Measuring CMYK Halftones: The Case for Spot Color Tone Value (SCTV) Formula
Mike Strickler MSP Graphic Services
70th Annual Technical Conference · Baltimore, MD · 2018
PROPOSAL:
Current density-based TVI should gradually be replaced with ISO 20654 (SCTV) colorimetric tone curves for CMYK printing.
§ 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
70th Annual Technical Conference · Baltimore, MD · 2018 Some background …
70th Annual Technical Conference · Baltimore, MD · 2018 Unlike CMYK: Spot color tints are usually undefined
50% Magenta (from profile)
L* 80 a* 20 b* -5
50% Pantone 239 C
L* ? a* ? b* ?
70th Annual Technical Conference · Baltimore, MD · 2018 Spot color gradients can be visually very nonlinear
70th Annual Technical Conference · Baltimore, MD · 2018 TVI varies widely among spot colors
Tone ramps of 15 spot inks screened and printed identically
(Courtesy John Seymour)
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 70th Annual Technical Conferencemeasure, · theBaltimore, 50% patch looks like aMD 98.6%. It· would 2018 be an understat ement 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. Density measurement of spot tints can be misleading
Color: Reflex Blue
Tonal compression in the “cyan” range
Courtesy John Seymour
70th Annual Technical Conference · Baltimore, MD · 2018 Density v Spot Color Tone (ISO 20654)
Where R is X, Y, or Z reflectance
Or, 1-10-Dt X 100 1-10-Ds
70th Annual Technical Conference · Baltimore, MD · 2018 Old Plot v New
TVI
30
25
20
Cyan
10
5
0 0 10 20 30 40 50 60 70 80 90 100
70th Annual Technical Conference · Baltimore, MD · 2018 Old Plot v New
SCTV
2
0 Cyan 0 10 20 30 40 50 60 70 80 90 100 Magenta -2 Yellow -4 Black
-6
70th Annual Technical Conference · Baltimore, MD · 2018 Why SCTV for CMYK?
Common visual reference for calibration of all devices and inks
70th Annual Technical Conference · Baltimore, MD · 2018 Avoidance of ambiguity in “mixed” TVI displays
CMYK (TVI) + Spot Color (SCTV)
Spot Color (SCTV) with correction
70th Annual Technical Conference · Baltimore, MD · 2018 Linear SCTV as a universal target?
SCTV = 0%
70th Annual Technical Conference · Baltimore, MD · 2018 Comparison: coated substrate
GRACoL2013
ISO Curve A (from profile)
SCTV Linear
70th Annual Technical Conference · Baltimore, MD · 2018 Comparison: Uncoated substrate
Fogra 52 (uncoated)
ISO Curve B (from profile)
SCTV Linear
70th Annual Technical Conference · Baltimore, MD · 2018 Comparison: black
GRACoL 2013
ISO Curve B (from profile)
SCTV Linear
Fogra 52
ISO Curve C (from profile)
SCTV Linear
70th Annual Technical Conference · Baltimore, MD · 2018 For backward compatibility, simplified correction curves
From SCTV to GRACoL 2013 CRPC6
1
0 Cyan 0 10 20 30 40 50 60 70 80 90 100 -1 Magenta -2 Yellow -3 Black -4
From SCTV to Fogra 52 Uncoated
6 Cyan 4 Magenta 2 Yellow 0 0 10 20 30 40 50 60 70 80 90 100 Black
70th Annual Technical Conference · Baltimore, MD · 2018 Gray balance and tonality still OK (GRACoL 2013)
✓
70th Annual Technical Conference · Baltimore, MD · 2018 Gray balance and tonality still OK (Fogra 52)
✓
70th Annual Technical Conference · Baltimore, MD · 2018 Overall match (GRACoL)
To GRACoL 2013 ✓ To “GRACoL” SCTV ✓
70th Annual Technical Conference · Baltimore, MD · 2018 Overall match
To Fogra 52 ✓ To “Fogra 52” SCTV ✓
70th Annual Technical Conference · Baltimore, MD · 2018 Why not G7 as a universal calibration target?
Not always visually linear!
100 100
90 90
80 80 Dye 70 70 Epson Inkjet Sublimation
60 Press 60 Cyan Cyan Magenta 50 50 Magenta Yellow Yellow Black Black 40 40
30 30
20 20
10 10
0 0 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100
Data courtesy of Bruce Bayne
70th Annual Technical Conference · Baltimore, MD · 2018 Recommendations:
§ 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
70th Annual Technical Conference · Baltimore, MD · 2018 Thank you
70th Annual Technical Conference · Baltimore, MD · 2018