Layman's guide to ISO print standards

Picture yourself at a dinner party with a collection of ISO printing standards people. They converse in a different language. Every sentence they speak is fully formed, is at least forty words long and is laced with phrases that normal people seldom use, like “characterization data set”, and “standardized printing conditions”. These folks often can be heard arguing about the distinction between “shall” and “should”.

Typical standards committee meeting

But the most unnerving part is they keep throwing four and five digit numbers around. They standards folks say that these are the numbers of the individual standards, but I understand them for what they are. These numbers are the passwords that can get you into these elite dinner parties. This paper will allow you to get past the gatekeeper, although I can’t promise that the party will be all that exciting!

The key standard

One standard is the mother of all the standards for measurement of color in the graphic arts world: ISO 12647. This standard defines how printing should be done. In terms of color measurement, there are two key specifications in this standard: First, there are target values for the color of the paper, the solids (C, M, Y, and K), and for the overprints, all measured as CIELAB values. Tolerances are given for these in terms of ΔE (delta E). The second color-related specification is for dot gain (they call it TVI, or Tone Value Increase). Again there is a target value and a tolerance.

There are several parts to this standard that refer to different types of printing. There are parts of the standard that pertain to web offset (part 2), newspaper (part 3), publication gravure (part 4), screen printing (part 5), flexo printing (part 6), and digital proofing devices (part 7).

This standard refers to a lot of other standards for support. To be compliant to ISO 12647, all of the other pieces must be adhered to. ISO 12647 references other standards that cover ink manufacture, viewing booths, and color measurement.

Ink manufacture

In order to comply with 12647 printing, you must use inks that comply with the ink standard, ISO 28461. This standard describes the target colors (CIELAB value) for each of the process inks, as well as a host of other properties. Like 12647, the ink standard has multiple parts for different types of printing.

Viewing booths

The color you see when you look at a print depends on the light that is shining on the print. A proof and a press sheet may match outdoors under sunlight, for example, but not in your living room under incandescent lamps. So, in order to assess whether there is a match, you must standardize on the illumination in the viewing booth. ISO 3664 defines this.

Color measurement

There are two key standards that cover color measurement, one of which is more or less irrelevant. The earliest of these color standards is ISO 5. (Note the low number!) This defines how a densitometer measures ink on paper. Years ago, when the color of print was specified in terms of density, ISO 5 was a critical standard.

Density is simple and easy to understand. Unfortunately, a density value does not uniquely define a color, so it is somewhat lacking when it comes to specification of color. Because of this, the mother of the print standards, ISO 12647, defines the color of patches in terms of CIELAB values instead of density.

This does not mean that density is unimportant. In fact, ISO 12647 recommends (but does not mandate) that the printer establishes a target density value for every combination of printing ink and substrate. With that target, density can then be used for process control. Density may not be used to demonstrate compliance.

Next we have ISO 13655, which defines how a color measurement device works, and how to compute color values. This second part might be a bit of a surprise, since there are two other standards for computing CIELAB values – CIE 15, and ASTM 2244.

Why does ISO 13655 need to define the computations?  CIE 15 is very broadly defined. It is like a set of Lego blocks that can be combined into whatever sort of color measurement is appropriate for a given application. The ASTM document was written so as to narrow down the choices, but it still leaves the reader with the choice between 72 different ways to calculate CIELAB from spectral data. ISO 13655 picks just one of these as the way to compute CIELAB in the graphic arts.

The problem with optical brighteners

The committee that writes the standards for printing (ISO Technical Committee 130) has recently been wrestling with the print assessment issues revolving around the use of optical brighteners. These are perhaps more accurately termed fluorescent whitening agents, but the acronym OBA (Optical Brightening Agent) seems to have stuck.

The use of OBAs to make paper white has increased steadily to the point where today, it is difficult to find paper without OBAs. This is a good thing in that a brilliant white paper can be manufactured cheaply, but not so good in that it causes problems with assessment of color. The brightness of the paper depends on how much ultraviolet light hits the paper. The larger the UV component in a viewing booth, the bluer the paper appears. The larger the UV component in the spectrophotometer light source, the bluer the paper is measured.

This has become something of an issue since, in the past, the standards for UV content in illumination (either in a viewing booth or in a spectrophotometer) have been somewhat loose. This meant that, if the proof and press sheet have different amounts of OBAs, they may match in some viewing booths and not in others. One spectro may say they match, and another may not.

Recent changes to the viewing booth standard (3664), the spectrophotometer standard (13655), and the printing standard (12647) have addressed this issue. They have more precisely defined the UV content of standard illumination so that all viewing booths and spectrophotometers will agree as to whether there is a match.

The transition for viewing booths has been relatively simple. For a viewing booth, replacement bulbs are widely available.

For a spectro, the road is not as smooth. The newest version of 13655 (from 2009) defines several so-called “conditions”, with the most relevant ones being M0 and M1. The M0 condition covers basically all existing spectros. The M1 condition is met when the illumination of the spectro provides a specific amount of UV light. The print standard (ISO 12647) has been updated to describe M1 as the preferred condition, with M0 also allowed.

Spectros that meet the new preferred standard “M1 condition” are not yet widely available, and it is likely that it will be expensive or impossible to retrofit old spectros from their current “M0 condition” to “M1”. This transition will likely be slow, since spectro owners will likely not be real keen on the idea of spending multiple thousands of dollars to get the M1 spectro, and updating all their internal standards and legacy data. I will be moderating a session at GraphExpo on October 8, 2012 about this subject.

In the works

The standardization of printing in 12647, with target colors of solids and overprints, and TVI for the halftones represents good process control. It is also a good first step toward making sure that a job is printed as expected. Most printing today goes a step further by using ICC profiles to set target CIELAB values for all combinations of inks. So long as the values in the ICC profile agree with the targets in 12647, this is not a problem. General ICC profiles are available from a number of places (like Idealliance, Fogra, and IFRA), but there are unfortunately no ISO standard profiles. None of the profiles are “international”.

Another issue with the printing standard for web offset (ISO 12647-2, 2004 version) is that the color of the paper is specified and has a tolerance. As paper color has gradually changed over time, the paper that is provided to the printer may or may not meet this requirement. To a lesser extent, the color of the paper has an effect on the color of the solids, so meeting the CIELAB values of these is also difficult.

A new standard is being developed that will address these issues. The key feature of ISO 15339 is that will include a set of data from which profiles can be built. There are currently seven of these, going from the smallest gamut, meant to apply to coldset newsprint on up to the largest gamut which encompasses digital printing and whatever else may be developed.

The printing standard under development also addresses the issue of the color of the paper. The current draft of 15339 has a bit more leeway in the color of the paper, and provides a way to adjust all the color targets based on a change in paper.

Summary

There are a number of key standards in the graphic arts when it comes to color. ISO 12647 is the big standard, since it standardizes everything about a print job, including color. Printing to ISO 12647 entails adherence to what is in this standard and also what is in the standards that 12647 references. The key standards that are referenced are ISO 3664 (for viewing booths) and ISO 13655 (for spectros). Knowing those three numbers can help you navigate through the maze of printing standards. (1) - The 2004 version of 12647 does not actually demand that the inks comply with ISO 2846. The current draft of the revised version does require this.