Matching Proof and Print under the Influence of OBA

Presented at the 40th IARIGAI Research Conference, Chemnitz, Germany, September 8-11, 2013 Matching Proof and Print under the Influence of OBA Robert Chung School of Media Sciences Rochester Institute of Technology 69 Lomb Memorial Drive, Rochester, NY 14623, USA rycppr@rit.edu Keyword: proof, print, color management, conformity assessment Abstract Color proofing standard, ISO 12647-7, stipulates that the digital proofing substrate be the same as the production substrate and have similar UV responses. The reality is often the opposite. This is because the use of optical brightening agent (OBA) in papermaking yields the bluish white of the paper that print buyers prefer. Brightened papers cause two concerns: (1) they affect printed colors, including greys and solids, and (2) they cause color mismatch between non-oba proofs and OBA prints. The former limits the usefulness of printing standards. The latter limits the trustworthiness of color proof between printer and print buyer. This case study, participated by four proofing solution providers, demonstrates the use of substrate-corrected dataset in M1 measurement conditions as a solution to bridge the above gaps. Substrate-corrected dataset or SCCA (substrate corrected colorimetric aims) is the adjustment of the color characterization data by means of tristimulus linear correction to account for the colorimetric difference between production paper and the reference substrate. When SCCA becomes the dataset and process control aims, printing by numbers are realistic goals. When SCCA becomes the proofing aims, proof matches print closely. 1. Introduction ISO/DIS 15339-1 (2010) specifies the tristimulus linear correction method to reconcile color differences between the white point difference of the target dataset and that of the actual printing condition. RIT conducted a press run by using its Goss Sunday 2000 web offset press, Sappi Opus paper (containing OBA), and demonstrated printing conformity to substrate-corrected SWOP3 dataset and CGATS TR 016 tolerances (ISO/TC130/WG3 N1259, 2012). Proofing to the substrate-corrected dataset is a new concept that addresses proof to print match under the influence of OBA. An RIT thesis experiment used the substrate-corrected characterization dataset as the common aims for proofing and printing. The psychometric analysis showed that substrate-corrected proof improved visual match to OBA print than ISO 12647-7 compliant proof where no substrate correction was applied (Carlos Carazo, 2012). To further verify that proofing to the substrate-corrected dataset is a viable solution, RIT invited four proofing vendors (referred to as Vendor A, B, C, and D) to participate in this project. They were asked to produce three color-managed proofs with each proof conforming to a specified dataset varying in their white points, i.e., Proof_1 conforms to the SWOP3 dataset as published, i.e., ignoring white point of the printing paper. Proof_2 conforms to the substrate-corrected SWOP3 dataset whereby the white point of Sappi Opus was measured by M0 illuminant. Proof_3 conforms to the substrate-corrected SWOP3 dataset whereby the white point of Sappi Opus was measured by M1 illuminant. There are three objectives in this project: (1) to assess proofing conformity according to the CGATS TR016 tolerance, (2) to visually examine the proofs with the OBA print as the reference under the standard viewing conditions, and (3) to discuss what substrate-corrected dataset aims produces the best proof and print match under the Influence of OBA.

