Device Color and ISO 10128

Transcription

1 Device Color and ISO Bob Chung, Professor RIT School of Media Sciences ISO specifies three printer calibration methods to align dissimilar printing devices to print alike.

2 2 Contents Device-dependent color Characterizing CMYK color Color gamut Tonal value increase Grey reproduction, midtone spread, and chromaticness difference ( C h ) ISO device calibration TVI adjustment via four 1-D curves Grey adjustment via four 1-D curves Multi-dimensional CMYK-to-CMYK transforms

3 3 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 (IT8.7/4) target Measuring selected color patches from control strips suffice for process standardization and calibration.

4 4 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.

5 5 Characterizing CMYK Color Process control parameters (ISO ) 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 E 00 Tonal value increase (TVI) - Colorimetric TVIs are specified in ISO TVI aims for CMYK are equal in ISO Grey reproduction - Midtone spread and C h are specified in ISO Midtone spread is derived from TVIs of C, M, and Y. - C h is derived from a* and b* of printed triplets.

6 6 Characterizing CMYK Color 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$ 80$ 60$ 40$ 20$ 0$!100$!80$!60$!40$!20$ 0$ 20$ 40$ 60$ 80$ 100$!20$!40$!60$ CRPC!6$!80$ $RIT!Print$!100$

7 7 Characterizing CMYK Color Colorimetric TVI are defined in ISO TVI M&K =100 * Y p Y t Y p Y s TVI Y =100 * Z p Z t TVI C =100 * X p C * Z p ( ) ( ) TV ( ) ( Z p Z s ) TV ( ) X t C * Z t ( X p C * Z p ) X s C * Z s Where X p, Y p, Z p are tristimulus values of paper X t, Y t, Z t are tristimulus values of halftone X s, Y s, Z s are tristimulus values of solid C (0.55) is a constant ( ) ( ) TV

8 8 Characterizing CMYK Color 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 TVI$ TVI$ 35" TVI_CRPC6" 30" TVI1RIT1Print" 25" 20" 15" 10" 5" 0" 0" 20" 40" 60" 80" 100" %TV$ 35" TVI_CRPC6" 30" YVI1RIT1Print" 25" 20" 15" 10" 5" 0" 0" 20" 40" 60" 80" 100" %TV$ TVI$ TVI$ 35" TVI_CRPC6" 30" TVI1RIT1Print" 25" 20" 15" 10" 5" 0" 0" 20" 40" 60" 80" 100" %TV$ 35" TVI_CRPC6" 30" TVI1RIT1Print" 25" 20" 15" 10" 5" 0" 0" 20" 40" 60" 80" 100" %TV$

9 9 Midtone Spread and C h 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.

10 10 Characterizing CMYK Color Grey reproduction CIELAB values of a set of pre-defined triplets (a* and b* vs. L*) ID C M Y K L* a*232crpc6 b*232crpc a*/b*% 8$ 6$ 4$ 2$ 0$!2$!4$!6$!8$!10$ a*$!$crpc6$ b*$!$crpc6$ a*$!$rit$ b*$!$rit$!12$ L*%

11 11 Characterizing CMYK Color Grey reproduction CIELAB values of a set of pre-defined triplets (a* and b* vs. %TV_Cyan) ID C M Y K L* a*232rit b*232rit a*$!$crpc6$ b*$!$crpc6$ a*$!$rit$ b*$!$rit$ 8$ 6$ 4$ 2$ a*/b*% 0$!2$!4$!6$!8$!10$!12$ %TV%(Cyan)%

12 12 ISO Device Calibration Calibration is the adjustment of device values such that the printing condition conforms to specifications. ISO/TS recommends three calibration methods: 1) Matching tone value curves 2) Using pre-defined near-neutrals 3) Using CMYK to CMYK multi-dimensional transforms

13 13 Matching Tone Value Curves Enter two tone value curves, i.e., reference & sample, for each channel. 1) Find %tone value pairs yield equal density. % AM 150Lpi FM 21µ AM 150Lpi FM 21µ % Dot Area

14 14 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 curve 100 AM 150Lpi FM 21µ #N/A Refrence Dot area

15 15 Using Pre-defined Near-neutrals 1) Print and measure the pre-defined triplets and a K-only ramp (P2P target). K 3C

16 16 Using Pre-defined Near-neutrals 2) Assess tone reproduction and gray balance of the initial printing condition using Curve2 s/w. L* for cmy (orange), L* for k (gray) %da %da a* (red), b* (blue), C h (green)

17 17 Using Pre-defined Near-neutrals 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.

18 18 Using Pre-defined Near-neutrals 5) Assess gray balance and tone reproduction of the G7 calibrated printing condition. L* for cmy (orange), L* for k (gray) %da Before After %da a* (red), b* (blue), C h (green)

19 19 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 CMYKto-CMYK transforms between the reference printing condition and the initial printing condition. Applicable to digital printing

20 20 Multi-dimensional Transforms Device link uses a pre-defined fourdimensional transform to convert CMYK data of the reference space to the sample space while preserving the integrity of single channel data. Standard Press STD CMYK Custom CMYK My Press

21 21 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.

22 22 Conclusions ISO 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.

23 23 References ISO/TS Graphic technology Methods of adjustment of the colour reproduction of a printing system to match a set of characterization data ISO Graphic technology Process control for the production of half-tone colour separations, proof and production prints Part 1: Parameters and measurement methods ISO Graphic technology Process control for the production of halftone colour separations, proof and production prints Part 2: Offset lithographic processes

24 24 Thank you. Q/A Bob Chung, Professor RIT School of Media Sciences