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”.
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Typical standards
committee meeting
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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!
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The key standard
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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.
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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).
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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.
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Ink manufacture
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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.
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Viewing booths
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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.
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Color measurement
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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.
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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.
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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.
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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.
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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.
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The
problem with optical brighteners
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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.
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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.
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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.
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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.
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The
transition for viewing booths has been relatively simple. For a viewing
booth, replacement bulbs are widely available.
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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.
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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
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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”.
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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.
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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.
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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.
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Summary
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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. |
Wednesday, September 5, 2012
Layman's guide to ISO print standards
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Although the standards provide good starting points, I find in my circle of work that printers are working more independently. Closed loop. You don't know what you have until you see it on their proof, correct in prepress and take that to press. Also, there are no standard proofing stocks being used. Every shop has it's own color. Good enough for most; can be difficult for the rest of us. Good info.
ReplyDeleteInteresting To reading this article .Thanks for sharing the information.
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