2. Methods There are three steps involved in the study: (1) inviting proofing vendors to produce color proofs to match specified datasets, (2) assess proofing conformity according to CGATS TR016, and (3) discuss the relationship between visual assessment and colorimetric analysis. The first step is implemented by proofing solution providers. RIT carried out the second and the third step. These three steps are further elaborated below: Step 1. Inviting proofing vendors to produce color proofs to match specified datasets Emails were sent to a number of proofing vendors explaining the objective of the study, i.e., color match between non-oba proof and OBA print, and anonymity of their participation. Once there was agreement, RIT sent each vendor (a) test forms (.pdf), (b) a Goss 2000 web offset press printed sheet, (c) examples of printed images on paper with and without OBA, and (d) proofing instructions, including the Excel template that calculates substrate-corrected dataset (.xlsx), and submission deadlines. The proofing instruction indicated that they needed to match three datasets with three different white points in each dataset. Table 1 shows CIELAB values of these white points. Proofing vendors would create three source ICC profiles or built-in substrate correction methods to produce these proofs. Table 1. White points of the three source ICC profiles Proof_ID Source white point CIELAB E ab Proof_1 SWOP3 93L*/0a*/0b* --- Proof_2 Opus (M0) 93.1L*/1.8a*/-4.8b* 5.1 Proof_3 Opus (M1) 93.4L*/1.7a*/-6.5b* 6.7 Proof_1 represents the white point of the source color space as if OBA does not exist; Proof_2 represents the white point of the Sappi Opus under M0 measurement conditions, and Proof_3 represents the white point of the Sappi Opus under M1 measurement conditions. A typical proofing substrate has a white point that matches the SWOP3 white point (93L*/0a*/0b*) closely. This is the only destination profile in the experiment. A proofing system uses its proprietary algorithm to modify the CMYK-to-CMYK transformation such that measured CIELAB values of the proof match the source dataset, including the white point of the source, closely. Step 2. Assessing proofing conformity according to CGATS TR016 RIT used the following procedure to assess the proofing conformity according to the CGATS TR016: (a) measured the IT8.7/4 target of the proof with an i1 isis (M0) spectrophotometer, and measurement data were saved as a.txt file, (b) the measurement file was assessed in a custom-built Excel template, for proofing conformity according to the CGATS TR 016. Step 3. Discuss the relationship between visual assessment and colorimetric analysis Proofing conformity assessment alone only answers the question if a proof passes or fails the established tolerances. More information can be obtained if we correlate visual assessment of pictorial printed colors with colorimetric analysis. In this regard, the ISO pictorial color reference image, Bar Set, was examined visually, and correlated with colorimetric analysis in terms of the five largest E 00 patches (out of 1,617 patches), white point, and grey reproduction. 3. Results and Discussion Four proofing vendors, i.e., Vendor_A, Vendor_B, Vendor_C, and Vendor_D, participated in the study. Results and discussion are highlighted in the following sections: (a) proofing conformity, (b) visual assessment, (c) white point analysis, and (d) grey ramp analysis between the proofs and press sheet. 2

3a. Proofing conformity CGATS TR 016 (2012) is a printing tolerance specification that (1) specifies substrate-corrected dataset and process control aims as conformity metrics, (2) defines three aspects of conformity assessment, i.e., deviation, within-sheet variation, and production variation, and (3) defines a multilevel tolerance schema with Level A being most stringent for critical color match, Level B for normal visual match, and Level C for pleasing color. Table 2 is a summary of the conformity assessment by vendor according to CGATS TR 016. Amongst the four proofing solution providers, Vendor_A scored the best in terms of deviation conformity. All vendors achieved Level A conformity in within-sheet variation. What follow are observations regarding dataset conformance of the three proofs from each vendor. Table 2. Deviation conformity of the three proofs by vendors Vendor_A - all three proofs achieved Level A in deviation conformity. Vendor_B - (a) Proof_1 achieved Level B deviation conformance (100Y patch was less chromatic), (b) Proof_2 achieved Level C deviation conformance (100Y patch was less chromatic and 50C patch was darker and less chromatic), and (c) Proof_3 failed (50C patch was darker and less chromatic). Vendor_C - (a) Proof_1 achieved Level B deviation conformance (both 100C and 100Y patch were darker and less chromatic), (b) Proof_2 achieved Level C deviation conformance (50Y patch was less chromatic), and (c) Proof_3 achieved Level C deviation conformance (100Y and 50Y patches were less chromatic; 100C and 50C patches were less chromatic). Vendor_D - (a) Proof_1 and Proof_2 achieved Level A in deviation conformance, and (b) Proof_3 achieved Level B in deviation conformance. 3b. Visual assessment Vendor_A Vendor_B Vendor_C Vendor_D Proof_1 A B B A Proof_2 A C C A Proof_3 A F C B When the three proofs from Vendor_A and the offset print (reference) were placed randomly in the ISO 3664 (2009) compliant viewing booth, several observers were asked to pick a proof that was most different in color appearance than the print reference. The SWOP conformed Proof_1 was picked all the time. This was because the yellowness of Proof_1, made from SWOP3 as the source profile, is most noticeable. When asked again, observers were likely to place Proof_2, made from SWOP3_SCCA(M0) dataset, between Proof_1 and the offset print. Finally, Proof_3, made from SWOP3_SCCA(M1) was placed next to the offset print (Figure 1). Figure 1. Simulation of the three proofs and the reference print 3

