Thursday 27 March, 2014

Seven pillars of wisdom -- an update on print quality standards

The question, whether process control data or color characterization data should determine printing aims and tolerances, has come in the forefront of ongoing efforts to develop universal print quality standards. And it looks like it is dividing those involved more than ever.

While the general secretariat of TC 130, ISO’s technical committee in charge of graphic technology, has shifted from Germany’s national standards organization DIN to the Standardisation Administration of China, its working groups met several times over the past twelve months. The most controversial discussion was and still is the relationship between the ISO 12647 and ISO 15339 sets of standards. WG3, the TC 130 working group looking into processes and metrics, is increasingly polarized between those primarily interested in the continual development of the ISO 12647 process control standard and those moving towards the process-independent ISO 15339 standard first drafted in 2010.

Ironically, both sets of standards apply to the separation and printing of our traditionally four process colors CMYK, relevant for full color reproduction mainly in commercial offset and publication printing, whereas packaging, signage and industrial decoration printers increasingly apply larger and specific color gamuts, not just by adding spot colors and special inks but by converting the three or four process colors into PMS as well. These applications require tightly controlled color management, proofing and monitoring procedures. But standards?

At any rate, a long awaited update of the ISO 12647 set of standards has been finalized. The offset and proofing parts, ISO 12647-1 through 3 as well as 7 and 8, have been published in December 2013. Parts 4, 5 and 6, for publication gravure, screen and flexo, are to be published shortly. Corresponding Fogra characterization sets and ICC profiles will be developed and released over the coming years. No agreement could be reached on the image quality measurements required for a separately planned standard for digital printing, ISO 15311. As a consequence, this project has momentarily been downgraded to the status of a prospective standard or, in ISO language, a ‘technical specification.’

Discussions on ISO 15339, the standard-to-be for printing from digital data across multiple technologies, are a bit in a limbo as well. A majority of WG3 participants, mainly the Germans and their European allies, is opposed to linking characterization data to press calibration and criticizes the substrate correction approach and tolerances proposed so far. The Americans decided to push ahead for their part and published their own standard, CGATS 21, largely reproducing the ISO 15339 specifications. New proposals for a three-part adaptation, including principles and procedures, a minimum set of reference characterization data, and a link-up to ISO 12647 specifications, are currently under scrutiny.

What all these standards have in common, is their emphasis on printing conditions. For ISO 12647, a printing condition is a set of primary process parameters describing the conditions associated to a specific output process, including the printing process, substrate type, ink, screening, surface finish, and printing sequence. The specifics of colorant descriptions (color gamuts) and tone value increase (dotgain) targets correspond to the different printing conditions. For ISO 15339, printing conditions are target references based on color characterization data defining the intended relationship between digital input data and the average achievable color gamuts in print.

ISO 12647-2, for sheetfed offset and web heatset, was slightly amended in 2007 to reflect advances in paper and ink developments enabling improved color gamuts. The standard’s present revision defines new TVI curves for periodic (AM) screening, TVI targets corresponding better to the characteristics of CtP plates, a better description of gray balance, and a standard method for processing non-standard substrates. The five paper types (PT) in the old standard have been replaced by eight printing substrates (PS) and their correlating colorant descriptions (CD), covering most common paper types in offset production.

ISO 12647-2:2013 Eight Printing Substrate Categories:
PS1 / CD1 Premium Coated, with one gamut for matt, silk and gloss coated wood-free paper; replaces PT 1 and 2 of the old standard, and the Fogra 39 dataset.
PS2 / CD2 Improved Coated, for MWC and Improved LWC, not covered in the old standard; will replace Fogra 45.
PS3 / CD3 Standard Glossy Coated, for standard LWC in heatset; corresponding to PT3 and Fogra 46.
PS4 / CD4 Standard Matte Coated, for LWC and MFC in heatset; not covered in the old standard; Fogra 41 has been used for this type of paper.
PS5 / CD5 Wood Free Uncoated, for uncoated paper in sheetfed and heatset; corresponds to PT4.
PS6 / CD6 Super Calendered Uncoated, for SC paper; not covered in the previous standard; Fogra 40 has been used for this type of paper.
PS7 / CD7 Improved Uncoated Newsprint, for improved newsprint in heatset presses; not in the previous standard but covered by the Fogra 48 dataset.
PS8 / CD8 Standard Uncoated, for standard newsprint in heatset, currently covered by Fogra 42.

