

Cyan

Magenta

Yellow

Black

From SCTV to GRACoL 2013 CRPC6

From SCTV to Fogra 52 Uncoated

-4

-3

-2

-1

0

1

0 10 20 30 40 50 60 70 80 90 100CyanMagentaYellowBlack


Gray balance and tonality still OK (GRACoL 2013)

✓


Gray balance and tonality still OK (Fogra 52)

✓


Overall match (GRACoL) To GRACoL 2013 To “GRACoL” SCTV✓ ✓


Overall match To Fogra 52 To “Fogra 52” SCTV✓ ✓


Not always visually linear!

Why not G7 as a universal calibration target?

Epson Inkjet

0

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30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100


0

10

20

30

40

50

60

70

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90

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0 10 20 30 40 50 60 70 80 90 100


Dye Sublimation

Press

Data courtesy of Bruce Bayne


§  Establish visually linear (SCTV) tone curve for a universal calibration standard

§  Provide “backward compatibility” with existing reference print conditions using SCTV adjustment curve sets

§  Continue to rely on ICC color management for precise matching to reference print conditions

Recommendations:


Thank you

Documents

Measuring CMYK Halftones: The Case for Spot Color Tone Value … Presentation.pdf · 2020. 11. 21. · 70th Annual Technical Conference · Baltimore, MD · 2018 TVI varies widely