The procedure, described below, was used to simulate the appearance of the three proofs and the reference print (Figure 1): (a) Assign the legacy CMYK file (Bar Set) to four ICC profiles: i. The SWOP3 ICC profile was assigned to the image (far left) and renamed as a tagged CMYK file, Proof_1. ii. The SWOP3_SCCA(M0) ICC profile was assigned to the image (second from the left) and renamed as, Proof_2. iii. The CMYK_SWOP3_SCCA(M1) ICC profile was assigned to the image (third from the left) and renamed as, Proof_3. iv. The Goss_Opus (M1) derived ICC profile was assigned to the image (far right) and renamed as Print. (b) These tagged files were converted to the Adobe RGB color space using the absolute colorimetric rendering intent, and placed in the Microsoft PowerPoint. (c) A screen capture of the PowerPoint (Figure 1) was placed in the Word document. If displayed in a calibrated display, these images resemble the hardcopy proofs and the offset print, including color of the substrates, closely. If displayed in a calibrated display, these images resemble the hardcopy proofs and the offset print, including color of the substrates, closely. 3c. White point analysis The white point of the three proofs (measured in M0) and the offset print (measured in M1) help explain the result of the visual assessment. Vendor_A s data, shown in Table 3, indicates that there is a color difference of 6.20 E 00 between the unbrightened (non-oba) Proof_1 substrate and the brightened (OBA) offset print. The color difference is visually seen as colorcast. Table 3. White point comparison between Vendor_A s proofs and the print Vendor_A Proof:White:Point:(M0) Printing:Paper:(M1) L* a* b* L* a* b* Proof_1 92.31 /0.25 /0.02 Proof_2 92.83 1.71 /3.87 93.4 1.7 /6.5 Proof_3 92.33 1.87 /4.75 E00 6.20 2.22 1.63 The color difference of the white point reduced to 2.22 E 00 between Proof_2 and the print, and to 1.63 E 00 between Proof_3 and the print. We can conclude that the colorcast reduction is the result of substrate-corrected colorimetric aims. The lesson learned is that color management can match colorimetrically specified color provided that the color is within the gamut of the destination color space. In this instance, Vendor_A s proofing system has done all it can to render Proof_3 with a bluish paper (-4.75 b*). This, being the proof that best resembles the offset print, still leaves the substrate as one of the largest color difference in relation to the SCCA dataset because of gamut clipping. Appendix A lists the five largest color differences ( E 00 ), out of 1,617 colors, between a proof and a print by vendor. When there is visual difference between a proof and a print, viewed side by side, a major cause is the color difference between the two white points, i.e., substrate colors. Based on observations from the Appendix, a capable color proofing system can (1) conform to CGATS TR016 Level A tolerance, and (2) simulate the white point of the source substrate closely (less than 2 E 00 ). 3d. Grey ramp analysis CGATS TR 015 (2011), based on a set of specific CMY triplets (also known as G7 grayscale), and an ink-paper-press condition (substrate color, CMY solid, and K solid), specifies tone reproduction and gray balance aims for that printing condition. When G7 adjustment curves are applied in raster image processing prior to CTP, different printing conditions will render the pre-defined CMY triplets the 4

same, thus, makes the shared neutral appearance of printed colors possible across multiple substrates and multiple printing conditions. We did not include the G7 triplets in the test form. However, we can simulate the gray reproduction of the G7 triplets using the A-to-B LUT of an ICC profile and the ColorThink Pro, a color management utility software. Figure 2 shows the gray reproduction of the SWOP3_SCCA(M1) reference (in dotted) versus the offset print (solid) measured under M1. Note that (a) there was an error in a* rendering of the gray ramp, and (b) the gray reproduction of the offset print deviates more towards the three-quarter tone. 10.0% 8.0% 6.0% SWOP3_SCCA_a*% Alpha7_CMY_M1_a*% SWOP3_SCCA_b*% Alpha7_CMY_M1_b*% 4.0% 2.0% a*# 0.0% b*# 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%!2.0%!4.0%!6.0%!8.0%!10.0% %dot#(c)# Figure 2. Grey ramp between the SWOP3_SCCA reference (dotted) and offset print (solid) Figure 3 compares grey ramp between the SWOP3_SCCA(M1) reference (dotted) and Vendor_A s Proof_1 (solid). Note that when Proof_1 ignores the printing paper, the starting point of the grey ramps are misaligned, and is perceived as a yellow cast in comparison with the brightened offset print. 10.0% 8.0% 6.0% A_proof1_a*% SWOP3_SCCA_a*% A_proof1_b*% SWOP3_SCCA_b*% 4.0% 2.0% a*# 0.0% b*# 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%!2.0%!4.0%!6.0%!8.0%!10.0% %dot#(c)# Figure 3. Grey ramp between SWOP3_SCCA reference (dotted) and Proof_1 (solid) Figure 4 compares the grey ramp between SWOP3_SCCA reference (dotted) and Vendor_A s Proof_2 (solid). Note that Proof_2 was aimed at matching the white point of the printing paper, measured by M0. Thus, the yellow cast had diminished in comparison with Proof_1. 5

