device color and iso 10128 - rochester institute of...

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Device Color and ISO 10128 Bob Chung, Professor RIT • School of Media Sciences

ISO 10128 specifies three printer calibration methods to align dissimilar printing devices to print alike.


Contents

§  Device-dependent color

§  Characterizing CMYK color –  Color gamut –  Tonal value increase –  Grey reproduction, midtone spread, and chromaticness

difference (∆Ch)

§  ISO 10128 device calibration –  TVI adjustment via four 1-D curves –  Grey adjustment via four 1-D curves –  Multi-dimensional CMYK-to-CMYK transforms

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Introduction

§  Printing from digital data, in its simplest sense, involves –  Converting job data into imageable pixels –  Converting imageable pixels into colorant amounts, and –  Transfering colorants to substrate

§  We can characterize a CMYK printing condition by measuring printed color patches. –  Full characterization is possible when printing and measuring

the ISO 12642-2 (IT8.7/4) target –  Measuring selected color patches from control strips suffice for

process standardization and calibration.

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Device-dependent Color

§  Device values, as specified in a color bar, contain CMYK data only. –  CMYK is tonal value,not color.

§  When the color bar is printed and measured, the printing condition is characterized by the relationship between CMYK values and their corresponding CIELAB values.

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Characterizing CMYK Color

§  Process control parameters (ISO 12647-2) –  Solid coloration or color gamut

-  Solid coloration is paper and ink dependent -  Solid ink aims are specified in CIELAB -  Tolerances are specified in ∆E*ab and ∆E00

–  Tonal value increase (TVI) -  Colorimetric TVIs are specified in ISO 12647-1 -  TVI aims for CMYK are equal in ISO 12647-2

–  Grey reproduction -  Midtone spread and ∆Ch are specified in ISO 12647-2. -  Midtone spread is derived from TVIs of C, M, and Y. -  ∆Ch is derived from a* and b* of printed triplets.

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§  Color gamut –  Expressed in the 2-D a*b*

diagram using single color solids (C, M, Y) and overprint colors (R, G, B)

–  The larger the area, the more colorful the printed colors may look.

–  Not sensitive to tone reproduction and grey reproduction.

!100$

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0$

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!100$ !80$ !60$ !40$ !20$ 0$ 20$ 40$ 60$ 80$ 100$

CRPC!6$

$RIT!Print$

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§  Colorimetric TVI are defined in ISO 12647-1

Where Xp, Yp, Zp are tristimulus values of paper

Xt, Yt, Zt are tristimulus values of halftone Xs, Ys, Zs are tristimulus values of solid C (0.55) is a constant

TVIM&K =100*Yp −Yt( )Yp −Ys( )

−TV

TVIY =100*Zp − Zt( )Zp − Zs( )

−TV

TVIC =100*Xp −C *Zp( )− Xt −C *Zt( )Xp −C *Zp( )− Xs −C *Zs( )

−TV

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§  TVI of CMYK ramps –  Based on the

colorimetric TVI formulas

–  Four single color ramp with tonal values from 0~100

–  TVI corresponds to midtone tone reproduction

–  The balance of TVI among C, M, and Y corresponds to midtone spread

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0" 20" 40" 60" 80" 100"

TVI$

%TV$

TVI_CRPC6"

TVI1RIT1Print"

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0" 20" 40" 60" 80" 100"

TVI$

%TV$

TVI_CRPC6"

YVI1RIT1Print"

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0" 20" 40" 60" 80" 100"

TVI$

%TV$

TVI_CRPC6"

TVI1RIT1Print"

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TVI$

%TV$

TVI_CRPC6"

TVI1RIT1Print"

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Midtone Spread and ∆Ch

MTS = Max [(Ac−Ac0),(Am−Am0),(Ay−Ay0)] − Min [(Ac−Ac0),(Am−Am0),(Ay−Ay0)]

where Ac is the measured tone value of the cyan process colour image; Ac0 is the specified tone value of the cyan process colour image; Am is the measured tone value of the magenta process colour image; Am0 is the specified tone value of the magenta process colour image; Ay is the measured tone value of the yellow process colour image; Ay0 is the specified tone value of the yellow process colour image.

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Characterizing CMYK Color §  Grey reproduction – CIELAB values of a set of pre-defined triplets

(a* and b* vs. L*)

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a*/b*%

L*%

a*$!$CRPC6$b*$!$CRPC6$a*$!$RIT$b*$!$RIT$

ID61636669727578

C M Y K L* a*232CRPC6 b*232CRPC60 0 0 0 95.02 0.98 34.012.7 2 2 0 92.79 1.13 33.9110.2 7.1 7.1 0 87.05 0.79 33.8524.7 18.8 18.8 0 75.58 0.88 32.8249.8 40 40 0 57.52 0.52 3274.9 65.9 65.9 0 39.48 0.21 30.78100 100 100 0 23 30.02 0

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Characterizing CMYK Color §  Grey reproduction – CIELAB values of a set of pre-defined triplets

(a* and b* vs. %TV_Cyan)

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a*/b*%

%TV%(Cyan)%

a*$!$CRPC6$b*$!$CRPC6$a*$!$RIT$b*$!$RIT$

ID61636669727578

C M Y K L* a*232RIT b*232RIT0 0 0 0 93.31 0.73 33.933.1 2.2 2.2 0 89.32 4.51 35.4810.2 7.4 7.4 0 75.18 2.8 39.725 19 19 0 67.23 1.28 38.550 40 40 0 49.3 1.02 310.475 66 66 0 35.7 1.28 39.82100 100 100 0 20.4 3.76 38.18

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ISO 10128 Device Calibration

§  Calibration is the adjustment of device values such that the printing condition conforms to specifications.