Seven pillars of wisdom
The ISO 15339 standard defines achievable color gamuts in printing conditions ranging from offset or gravure printing on premium coated paper to coldset printing on newsprint, while adding an extra large gamut for digital printing and special applications. Acknowledging that there is significant overlap of process, substrate and ink combinations, a choice was made for intermediate gamuts representing seven widely used printing conditions, defined by reference characterization data specified in the standard. The standard includes a method to change the data in case the printing substrate to be used has a color that differs from the reference printing condition.

Because these data are to be used as the reference for any printing process, they are not aligned with TVI and trapping settings associated with a specific process. According to the draft standard, they need to be ‘middle-of-the-road’ values representing a compromise between all processes: ‘in effect, virtual printing on a virtual printing system.’ The seven characterized reference printing conditions (CRPC) proposed for the ISO 15339 standard have been adopted in the USA as their own CGATS 21, and the initial datasets and profiles have been made available for testing on the CGATS and ICC websites, respectively. (http://www.npes.org/programs/standardsworkroom.aspx and http://www.color.org/registry/index.xalter)

ISO 15339 Seven Reference Printing Conditions
RPC0 (CGATS 21-2-7) Extra Large, for digital printing, special inks, and potentially other large gamut printing processes.
RPC1 (CGATS 21-2-6) Premium Coated, large gamut using sheetfed offset or gravure; corresponding to GRACoL #1-2 coated paper types.
RPC2 (CGATS 21-2-5) Publication Coated, for magazine publication printing; corresponding to SWOP #5 coated.
RPC3 (CGATS 21-2-4) Super Calendered, for general printing on super calendered paper.
RPC4 (CGATS 21-2-3) Premium Uncoated, for utility printing on matt uncoated paper.
RPC5 (CGATS 21-2-2) Heatset News, moderate gamut for heatset or similar printing on improved newsprint.
RPC6 (CGATS 21-2-1) Coldset News, small gamut for coldset, flexo or letterpress printing on newsprint.

Although some experts argue that creating color characterization sets for reference printing conditions would be easier than developing process-specific press targets, and could thus be more accessible for printers, others are more concerned with the different output results of the standards existing so far. Harmonization of printing aims between the different standards, each with their own and conflicting approaches, still appears to have a long way to go.

Substrate corrected colorimetric aims (SCCA) for new and non-standard paper categories, with or without optical brighteners, also remain an unresolved question. Standards may be useful for equipment and material manufacturers, as well as for print buyers, but they have to be reasonably and reliably applicable for printers as well. A group of paper manufacturers heavily involved in the development of the ISO standards itself, comprising Norske Skog, Sappi, Stora Enso and UPM, commented: “We develop papers for customers, not for standards.”

In the meantime, who can blame the printers who apply their own standards – mostly with device link profiles, standard software, and the rule-of-thumb Delta-E 3 trick. Nevertheless, some 2,800 printing houses and 1,000 individuals worldwide have had the output quality of their printing processes certified one way or another already. Some of these certifications or qualification proofs are referring to the ISO 12647 series, such as RIT’s PSA scheme, the Process Standard Offset, Digital and Gravure (PSO, PSD, PSG) procedures of Fogra, Ugra and its European partners, Japan Color and equipment suppliers like Heidelberg, Esko and X-Rite. The majority, more than 65%, refer to less ‘universal’ standards and specifications such as SNAP for coldset, FIRST for flexo, and IDEAlliance’s SWOP, GRACoL and G7, all largely based on CGATS standards and guidelines developed in the US.

Ron Augustin, March 2014 issue Indian Printer and Publisher

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