10.0% 8.0% 6.0% A_proof2_a*% SWOP3_SCCA_a*% A_proof2_b*% SWOP3_SCCA_b*% 4.0% 2.0% a*# 0.0% b*# 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%!2.0%!4.0%!6.0%!8.0%!10.0% %dot#(c)# Figure 4. Grey ramp between and SWOP3_SCCA reference (dotted) and the Proof_2 (solid) Figure 5 compares grey ramp between Proof_3 (solid) and SWOP3_SCCA (dotted). Not that Proof_3 is aimed at matching the white point of the printing paper, measured by M1. Despite the gamut clipping in the highlight region, it visually matches the brightened offset print the best. 10.0% 8.0% 6.0% A_proof3_a*% SWOP3_SCCA_a*% A_proof3_b*% SWOP3_SCCA_b*% 4.0% 2.0% a*# 0.0% b*# 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%!2.0%!4.0%!6.0%!8.0%!10.0% %dot#(c)# Figure 5. Grey ramp between SWOP3_SCCA reference (dotted) and the Proof_3 (solid lines) The grey ramp analyses further indicate that (1) when printing and proofing to the same substratecorrected dataset under M1 measurement conditions, visual agreement between proof and print in the ISO 3664 compliant viewing condition improves, (2) gamut clipping, e.g., paper and light cyan, causes colorcast, and (3) using a source ICC profile, constructed from a specific press sheet, instead of the substrate-corrected dataset, will further improve the proof-to-print visual match under the influence of OBA. The lessons learned are that SCCA (substrate corrected colorimetric aims) is the adjustment of the color characterization data to account for the colorimetric difference between production paper and the reference substrate. When SCCA is applied to printing conformance, it reconciles the color difference between the dataset white and the paper white. When SCCA is applied to proofing using M1 measurement, proof matches print closely under the influence of OBA. One caveat is that there should be no gamut clipping, particularly in the highlight region. In this case study, when the color difference between the OBA substrate and the dataset white point is greater than 6 E 00, the user is cautioned that proof and press match may be compromised due to gamut clipping. 4. Conclusions OBA is a blessing to print buyers who want brighter and colorful appearance of printed colors while controlling costs. OBA is a variable in paper that affects the appearance of printed color that causes two concerns to printers, i.e., print-to-numbers and proof-to-print match. If the RIT thesis was a proof of concept (Carlos Carazo, 2012), this case study is a feasibility study to see if the problems of print-to-numbers and proof-to-print match can be implemented by today s color proofing 6

technologies. In conclusion, printer can meet the challenges of printing on OBA papers that his/her customers prefer and, at the same time, print-to-numbers and proof-to-match-print with the following key steps: Use M1 color measurement mode (ISO 13655, 2009) for press sheet measurement. Use substrate-corrected printing aims to verify printing conformance (CGATS TR 016, 2012). Use substrate-corrected dataset as proofing aims. Use ISO 3664 (2009) compliant viewing booth to assess proof-to-print visual match. Acknowledgments The author wishes to thank the following organizations, listed in alphabetical order, for their participation and supports in this project: CGS, Epson America, Just, Global Graphics, GMG, GTI, Konica-Minolta, and X-Rite. The author also wishes to thank Mr. David McDowell (CGATS) and Mike Rodriguez (CGATS) for their reviews and comments, and the RIT PSA (Printing Standards Audit) team for their continuing supports and encouragement. Literature Carlos Carazo (2012), Print-to-Proof Visual Match Using Papers with Optical Brightening Agents, an RIT School of Print Media thesis CGATS TR 015 (2011) Graphic technology Methodology for Establishing Printing Aims Based on a Shared Nearneutral Grey-scale CGATS/TR016 (2012) Graphic technology Printing Tolerance and Conformity Assessment ISO/DIS 12647-1 (2012) Graphic technology Process control for the production of half-tone colour separations, proof and production prints Part 1: Parameters and measurement methods ISO 13655 (2009), Graphic technology Spectral measurement and colorimetric computation for graphic arts images ISO/DIS 15339-1 (2012) Graphic technology Printing from digital data across multiple technologies Part 1: Principles and characterized reference printing conditions ISO/TC130/WG3 N1259 (2012) Conformance to Substrate-corrected Dataset, a Case Study, Bob Chung 7