§  ISO/TS 10128 recommends three calibration methods: 1)  Matching tone value curves 2)  Using pre-defined near-neutrals 3)  Using CMYK to CMYK multi-dimensional transforms

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Matching Tone Value Curves

§  Enter two tone value curves, i.e., reference & sample, for each channel. 1)  Find %tone value pairs yield equal density.

0.0

0.1

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0.70.8

0.9

1.0

1.1

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0 10 20 30 40 50 60 70 80 90 100% Dot Area

AM 150Lpi

FM 21µ

% AM 150Lpi FM 21µ1 0 0.000 0.0002 3 0.030 0.0183 7 0.070 0.0394 10 0.100 0.0705 15 0.145 0.1236 20 0.190 0.1847 25 0.240 0.2638 30 0.280 0.3429 40 0.390 0.587

10 50 0.510 0.80611 60 0.620 1.04312 70 0.750 1.22713 80 0.910 1.34114 90 1.140 1.39415 100 1.420 1.42016 100 1.420 1.42017 100 1.420 1.42018 100 1.420 1.42019 100 1.420 1.42020 100 1.420 1.42021 100 1.420 1.42022 100 1.420 1.420

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Matching Tone Value Curves 2)  Send transfer curve (%TV_in vs. %TV_out) to RIP to alter the

device values for each channel during the CTP operation.

Transfer curveAM

150Lpi FM 21µ1 0 0.02 3 5.33 7 10.04 10 12.85 15 16.86 20 20.47 25 23.58 30 26.19 40 32.0

10 50 36.911 60 41.512 70 47.413 80 54.414 90 65.315 100 100.016 100 #N/A17 100 #N/A18 100 #N/A19 100 #N/A20 100 #N/A21 100 #N/A22 100 #N/A

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Refrence Dot area

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Using Pre-defined Near-neutrals

1)  Print and measure the pre-defined triplets and a K-only ramp (P2P target).

K

3C

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2)  Assess tone reproduction and gray balance of the initial printing condition using Curve2 s/w.

∆a* (red), ∆b* (blue), ∆Ch (green)

∆L* for cmy (orange), ∆L* for k (gray)

%da

%da

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3)  Determine gray balance and tone reproduction requirements based on paper white and neutral print density.

4)  Derive transfer curves for each channel via Curve2 s/w.

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5)  Assess gray balance and tone reproduction of the G7 calibrated printing condition.

∆a* (red), ∆b* (blue), ∆Ch (green)

∆L* for cmy (orange), ∆L* for k (gray)

%da

%da

Before After

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Multi-dimensional Transforms

§  Four 1-D transfer curves assume similar color and transparency of the inks between the reference printing condition and the initial printing condition. –  Applicable to conventional printing where CTP is used.

§  The above assumption is not required when ICC device-link profiles are used to implement CMYK-to-CMYK transforms between the reference printing condition and the initial printing condition. –  Applicable to digital printing

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Multi-dimensional Transforms

§  Device link uses a pre-defined four-dimensional transform to convert CMYK data of the reference space to the sample space while preserving the integrity of single channel data.

STD CMYK

Custom CMYK My Press

Standard Press

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Conclusions

§  CMYK only defines dot area combinations, and not color. –  This is the reason why we refer CMYK as device-dependent

color.

§  Color of the CMYK separation depends on the paper, solid coloration, TVI, and midtone spread. –  The same CMYK file, printed in different printing conditions, will

produce different color.

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Conclusions

§  ISO 10128 specifies three printer calibration methods to align dissimilar printing devices to print alike. –  TVI adjustment and grey adjustment are applicable to analog

printing.

–  There is a regional preference in terms of the calibration technique. TVI calibration is popular in European. Grey (G7) calibration is popular in North America. Both methods are used by printers in Greater China to meet their oversea print buyer’s requirements.

–  Device link is applicable to digital printing.

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References

§  ISO/TS 10128 Graphic technology — Methods of adjustment of the colour reproduction of a printing system to match a set of characterization data

§  ISO 12647-1 Graphic technology — Process control for the production of half-tone colour separations, proof and production prints — Part 1: Parameters and measurement methods

§  ISO 12647-2 Graphic technology — Process control for the production of halftone colour separations, proof and production prints — Part 2: Offset lithographic processes

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Thank you. Q/A

Bob Chung, Professor Email: [email protected]

RIT • School of Media Sciences

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