Appendix A. Five Largest E 00 Patches When there is visual difference between a proof and a print, viewed side by side, a major cause is the color difference between the two white points, i.e., substrate colors. This Appendix lists the five largest color differences ( E 00 ), out of 1,617 colors, between a proof and a print by vendor. Vendor_A These patches (Table A1) include outer gamut patches with 100K overprints in Proof_1 and Proof_2, and paper (Patch 1367) in Proof_3. The latter means that the bluish paper white of the source in Proof_3 is outside of the proofer gamut. Table A1. The five largest E 00 patches in Vendor_A proofs Vendor_A Patch.ID C M Y K E00 1271 0 100 40 100 1.95 Proof_1. ( Proof_2. (M0.SCCA. Proof_3. (M1.SCCA. 1281 40 0 100 100 2.42 1273 40 40 40 100 2.66 1261 0 40 0 100 3.20 1262 0 100 0 100 3.98 1265 40 100 0 100 2.32 1282 40 40 100 100 2.61 1247 0 100 100 80 2.64 1273 40 40 40 100 2.91 1262 0 100 0 100 4.31 1367 0 0 0 0 1.74 1399 100 85 85 100 1.75 1284 100 0 100 100 1.98 1268 100 100 0 100 2.04 1269 0 0 40 100 2.12 Vendor_B - These patches (Table A2) include patches with 100C and yellow overprints in Proof_1; paper (Patch 1367 and Patch 1) and light cyan patches in Proof_2 and Proof_3. Again, the bluish white point of the print is clipped by the limitation of the proofer gamut. Table A2. The five largest E 00 patches in Vendor_B proofs Vendor_B Patch.ID C M Y K E00 640 100 0 85 0 2.02 Proof_1. ( Proof_2. (M0.SCCA. Proof_3. (M1.SCCA. 649 0 0 100 0 2.02 568 0 0 85 0 2.13 478 100 0 55 0 2.28 479 100 10 55 0 2.35 1306 2 0 0 0 3.22 1367 0 0 0 0 3.22 1305 3 0 0 0 3.25 1304 5 0 0 0 3.38 1303 7 0 0 0 3.41 1367 0 0 0 0 3.98 1305 3 0 0 0 4.02 1306 2 0 0 0 4.08 1 0 0 0 0 4.09 1303 7 0 0 0 4.11 Vendor_C - These patches (Table A3) include yellow highlight patches in Proof_1; paper (Patch 1367 and Patch 1) and cyan highlight patches in Proof_2 and Proof_3. 8

Table A3. The five largest E 00 patches in Vendor_C proofs Vendor_C Patch.ID C M Y K E00 173 10 10 20 0 2.29 Proof_1. ( Proof_2. (M0.SCCA. Proof_3. (M1.SCCA. 1338 0 0 30 0 2.30 1416 0 3 3 0 2.30 1488 0 0 20 10 2.35 1262 0 100 0 100 2.51 1302 10 0 0 0 4.89 1 0 0 0 0 5.11 1367 0 0 0 0 5.14 1305 3 0 0 0 5.19 1306 2 0 0 0 5.30 10 10 0 0 0 6.30 1305 3 0 0 0 6.34 1306 2 0 0 0 6.44 1 0 0 0 0 6.51 1367 0 0 0 0 6.56 Vendor_D - These patches (Table A4) include lighter patches with two or more overprints. It is interesting to note that paper white is not in the five largest Es. Table A4. The five largest E 00 patches in Vendor_D proofs Vendor_D Patch.ID C M Y K E00 1577 40 40 10 10 1.81 Proof_1. ( Proof_2. (M0.SCCA. Proof_3. (M1.SCCA. 50 55 40 0 0 1.91 1457 40 40 3 3 1.98 384 70 55 40 0 2.00 131 55 40 10 0 2.01 281 40 10 30 0 1.63 864 70 20 40 20 1.64 632 70 10 85 0 1.64 1432 0 0 7 7 1.74 815 20 10 20 20 1.79 1101 20 0 20 60 1.72 1516 10 10 0 10 1.73 1487 0 0 10 10 1.75 553 85 30 70 0 1.85 1512 10 0 10 10 1.97 Based on observations from Table A1 A4, a capable color proofing system can (1) conform to CGATS TR016 Level A tolerance, and (2) simulate the white point of the source substrate (less than 2 E 00 ). 9